U.S. patent application number 11/921362 was filed with the patent office on 2010-03-25 for grid responsive control device.
Invention is credited to David R Hirst.
Application Number | 20100072817 11/921362 |
Document ID | / |
Family ID | 34835108 |
Filed Date | 2010-03-25 |
United States Patent
Application |
20100072817 |
Kind Code |
A1 |
Hirst; David R |
March 25, 2010 |
Grid responsive control device
Abstract
A load control device which is responsive to a physical variable
representing the balance between load and generation on an
electricity grid. The control device varies the energy consumption
of the load based on the current value of the physical variable of
the grid relative to a central value of that physical variable,
which is derived from past readings of the physical variable of the
grid. The grid responsive control device also takes into account
the time since the load last varied its energy consumption in
determining whether or not the grid variable load control should be
provided.
Inventors: |
Hirst; David R; (Brighton,
GB) |
Correspondence
Address: |
ALIX YALE & RISTAS LLP
750 MAIN STREET, SUITE 1400
HARTFORD
CT
06103
US
|
Family ID: |
34835108 |
Appl. No.: |
11/921362 |
Filed: |
June 1, 2006 |
PCT Filed: |
June 1, 2006 |
PCT NO: |
PCT/EP2006/005252 |
371 Date: |
November 30, 2007 |
Current U.S.
Class: |
307/31 ; 700/295;
700/296 |
Current CPC
Class: |
H02J 2310/14 20200101;
H02J 3/14 20130101; Y02B 70/30 20130101; H02J 3/24 20130101; Y04S
20/242 20130101; Y02B 70/3225 20130101; Y04S 20/222 20130101 |
Class at
Publication: |
307/31 ; 700/296;
700/295 |
International
Class: |
H02J 3/14 20060101
H02J003/14; G06F 19/00 20060101 G06F019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 3, 2005 |
GB |
0511361.8 |
Claims
1-26. (canceled)
27: A control device for controlling an energy consumption of a
load on an electricity grid, said control device comprising: means
for sensing over a period of time values of a physical variable of
the grid, said physical variable varying in dependence on a
relationship between electricity generation and load on the grid;
means for determining a historically based value of the physical
variable of the grid from past readings of said values of the
physical variable of the grid; and means for increasing or
decreasing the energy consumption of said load, said varying
dependent upon a current physical variable of said grid, relative
to said historically based value.
28: The control device of claim 27, comprising: means for
increasing or decreasing the energy consumption of said load when a
current value of said sensed physical variable of the grid reaches
a trigger value; and means for determining said trigger value, said
determining of said trigger value dependent upon said historically
based value, and wherein a) said means for determining said trigger
value comprises means for randomly providing said trigger value
between a determined upper or lower value of the physical variable
of the grid and said historically based value, and/or wherein b)
said control device comprises means for sensing a value of a
physical variable of the load, said physical variable
representative of the energy stored by the load; said determining
of the trigger value further dependent upon said sensed physical
variable of the load.
29: A control device for controlling the energy consumption of a
load on an electricity grid, said control device comprising: means
for sensing a value of a physical variable of the grid, said
physical variable varying in dependence on a relationship between
electricity generation and load on the grid; means for sensing a
value of a physical variable of the load, said physical variable of
the load representative of the energy stored by the load; means for
varying the energy consumption of said load when a value of said
physical variable of the grid reaches a trigger value; and means
for determining the trigger value, said determining of the trigger
value dependent upon said sensed physical variable of the load and
further based upon a random value.
30: The control device of claim 29, wherein said means for varying
comprises means for comparing said trigger value with the current
sensed physical variable of the grid.
31: The control device of claim 29, wherein the means for varying
the energy consumption of the load is configured to vary the energy
consumption of the load so as to maintain the sensed physical
variable of the load within central limits and is further
configured to vary the energy consumption when said value of the
physical variable of the grid reaches the trigger value.
32: The control device of claim 31, said means for determining the
trigger value configured to determine the trigger value in
dependence on the value of the sensed physical variable of the load
relative to its minimum or maximum values.
33: The control device of claim 31, said means for determining said
trigger value comprising means for defining a trigger value profile
varying with said physical variable of the load, said profile such
that the more recently the energy consumption of the load has
varied the further the trigger value is from a historically based
value from past readings of the physical variable of the grid.
34: The control device of claim 29, said means for determining said
trigger value comprising means for defining a trigger value profile
varying with said physical variable of the load, said profile being
influenced by a random value.
35: The control device of claim 29, wherein said sensed physical
variable of the grid is a sensed frequency of the grid.
36: The control device of claim 29, said means for varying
configured to vary said energy consumption by switching the energy
consumption between a first state of increasing the energy stored
by the load and a second state of decreasing the energy stored by
the load.
37: A control device for controlling an energy consumption of a
load on an electricity grid, said control device comprising: means
for determining a random delay; means for delaying the starting of
energy consumption of said load by a randomly generated amount of
time after power is initially provided to the control device.
38: A control device for controlling an energy consumption of a
load on an electricity grid to maintain a physical variable of the
load within upper and lower limits, said control device comprising:
means for sensing the physical variable of the load; means for
providing the upper and lower limits of the sensed physical
variable of the load; and means for increasing the upper and/or
lower limit of the sensed physical variable at a rate less than a
maximum energy consumption of the load after power is initially
provided to the control device.
39: A method of controlling an energy consumption of a load on an
electricity grid, said control device comprising: sensing over a
period of time values of a physical variable of the grid, said
physical variable varying in dependence on an relationship between
electricity generation and load on the grid; determining a
historically based value of the physical variable of the grid from
past readings of said values of the physical variable of the grid;
and increasing or decreasing the energy consumption of said load,
said varying dependent upon a current physical variable of the grid
relative to said historically based value.
40: The method of claim 39, comprising: varying the energy
consumption of said load when a current value of said sensed
physical variable of the grid reaches a trigger value; and
determining said trigger value, said determining of said trigger
value dependent upon said central value.
41: The method of claim 40, said determining said trigger value
comprising a function for randomly providing said trigger value
between a determined upper or lower value of the physical variable
of the grid and said central value.
42: The method of claim 40, comprising sensing a value of a
physical variable of the load, said physical variable
representative of the energy stored by the load; said determining
of the trigger value further dependent upon said sensed physical
variable of the load.
43: A method of controlling the energy consumption of a load on an
electricity grid, said method comprising: sensing a value of a
physical variable of the grid, said physical variable varying in
dependence on a relationship between electricity generation and
load on the grid; sensing a value of a physical variable of the
load, said physical variable of the load representative of the
energy stored by the load; varying the energy consumption of said
load when a value of said physical variable of the grid reaches a
trigger value; and determining the trigger value, said determining
of the trigger value dependent upon said sensed physical variable
of the load, and further based upon a random value.
44: The method of claim 43, wherein said varying comprises
comparing said trigger value with the current sensed physical
variable of the grid.
45: The method of claim 43, wherein the varying the energy
consumption of the load comprises varying the energy consumption of
the load so as to maintain the sensed physical variable of the load
within control limits and further varying the energy consumption
when said value of the physical variable of the grid reaches the
trigger value.
46: The method of claim 43, said determining the trigger value
comprising determining the trigger value in dependence on value of
the sensed physical variable of the load relative to its minimum or
maximum values.
47: The control device of claim 45, said determining said trigger
value comprising defining a trigger value profile varying with said
physical variable of the load, said profile such that the more
recently the energy consumption of the load has varied the further
the trigger value is from a historically based value determined
from past readings of the physical variable of the grid.
48: The method of claim 47, wherein said sensed physical variable
of the grid is a sensed frequency of the grid.
49: The method of claim 48, said varying comprising varying said
energy consumption by switching the energy consumption between a
first state of increasing the energy stored by the load and a
second state of decreasing the energy stored by the load.
50: A method of controlling an energy consumption of a load on an
electricity grid, said method comprising: providing a control
device, said control device determining a random amount of time;
delaying the starting of energy consumption of said load by a
randomly generated amount of time after power is initially provided
to the load.
51: A method of controlling an energy consumption of a load on an
electricity grid to maintain a physical variable of the load within
upper and lower limits, said method comprising: sensing the
physical variable of the load; providing the upper and lower limits
of the sensed physical variable of the load; and increasing the
upper and/or lower limit of the sensed physical variable at a rate
less than a maximum energy consumption of the load after power is
initially provided to the control device.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a means and method for
controlling the balance between supply and generation on an
electricity grid.
BACKGROUND OF THE INVENTION
[0002] A reliable source of electricity is essential for almost all
aspects of modern life.
[0003] Providing reliable electricity is, at present, an enormously
complex technical challenge. It involves real-time assessment and
control of an electricity system consisting of generation, of all
types (nuclear, coal, oil, natural gas, hydro power, geothermal,
photovoltaic, etc.), and load e.g. the appliances, instruments etc.
using the electricity.
[0004] The electricity is supplied over a distribution network
consisting of transmission lines interconnected by switching
stations. The generated electricity is generally `stepped-up` by
transformers to high voltages (230-765 kV) to reduce transmission
losses of electricity (through heating). The generators,
distribution networks and loads comprise an electricity power
grid.
[0005] Reliable operation of a power grid is complex as, at
present, electricity must be produced the instant it is used,
meaning power generation and demand must be balanced continuously.
In existing power management systems, the supply of electricity is
balanced to demand by planning, controlling and coordinating the
generation of electricity.
[0006] Failure to match generation to demand causes the frequency
of an AC power system to increase when generation exceeds demand
and decrease when generation is less than demand.
[0007] In the UK, the electricity boards must maintain a nominal
frequency of 50 Hz and are allowed a variation of .+-.1/2%. In the
US, this nominal frequency is 60 Hz. In some closed loop systems,
such as an aeroplane, the nominal frequency is 400 Hz. The nominal
frequency is the frequency of the AC power that a grid was designed
for and it is intended to keep this frequency controlled and
stable.
[0008] Random, small variations in frequency are normal, as loads
come on and off and generators modify their output to follow the
demand changes. However, large deviations in frequency can cause
the rotational speed of generators to move beyond tolerance limits,
which can damage generator turbines and other equipment.
[0009] The variation in frequency can also damage loads.
[0010] A frequency change of just .+-.1/2% is a large signal in
terms of the precision of modern semiconductor instrumentation.
[0011] There are problems with the present supply-side style
architecture of matching generation to demand. At present, when
extreme low-frequencies can not be dealt with, i.e. demand
out-strips generation, automatic under-frequency load shedding may
be triggered, which takes blocks of customers off-line in order to
prevent a total collapse of the electricity system. This may have
the effect of stabilising the system, but is extremely inconvenient
and even hazardous to the user.
[0012] After a blackout the grid is at a particularly sensitive
stage and recovery is slow. Large generators generally require
other generators to make some power available to start or re-start
it. If no power is available, such generator(s) cannot start. So
grid systems have services, known as "black start" services,
whereby a subset of generation has the capacity to start and
continue generating, even when the rest of the grid is inactive (ie
black). Grid operators have prepared planned sequences for
restoring generation and load. These ensure that the limited
initial supplies are used first to provide communication and
control, then to start up bigger generators, and thereafter the
load is progressively connected to match the increasing
availability of generation.
[0013] The entire process of black start is a fraught one. A
blackout is a very rare event, and not one that can be practiced
except in an actual crisis. Everybody involved is under severe
pressure, and the systems are being operated quite outside their
normal operating range (and sometimes outside their design range).
Every step when load or generation is added is a shock to the
system and the grid can take seconds or minutes to stabilise after
it happens. Prudence would suggest making changes in small
increments. This inevitably slows down the overall process,
prolonging the blackout for those who are still to be
reconnected.
[0014] In order to insure as much as possible against
load-shedding, a power system will be operated at all times to be
able to cope with the loss of the most important generator or
transmission facility (i.e. the most significant single
contingency). Thus, the grid is normally being operated well below
its capacity such that a large random failure does not jeopardise
the system as a whole. This, however, means that the generation is
not operating as efficiently as possible, with a resulting increase
in electricity supply costs.
[0015] High air conditioning and other cooling loads in the summer
and high space heating loads in the winter are a normal cause of
peak-loads. Grid operators, however, use rigorous planning and
operating studies, including long-term assessments, year-ahead,
season-ahead, week-ahead, day-ahead, hour-ahead and real time
operational contingency analyses to anticipate problems.
[0016] The unexpected can still occur, which is why the system
operates with headroom for compensating for the largest
contingency. Utilities can use additional peaking generators, which
have high running cost, to provide additional electricity when
needed or, alternatively do not operate main generators at capacity
so as to leave some potential for extra generation to satisfy
excess loads. Both of these methods result in a higher unit cost of
electricity than if the system was operating nearer to
capacity.
[0017] There has been proposed an alternative architecture for
matching load and generation to that presently used. The general
idea is to compensate for differences between load and generation
using the demand-side by way of load management.
[0018] Limited literature exists on the concept of using load, or
demand, to contribute (at least) to grid stability.
[0019] U.S. Pat. No. 4,317,049 (Schweppe et al.) proposes such a
different basic philosophy to existing electric power management in
which both supply and demand of electricity respond to each other
and try to maintain a state of equilibrium.
[0020] This document identifies two classes of usage devices. The
first type are energy type usage devices characterised by a need
for a certain amount of energy over a period of time in order to
fulfil their function and an indifference to the exact time at
which the energy is furnished. Examples were space conditioning
apparatus, water heaters, refrigerators, air compressors, pumps,
etc.. The second class was a power type usage device characterised
by needing power at a specific time. Such devices would not be able
to (fully) fulfil their function if the power was not supplied at a
designated time and rate. Examples include lighting, computers,
TVs, etc..
[0021] The Frequency Adaptive, Power-Energy Re-scheduler (FAPER) of
the Schweppe et al. patent provided its power management by
application of a FAPER to energy type usage devices. The Schweppe
et al. patent particularly discusses application of the FAPER to a
water pump for pumping water into a storage tank.
[0022] The water level in the water tank has a minimum allowable
level Y.sub.min and a maximum allowable level Y.sub.max.
Ordinarily, the water pump will be switched on to pump water into
the storage tank when the level falls to or below the minimum level
and turns the pump off when the maximum level is reached.
Otherwise, the pump is idle.
[0023] The FAPER modifies these limits (Y.sub.max, Y.sub.min)
depending upon the system frequency. Thus, in a period of high
frequency (electricity demand shortage), i.e. when the grid
frequency increases above nominal, the minimum water level causing
the pump to activate (Y.sub.min)is increased and the maximum water
level (Y.sub.max) is also raised. Thus, the pump switches on at a
higher level and stops at a higher level than operation not under
the control of a FAPER. This means that the excess in generation is
being taken up. Using the same principle, as the electricity
frequency drops below the grid nominal frequency (a generation
shortage), the minimum and maximum water levels are lowered. This
lowering results in ON pumps being switched off sooner and OFF
pumps coming on later than usual, and so using less electricity,
thereby reducing the load.
[0024] According to Schweppe, the raising of the limits
(particularly the maximum) and the lowering of the limits
(particularly the minimum) should have an extent cap, defined by
either user desires or safety requirements. Thus, the limits should
be extendable, but only to a certain extent, as otherwise the tank
could unacceptably empty or overflow.
[0025] The broad concept uncovered by Schweppe in this patent is
that consuming devices, which incorporate some sort of energy store
and operate to a duty cycle, are useful in providing grid
responsive behaviour. When running, the energy store is being
replenished or filled and, thus, the potential energy of the store
is increasing. When the devices are not running, their function is
preserved because of the load's ability to store energy.
[0026] The FAPER modifies the timing of the load's consumption,
without detriment to the service provided by the device, using the
grid frequency as a guide. Thus, the potential energy of the device
is increased when the grid frequency is high in order to maximise
the amount of energy fed into the device which is stored. This
compensates for any excess. During times of insufficient generation
(high frequency), the potential energy of the device is lowered,
thereby releasing energy to the grid and compensating for the
shortage.
[0027] Moving on from the FAPER, a different and improved
"responsive load system" was disclosed in GB 2361118 by the present
inventor. The responsive load system was based on the same
underlying principle as the FAPER devices, that grid stability can
be at least aided by using demand-side grid response, and built on
the response method and offered a further enhancement of using
probabilistic methods as to the ON/OFF switching timing for the
load.
[0028] One problem with the FAPER device is that, without any
randomisation, the smallest movement of the frequency could result
in all loads with FAPERs applied responding in the same way and
doing so simultaneously. This could result in a destabilising
influence on the grid. A more gradual response is needed and the
responsive load system offered this by distributing the frequencies
to which each device is responsive by using a randomised
function.
[0029] As mentioned above, the responsive load system of GB 2361118
defines a probability based method for choosing the frequency to
which a device is sensitive. In this way, a progressively larger
proportion of the responsive load device population changes the
load as the system frequency departs from the nominal frequency of
the grid.
[0030] In more detail, the responsive load system uses a randomiser
to choose both a high frequency and a low frequency to which the
device is sensitive. This is advantageous over the FAPER device as
more and more load is switched on or off progressively as the
frequency increases or decreases, respectively.
[0031] The random inputs for the high and low frequencies to which
the devices are sensitive are revised from time to time. This step
has the advantage of distributing any disadvantages of responsive
devices among the population and ensuring that no one device was
stuck with unfavourable frequency triggers. For example, it would
not be appropriate if a particular device was constantly sensitive
to the slightest change in frequency whereas another device had
such broad trigger frequencies that it only provided frequency
response in extreme grid stress situations.
[0032] One problem with this system is that the controller is not
tamperproof. Users, such as users of air conditioning, might choose
to turn up their controls because of the slight heating/cooling of
a room beyond that desired as a result of a frequency responsive
load change being noticed. Thus, if the air conditioner is
generating in a lower temperature range, that is the air
conditioner is working harder and is on more frequently, because of
an increase in grid frequency, and a user notices this and turns
the air conditioning down, before the frequency returns to an
acceptable level, then the response has been lost.
[0033] Partially because of the above problem, the Grid Stability
System of UK patent application number 0322278.3 was formed. The
grid stability system prevents an end user from overriding the
frequency response function by fixing the frequency triggers at
pre-grid stress settings. In this way, manipulation of a set point
controller, such as a thermostat, is made ineffective for the
duration of the period of high stress.
[0034] The grid stability system also defines three states of the
system, normal, stress and crisis. The stress level of the grid
determines which of the above three grid states are relevant.
[0035] The stress level of the grid can be determined by comparing
the present grid frequency to limit values for the frequency and
determining whether the current frequency falls inside limits
chosen to represent a normal state, a stressed state or a crisis
state.
[0036] Rapid changes in frequency, whatever their absolute value,
are also used as indicators of grid stress level by defining limits
for the rate of change of the grid frequency.
[0037] The grid stress level can also be indicated by an
integration, over time, of the deviation of the grid frequency from
the nominal grid frequency. Thus, even if the extent of frequency
departure is very small, if it departs for a long enough time, then
a grid stress or crisis condition is still determined.
[0038] The grid status is, therefore, determined, according to the
grid stability system, by taking into account instantaneous large
frequency departures from nominal, rapid changes of frequency and
accumulatively large, but not necessarily outside a preferred
frequency change at any given time, departures all being signs of
grid stress. Each of these possible types of grid indicators has an
associated set of limits that individually or in combination
determine whether the grid is in a normal state, a stressed state
or a crisis state.
[0039] Having determined the status of the grid, that is whether
the grid is in a normal state, a stressed state or a crisis state,
the controller of the grid stabilising system adapts its grid
responsive behaviour depending upon the determined grid status. If
a normal status is determined, the device provides response to
frequency changes in the same way as the original responsive load
device. So, as grid frequency rises above the temperature
determining trigger frequency, off devices will switch on in order
to "take up" the extra generation. In the case that the grid
frequency falls below a low frequency trigger value, "on" devices
will switch off to reduce the load upon the grid.
[0040] If operated according to the FAPER invention, a physical
variable associated with the load (water level, temperature) is
still controlled within minimum and maximum values during this
time, but the limits are extended so that devices switched on and
devices already on will stay on for longer than if the controlled
device was operating within the normal frequency limits. Similarly,
in periods of overly high grid frequency, off devices will stay off
for longer as the lower limit of the physical variable has been
extended as well.
[0041] Using the example of the water tank, as grid frequency
increases above the higher frequency limits, off devices will
switch on and on devices will stay on until the physical variable
reaches its extended limit or until the frequency returns below the
higher frequency limits. If the normal range for the water tank
depth lies between 1 and 1.5 meters, for example, if the grid
frequency rises above the higher frequency limits, off devices will
switch on and on devices will stay on up to an extended water depth
of 1.7 meters, for example. Thus, the potential maximum level of
the water tank has been raised above its normal level. Further, the
potential energy of a population of water pumps controlled in this
way will have increased their average depth of water. This serves
to compensate the excessive generation, which produced the high
grid frequency, and stored the excessive grid energy, which will
compensate, to some extent, the higher frequency. When the
frequency drops below the lower frequency limits, this energy is
repaid to the grid by switching on devices off and keeping off
devices off up to a lower extended physical variable limit of, for
example, 0.8 meters. This allows a large population of devices to
reduce their potential energy and supply the energy difference into
the grid. This serves to compensate for the lack of generation that
resulted in the low frequency.
[0042] If operated according to the responsive load system of
GB2361118, the control limits remained unchanged, but the device
could be switched on or off if the system frequency moved beyond
the frequency to which the device was sensitive. So the device
could be switched before it reached its control limits, and this
extra switching modified the load and so contributed the change of
load necessary to balance the system.
[0043] Using the example of the water tank again, low frequency
would cause an on device to switch off at, for example, 1.4 m and
so earlier than if the limit of 1.5 m was reached, and, conversely,
high frequency would cause the device to switch on at, for example,
1.1 m and so earlier than if the lower limit of 1 meter was
reached.
[0044] Together, these cause the average water level in a
population of devices to become lower when the frequency is low,
and to become higher when the frequency was high, although each
individual device would operate within its control limits.
[0045] The frequency limits for a particular device are chosen to
fall within an upper frequency range and a lower frequency range.
As with the Responsive Load previously discussed, a randomiser is
used to choose the particular high trigger frequency and the
particular low trigger frequency such that a population of devices
have high trigger frequencies and low trigger frequencies
distributed within the upper frequency range and the lower
frequency range, respectively. Thus, a window is provided between
the distribution of high trigger frequencies and low trigger
frequencies. This window is centred around the nominal frequency.
The window allows the controlled load, e.g. a water tank,
refrigerator or air conditioner, to operate entirely as normal,
i.e. as though it did not have a frequency responsive controller
applied to it, when the frequency of the grid is close enough to
the nominal grid frequency to lie within the window. Response is
provided only when the grid frequency extends outside this
window.
[0046] In the case that a stressed state is determined, the control
limits of the device are frozen at pre-stress settings so that
manipulation of a control panel to adjust a set point for the
sensed physical variable (e.g. temperature) is ineffective. Thus,
the user of the controlled load cannot adjust the loads settings,
for example by using a thermostat control. If the responsive device
is controlling an air conditioner, a grid responsive induced change
in room temperature could be noticed. A user may decide to attempt
to counter the change in temperature by adjusting the thermostat.
The responsive load device of the grid stabilising system overrides
such an adjustment of the set point when the grid is determined to
be in a stressed condition. This is important as the grid is
particularly sensitive during a period of grid stress and users
negating the response provided could worsen the destabilisation of
the grid.
[0047] In extreme circumstances, when a risk of blackout exists, a
grid crisis state may be determined. In the grid crisis state, the
grid stabilising system relaxes the control of the physical
variable limits and allows them to move outside of a preferred
range. In a high frequency grid state, the loads are switched on
until the grid crisis state is exited and in a low frequency grid
crisis state, the responsive load (e.g. fridge) is switched off
until the crisis state is exited. The switching on and switching
off is carried out irrespective of the control limits, so a fridge,
for example, could continuously cool down to well below a preferred
minimum or the fridge could be allowed to warm up to an ambient
condition well above a preferred maximum temperature. These extreme
measures are only taken in the most serious of grid conditions,
when the alternative is a blackout.
[0048] Modelling of the prior art frequency and responsive control
devices has uncovered previously unknown problems with the above
described prior art grid responsive loads.
[0049] It has been found that after response has been affected for
a period of time, a population of the devices will tend to approach
the physical variable control limits, and start switching at an
excessive rate. For example, a refrigeration unit controlled by a
frequency responsive device will reach its extended temperature
limits after a sustained period of high or low frequency. Using the
example of a higher than nominal grid frequency, devices will
switch on until the low temperature limit has been reached and will
then switch off, but as soon as the temperature passes back over
the low temperature limit the device will again check whether the
grid frequency is above its higher frequency limits, and if so will
switch on again immediately. This results in very frequent
switching as the device is attempting to provide frequency response
to a unit close to its physical variable limits. This is not
desired behaviour as it could damage the controlled loads.
Excessive oscillating on and off switching of the load will reduce
the lifespan of the device.
[0050] Also, modelling of the prior grid frequency responsive loads
have been found to have an unexpected effect on the grid frequency.
It was assumed that the responsive devices would smooth the grid
frequency to provide a far clearer, less noisy, grid frequency.
This did not, however, entirely bear out during modelling, and some
previously unknown strange behaviour of the grid frequency was
observed as a result of the responsive loads.
[0051] The prior art grid responsive control devices do not provide
any special assistance to a grid recovering from a blackout, but
the stabilising effect of responsive loads are needed more than
ever at this time.
[0052] Amongst other objects, the present invention aims to have an
improved stabilising effect on a power grid.
[0053] The present invention also aims to reduce the switching of
powering of an energy store during operation of a grid responsive
device controlling the powering of the energy store.
[0054] The present invention also aims to aid the grid start-up
after a blackout. In particular, the present invention aims to
soften the shocks to the system during the black start process. The
loads and generators can be reconnected more quickly, so speeding
recovery.
[0055] The device of the present invention also aims to overcome
the above identified problems with prior art grid responsive
control devices.
SUMMARY OF INVENTION
[0056] According to a first aspect, the present invention provides
a control device for controlling an energy consumption of a load on
an electricity grid, said control device comprising: [0057] means
for sensing over a period of time values of a physical variable of
the grid, said physical variable varying in dependence on an
relationship between electricity generation and load on the grid;
[0058] means for determining a central value of the physical
variable of the grid from said values of the physical variable of
the grid; and [0059] means for varying the energy consumption of
said load, said varying dependent upon said central value.
[0060] According to a second aspect, the present invention provides
a corresponding method of controlling an energy consumption of a
load on an electricity grid
[0061] Conventionally, a nominal frequency of the grid and a
current value of the physical variable is used for controlling the
energy consumption of the load. The present invention, however,
uses some function of the past readings of the physical variable.
This gives a long-term past value for the central value and it is
this central value that is taken into account for controlling the
energy consumption of the load. Modelling of the present invention
has shown the use of a historically based central value for
controlling the energy consumption of the load removes the strange
effects on the grid frequency found to exist with prior art grid
responsive control devices.
[0062] The first and second aspects of the present invention can be
used in combination with prior art grid responsive control devices
as discussed above. Alternatively, the preferred form of the
present invention encompasses a comprehensive grid responsive
control device combinable with any of the below described other
aspects of the invention or any of the below described preferred
features.
[0063] In a third aspect, the present invention provides a control
device for controlling the energy consumption of a load in an
electricity grid, said control device comprising: [0064] means for
sensing a value of a physical variable of the grid, said physical
variable varying in dependence on a relationship between
electricity generation and load on the grid; [0065] means for
sensing a value of a physical variable of the load, said physical
variable of the load representative of the energy stored by the
load; [0066] means for varying the energy consumption of said load
when a value of said physical variable of the grid reaches a
trigger value; and [0067] means for determining the trigger value,
said determining of the trigger value dependent upon said sensed
physical variable of the load.
[0068] In a fourth aspect, the present invention provides a
corresponding method of controlling an energy consumption of a load
on an electricity grid.
[0069] The third and fourth aspects of the present invention
control the load based upon a value of the grid variable, which is
selected in dependence upon the variable of the load. Thus, these
aspects of the invention allow the energy consumption of the loads
to be changed in a way that varies with the sensed physical
variable of the load. By taking into account the variable of the
load in this way, the energy consumption of the load can be
controlled to minimise the rate of changes in the energy
consumption of the load. This is so because loads closer to their
natural switching points (which is determined by the variable of
the load) are favoured for grid responsive control.
[0070] Again, the invention provided by the third and fourth
aspects are advantageous when used in combination with prior art
grid responsive control devices. These aspects are especially
advantageous when combined with the first and second aspects of the
invention described above and provide further advantages when
combined with the preferred embodiments detailed below.
[0071] The preferred embodiments described below are applicable as
preferred embodiments of the methods of the present invention or
the apparatus. Thus, the features of the preferred embodiments may
be adapted to include the means of a control device for performing
the feature or may be adapted to comprise method steps. The
preferred features are generally worded in apparatus terms, but are
applicable to all aspects of the present invention.
[0072] In a preferred embodiment of the first and second aspects of
the invention, the control device is adapted to determine a trigger
value of the physical variable of the grid based upon said central
value and to vary the energy consumption of the load when a current
value of the sensed physical variable of the grid reaches the
trigger value.
[0073] The control device may determine a trigger value based upon
just the sensed physical variable of the load or both the sensed
physical variable of the load and the sensed physical variable of
the grid, or just the sensed physical variable of the grid. This
combination of features of the present invention is advantageous as
set out in more detail below.
[0074] According to a preferred form of the aspects of the
invention, the means for determining the trigger value comprises a
function for randomly providing the trigger value between a
determined upper or lower value of the physical variable of the
grid and the central value.
[0075] The control device may also preferably be adapted to
generate a random value and to determine the trigger value based
further upon said random value and to control the energy
consumption of the load dependent upon the trigger value.
[0076] Thus, all aspects of the present invention may
advantageously utilise a random value in determining the trigger
value as this will provide a randomised element to the trigger
value, meaning that a population of loads controlled in this way
will not all change their energy consumption in a synchronised way,
which would destabilise the grid.
[0077] According to a further preferred feature, the control of the
energy consumption of the loads is performed by comparing the
trigger value of the physical variable of the grid with the current
sensed physical variable of the grid.
[0078] In a preferred embodiment, the physical variable of the grid
is a frequency and so it is the frequency of the grid which is
sensed. Alternatively, an amplitude of the supply voltage could be
sensed, which also shows dependence upon the balance between
generation and load of the grid.
[0079] Thus, according to one preferred embodiment of the present
invention a central frequency is determined from past readings of
the frequency of the grid and the control device tends to resist
any change in frequency, up or down, to some extent regardless of
the absolute frequency of the grid. Thus, while in prior art grid
frequency responsive control devices it is the nominal frequency of
the grid which is used as a reference point for determining whether
to provide response, the present invention, differs in using a
historical value, around which the response trigger frequency is
set.
[0080] The basic concept is that even during a period in which the
frequency drops below nominal, if the frequency starts to rise,
then the responsive control device will function to resist this
change, despite the frequency actually moving closer to nominal,
which conventionally was considered favourable.
[0081] During periods of low frequency, the average input energy in
a population of loads drops in order to reduce the energy
extraction from the grid and, therefore, compensates for the
excessive load causing the frequency drop. Energy is, in effect,
being loaned to the grid.
[0082] Ideal behaviour would be to recover this energy, and restore
the total energy store, before the frequency again returns to the
nominal grid frequency. So a frequency rising from a below nominal
value is the most favoured time to repay the energy to the
grid.
[0083] Similarly, but symmetrically, during a period of above
nominal frequency on the grid, the loads are controlled to borrow
energy from the grid in order to take up some of the excess
generation. The favoured behaviour is to return this energy before
the frequency again reaches the grid nominal frequency.
[0084] The behaviour of the control device reinforces the natural
emergent property of grids by which the frequency is an indication
of excesses or deficits of energy in the grid. If the frequency is
low, there is an energy deficit, and if high, there is a surplus.
If the deficit or surplus is largely absorbed by the loads, then
the frequency signal is made clearer.
[0085] The central value of the variable, e.g. frequency, is
preferably provided by calculating a moving average from past
readings of the physical variable of the grid.
[0086] The trigger value is a value, e.g. frequency, at which
responsive control devices will either increase or decrease their
energy consumption and is determined based upon this central value.
Thus, for example, for a population of such control devices, if the
current frequency is above the central frequency, the energy
consumption of the load will tend to increase, and if below, the
energy consumption of the load will tend to decrease.
[0087] A random element is also preferably included in the
determination of the trigger value to ensure that the increasing or
decreasing of loads is gradual so as to not burden the grid with a
population of loads all switching at the same time, thereby
negating the stabilisation object of the control device. Thus,
large scale synchronised switching is avoided.
[0088] The overriding effect of the use of the central value to
determine the trigger value, at which the energy consumption of the
load is changed, means that a population of loads controlled by
such devices, actively and continuously damp grid frequency
variations.
[0089] In a preferred embodiment, the device is further adapted to:
sense a physical variable associated with the load; determine upper
and lower limits for the physical variable associated with the
load; and change the energy consumption of the load when the
physical variable associated with the load reaches its upper or
lower limits.
[0090] This feature ensures the load still performs its main
function, which is to maintain a variable associated with the load
within certain limits. These limits may be derived from a user
selection. For example, the set point of a thermostat for
air-conditioning or a refrigerator setting would lead to limits
being defined. The temperature of the space being conditioned or
refrigerated should not exceed or go outside of these limits. The
temperature is kept around a desired temperature. A refrigerator,
for example, would operate to a duty cycle such that when the
temperature reaches its upper limits, the cooling mechanism of the
refrigerator will be switched on so as to lower the temperature. Of
course, once the temperature reaches its lower limits, the
refrigerator will switch off.
[0091] The majority of the description that follows is concerned
with the loads that control the physical variable of the load
within the control limits by turning the energy consumption on or
off. However, loads in which this control is achieved by increasing
or decreasing the energy continuously are also applicable with the
control device of the claimed invention.
[0092] The preferred embodiment provides two layers of control, the
first is to increase or decrease the energy consumption of the load
to keep the physical variable associated with the load within its
control limits and the second layer is to further control the
energy consumption of the load depending upon relative rises or
falls of the grid variable from a central value.
[0093] As described above, one of the problems with prior art grid
responsive devices was that this two layer control tended to
increase switching rates after a prolonged frequency deviation. The
present invention aims to combat this switching rate increase and
the third and fourth aspects of the present invention, and
preferred embodiments of the first and second aspects of the
invention, are directed to encompass the achievement of this
objective.
[0094] In a preferred embodiment, this objective is also achieved
by the trigger value (or trigger frequency) being based upon the
sensed physical variable of the load. In a preferred form, the
means for determining the trigger value is configured to determine
the trigger value in dependence on the sensed physical variable of
the load and the control limits so as to reduce the rate of
variation of the load.
[0095] In another preferred form, the means for determining the
trigger value comprises a function which returns the trigger value
in dependence upon the sensed physical variable of the load, said
function defining a trigger value profile varying with said
physical variable of the load, said profile such that the more
recently the energy consumption of the load has varied, the further
the trigger value is from a central value of the physical variable
of the grid.
[0096] More specifically, in a further preferred embodiment the
provision of a trigger value (e.g. frequency) is further based upon
a ratio of a value representing said sensed physical variable
relative to the upper or the lower limit of the sensed physical
variable associated with the load to a range between the upper
limit and the lower limit.
[0097] The above defined ratio is an indication of how much energy
is stored in the load compared with the maximum or minimum defined
by the control limits. Again, in the case of a refrigerator, when
the refrigerator has been on for 50 percent of the on portion of
the duty cycle of the refrigerator, then the sensed variable
associated with the load will be half way to its lower temperature
limit or, in other words, the refrigerator is half way to its
maximum input energy. In determining the trigger frequency for the
preferred embodiment, the controlling device takes into account how
full the energy store is and, therefore, how close it is to a
natural switching point.
[0098] In an extension of this embodiment, the trigger value varies
with the ratio such that the likelihood of the energy consumption
of the load being changed increases as the ratio increases. Thus,
the ratio increases depending upon the length of time the load has
been in a particular consumption state. For example, in the case of
a refrigerator, the cooling provision means being in an off state
is one particular energy consumption state and the cooling
provision means being in an on state is another particular energy
consumption state. In a preferred form, a first consumption state
is one in which the energy stored by the load is increasing and a
second consumption state is one in which the energy stored by the
load is decreasing.
[0099] The ratio can be any function representative of how long the
load has been in a particular energy consumption state. Thus, in a
preferred embodiment a ratio is defined representing the length of
time a load has been in a particular energy consumption state
relative to a maximum time for that state. Preferably, this
representation is derived from the physical variable associated
with the load and its upper and lower limits.
[0100] The ratio is defined such that it will increase the longer
the refrigerator is on and is also defined such that it will
increase the longer the refrigerator is switched off. If the
likelihood of the energy consumption state of the load changing
increases as this ratio increases then the switching of the load
between energy consumption states is minimised. This is, as
mentioned before, important for preventing long term damage to the
load equipment, which would be unacceptable to the consumer.
[0101] It is an important feature of preferred embodiments that the
determined trigger value takes into account how close the load is
to a natural switching point or how long the load has been in a
particular energy consumption state as compared to a maximum length
of time as determined by how close the physical variable associated
with the load is to the loads maximum and minimum values for that
variable. A refrigerator in a "cooling on" state is closer to its
natural switching point as the sensed physical variable approaches
a lower limit for the temperature of the refrigerator. Conversely,
the refrigerator in a "cooling off" state is closer to its natural
switching point as the sensed physical variable approaches an upper
limit for the temperature of the refrigeration space.
[0102] Thus, some ratio representing the sensed physical variable's
relative position between the maximum and minimum limits for the
physical variable associated with the load is the preferred way for
determining the load's natural switching point. The ratio is taken
into account by the function calculating the device's trigger
frequency.
[0103] In a preferred embodiment, the control device is adapted to
determine an upper and a lower limit for the physical variable of
the grid; wherein the provision of a trigger value is further based
upon said upper and lower limits for the physical variable of the
grid. In this way, the control device appropriately distributes the
trigger frequency of a population of the devices between the upper
and lower limit in order to provide response when it is needed.
[0104] In a preferred embodiment, the value of the trigger
frequency is such that loads remaining in a particular state for a
longer time than others are more sensitive to changes in the sensed
variable of the grid by providing an appropriate function for
calculating the trigger value biased in this way.
[0105] More particularly, the provision of a trigger value
preferably first involves the control device being adapted to
provide a base value of the physical variable grid based upon said
random value and said central value, for example to randomly
provide said base value between said central value and said upper
or lower control limits; the control device is further adapted to
provide a trigger value function from said base value; and then
determine the trigger value from the trigger value function.
[0106] Thus, the randomisation provided by the random value is
directed to the provision of a base value, which is, in turn,
determinative of a particular function used to provide the trigger
value. In a particularly preferred embodiment, the trigger value
function defines a trigger value varying with the length of time
that the load has been in a particular energy consumption state.
More preferably, the trigger frequency is provided from the trigger
value function varying as described above.
[0107] Thus, each device is first provided with a randomised base
value, from which is provided a trigger value function. The
particular form of the function, i.e. how it varies with the ratio,
is dependant upon the value of the base value. Thus, the increase
or decrease in likelihood of the load changing its energy
consumption state is different depending upon the base value.
[0108] According to the preferred embodiments of the present
invention, each control device in a population will determine its
own base frequency. The base frequencies will be distributed
randomly across the population so that the changing of the energy
consumption of the loads or the switching of the loads is
progressive across the population.
[0109] According to the preferred embodiments, once this base
frequency has been determined, the exact frequency to which the
device is responsive depends upon the triggering frequency
determined from the triggering value function. This function is
defined such that the willingness of the load's response varies
according to its internal state. If it is in a very low energy
state, and the device is on, or in a first state of increasing the
energy stored by the load, it will not wish to switch off or to
switch to a second state of decreasing the energy stored by the
load except in the most extreme of grid states (as represented by
the physical variable of the grid, i.e. the frequency) but if its
energy store is approaching the upper limit, it is very willing to
switch off or to the second state. This changing willingness is
reflected by the extent to which the trigger frequency departs from
the central frequency.
[0110] Thus, the trigger frequency is provided with a nonlinear
trajectory as the energy state of the load varies. In order to
preserve the random distribution of likelihood of switching across
the population, the form of the trigger value function changes
depending upon the randomly provided base value.
[0111] By changing the willingness in this way, switching will be
as rare as possible, and the switching load is distributed across
the loads. This also serves to maintain the diversity of the load,
by avoiding building up a sub-population that is very close to the
limits.
[0112] In a preferred embodiment the random value is provided from
a randomiser configured to provide a distribution of base values
for a population of said control devices, said distribution
extending from a limit of the physical variable of the grid to the
central value of the physical variable of the grid. This is in
contrast to prior art devices where a window is defined in which
grid response is not provided and in which the device is allowed to
behave as normal, as though it had no responsive control device
installed.
[0113] The present invention, however, distributes the population
of trigger values from a central value to a limit and, thus,
response is provided at all frequencies between the determined
upper and lower limits for the frequency of the grid. In this way,
borrowing of energy from the grid or repayment of energy so
borrowed from the grid occurs throughout the determined frequency
spectrum of the grid. This is influential in providing a damping to
all movements of grid frequency from the central frequency.
[0114] It is also preferred that the randomizer is such that a
population of the control devices will have trigger values having a
distribution extending between the upper and lower limit of the
physical variable of the grid. In a preferred embodiment, the
trigger value varies from a limit of the physical variable of the
grid to the central value as the ratio moves from a minimum ratio
to a maximum ratio. In this way, the trigger value is closer to the
central value the longer the load has been in a particular energy
state. Hence, the energy consumption of the load is more likely to
change the longer the device has been in a particular energy
consumption state.
[0115] In a preferred embodiment, the change of the energy
consumption of the load involves either switching the load on or
switching the load off. A load is defined as the energy consumption
associated with the main function of the load. For example, in the
case of the refrigerator, the load is the energy consumption of the
cooling provision means. Thus, using this definition, background
operation of a refrigerator, such as lighting or other peripheries
to the main function of the load, is not considered the load in the
context of the specification.
[0116] It should be clear that the ratio described above is a
representation of how long the device has been on or how long the
device has been off. Preferably, the ratio is at a maximum as the
sensed physical variable of an off device approaches the device's
limit associated with the off state of a load or the ratio is
defined for an on device such that it is at a maximum when the
sensed physical variable approaches the limit for the on state of
the device.
[0117] In a further preferred embodiment, the provision of a
trigger value is further based upon the particular energy state of
the load (e.g. whether the load is an on or off state). Also
preferably, the ratio representing how close the device is to the
sensed physical variable reaching a limit, is dependent upon the
particular energy consumption state of the load. Thus, according to
the preferred embodiments of the present invention, the ratio is
defined differently depending upon the particular energy state of
the load (e.g. whether the load is on or off or in the first state
or second state).
[0118] This is advantageous as an off load, for example, will
switch on normally at a low load variable limit (minimum stored
energy). An on load, on the other hand, is approaching its natural
switching point at a high load variable limit (maximum energy
stored). It is, therefore, preferred to take the energy consumption
state of a load into account when defining the trigger
frequency.
[0119] In yet another preferred embodiment, the upper and lower
limits associated with the load are derived from a setpoint of the
physical variable associated with the load. A set point could, for
example, be defined by a thermostat setting or the particular
setting of a refrigerator. It is an advantageous feature of the
present invention that not only is a good stabilising effect
achieved by providing grid frequency response, but also that the
primary function of the load, for example, cooling, heating,
pumping etc. is carried out.
[0120] There are certain grid conditions in which the limits of the
sensed physical variable associated with the load are controlled to
be changed for an extended period of time. These changing of the
limits is not usually related to the provision of normal grid
responsive behaviour, nor is it due to a change in a setpoint for
the physical variable. The extended change of the limits is more
usually due to a grid condition.
[0121] According to a preferred embodiment of the invention, the
upper and/or lower limit of the sensed physical variable are
increased or decreased at a rate less than or more than,
respectively, a maximum rate of increase or decrease of the sensed
physical variable of the load.
[0122] In this way, the limits are moved at a rate less than the
physical variable could theoretically move. The lower rate of the
limit movement means that there is still some provision for the
load to be grid responsive even while variable limits are being
changed.
[0123] One example of a grid condition where this is useful is
during start-up after a power outage. As discussed above, the grid
is particularly delicate at this stage. Normally, the sensed
physical variable will be outside its normal range after a power
cut and the load will need to be operated to bring the variable
back within its preferred control limits. According to a preferred
aspect of the present invention, the upper and/or lower limit of
the sensed physical variable is increased at a rate less than a
constant maximum energy consumption of the load.
[0124] Thus, there is potential during the increase in the limits
to provide response. This ability of the load to provide grid
responsive behaviour is especially important during black start as
the grid is especially delicate at this time.
[0125] In another preferred embodiment, the present invention
defines a black start assistance mode in which a random delay is
provided before the load draws energy from the grid. This preferred
feature means that loads will start drawing energy from the grid
gradually, rather than them all coming on-line at the same time and
severely stressing the grid.
BRIEF DESCRIPTION OF THE DRAWINGS
[0126] Preferred forms of the present invention will be described
below with reference to the following drawings.
[0127] FIGS. 1A to 1C show a preferred form of how the trigger
frequency varies with energy stored in the load.
[0128] FIG. 2 shows an example population of the loads controlled
according to a preferred form of the present invention.
[0129] FIGS. 3A to 3B show an example of a profile of the trigger
frequency function.
[0130] FIG. 4 shows an overview of the various states of a
preferred responsive control device.
[0131] FIG. 5 shows a block diagram of the preferred operation of
the responsive control device of the present invention.
[0132] FIG. 6 shows a block diagram broadly outlining the operation
of a PID controlled load.
[0133] FIG. 6A shows a block diagram broadly outlining the
operation of a set point adjusting grid responsive control device
for a PID controlled load.
[0134] FIG. 6B shows a block diagram broadly outlining the
operation of a motor power adjusting grid responsive control device
for a PID controlled load.
[0135] FIG. 7 discloses a grid responsive controlling operating
with a price as an indicator of the balance between load and
generation on the grid.
DETAILED DESCRIPTION OF THE INVENTION
[0136] Specific embodiments of the present invention will now be
described in order to aid in the understanding of the present
invention.
[0137] The control device of the present invention is applicable to
energy storage loads on a grid, which consume intermittent or
variable energy.
[0138] The control device requires two main inputs, the first is a
frequency of the grid, or another parameter representative of the
balance between power generation and power requirement, and the
second some physical variable associated with the energy storage
load. Generally, the primary function of the load is to maintain
the physical variable within specified control limits.
[0139] The loads will generally operate on a duty cycle of some
kind, usually with a period in which the load is on and with a
period in which the load is off. Thus, a duty cycle of 50% means
that the load will be on and off for an equal amount of time.
Specific loads of this kind to which the present invention are
applicable include space conditioners (e.g. heating or cooling),
refrigerators and water storage pumps, amongst others.
[0140] However, modern power electronic control also makes it
feasible to vary the power consumed by a motor. This can make the
motor more efficient, and also means the motor is running
continuously or nearly continuously, with the power varied
according to the demands of the device. So in a fridge, for
example, the motor will reduce its power when the temperature has
reached its desired set point, will increase if the temperature
rises, and will reduce further if the fridge gets too cool. For
fridges this also has some benefits in perception of noise.
[0141] The motor will generally need to operate over quite a wide
power range, as, in a fridge or freezer, for example, it will have
to have the capacity to cool a warm fridge rapidly when it is
switched on or when a warm mass is put into it. So there remains
scope for temporarily changing the power demands of the device from
inputs other than the temperature--such as the frequency.
[0142] The present invention provides a control device operable to
vary the energy consumed by both types of loads, i.e. by binary
on/off control and by graduated or continuous increase and decrease
of the energy consumption.
[0143] For the remaining description, a refrigerator will serve as
the main example for use with the control device of the present
invention.
[0144] The present invention operates, up to a point, in common
with grid responsive control devices known from the prior art. The
present invention utilises the principle that energy store loads,
as described above, can perform their function without requiring
input energy from the grid at a specified time. Unlike lighting and
other such loads, energy store loads can receive input energy at
varying levels or varying intervals and still operate in a fully
satisfactory manner, provided they are controlled so as to keep the
physical variable of the load within the specified control limits
of the particular load.
[0145] The amounts of energy stored by the above described energy
storage devices is determined by the control limits of the physical
variable. In the case of a refrigerator, the maximum amount of
energy stored by the load is defined by the lower temperature limit
for the current setpoint setting of the refrigerator and the
minimum amount of energy stored is the higher temperature
limit.
[0146] In the following description, y is the normalised
measurement of the physical variable of the load being controlled
by the grid responsive control device of the present invention. A
larger y implies more input energy is stored (i.e. the refrigerator
is coolest) than a smaller y. If x represents the energy in the
store, then y is a function of x, i.e. y=f(x). A normalised y can
range from 0, with no energy stored, to 1, a critical maximum level
of energy stored. The function is normally sufficiently close to
linear to make this a useful approximation.
[0147] In the case of a refrigerator, the input energy is directed
towards cooling. So y is greatest, 1, when the fridge is at its
coldest possible, and 0 when the internal temperature rises to
ambient. In the case of a tank, y is 0 when the tank is empty, and
1 at a level when the tank overflows. Normally, of course, it is
controlled to within narrower limits, and these are referred to as
the upper and lower limits of the physical variable, or y.sub.max
and y.sub.min.
[0148] According to known principles of grid responsive loads, at a
particular setting of the load, the input energy is varied to keep
the physical variable y within the limits set by y.sub.min and
y.sub.max, in the same way as the load would normally be operated,
except the frequency of the grid (or some other parameter
associated with the balance between generation and load on the
grid) is taken into account.
[0149] Speaking generally, a load, of the type applicable with the
present invention, operated without a grid responsive controller
would switch the load on when the minimum value of y (y.sub.min) is
reached and switch the load off when its maximum value (y.sub.max)
is reached.
[0150] According to the grid responsive controller of the preferred
embodiment, the timing of the switching, when an on load is
switched off or when an on load is switched on, is adjusted
depending upon the frequency of the grid. During a period of low
frequency, for example, there is too much load on the grid and not
enough generation to match it, and a grid responsive device which
is on will react by switching off (or switching to a decreased
energy consumption state) before it would normally have switched
off, i.e. before y reaches y.sub.max. Likewise, during a period of
high frequency, more load is needed to take up the excess in
generation and the loads will be switched on (or switched to an
increased energy consumption state) before y.sub.min is
reached.
[0151] Further, an extended set of upper and lower limits for the
sensed variable can be determined in order to improve the amount of
response provided. So, during a period of high frequency, grid
responsive loads will be switched on and the maximum value for the
sensed variable (y.sub.max) can be increased such that the loads
having been switched on remain on for a longer than normal period
of time, as will loads that were already on. A similar provision is
utilised during periods of low frequency.
[0152] The preferred control device of the present invention
defines a status for the grid, so as to determine the exact type of
response to changes in frequency provided by the grid responsive
control device. The grid responsive control device has three modes
of operation, a "normal" mode, a "stress" mode and a "crisis" mode,
in a similar way to the system described in UK patent application
no. 0322278.3.
[0153] The preferred embodiment of the present invention determines
the mode of operation of the controller and the associated grid
status from a defined function of the frequency, hereinafter called
h. The function h determines from the behaviour of the grid
frequency the current status of the grid. Ideally h represents to
some extent a measure of how much energy has been borrowed from or
loaned to the energy store loads.
[0154] The function h preferably includes three principal terms, a
proportionate term, an integral term and a derivative term. These
three terms will give a good indication of the stability state of
the grid.
[0155] The proportionate term is the current frequency deviation
from the nominal frequency of the grid or some other central value
that represents how much the frequency needs to be corrected to
return to the desired central value.
[0156] The integral term represents a longer term (as compared to
the instantaneous proportionate term) view of the frequency error.
This term is useful, as a small error for a long time, will
influence the function h and, thus, be taken into account in
providing grid stability response. The integral term can be a sum
of a set amount of past frequency deviations or can be a moving
average of past frequency deviations. Rather than from time zero,
the integral term can be measured since the last time the frequency
deviation was zero.
[0157] The derivative term is related to the current instability of
the grid. It can be a rate of change of frequency deviation. Thus,
large swings in frequency will also affect the function h and can
be an indication of an instable grid, even if the actual present
deviation of the grid frequency is not outside preferred
limits.
[0158] In equation form, the function h can be written
h=Pf.sub.c+IC.sub.c-Df'.sub.c
where f.sub.c is the proportionate term, C.sub.c is the integral
term and f'.sub.c is the derivative term. P, I and D are constants
for influencing the degree of importance to the function h of each
of the terms.
[0159] The integral term C.sub.c may be calculated by (f.sub.cS),
where S is the sample integral.
[0160] The three parameters, P, I and D should be enough for the
control device to derive h, but for completeness and flexibility,
it may be appropriate to extend this to quadratic or cubic
terms.
[0161] According to the preferred implementation of the present
invention, the grid status is inferred from the function h. For
example, if h is below a first limit, then a "normal" status of the
grid is determined. If h is between the first limit and a second,
greater limit, than a "stress" condition for the grid is
determined. If h is between the second limit and a third, greater,
limit, then a "crisis" condition is determined. The difference
between the modes of operation associated with each of these grid
states is similar to that described in UK patent application no.
0322278.3.
[0162] The function h is a useful way of determining the stress
under which the grid is operating. Appropriate setting of the
parameters P, I and D of h enable the function to appropriately
distinguish the three general states of the grid.
[0163] During the normal mode of operation, the grid responsive
control device of the present invention will operate as fully
described below. During stress mode of operation, a user of the
energy store load is not allowed to adjust a setpoint of the
physical variable associated with the load. Thus, negation of the
grid responsive compensation provided by the present invention is
not possible. During a crisis state, the energy store load is
operating without regard to the desired range of the physical
variable associated with the load. The load's physical variable is
allowed to reach the absolute limits of y rather than the preferred
range represented by y.sub.max and y.sub.min. For example, in a
crisis state, a refrigerator could be allowed to reach an ambient
temperature, or be allowed to go to the lowest temperature that the
refrigerator is capable of achieving. Similarly, in the case of a
water tank, the water level could be allowed to reach empty or
extend up to a full tank level.
[0164] A main mode of practising the principles of the present
invention is now described. Other preferred embodiments of the
invention follow.
[0165] The grid responsive controller of the present invention
includes a control mechanism for actively and continuously damping
grid frequency variation. The grid responsive control device of the
present invention is responsive to all frequency variations from a
central value, which is defined as an average value over a
predetermined sample period of historical frequency readings.
[0166] When the control device is first used, the central value
will be set to the current frequency. The central value will then
evolve as past samples of the grid frequency are incorporated into
the moving average. The central frequency is the average value of
the grid frequency since the start of the sample period.
[0167] Any movement of the grid frequency from the central
frequency is resisted by the population of responsive control
devices of the present invention. If the current frequency is above
the central value, then the responsive control devices will tend to
switch on their loads to compensate for the increase. If the
current grid frequency falls below the central value, then on
devices will tend to switch off to compensate for the deficit in
generation. This provides an overriding stabilising effect on the
grid, as represented by a clearer, or less noisy, grid frequency
signal.
[0168] The loads will not all change energy consumption status at
the same time. The control device of the present invention is
adapted to ensure the loads are switched in a progressive way such
that greater deviation from the central value results in more loads
tending to switch on/off. This progressive switching is important
in order to ensure that the response of a population of loads is
not simultaneous, which would provide a destabilising influence to
the grid. The randomisation is described in more detail below.
[0169] In the preferred implementation of the responsive control
device of the present invention, the sample period of calculating
the central frequency value is taken as the period since the
central frequency last crossed the nominal frequency of the
grid.
[0170] The present invention defines high frequency excursions,
when the central frequency moves above nominal, and low frequency
excursions, when the central frequency is below nominal. The end of
one of these types of excursions marks the start of the other.
These cross-overs have been found to be a convenient time for
beginning accumulation of frequency readings for calculation of the
central frequency. Thus, the central frequency will be calculated
for each high excursion (above nominal) or low excursion (below
nominal) of the central frequency. The central frequency will,
therefore, be calculated as a moving average of the frequency
during the current excursion and is reset once the central
frequency crosses nominal and a change of excursion (e.g. high to
low excursion or vice versa) occurs.
[0171] An advantage of choosing above nominal or below nominal
excursions for the sample period is that devices will end-up having
a shared common view of the central frequency. Loads that are
recently connected to the grid, and so have no history, will soon
come to see the same recent history as other devices, since the
central frequency crossing over the nominal frequency of the grid
is expected to occur frequently enough. It is useful for the
devices to appreciate a common central frequency as it allows their
behaviour to be coordinated (but not synchronised) in an intended
manner.
[0172] This sample period may not always be appropriate. If the
excursion lasts for a period that approaches the average on or off
cycle of the energy storage device, the devices may well be called
upon to provide grid responsive behaviour without having had the
opportunity to reach their maximum or minimum energy store. This
could have an adverse effect on switching rates of the energy
storage loads. Further, if the load does not reach its maximum
energy store, and fully replenish itself, then the population of
such loads will, on average, be depleted. It may be that the
control device will have to be adapted slightly in order to be
useful in such circumstances.
[0173] It is envisaged that the moving average for obtaining the
central frequency could be a weighted moving average, such that the
most recent frequency terms are given more importance. In this way,
movements of frequency from recently obtained values will more
likely provide load response and could be compensated for. This
will further help to stabilise any frequency movement of the
grid.
[0174] The grid responsive control device of certain aspects of the
present invention also includes a further improvement aimed to
minimise switching of a load and to distribute energy variations
across the available population of the loads. As described in
further detail below, this is achieved by varying the trigger
frequency of the device as it progresses through the current on or
off state.
[0175] A trigger frequency is the frequency of the grid at which
the load will be controlled to switch from an on state to an off
state or an off state to an on state. The loads will also be
switched on or off when the sensed variable associated with the
load reaches its current minimum or maximum, as defined by
y.sub.min and y.sub.max.
[0176] The grid responsive control device is adapted to determine a
target (or base) frequency in a random way. In a population of such
devices, the target frequencies will be distributed randomly across
the population so that the above described progressive response is
achieved.
[0177] According to a preferred embodiment of the present
invention, the device's target frequency is the frequency to which,
on average, the device will respond. The current triggering
frequency, however, which is the frequency of the grid at which the
load will switch between on and off states, is not usually the same
as the target frequency. The target frequency is a randomly chosen
frequency, from which a unique profile for determining the trigger
frequency, the grid frequency that will cause the device to trigger
between states, is derived.
[0178] So, the profile for the trigger frequency is derived from a
function, which, in turn, is dependent upon the randomly chosen
target frequency. The actual trigger frequency used by the control
device is derived from this function, which is preferably a
function of how long the device has been in its current energy
consumption state, i.e. how long it has been on for or off for.
[0179] How long a device has been on or off for, is determined
relative to a natural switching point, which is the point at which
the sensed physical variable will reach its current maximum or
minimum values for the sensed physical variable (y.sub.max and
y.sub.min) and would, therefore, switch anyway. Thus, the function
for determining the device's trigger frequency is also a function
of the value of the sensed variable relative to its minimum or
maximum values.
[0180] The current trigger frequency is, therefore, dependent upon
the current value of y. According to the preferred embodiment of
the present invention, the trajectory of the trigger frequency is
biased such that the further away a load is from its natural
switching point, the trigger frequency will be a less likely
frequency of the grid, i.e. the trigger frequency will be further
away from nominal. Thus, the device is less likely to switch the
further away it is from a natural switching point.
[0181] Preferably, the trajectory of the trigger frequency is
biased in such a way that half the time the device is less
sensitive than the randomly chosen target frequency, and half the
time it is more sensitive. Thus, preferably, the average of the
trigger frequency is the target frequency.
[0182] In the preferred embodiments, the length of time in which a
load has been in either the on or the off state is calculated from
the current value of the sensed variable as compared to a range
allowed for that variable as defined by the current values of
y.sub.max and y.sub.min. This could, for example, be expressed as a
percentage. For the sake of illustration, a load device which is in
an on state with the sensed variable close to reaching a maximum of
the sensed variable could have been on for 80% of its normal on
period. This can be expressed formulaically as
t.sub.on=(y-y.sub.min)/(y.sub.max-y.sub.min)
where t.sub.on is the amount of time that the load has been
switched on relative to its expected on time and y is the current
value of the sensed variable.
[0183] How long the device has been off for is defined using a
different formula, but the same principle applies. The closer an
off device is to its lower limit y.sub.min, the longer it has been
off for. Thus, the appropriate formula is as follows:
t.sub.off=(y.sub.max-y)/(y.sub.max-y.sub.min)
where t.sub.off is the relative amount of off time as compared to
the expected amount of off time for the load.
[0184] FIGS. 1A, 1B and 1C show example forms of the profile of the
trigger frequency function. Frequency is plotted up the y-axis and
percent fullness/emptiness, in terms of energy, of the energy store
load is plotted along the x-axis. FIG. 1A shows the frequency at
which an on device will switch off. As is unique to the present
invention, the trigger frequency is dependent upon the time for
which the device has been on, as compared to the expected time
(y.sub.max reached). As can be seen from FIG. 1A, for 50% of the
time, the trigger frequency is relatively close to the central or
nominal frequency, for the other 50% of the time, the trigger
frequency is further away from these frequencies. Thus, it is only
during the more extreme grid circumstances that the devices which
have only been on for 50% or less of their expected on time will be
triggered. This is based on the assumption that the grid frequency
will, for the majority of the time, reside around the central or
nominal frequency and, thus, trigger frequencies that are closer to
this are more likely to be achieved by the grid. Thus, switching
the load is more likely to take place the closer the triggering
frequencies are to the nominal or central frequency.
[0185] It is also important to appreciate that the exact form of
the trigger frequency's dependency upon time on or off compared to
the expected time on or off is chosen by the target frequency,
which is randomly chosen. In this way, a population of loads will
provide a diversitised grid frequency response.
[0186] Comparing FIG. 1B with FIG. 1A illustrates the profile
dependence upon the target frequency chosen. It can be concluded
that while the trigger frequency is always varied with the percent
of expected on or off time of the load, the form of this variance
is determined by the randomly chosen target frequency.
[0187] FIGS. 1A and 1B show the trigger frequency for an on device.
FIG. 1C, conversely, shows the profile for an off device. The
principles are the same. Namely, the frequency at which an off
device will switch on varies depending upon the percent of expected
off time, as defined by the above formula. As can be seen from FIG.
1C, the trigger frequency approaches the central frequency or the
nominal frequency of the grid as the device approaches its natural
switching on point. Generally, the profile requires that the closer
the device is to its natural switching on point, the closer the
frequency to the damping or nominal frequency and, therefore, the
more likely the load will be used for providing grid frequency
response.
[0188] According to the preferred implementation of the invention,
any movement of sensed grid frequency above or below the central
frequency will result in loads being switched. The further the
sensed grid frequency is from the central frequency, the
progressively more loads that will switch. Since the central
frequency is a moving average of past frequency ranges, the central
frequency will tend to "follow" the sensed grid frequency, although
in a damped manner. This will provide a smooth central value for
using to determine whether to perform high frequency (above
nominal) or low frequency (below nominal) response.
[0189] The sensed frequency may well change direction and go above
or below the central frequency. The device of the present invention
will resist any rapid increases or decreases in the grid frequency
above or below the central frequency by borrowing or repaying
energy from or to the grid as soon as the grid frequency starts to
move. This is the appropriate time for the energy borrowing or
repayment, as discovered by the present inventor, and provides a
far more stable grid frequency, as compared to the prior art grid
responsive control frequencies.
[0190] At first, any movement above or below the grid central
frequency only switches loads that are near to their natural
switching points. This is because of the trigger frequency being
variable for a particular device with on or off time for a device.
All loads that have been on or off for greater than 50% of their
expected on or off time are favoured. It is only when the grid
frequency moves dramatically away from the central frequency that
devices that are less than 50% of the time away from their previous
switching point will switch.
[0191] Thus, the preferred implementation of the present invention
provides a more stable grid frequency, thereby inherently resulting
in less switching of the responsive load. Furthermore, switching of
devices that have already been switched is disfavoured, thereby
further decreasing the switching burden on the load.
[0192] A system consisting of a population of energy store loads
controlled by the grid responsive control devices of the present
invention provides a population of loads ready to switch in
response to any change in the grid frequency. The larger the change
in frequency, the larger the population of loads providing
response. This should be a linear relationship.
[0193] FIG. 2 shows an example of a system being controlled in
accordance with the present invention, in a stable state and
running at the grid's normal frequency. As shown in FIG. 2, in this
state the portion of devices that are off [1] and the portion of
devices that are on [2] corresponds to the expected duty cycle. So,
if the load is run at a 50% duty cycle, the population is evenly
divided.
[0194] If the system moves into a low (below nominal) frequency
excursion, on loads will be triggered off [3] in order to reduce
the load. They will become unlikely to switch on again for a
while.
[0195] During this low frequency excursion, some off loads will be
switched on [4], despite the current excess of load on the
frequency, because of the fact that they have reached their minimum
energy store state and proper function of the load requires it to
switch on. These loads were not called on to provide high frequency
response, and are lost from the population of loads capable of
providing high frequency response, even though they were actually
the most sensitive. Again, these recently switched loads are
unlikely to switch on again for a while.
[0196] Some loads will reach their maximum energy state, and will
need to be switched off [5]. If the duty cycle is 50% the number of
devices reaching their maximum energy state [5] will tend to be the
same as the number of devices reaching their minimum energy state
[4].
[0197] The remaining devices capable of providing low frequency
response [7] is the population that had the less sensitive
frequency settings, since those close to the nominal grid frequency
have been "used up".
[0198] If the frequency now rises above the central frequency,
then, despite the frequency still being below the nominal grid
frequency, it is desired that the loads begin to switch on and
start recovering the energy loaned to the grid earlier.
[0199] As the frequency rises above the central frequency, some
devices will be triggered on [8] in order to increase their load.
These are most likely to be drawn from the population remaining
from [1], as the loads [3] will be in a minimum switching mode
since they were only recently switched.
[0200] As before, some on devices will come off [10], and some off
devices will come on [9] because they have reached their maximum or
minimum energy state, respectively, without being called upon to
provide high frequency response. While the on devices coming off
[10] were the most sensitive for providing response of the
population, they were lost to the population for providing low
frequency response, without being used. This population of loads
reaching their minimum or maximum energy states will be quite
small.
[0201] The population of devices continuing to be able to provide
high frequency response is as desired, in as much as they are
reasonably evenly distributed amongst the frequency between the
central frequency and a maximum limit frequency. The population of
devices sensitive to frequencies immediately below the central
frequency has, however, become depleted. So a downturn in frequency
again will trigger less load reduction than before resulting in an
average frequency that will fall even further until an undepleted
zone is reached, or the natural migration of trigger frequencies
repopulate the depleted zone.
[0202] This is desired behaviour. During a low frequency excursion
where the frequency is undulating, the frequency will tend to fall
more easily than it rises (or, more generally, move further from
nominal more easily than it approaches nominal). This reflects the
fact that the loads are lending energy to the grid and resisting
rising frequency as the loan is repaid. Ideally, it is only when
the loan is fully repaid that the frequency returns to nominal.
[0203] One possible manipulation of the low frequency population
shown in FIG. 2 is to distribute remaining on loads across the
range between the central frequency and the minimum frequency
(rather than between the nominal frequency and the minimum
frequency). This has the effect of leaving the frequencies
immediately below the central frequency additionally depleted (as
devices that would have chosen target frequencies above the central
frequency, between the nominal frequency and the central frequency
now have them below the central frequency) so the frequency has a
greater tendency to drop. This may not be desirable.
[0204] In an alternative to this manipulation, this change could be
made only for some loads, such as those that have switched on since
the start of the low frequency excursion. The logic of this arises
because the devices which have switched on since the start of the
excursion would have done so because they are low in energy, and so
need an opportunity to replenish this energy before they provide a
response. One way to affect this is to systematically lower (make
more extreme) the frequency at which they will switch on. This, in
turn, will tend to allow the grid frequency to fall further. It
will, in extreme circumstances, also tend to distribute the on time
more evenly among devices. The on time is, without this
modification, already evenly distributed across devices by the
trajectory of the trigger frequency.
[0205] The example shown in FIG. 2 is for a low frequency
excursion, the behaviour of a population of loads during a high
frequency excursion is symmetrical.
[0206] In an ideal system, where all grid response is provided by
devices controlled according to the present invention, frequency
excursions will not end until energy borrowings have been repaid.
If the response to grid frequencies comes from other sources as
well, (i.e. by generators), the excursion may end before borrowings
have been fully repaid, but the loads will nonetheless retrieve the
energy required to replenish their energy store.
[0207] The central frequency, derived from a moving average of the
frequency is the frequency above which the overall load derived
from devices controlled according to the present invention will
increase, and below which the load will decrease. This is,
effectively, a target frequency for the whole system. It could be
that a system target frequency different from this could be further
derived. The possibility is to move the system target frequency
closer to the nominal frequency, so as to provide some bias to
influence the grid frequency towards nominal.
[0208] Below is described, in further detail, a complete procedure
for obtaining the triggering frequency for a particular control
device.
[0209] First, the central frequency is calculated. Each reading
from the first recorded frequency reading since the current
excursion above or below nominal began is taken into account. The
obtained central frequency may then be further manipulated to bias
it towards the nominal frequency, but this may not be necessary
since such a bias is an inherent feature of the control devices of
the present invention.
[0210] A device base or target frequency is then determined. To do
this, a range within which the base frequency is to be placed is
determined and then a random target frequency is chosen within this
range. Each device has both a high target frequency and a low
target frequency, which are preferably provided from separate
random values called the low random value and the high random
value. The high target frequency is for a high frequency excursion
and the low target frequency is for a low frequency excursion.
[0211] When choosing the random number which distributes the target
frequency between the nominal and low limit or high limit of the
permissible frequency range, it is preferred that one random number
is used for high frequency excursions, and another random number is
used for low frequency excursions. The random numbers are
preferably provided between 0 and 1 so that the target frequency
can be positioned anywhere between the full range (as defined
above) of possible frequencies. It is preferred that the two random
numbers are regenerated after an opposite excursion begins.
[0212] Thus, the low frequency excursion random number is chosen at
the start of the high frequency excursion and the high frequency
excursion random number is chosen at the start of a low frequency
excursion, thereby ensuring the readiness of the random number upon
a frequency excursion changeover.
[0213] It is important to regenerate the random numbers at regular
intervals, as the sensitivity to grid frequency changes for a
particular control device depending upon the random number. As will
become clearer below, a refrigerator with small random numbers will
tend to carry a greater switching burden than one with larger
random numbers. This is because the target frequency generated from
a large random number will be more likely to provide a triggering
frequency closer to the outer frequency limits, which are more
rarely realised by the grid, than frequencies closer to the nominal
frequency of the grid.
[0214] It is also important that the random number is not
regenerated during a particular excursion. This could result in an
unpredictable impact upon the grid stability. Other strategies are
possible, however. For example, the random numbers may be generated
during a first change following a 24 hour period or other such
chosen period.
[0215] There are four possible ranges within which the target
frequencies should be provided:
[0216] (1) The grid is in a low frequency excursion (central
frequency below nominal) condition and the load is currently on.
This is shown in the left hand side of FIG. 3A. In this case, the
target frequency is provided between a low frequency limit (the
selection of the low frequency limit for the grid is discussed
below) of the grid and the nominal grid frequency. Since the grid
is currently in a low frequency excursion, the central frequency
will also be provided between the nominal frequency and the above
low frequency limit.
[0217] (2) In the case of a low frequency excursion when the load
is off (FIG. 3A right hand side), the target frequency will be
randomly positioned between a high frequency limit (the selection
of the high frequency limit for the grid is discussed below) of the
grid and the central frequency (different from the nominal
frequency).
[0218] (3) In the case of a high frequency excursion (central
frequency above nominal) and the load is off (FIG. 3B left), the
target frequency is randomly provided between the high frequency
limit and the nominal grid frequency.
[0219] (4) In the case of a high frequency excursion and the load
is on (FIG. 3B right), the target frequency is provided between the
low frequency and the central frequency value.
[0220] FIGS. 3A to 3B show example positions of the frequencies in
each of these four possibilities. These figures also show the
trigger frequencies, at which point the grid frequency will be such
that it triggers a particular device off if it was already on or on
if it was already off. FIGS. 3A to 3B show that the triggering
frequencies are provided within the same range of frequencies
provided for the random placement of the target frequency.
[0221] As shown in FIGS. 3A to 3B, the device target frequency is
determinative of the form of the triggering frequency profile.
Thus, the randomisation of the target frequency is carried through
to the triggering frequencies.
[0222] In the preferred implementation of the control device of the
present invention, once the high or low target frequency has been
calculated for a particular device, the devices specific trigger
frequency needs to be calculated. When the device is on, only the
low target frequency is relevant and when the device is off only
the high target frequency is relevant. From the value of the
particular target frequency, the form of the function can be
derived. The function is different, not only depending on the
target frequency for the device, but also on whether the device is
on or off. From this function, using the current value of the
sensed variable, the device's current triggering frequency can be
obtained. This triggering frequency is then determinative of
whether the device will switch on or off by comparing it to the
sensed frequency.
[0223] The value of the triggering frequency shown in FIGS. 3A to
3B is calculated as outlined below. The proportion referred to
below is a representation of how close the energy store is to being
at its maximum or minimum depending on whether the device is on or
off, respectively. The proportion is preferably t.sub.on or
t.sub.off, the calculation of which is described above.
[0224] (1) If the proportion is less than 0.5, then [0225] (i.e. is
the time since the load switched last less than 50% of the time it
takes to reach minimum or maximum)
[0226] (2) "Offset"=(the target frequency-"StartPoint") * the
proportion [0227] where the StartPoint is the low frequency limit
for on devices and the high frequency limit for off devices. Thus,
the (target frequency-StartPoint) is the difference between the
high frequency limit or the low frequency limit and the target
frequency. Since the proportion always runs between 0 and 0.5 (as
per step (1)), this difference is made smaller by the proportion
term. Thus, in this step, the value of the sensed variable is
influencing the triggering frequency as is the target
frequency.
[0228] (3) the trigger frequency=Startpoint+OffSet [0229] thus, for
on devices the trigger frequency is offset from low frequency limit
and for off devices, the trigger frequency is offset from the high
frequency limit.
[0230] (4) If the proportion is greater than or equal to 0.5, then
[0231] (i.e. is the time the device has been on or off more than
half way towards its natural switching point? If so, then the load
needs to operate in a higher probability switching zone than
above).
[0232] (5) Offset=("Endpoint"-the target frequency) * the
proportion [0233] where the Endpoint is the central frequency for
off devices during low excursions and for on devices during high
excursions and is the nominal frequency for on devices during high
excursions and off devices during low excursions. The offset is the
difference between the target frequency and the endpoint, with the
difference factored by the proportion. Since the proportion is
always between 0.5 and 1, the offset is somewhere between being all
or half of this difference. Again, this step shows that how long
the device has been on or off and the target frequency both
influence the value of the offset.
[0234] (6) the trigger frequency=the target frequency+Offset [0235]
thus, the trigger frequencies are provided between the target
frequency and the central or the nominal frequency.
[0236] A load control device having the triggering profiles shown
in FIGS. 3A will now be described.
[0237] During a low frequency excursion, the central frequency is
provided between the nominal and the low limit for the grid
frequency, as shown in FIGS. 3A. During such a low frequency
excursion, the overall desired behaviour is for on devices to tend
to switch off in order to eventually bring the system frequency
back towards nominal.
[0238] FIG. 3A shows the evolution of a load which is initially in
an on state while the grid is in a low frequency excursion. The
trajectory of the energy state 1 (left hand axis) shows it moving
from a minimum energy state towards a maximum energy state. If no
response is provided, the load will switch off at the maximum
energy state from its limit setting, and the energy state will then
move from the maximum to the minimum.
[0239] For each reading of the grid frequency and the physical
variable associated with the load, the central frequency is
recalculated. For clarity, the diagram shows a fixed central
frequency, but it will actually vary with grid conditions.
[0240] While the device is on, the target frequency for off is then
calculated using the low frequency random number 2. This will lie
over the range 3 shown on the left of the diagram (FIG. 3A), which,
in this state, is chosen to be between the low frequency limit and
the nominal frequency. The physical variable associated with the
load is then used to calculate the trigger frequency for off 4. The
triggering frequency for on devices thus takes into account the new
central frequency and the new sensed variable associated with the
load. For on devices, when the grid frequency is below the target
off frequency, then the load will be switched off. For off loads,
when the measured grid frequency is greater than the triggered on
frequency, then the load will switch on.
[0241] When the grid frequency becomes lower than the trigger
frequency 5 the device will switch off, and the energy trajectory
will change direction, even though the maximum energy store has not
been reached. This has the effect of lowering the average energy
stored in the device without changing the physical variable limits.
In a large population of devices, this has the effect of raising
the average temperature of the population of devices.
[0242] When the device has switched off, its further behaviour is
shown on the right hand side of FIG. 3A. The portion of the duty
cycle that is missed 6 is shown hatched.
[0243] In this case the range over which the target on frequency is
chosen lies between high frequency limit and the central frequency,
and an example trajectory 8 of the trigger frequency for on is
shown. If the central frequency stays unchanged, then the device
will not switch on again 9 until the energy state has once again
reached its minimum.
[0244] According to FIG. 3A, any movement of the measured grid
frequency away from the nominal will result in loads being switched
off. Clearly, the further the grid frequency is from nominal, the
progressively greater number of devices that are switched off.
Also, it can be seen that the further the sensed frequency is from
nominal, the earlier the load will tend to be switched off during
its on cycle.
[0245] According to FIG. 3A, any movement above the central
frequency will tend to result in the off loads being switched on.
Thus, the triggering frequencies provided by the present invention
resist all grid frequency movements about the central
frequency.
[0246] A similar discussion is applicable for high frequency
excursions, as shown in FIGS. 3B.
[0247] In a manipulation of the shown embodiments, all four of the
ranges for provision of the target frequency, described above,
could be provided between the central frequency and a maximum or
minimum limit, rather than two of the ranges being between the grid
nominal frequency and a high or low limit (as in FIGS. 3A to 3B).
In this alternative form of the control device, FIG. 3A will be
adjusted such that only decreases in the sensed frequency below the
central frequency will result in loads being switched off (rather
than decreases below central and increases above central up to the
nominal frequency, as is shown). This will still provide the
desired response, as a reduction of frequency means too much load
and, therefore, devices switching off. Similarly, the profile of
FIG. 3B could be modified so only increases above the central
frequency will result in off devices coming on (rather than
increases above central and decreases below central up to the
nominal value, as is shown). Again, the response provided in this
modified form is still as desired since a rise in frequency
represents an increase in generation, which needs to be taken up by
switching loads on.
[0248] According to FIG. 3A, during a low frequency excursion, if
the current system frequency falls below a central frequency, then
the off devices cannot switch on, since no triggering frequencies
are provided below this point. The only way in which off devices
will be switched on would be if the physical variable of a load
reaches its lower limit. Thus, in the case of a decrease below the
central frequency, response is only provided for on devices to
switch off, as can be determined from FIG. 3A, which is exactly as
required to compensate for the excess load causing the frequency
drop.
[0249] Again with reference to FIGS. 3B, during a rise in frequency
above the central value, it is desired that off devices begin to
switch on. This behaviour is provided according to FIG. 3B. FIG. 3B
also shows how devices approaching natural switching on points are
favoured by having their triggering frequencies closest to the
central frequency. The figure also shows how the triggering
frequencies of the population of the off devices are spread between
the central frequency and the high frequency limit so as to provide
progressive response behaviour.
[0250] There is the possibility that the frequency of the grid will
repeatedly move up and down within a narrow frequency range close
to the central frequency. In these circumstances the population of
devices sensitive to the experienced frequencies will become
depleted. That is, when the frequency falls, the most sensitive
devices will switch off, and when it rises the most sensitive
devices will switch on. The devices that switch in this way will
become unavailable for providing further response until they have
completed the remainder of their cycle. In due course, the
population of sensitive devices will be restored as devices
approach the state in their cycle, which may be shorted by the
response it provides, where they are again willing to switch.
[0251] The rate at which a depleted frequency zone is replenished
is influenced by the range over which the target frequency is
chosen. Including the frequency zone that is depleted of sensitive
devices into the target frequency range, increases the rate at
which depleted zone is replenished from the population of devices
that are approaching sensitive points.
[0252] The increased replenishment is achieved by spreading the
depletion across a wider frequency range, which is not currently
being experienced on the grid. While this does reduce the total
response still available, this properly reflects the physical fact
of using up the finite response provided by the population of
fridges.
[0253] Although it appears that there is a zone between the Nominal
frequency and the central frequency where the action of on devices
and the action of off devices appear to overlap and so negate each
other, in practice, these actions will not in fact take place at
the same time, but are separated by the time taken for the grid
frequency to change direction and serve to damp small and up and
down frequency changes.
[0254] The degree of Response available as the grid frequency
passes through a depleted zone will be less, so the change in load
available to slow the change in frequency will tend to be less.
This tendency makes the frequency a more accurate indicator of the
extent to which energy has been loaded to or borrowed by the fridge
population and is the intended desired behaviour.
[0255] As can be seen from FIG. 3A, when the sensed grid frequency
increases above the central frequency, off devices come on
according to FIG. 3A. Only if the frequency then again decreases
will on devices turn off as there remains a population of devices
with the trigger off frequencies between the central frequency and
the nominal frequency. This means that while movements of the grid
frequency below the central frequency will result in only on
devices being switched off (excluding off devices reaching their
minimum limits of the physical variable associated with the load),
the response for the grid frequency moving above the central
frequency is provided by off devices switching on, as desired to
stabilise the grid frequency movements.
[0256] A similar discussion to the one given above concerning low
frequency excursions with regard to FIGS. 3B, is symmetrically
applicable to a high frequency excursion (above nominal) of the
central frequency.
[0257] In a real grid, changing between high and low excursions as
the load and generation varies, the population of fridges in each
state will be dynamic and the behaviour of individuals fridges less
determined than in these descriptions.
[0258] The maximum and minimum frequency limits are used by the
control device for determining the ranges over which the target and
triggering frequencies should be spread. These frequency limits can
be determined by experience over time of the frequency behaviour of
the grid or can be set at installation depending upon the grid with
which it is intended to be used.
[0259] For example, in the US, the grid frequency is intended to be
kept within plus and minus 0.5% of the nominal grid frequency, i.e.
the grid frequency should always fall between 59.7 hertz and 60.3
hertz. This would be the default value for a control device
intended to be operated on the US grid. These default values could
be set or could be self-optimising based on the device's experience
with the grid. The possibility of a self-optimising control device
for providing these frequency limits will now be discussed.
[0260] The control device of the present invention will be
preferably provided with a default set of parameters related to the
grid with which it is expected to be used. As can be seen from
FIGS. 3A to 3B, if the grid frequency passes outside the maximum or
minimum range, the entire population of devices will be in the same
switched state, i.e. either off or on. No further grid response is
available from the load. Thus, it is important to perform a
self-tuning of the frequency limits correctly and carefully.
[0261] Ideally, the frequency control limits are chosen to lie just
beyond the frequency deviation tolerable by the grid. It is also,
however, desirable to keep the rate at which the grid frequency
varies fairly low. The method adopted by the grid responsive
control device of the present invention is to balance these
requirements to monitor the frequency extremes experienced, and to
use these to adjust the frequency limits stored. Two core
adjustment processes are used.
[0262] First, if the extreme frequency experienced during an
excursion is greater than the limit used, then, in subsequent
excursions, the extreme will become the new limit. So on a grid
with big variations, the grid responsive control device will adjust
to distribute its service across the full range of frequencies
experienced. The grid responsive control device has the capacity to
analyse the events leading up to the extreme, and can use this to
moderate the extent to which the limits are widened.
[0263] In the second process, if the extreme frequency experienced
within a period is less than the currently stored frequency limits
then the frequency limits will be adjusted to be closer to the
frequency extremes experienced. The responsive control device will,
however, only bring the limits closer by a small proportion of the
difference between the extremes and the limits (a moving average
technique). In this way, it will take numerous cycles of adjustment
before the frequency limits become significantly narrower. The
tendency for the limits to narrow could also be countered by
ignoring all excursions outside the stored frequency limits that
are shorter than a defined period (for example in minutes).
[0264] So, if the device experiences more extreme frequencies than
its defaults lead it to expect, it will rapidly widen its behaviour
to suit the circumstances. If, on the other hand, the grid is more
stable than the defaults lead it to expect, it will only slowly
migrate towards narrower limits, and will still react quickly if
the grid behaviour again becomes more volatile.
[0265] Further, the limits are provided with a margin, a so-called
rare event margin, such that the grid responsive control device
will assume that the biggest frequency excursion is not rare, and
so the frequency limits actually chosen are adjusted to provide
spare capacity proportionate to the rare event margin. The rare
event margin could be provided, at manufacture, in two ways.
[0266] The rare event margin could be set to be less than unity
meaning that grid response behaviour will not be possible whilst
normal extremes of the network are experienced. This is because the
rare event margin will define the control device's frequency limits
to lie within the grid's frequency extremes. In a grid where grid
responsive behaviour is predominantly provided by fossil fuel
plants and not by the loads, substantial emissions benefits can be
achieved with a rare event margin of less than unity.
[0267] Alternatively, the rare event margin may also be set to
greater than unity. Thus, the grid responsive control device will
tune itself so that even during grid extremes, there is a margin
for exceptional events. This mode is essential when the grid
responsive control devices of the present invention are the
predominant provider of grid responsive behaviour, as some
responsive behaviour in all grid circumstances will be needed.
[0268] Thus, the rare event margin of less than one will be used at
the early stages of implementation of the grid responsive control
device and as the population grows, a rare event margin of greater
than unity will become the normal standard.
[0269] The emissions benefit of having a rare event margin of less
than one arises because providing response at the load end will not
have any impact on emissions, whether of carbon dioxide or other
pollutants. This is in contrast to providing response at the
electricity supply end, where the generation plant will have to be
operated at less than capacity and be able to operate with frequent
dynamic changes (making efficiency and pollution control
harder).
[0270] In order to conclude when an extreme frequency or rare event
has occurred, the grid responsive control device of the present
invention needs some definition of "rare" to use. Extreme grid
events include a failed generation plant or a failed important
transmission line. Such an event is most unlikely to happen any
more than extremely infrequently and it is the sort of event that
the rare event margin of greater than one is intended to cover. On
the other hand, if a transient peak load occurs, such as a TV break
in winter, is not covered by the extreme frequency limits, then the
limits should usefully be adjusted to cover such an event, which is
an indication of grid stress, but not a rare failure.
[0271] It may also be worth considering having different frequency
control limits for different various periods within a day or a week
(many grids use half hours as metering boundaries, and this may be
useful here). The range of the limits may be wider at times when
the demand is changing rapidly, as indicated by the stress status
function (h) defined earlier. Minimum demand times or a low stress
state of the grid could have a narrowed range of frequency control
limits. The times during a day when the grid is most likely to be
stressed could be learned from experience with the grid and the
intervals at which the frequency control limits need to be widened
could be timed by the control device. Since, however, the control
device will not have access to an external clock, this tuning will
need to be discarded whenever the power is switched off.
[0272] In overview, the present invention provides a grid frequency
response control device that minimises switching of loads, resists
all changes of frequency about a historical moving average of the
current frequency and biases the system frequency towards nominal
to some extent. Thus, the grid is stabilised and overworking of the
loads is prevented. A clear frequency signal is also provided that
is less noisy, is smoother and which is gradually and continuously
biased towards an ideal nominal frequency of the grid.
[0273] In the above, switching the energy consumption of the load
between on and off states is performed by directly controlling the
energy consuming device of the load. However, an alternative
implementation of the present invention is to adjust the set point
or the central limits of the parameter of the load. In this way,
the load will adjust its energy consumption to keep the sensed
variable of the load within the control limits.
[0274] In the example of a refrigerator, when the frequency sensed
is such that the refrigerator should switch on, the control limits
can be shifted below the present value of the temperature of the
refrigerator's cooling space. This, the control mechanism of the
refrigerator will detect that the temperature is too high and
respond by switching the cooling means of the refrigerator into an
on state. The opposite direction of moving the control limits can
be performed when the frequency is sensed as being such that the
refrigerator should switch off.
[0275] Instead of adjusting the control limits, the set point
itself can be adjusted by the control device of the present
invention. The control mechanism of the load will receive the new
set point and derive the control limits itself.
[0276] The control device controlling the setpoint or the control
limits in this way may be advantageous. Such a control device will
not need to be integrated into the control circuitry of the load so
as to be able to directly communicate with the energy consuming
means of the load. Instead, it merely needs to provide a central
signal to the control circuitry of the load and the varying of the
energy consumption is performed in the normal way.
[0277] We have up to yet discussed preferred embodiments where grid
responsive control is performed by switching the energy consumption
either on or off. Some loads, however, control a physical variable
of the load within control limits by adjusting the level of energy
consumption. Thus, the load may be controlled between a first state
of increasing the energy stored by the load and a second state of
decreasing the energy stored by the load, as has previously been
discussed. Below is described an example implementation of the
control device of the present invention with a refrigerator using
such continuous control of the energy consumption to maintain the
temperature of the cooled space within control limits.
[0278] A pure temperature controller will likely aim to become a
classic three term controller, with parameters influencing the
extent to which variations from the set point influence the power.
Classically, these are Proportionate error (how big is the error
now); the Integral error (accumulating smaller error over time),
and the Derivative error (so that, if the error is reducing rapidly
its overshoot is minimised). This is known as the physical PID
controller, although the control may, in fact, not include all
three terms and so be simpler than this.
[0279] In general, the PID controller actually drives a motor power
controller, which in turn drives the power electronics of the motor
controller that actually drives the motor or load. FIG. 6 gives
further detail: [0280] The Manual Controller provides input to a
Set Point Controller that provides the set point signal to the PID
controller in a suitable form. The PID controller also has as input
the current state of the variable being controlled, so, in a
fridge, for example, this would be the temperature. [0281] The
output from the PID controller is a desired motor power level. This
is the power level considered appropriate to keep the controlled
variable at its set point. [0282] This desired power level is often
used by a further controller to make adjustments to the actual
power flowing to the motor, as the rate of change of the actual
power may be slower than the rate at which the desired set point
can change. So a further feedback control may be implemented to
ensure that the (electronic) motor controller is set as accurately
as possible.
[0283] Two methods are described by which the desired grid
responsive services of the control device of the present invention
can be enabled in such a load. In a particular implementation
either or both may be used.
[0284] A set point modification approach, as described above,
influences the power consumed by the device by modifying the set
point or control limits used by the PID controller to make its
control decisions. So that, in a fridge, the lower the frequency,
the lower the temperature set point (i.e. increased energy stored),
and the higher the frequency, the higher the temperature set point
(i.e. decreased energy stored). More generally, the lower the
frequency, the higher the internal energy stored, as indicated by
the Physical Variable of the Load, that the device aims to
achieve.
[0285] FIG. 6A shows a block diagram outlining the proposed control
device. As for a conventional controller, a manual input is used to
define the normal set point for the PID. For this controller, this
sets the target internal energy level that will apply when the
actual frequency is the same as the central frequency. That is, it
will apply when no further control over the frequency is
necessary.
[0286] In this controller, a Set Point Adjusting Frequency Function
feeds an adjustment to the Set Point Controller. This signal is
scaled such that: when it is at its maximum positive value, the
internal energy level set point is set to the highest permitted
value; when it is zero, the internal energy level set point is set
to the manual control; and when it is at its maximum negative value
the energy level set point is set to the lowest permitted
value.
[0287] The Set Point Adjusting Frequency Function has two
inputs:
[0288] 1. The central frequency, derived as described above.
[0289] 2. The current value of the sensed frequency.
[0290] At its simplest, the Set Point Adjusting Frequency Function
may operate by comparing the two frequencies, multiplying this by a
parameter, and feeding the result as input to the Set Point
Controller.
[0291] A flaw in this simple approach concerns the possibility
that, if the parameters of the PID and the function (or simple
multiplier) to relate the frequency change to the change in set
point were not specifically tuned for the specific circumstances of
the specific grid, then there is the possibility that the
population of fridges will over or underestimate the change on
output necessary to achieve stability. In correcting this change,
the devices could cause the frequency to oscillate.
[0292] Such oscillation (which arises from loss of what is known as
Small Signal Stability) does sometimes occur in existing grids,
and, if not detected early and corrected, can have severe
consequences. When detected, the normal method of correction is to
reconfigure the grid and generation so the particular frequency of
oscillation is no longer resonant (a fairly hit and miss approach).
It can also be resolved by retuning some of the controllers of the
large gensets that participate in the oscillation. Analysing grids
to detecting and correct and retuning control is demanding of
information and computational capability.
[0293] However, future grids, with very large numbers of grid
responsive control devices according to the present invention,
cannot so easily be deliberately reconfigured (it can happen
accidentally as the oscillations trigger failures!)
[0294] Hence it is important to include in the automatic control
system an element of diversity in the sensitivity of response among
the population of devices. With such diversity, there is a smooth
progression of response from the most sensitive devices to the
least, so making the change in load monotonic with increasing
departure from nominal frequency.
[0295] The achievement of this diversity is described below by
incorporating a probability element to the set point control.
[0296] The controller uses two random numbers, chosen as described
above, one for low frequency, and one for high frequency.
[0297] If the current frequency is below the central frequency,
then the set point adjusting function will: [0298] 1. Derive a
negative value of the frequency difference (e.g. by current
frequency-central frequency).
[0299] 2. Make this value proportionate to the range over which the
controller will operate (Minimum frequency to Nominal
frequency)
[0300] 3. Multiply this value by the low frequency random
number.
[0301] 4. Multiply the result by a sensitivity parameter defining
the sensitivity of the system.
[0302] 5. Feed the result to the Set Point controller, which will
use this to adjust the set point and reduce the energy level it
seeks.
[0303] If the actual frequency is above the central frequency, the
procedure is similar, but uses the high frequency random number,
and may use a different sensitivity parameter.
[0304] The sensitivity parameter will be set in the light of
expected grid behaviour, and may be adjusted in the light of the
experience of the device in use.
[0305] An alternative to set point modification for a PID
controller is an output responsive PID controller which controller
adjusts the normal output of the PID controller to modify the
actual energy consumed by the device according to the
frequency.
[0306] With reference to FIG. 6B, the output of the PID controller
is used by the motor power controller to increase or reduce the
power consumed by the motor.
[0307] If the central frequency is the same as the actual
frequency, the behaviour of the motor power controller continues to
function as normal to keep the control variable within the central
limits.
[0308] FIG. 6B shows a block diagram outlining the operation of
such a control device for use with a PID controlled load.
[0309] If the central frequency and the actual frequency are
different, then the increase or reduction in the power level of the
motor is modified by the signal from an output adjusting frequency
function. With both these signals normalised to reflect the range
over which the devices operate, the four possible control actions
are each discussed: [0310] 1. If the PID controller signal is for
an increase in the motor power level, and the actual frequency is
above that of the central frequency. The desired of both control
signals are in the same direction. In this case the output
adjusting frequency function will enlarge the increase in power
level sought by the PID controller. The calculation will be:
[0310] adjusted power output level increase=PID output power revel
increase+(PID output power level increase*high frequency random
number*high frequency increase parameter*(actual frequency-central
frequency)). [0311] 2. If the PID controller signal is for an
increase in the motor power level and the actual frequency is below
that of the central frequency. In this case the desires of the two
control signals are in conflict. In this case the output adjusting
frequency function will reduce the increase in power level sought
by the PID controller. The calculation will be:
[0311] adjusted power output level increase=PID output power level
increase-(PID output power level increase*low frequency random
number*low frequency reduction parameter*(central frequency-actual
frequency)). [0312] 3. If the PID controller signal is for a
reduction in the motor power level, and the actual frequency is
below the central frequency. The desires of both control signals
are in the same direction. In this case the adjusted output
adjusting frequency function will enlarge the increase in power
level sought by the PID controller. The calculation will be:
[0312] adjusted power output level reduction=PID output power level
reduction+(PID output power level reduction*low frequency random
number*low frequency reduction parameter*(actual frequency-central
frequency)). [0313] 4. The PID controller signal is for a reduction
in the motor power level, and the actual frequency is above the
central frequency. In this case the desires of the two control
signals are in conflict. In this case the adjusted output adjusting
frequency function will reduce the reduction in power level sought
by the PID controller. The calculation will be:
[0313] adjusted power output level reduction=PID output power level
reduction-(PID output power level reduction*high frequency random
number*high frequency reduction parameter*(central frequency-actual
frequency)).
[0314] The four parameters: high frequency increase parameter, low
frequency increase parameter, low frequency reduction parameter,
and high frequency reduction parameter are set in the light of the
desired grid response, and may be adjusted by the controller in the
light of actual grid experience.
[0315] There are many examples of loads having intermittent or
variable energy consumption in order to control a variable within
central limits. Further, there are many devices that can benefit if
they operate to longer term cycles than those discussed up to now.
One example from the water industry is that of "reservoir
profiling". This is used when there are, for example, water
reservoirs that have capacity to meet their needs for a period of a
day or so, or for long enough to span at least one "off-peak"
pricing period.
[0316] In such circumstances, it is possible to let the reservoir
empty below the preferred level when demand for electricity is
high, and replenish it when the cost of electricity is lower. So,
for example, during the morning peak demand period of electricity,
which also corresponds to the morning peak period for water demand,
cost savings are possible by postponing the replenishment of the
reservoir until electricity demand is lower.
[0317] Yet the intermittent nature of reservoir replenishment makes
it an ideal candidate for use with a grid responsive control
device.
[0318] The present example control device makes use of a price
parameter to provide grid responsive control. The current price of
electricity is, like frequency, also representative of the balance
of generation and load on the grid.
[0319] The detection and use of a real time electricity price is
discussed in GB 2407947.
[0320] The price is then used within a central limits or set point
controller to adjust the central limits of the physical variable f
the load.
[0321] The principle is that, as the price rises, the limits (or
set point) for the internal energy store are lowered, and, as the
price falls, the limits (or set point) for the internal energy
store are raised.
[0322] A simple, proportionate control, with the limits chosen to
be proportionate to price is used.
[0323] A refinement of this is to have the price modify the "rate
of change" of the limits. So that, if the price is high, or above a
threshold set by those you pay, then the rate at which the limits
(of internal energy) are reduced is increased. The limits are
prevented from passing extremes set by operational and safety
requirements.
[0324] Similarly, if the price is low, or below a threshold set by
those who pay it, then the rate at which the limits (of internal
energy) are increased is itself increased.
[0325] The ideal tuning for this is to enable a population of such
loads to be able to provide some of both high frequency and low
frequency response at all times, but also to benefit from the
longer term storage by minimising the cost of the electricity.
[0326] The present invention also provides a black start assistance
feature, which allows the energy store loads to provide grid
responsive behaviour during black starts, after a blackout has
occurred. As previously mentioned, the grid is particularly
sensitive at this time and the provision of grid frequency response
loads is necessary to ensure grid stabilisation at this most
important of points and also to speed up the recovery of the
grid.
[0327] Thus, in accordance with a fifth aspect, the present
invention provides a control device for controlling an energy
consumption of a load on an electricity grid, said control device
comprising: [0328] means for delaying the starting of energy
consumption of said load by a randomly generated amount of time
after power is initially provided to the control device.
[0329] A corresponding method for the fifth aspect is provided in a
sixth aspect of the present invention.
[0330] In accordance with a seventh aspect, the present invention
provides a control device for controlling an energy consumption of
a load on an electricity grid to maintain a physical variable of
the load within upper and lower limits, said control device
comprising: [0331] means for sensing the physical variable of the
load; [0332] means for providing the upper and lower limits of the
sensed physical variable of the load; and [0333] means for
increasing the upper and/or lower limit of the sensed physical
variable at a rate less than a maximum energy consumption of the
load after power is initially provided to the control device.
[0334] A corresponding method for the seventh aspect is provided by
an eighth aspect of the present invention.
[0335] The features of the aspects of the invention associated with
the black start mode are combinable to provide a particularly
advantageous control device. They may be used with grid responsive
control devices of the prior art or with the grid responsive
control devise hereinbefore described and, particularly combinable
with the previously set-out aspects and preferred aspects of the
invention. The black start assistance (BSA) aspects of the
invention will now be described in more detail.
[0336] When a load is powered off, this could be due to a power cut
or blackout. The control device of the present invention is adapted
to recognise this possibility.
[0337] In such circumstances the grid may be delicate, and it is
desirable for the device: 1), to start providing both high and low
frequency response as soon as possible; 2), to avoid behaviour
synchronised with other grid responsive control devices; and 3), to
re-establish the sensed physical variable of the energy store load
within its maximum and minimum limits. Since, however, a blackout
could already have moved the sensed physical variable for the load
outside of its control limits, a slight delay of the time to
re-establish the load into its preferred operating range will
generally have a lower priority than keeping the recovering grid
stable.
[0338] The control device of the present invention offers a Black
Start Assistance (BSA) mode upon power up to aid a recovering grid
during reconnection of load.
[0339] In one aspect of the BSA mode, the grid responsive control
device determines a random delay before starting. This delay is
both to prevent a peak load arising upon restoration of power, due
to all the loads switching on as soon as the cut portion of the
grid is reconnected, and to minimise the synchronisation (maximise
the diversity) of the control devices as soon as possible. The
random delay in starting-up after re-connection in black start mode
provides a gradual increase in load on the grid after blackout.
[0340] Upon re-connection, a conventional refrigerator will set a
100 percent on duty cycle for the energy store load until the
sensed physical variable of the load reaches its maximum control
limit (y.sub.max) and will then shut-off immediately. In a second
advantageous aspect of the present invention, however, response is
provided by the load even when the load is being operated at an
accelerated rate to re-establish the load within its preferred
operating parameters.
[0341] According to this second advantageous aspect of BSA, the
load is ramped up to its proper operating condition, i.e. when the
sensed physical variable is within the load's control limits for
the variable, with some duty cycle maintained. The provision of a
duty cycle during this ramping up process allows some response to
be provided, thereby aiding black start. In order to accelerate the
load to its proper operating condition, the energy store load's
limits for the sensed physical variable must be increased. Thus,
the duty cycle is adapted such that the device will operate to a
longer on portion than for normal operation. The load is
controlled, though, so it does still retain a duty cycle. One
example method for achieving this is as follows.
[0342] The first step is to choose a time over which the device
will reach its proper operation. This would be some factor (greater
than 1) of the time that the load would reach this proper operation
if it was not interrupted. This factor will provide periods of no
load during the black start process. In this way, the load can both
switch off in light of a low frequency and on in light of a high
frequency. Thus, the load is able to provide response during the
ramping up of the load's control limits. This factor could, for
example, be the ratio of the expected overall cycle time of the
load to the expected on portion of that cycle.
[0343] In the case of a refrigerator currently at ambient
temperature because of a recent blackout, the normal, 100 percent
on duty cycle time for reaching its maximum temperature limits, say
0 degrees Celsius, could be 30 minutes. Using a factor of two, the
time for increasing the load to its normal operating temperature
range will be 60 minutes.
[0344] The factor chosen can be altered by a randomisation function
to encourage further diversification of the load control
devices.
[0345] The next step is to assess the expected on time for
restoration of normal operation. One way to estimate this is to
extrapolate from the normal temperature change for a unit of on
time of the load to determine how long the load will need to be on
starting from the current energy store level. If necessary, this
estimation of the expected on time can be made more sophisticated
than a linear extrapolation.
[0346] A rate of change of the target energy store level in view of
how long the device will need to be on for and in view of the
period chosen for restoration to the desired level can then be
determined.
[0347] After the random delay has passed, the low energy limit is
set to the current value of the sensed variable and the upper
energy limit is defined to be a normal amount of offset from the
lower limit. The load is started and moved to normal grid
responsive operation.
[0348] The limits are incremented according to the chosen rate of
change of the energy store level.
[0349] An overview of the operation of the grid responsive
behaviour incorporating preferred embodiments of all aspects of the
invention in a single system combined, with reference to FIGS. 4
and 5 will now be given.
[0350] As shown in FIG. 5, the grid responsive controller is
preferably integrated with a load for drawing energy from the grid.
When the load is first plugged into or connected to the grid, the
responsive load control device is adapted to determine the current
frequency of the grid. This frequency measurement is performed
periodically based upon a central processor clock cycle or some
other processing cycle of the responsive control device, or a
predetermined number of such cycles. These consecutive frequency
readings will be accumulated so as to calculate the central
frequency of the grid, amongst other uses, and are critical to the
operation of the grid responsive control device of the present
invention. Apart from the frequency measurement, the grid
responsive control device also requires a physical variable to be
sensed from the load.
[0351] FIG. 4 shows a representation of various states and state
transitions in which the responsive control device can operate. As
can be seen from FIG. 4, the grid responsive control device
preferably starts up in a black start assistance mode, as described
above. In this way, all the loads recently connected to the grid
will provide grid responsive behaviour from the beginning, which,
as already described above, is especially useful after a
blackout.
[0352] As part of the black start assistance features offered by
the present invention, the control device could be provided with an
attended restart actuator (as shown in FIG. 5), which results in
the sensed variable of the load being brought within normal control
limits as soon as possible if actuated. Thus, if the attended
restart control is activated, then the black start assistance mode
is overridden and the load is operated at maximum energy
consumption until the sensed physical variable is provided within
its control limits. This feature is useful as often the load is
simply being switched on for the first time or perhaps after being
serviced. In these circumstances, the grid is relatively stable and
on devices being operated without response for a brief period
during start-up is inconsequential in terms of grid stability.
[0353] The attended restart actuator could be a button provided on
the load. The button should be fitted where a service engineer
would be aware of it, but where it would be inconvenient for a
technically aware load owner to press. If the button was such that
many load owners were aware of the attended restart button, then
the black start assistance function of the grid responsive control
device of the present invention could be overridden.
[0354] Once the sensed variable of the load is within its specified
control limits, the state of the grid will be determined, so as to
derive the mode of operation for the control device. The state of
the grid is determined from the h function defined above and a
measured grid frequency as shown in FIG. 5. As previously stated
and as shown in FIG. 4, the grid can be in a high or low crisis
state, a high or low frequency stress state or a normal state,
depending on the value of the function h.
[0355] A general principle of the responsive control device of the
present invention is that the maximum and minimum limits for the
sensed physical variable (y.sub.max, y.sub.min) is dependent upon
the mode of operation of the control device, as outlined below.
[0356] During black start assistance mode, the current limits for
the sensed physical variable are set around the value of the sensed
physical variable measured upon initial power-up of the load. This
setting of the initial Black Start Assistance limits for the
physical variable is shown in FIG. 5. These limits are incremented
at a predetermined rate until the normal limits for proper
operation of the load are reached, as described more fully above.
It is an advantageous feature of the invention that the
predetermined rate of limit increment provides for the device to
have some duty cycle. Having a duty cycle will allow the load to
provide response, rather than the alternative of having the load
continuously on.
[0357] The increment of the limits during BSA mode is always
performed unless: a period of low frequency stress or crisis is
determined, in which case the limits are frozen; or a period of
high frequency stress or crisis is determined when the rate of
increment is increased. During a low frequency stress or crisis
state, there is too much load on the grid, and so continuing to
increase the energy consumption of the responsive loads is not
appropriate. During a high frequency stress or crisis state, there
is too much generation, so it will be beneficial to the grid to
increase the rate of increment.
[0358] During lower frequency crisis, the limits of the physical
variable of the load are decremented, until they reach a minimum
energy state (y=0). The rate of decrement is chosen to be
approximately half the on running time of the load, so some
response will remain as the limits are reduced towards zero.
[0359] During low frequency stress, the current limits of the load,
as defined by the set point of the load, are frozen so as to
prevent adjustment of the set point by the user. The exception to
this freezing of the limits is in the case of recovery from low
frequency crisis, during which time the limits are incremented to
bring them back towards their value before the crisis state was
entered.
[0360] Once the normal mode of operation after BSA has been
reached, the limits of the sensed physical variable are preferably
controlled depending upon whether the grid is facing high frequency
stress or crisis or low frequency stress or crisis. During high
frequency stress or crisis, off devices are preferably turned on in
order to take up the excess generation. Thus, the value of
y.sub.max is preferably increased such that on devices will remain
on for a longer period of time and previously off devices that have
just been switched on because of the high frequency remain on for
an extended period of time as well. During a low frequency stress
or crisis, the opposite is true, and there is too much load on the
grid. This means that the lower limit of the sensed physical
variable (y.sub.min) is decreased to ensure off devices remain off
for an extra amount of time.
[0361] During high frequency crisis, the limits are incremented
until they reach a maximum energy store level (y=1). The increments
are chosen to approximately double the on portion of the duty cycle
of the load, so as to reduce the energy store level, but still
maintain some response.
[0362] During high frequency stress, the minimum and maximum limits
for the sensed physical variable are frozen for the same reason
that they are frozen during a period of low frequency stress--to
prevent set point adjustment. An exception to these limits being
prevented from being changed occurs when the grid is in recovery
from high or low frequency crisis, when the limits are moved in
small steps until they have become those used before the grid
entered a crisis state.
[0363] The increment of the limits during high frequency crisis or
black start assistance mode of operation of the grid responsive
control device and the decrement of the limits during low frequency
crisis are illustrative of another novel and advantageous feature
of the present invention over the prior art. According to the
present invention, even during such rare grid events, some grid
response behaviour is still given. This response is particularly
beneficial during these grid states if grid stability is to be
recovered.
[0364] The dashed lines in FIG. 4 show illegal transitions which
represent strange behaviour of the grid. For example moving
directly from a low frequency crisis state to a high frequency
crisis state should not occur. In general if such a transition does
happen, an intermediate state is chosen by the control device to
make the state transition of the grid responsive control device
less abrupt.
[0365] While the minimum and maximum limits of the sensed variable
are changed depending upon the mode of operation of the device, the
determination of the trigger frequency is as previously described.
The only difference being that the average temperature level in a
population of such devices will be extended over a larger
temperature range depending on the mode of operation. Thus, in a
crisis mode, the load's variable limit (y.sub.min or y.sub.max)
will be extended as compared to the limits during normal operation.
This will result in the population of the devices providing
response over an extended range of the physical variable of the
load.
[0366] With reference to FIG. 5, once the device has started-up in
black start mode and once the grid status has been determined, the
target and trigger frequencies will be calculated using the
adjusted y.sub.max and/or y.sub.min, which are adjusted depending
on the grid status, and the current value of the physical variable
of the load, as sensed. Having sensed the grid frequency and the
physical variable of the load and having obtained a value of the
sensed frequency to trigger the load on or off, a decision can be
as to whether to switch the device. This decision is made by
comparing the sensed frequency to the trigger frequency and by
comparing the sensed variable of the load to the current limits for
the load's sensed variable.
[0367] Further steps are also shown in FIG. 5. These steps involve
capturing data concerning device operation and using this data to
tune the operation of the device. This capturing and tuning has
already been discussed above with respect to the provision of the
grid frequency limits and their adjustment depending upon
experience of the grid. Further possibilities for tuning the device
are discussed below. The tuned variables could potentially be
stored and re-used advantageously.
[0368] FIG. 5 also shows the possibility of communicating data
captured and this is discussed below.
[0369] The present invention also encompasses a grid responsive
control device as discussed above with certain modifications. These
modifications are optional features that may offer particular
improvements to the control device already discussed.
[0370] The control device of the present invention aims to prevent
rapid switching of the energy store loads, but there may still be
certain grid conditions that result in an excessive switching rate,
particularly when the grid is under stress. Such rapid switching
rates may, in the case of a refrigerator for example, make its
compressor ineffective as well as damaging it. The ineffectiveness
of the compressor may result from a minimum time needed for
internal pressure in the compressor to dissipate after being
switched off. If it is switched on again before this has happened,
the high pressure in the compressor cannot be overcome (it needs an
extra push from the inertia of a running pump), so it will stall.
This can create a high electrical load, dissipated as heat, putting
the whole device at risk. Refrigerators usually have stall or
thermal detectors which disconnect power and so protect the device
from this damage.
[0371] The responsive control device of the present invention may
include a hysterisis feature, such that an on or off state is
maintained for a minimum period, and this can be set to suit the
device. This hysterisis feature is a backup, as the trigger
frequency trajectory being biased to minimise switching should
normally prevent any rapid switching. It will only be in the most
extreme grid conditions that the switching rate will become
excessive and the hysterisis feature is required.
[0372] The grid responsive control device of the present invention
should be capable of operating without any external input, apart
from the frequency and the sensed variable. The grid responsive
control device should also be autonomous over the whole life of the
energy store.
[0373] In order to achieve such autonomy requirements, the grid
responsive control device of the present invention is preferably
adapted to detect the nominal frequency (and this step is shown in
FIG. 5) of the grid itself. As described above, it is important for
the present invention to be aware of the nominal frequency so that
the control device can bias its grid response behaviour so as to
urge the system frequency towards the nominal frequency.
[0374] There are other grid particular settings which the present
invention makes use of, and which, the grid responsive control
device should be able to ascertain from experience of the grid to
which it is connected and not from additional inputs. One other
example is the detection of the upper and lower frequency limits,
as described above.
[0375] In view of the above requirements of the present invention,
the grid responsive control device is adapted to determine the
nominal frequency after taking a series of measurements. For each
of these measurements, a set of "standard nominal frequencies"
stored in a memory of the control device are interrogated and the
closest standard nominal frequency to the grid frequency
measurement is taken as the standard frequency for that
measurement. Once the same standard frequency has been determined
from a consecutive number of frequency measurements, the value
determined is chosen as the nominal frequency of the grid. The
responsive control of the present invention is, therefore, required
to keep a list of possible normal frequencies, such as 50 hertz, 60
hertz and 400 hertz.
[0376] The control device of the present invention may also be
configured to be aware of certain pre-established periods of time,
which are employed in saving any current settings learned from the
grid. Any of these grid experience determined parameters can be
saved in long term non-volatile memory at the end of an appropriate
time period. In this way, key features of the grid behaviour can be
recorded onto long term memory.
[0377] The ability to store data and update this data as the device
learns from the grid's behaviour and the load's behaviour is an
important feature of the present invention (and is shown In FIG. 5)
as it is very possible that a particular load could be moved
between grids. For example, in Denmark the load will not even need
to be moved internationally to change grids. Each grid will behave
differently and the grid responsive control device will need to
react to this and adapt accordingly.
[0378] The control device will also need to tune itself to the
grid's behaviour because it is possible that this behaviour could
change with time, particularly as more and more of the grid
responsive control devices are applied to the energy store loads on
the grid. The self-tuning, however, needs to be performed carefully
as it would not be helpful if, for example, a sustained period of
grid instability caused self-tuning that damaged the device's
ability to respond during a rare crisis.
[0379] The responsive control devices may also need to tune their
parameters to take into account the behaviour of the load. For
example, a very full refrigerator does not behave in quite the same
way as a nearly empty one.
[0380] The possibilities for self-tuning are presently envisaged to
include optimisation taking account of variation in the expected
duty cycle time, optimisation of the maximum and minimum frequency
limits in light of grid experience (as discussed above) and
optimising the use of historical frequency behaviours within the
adaptation parameters.
[0381] If a load is recovering from a blackout, it will be
desirable to retain any tuned parameters achieved before the
blackout. This requires storing of the tuned parameters and other
captured data, as shown in FIG. 5. The control device, however,
also needs to take into account that the device could be being
switched on for the very first time and there are not any
previously tuned parameters to recover. The general principle to
which the grid responsive control devices will be operated is that
the device will aim to recover earlier tuning, unless the device
has been disconnected for so long that it cannot be a blackout, or
the grid nominal frequency has changed.
[0382] A hardware feature could be used to determine whether the
device has been disconnected for longer than a blackout, such as a
leaky capacitor, which, when this is discharged, suggests the load
is in a y=0 state.
[0383] Thus, the controller is provided with some means of
determining whether the load was switched off because of a blackout
or simply because the user had switched it off. In both cases,
recovery of previously tuned parameters is appropriate. If,
however, the load is being switched on for the first time, or is
likely to have been moved between grids, then, loading of
previously determined parameters from memory will not be
performed.
[0384] Recovery from a blackout also realises the possibility of
all of the data capture periods of a population of the loads
connected to the grid becoming synchronised. None of the currently
envisaged processes depends critically upon diversified periods,
but the possibility of rapid change in grid behaviour from
simultaneous identical self-tuning is removed if they are. Thus,
upon initial switch on, the grid responsive control devices of the
present invention are preferably adapted to choose a random time
for any periods that the device makes use of.
[0385] The responsive control device of the present invention often
makes use of the time which the device is expected to be on, and
the time which the device is expected to be off, for example in
determining the rate of increment or decrement of the sensed
variable control limits during black start assistance or high or
low crisis operation. The time the device is expected to be on or
off is the time the load is expected to take in moving from one
sensed variable value to another. Tuning of this expectation time
is possible based on experience of how the sensed variable of the
load reacts to a particular energy consumption level.
[0386] One way of optimising the load's response to energy
consumption is as follows. After each change of state, i.e.
switching from an on status to an off status or vice visa, it is
possible to note how long the load has run, and the extent of
change of the sensed variable in that time. For estimating an
expected on time or off time for a particular variable change,
these noted values can be extrapolated. How the sensed variable
will change with on/off time depends upon its current use, e.g. how
full it is and how often it has been opened. The expected on or off
time calculations could be performed at each switching point, for
example.
[0387] The responsive control device may also make use of a
prediction of how long it will be in an on state or an off state.
This can be determined from a moving average of the actual times of
previous states.
[0388] It is clear from inspection of frequency charts that
different grids have real differences in their frequency behaviour.
The range over which the frequency varies is one important aspect,
but there are also more subtle differences such as its tendency to
fluctuate, the usual length of excursions above nominal, etc.. It
is possible that these features can be used to modify some of the
parameters, such as the parameters adjusting the rate at which
frequency limits narrow. Thus it is important for the responsive
control device of the present invention to capture information on
the behaviour of the grid frequency, particularly at the end of
natural periods, such as a frequency excursion, and of a particular
grid state (normal, stressed, or crisis), the end of particular
state of the load (on or off) or the end of an operation cycle (one
cycle of the processor controlling the timings of the major
functions of the control device). All of the information captured
could be used for the input in tuning the responsive control device
so as to optimise operation with respect to the grid to which it is
connected.
[0389] The responsive control device of the present invention may
also include some form of communication means, and a communication
step is shown in FIG. 5, so that the data collected can be
transferred. The transference of data will normally be provided by
maintenance personnel. The communication means may also be
available such that the software of the responsive control device
or the grid parameters may be updated upon a maintenance visit. The
communication means will also make it possible to capture
measurements of the grid behaviour during the working life of the
load and also the loads contribution to the grid. Thus, some
measure of load's value to the grid can be determined.
* * * * *