U.S. patent application number 10/002847 was filed with the patent office on 2002-06-20 for voltage regulation of a utility power network.
This patent application is currently assigned to American Superconductor Corporation, a Delaware corporation. Invention is credited to Buckles, Warren Elliott, Folts, Douglas C., Hubert, Thomas Gregory.
Application Number | 20020075701 10/002847 |
Document ID | / |
Family ID | 22607129 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020075701 |
Kind Code |
A1 |
Hubert, Thomas Gregory ; et
al. |
June 20, 2002 |
Voltage regulation of a utility power network
Abstract
Power compensation is provided from a power compensation device
to a utility power network carrying a nominal voltage. The power
compensation device has a steady-state power delivery
characteristic. The power compensation is provided by detecting a
change of a predetermined magnitude in the nominal voltage on the
utility power network and controlling the power compensation device
to deliver, for a first period of time and in response to the
detected change in the nominal voltage, reactive power to the
utility power network. The power compensation device is controlled
to deliver, for a second period of time following the first period
of time, reactive power to the utility power network at a level
that is a factor N (N>1) greater than the steady-state power
delivery characteristic of the power compensation device.
Inventors: |
Hubert, Thomas Gregory;
(Necedah, WI) ; Folts, Douglas C.; (Baraboo,
WI) ; Buckles, Warren Elliott; (Madison, WI) |
Correspondence
Address: |
FRANK R. OCCHIUTI
Fish & Richardson P.C.
225 Franklin Street
Boston
MA
02110-2804
US
|
Assignee: |
American Superconductor
Corporation, a Delaware corporation
|
Family ID: |
22607129 |
Appl. No.: |
10/002847 |
Filed: |
November 14, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10002847 |
Nov 14, 2001 |
|
|
|
09718672 |
Nov 22, 2000 |
|
|
|
60167377 |
Nov 24, 1999 |
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Current U.S.
Class: |
363/35 |
Current CPC
Class: |
Y02E 40/20 20130101;
H02J 3/1842 20130101; Y02E 40/22 20130101 |
Class at
Publication: |
363/35 |
International
Class: |
H02J 003/36 |
Claims
What is claimed is:
1. A system for use with a reactive power compensation device
connected to a utility power network carrying a nominal voltage,
the system comprising: a controller which controls the reactive
power compensation device to deliver, for a first period of time
and in response to a detected change in the nominal voltage,
reactive power to the utility power network; wherein, in a second
period of time following the first period of time, the controller
controls the reactive power compensation device to provide reactive
power to the utility power network at a level that is a factor N
(N>1) greater than a maximum power capability characteristic of
the reactive power compensation device.
2. The system of claim 1, wherein, during the first period of time,
the reactive power compensation device provides real power and
reactive power to the utility power network.
3. The system of claim 2, wherein the controller controls the
reactive power compensation device to non-discontinuously decrease
the reactive power to a steady-state power delivery characteristic
after the second period of time.
4. The system of claim 3, wherein a slope of the
non-discontinuously decreasing reactive power is determined on the
basis of a characteristic impedance of the utility power
network.
5. The system of claim 1, wherein the factor N is determined on the
basis of a transient thermal capacity characteristic of the
reactive power compensation device.
6. The system of claim 5, wherein the transient thermal capacity
characteristic is represented by an I.sup.2t rating of the reactive
power compensation device.
7. The system of claim 1, wherein a sum of the first period of time
and the second period of time is determined on the basis of the
ability of the reactive power compensation device to absorb thermal
energy.
8. A method of providing power compensation from a power
compensation device to a utility power network carrying a nominal
voltage, the power compensation device having a steady-state power
delivery characteristic, the method comprising: detecting a change
of a predetermined magnitude in the nominal voltage on the utility
power network; controlling the power compensation device to
deliver, for a first period of time and in response to the detected
change in the nominal voltage, reactive power to the utility power
network; and controlling the power compensation device to deliver,
for a second period of time following the first period of time,
reactive power to the utility power network at a level that is a
factor N (N>1) greater than the steady-state power delivery
characteristic of the power compensation device.
9. The method of claim 8, wherein, during the first period of time
the power compensation device provides real power and reactive
power to the utility power network.
10. The method of claim 9, further comprising, after the second
period of time, non-discontinuously decreasing the reactive power
from the power compensation device to the steady-state power
delivery characteristic.
11. The method of claim 10, further comprising determining a slope
of the nondiscontinuously decreasing reactive power on the basis of
a characteristic impedance of the utility power network.
12. The method of claim 1 1, wherein the characteristic impedance
of the utility power network is determined on the basis of known
characteristics of the utility power network.
13. The method of claim 11, further comprising determining the
characteristic impedance of the utility power network by applying a
stimulus to the network and measuring a response to the
stimulus.
14. The method of claim 8, wherein the factor N is determined on
the basis of a transient thermal capacity characteristic of the
power compensation device.
15. The method of claim 14, wherein the transient thermal capacity
characteristic is represented by an I.sup.2t rating of the power
compensation device.
16. The method of claim 8, wherein the second period of time is
determined on the basis of the ability of the power compensation
device to absorb thermal energy.
Description
CLAIM TO PRIORITY
[0001] This application claims priority from U.S. Provisional
Application No. 60/167,377, entitled "Voltage Regulation of a
Utility Power Network" and filed on Nov. 24, 1999.
INCORPORATION BY REFERENCE
[0002] The following applications are hereby incorporated by
reference into this application as if set forth herein in full: (1)
U.S. patent application Ser. No. 09/240,751, entitled "Electric
Utility Network With Superconducting Magnetic Energy Storage" and
filed on Jan. 29, 1999; (2) U.S. Provisional Application No.
60/117,784, entitled "Electric Utility Network With Superconducting
Magnetic Energy Storage" and filed on Jan. 29, 1999; (3) U.S.
patent application Ser. No. 09/449,505, entitled "Method and
Apparatus for Discharging a Superconducting Magnet" and filed on
Nov. 24, 1999; (4) U.S. patent application Ser. No. 09/449,436,
entitled "Method and Apparatus for Controlling a Phase Angle" and
filed on Nov. 24, 1999; (5) U.S. patent application Ser. No.
09/449,378, entitled "Capacitor Switching" and filed on Nov. 24,
1999; (6) U.S. patent application Ser. No. 09/449,375, entitled
"Method and Apparatus for Providing Power to a Utility Network" and
filed on Nov. 24, 1999; (7) U.S. patent application Ser. No.
09/449,435, entitled "Electric Utility System with Superconducting
Magnetic Energy Storage" and filed on Nov. 24, 1999; and (8) U.S.
Provisional Application No. 60/167,377, entitled "Voltage
Regulation of a Utility Power Network" and filed on Nov. 24,
1999.
BACKGROUND
[0003] This invention relates to electric power utility networks
including generating systems, transmission systems, and
distribution systems serving loads. In particular, the invention
relates to controlling the transfer of energy to and from a utility
power network. Energy storage devices, including capacitor banks
and superconducting magnetic energy storage devices (SMES), are
used to provide power to a utility power network in order to
compensate for power shortfalls or voltage instability problems on
the network. For example, in the event of a fault or outage on the
network, power may be transferred from an energy storage device to
the network to ensure that the amount of power on the network
remains within acceptable limits.
SUMMARY
[0004] The invention features a system for controlling a power
compensation device, such as an inverter connected to a utility
power network, to operate in an "overload" mode. Operating in an
overload mode means operating the power compensation device in
excess of its maximum steady-state power delivery characteristic
(e.g., power delivery rating). This reduces the cost of heat
dissipation elements in the compensating device and reduces the
number of solid state switching devices required therein.
[0005] In one aspect, the invention is a system that includes a
controller which controls a reactive power compensation device to
deliver, for a first period of time and in response to a detected
change in a nominal voltage, reactive power to the utility power
network. In a second period of time following the first period of
time, the controller controls the reactive power compensation
device to provide reactive power to the utility power network at a
level that is a factor N (N>1) greater than a maximum power
capability characteristic of the reactive power compensation
device.
[0006] In another aspect, the invention is directed to providing
power compensation from a power compensation device to a utility
power network carrying a nominal voltage, the power compensation
device having a steady-state power delivery characteristic. This
aspect features detecting a change of a predetermined magnitude in
the nominal voltage on the utility power network, and controlling
the power compensation device to deliver, for a first period of
time and in response to the detected change in the nominal voltage,
reactive power to the utility power network. The power compensation
device is controlled to deliver, for a second period of time
following the first period of time, reactive power to the utility
power network at a level that is a factor N (N>1) greater than
the steady-state power delivery characteristic of the power
compensation device.
[0007] Having detected and reacted to a change of a predetermined
magnitude in the nominal voltage on the utility power network by
increasing injected power to a level that is as much as N times
higher than the maximum steady-state power delivery characteristic
of the compensation device, power injection of the compensating
device can be purposefully and gradually reduced to the maximum
steady-state value so as not to include a transient response by the
network that could result in voltage instability and/or other
undesirable events.
[0008] Among other advantages, these aspects of the invention
provide an approach for operating a reactive power compensation
device in an overload mode for a maximum period of time without
incurring an abrupt, step-like change in inverter current at the
time the overload capability of the compensating device has been
expended, thereby forcing the compensating device's current to be
at or below a specified level. Thus, as noted, the invention
reduces the possibility of undesirable transients (e. g., ringing
oscillations) in the utility power network. Furthermore, a
substantially optimum ramp down profile can be determined on the
basis of the characteristic impedance of the network.
[0009] Embodiments of the foregoing aspects of the invention may
include one or more of the following features. During the first
period of time, the compensation device provides real power and
reactive power to the utility power network. After the second
period of time, the reactive power from the compensation device is
non-discontinuously decreased to the steady-state power delivery
characteristic. The factor N is generally determined on the basis
of a transient thermal capacity characteristic (e. g., a 1% rating)
of the compensation device. The second period of time is determined
on the basis of the ability of the compensation device to absorb
thermal energy. The ramp down profile may be determined on the
basis of the characteristic impedance of the network. The
characteristic impedance of the network may be determined using
known characteristics of the network. Alternatively, the reactive
power compensation device can apply a stimulus to the network and a
response measured.
[0010] These and other features and advantages of the invention
will be apparent from the following description, drawings and
claims.
DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a block diagram showing a reactive power
compensation device, here an inverter, and an energy storage unit
connected to a utility power network through the inverter.
[0012] FIG. 2 is a block diagram of the inverter and energy storage
unit of FIG. 1.
[0013] FIG. 3 is a flow diagram illustrating operation of the
inverter of FIG. 1.
[0014] FIG. 4 is a graph illustrating real and reactive output
power characteristics of the inverter as a function of time.
[0015] FIG. 5 is a graph illustrating the thermal capacity
characteristic of the inverter as a function of time for the output
power characteristic.
[0016] FIG. 6 is a graph modeling the thermal capacity
characteristic of the inverter during a ramp-down process.
DETAILED DESCRIPTION
[0017] Referring to FIG. 1, a power compensation system 30 is shown
connected in shunt with a distribution line 20 of a utility power
network. Distribution line 20 is connected to a transmission line
18 on a transmission line network through a first transformer 22a,
which steps-down a relatively high voltage (e. g., greater than
24.9 kV carried on transmission line 18) to a lower voltage, here 6
kV. A second transformer 22b steps-down the 6 kV voltage to a
voltage suitable for a load 24, here 480 V.
[0018] Power compensation system 30 includes an energy storage unit
32, an inverter system 44, and a controller 60. Energy storage unit
32 may be a part of a D-SMES (Distributed SMES) module which,
together with inverter system 44, is capable of delivering both
real and reactive power, separately or in combination, to
distribution line 20. In this embodiment, the DSMES module is sized
at 3.0 MVA and is capable of delivering an average of 2 MWatts for
periods as long as 400 milliseconds, 7.5 MVA for a full second, and
3.0 MVAR of reactive power for an indefinite period of time. As
described below, inverter 44, under the intelligent control of
controller 60, transfers reactive power to and from the utility
power network.
[0019] Referring to FIG. 2, inverter 44 converts DC voltage from
energy storage unit 32 to AC voltage and, in this embodiment
includes four inverter units 46. Inverter 44 can act as a source
for leading and lagging reactive power. In general, inverter 44 can
only source real power from energy storage unit 32 for as long as
real power is available. However, inverter 44 can source reactive
power indefinitely assuming it is operating at its nominally rated
capacity. Thus, inverter 44 can provide reactive power without
utilizing power from energy storage unit 32. One example of an
inverter that may be used in conjunction with the processes
described herein may be obtained from Integrated Electronics, a
division of American Superconductor Corp. (Part No. A0016701CH).
However, the invention is not limited to use with this type of
inverter and any other type of inverter may instead be used.
Further details regarding the arrangement and operation of the
D-SMES module and inverter 44 can be found in co-pending U.S.
patent application Ser. No. 09/449,435, which was incorporated
herein by reference above.
[0020] Each of the four inverter units 46 is capable of providing
750 KVA continuously and 1.875 MVA in overload mode for one second.
The outputs of each inverter unit 46 are combined on the
medium-voltage side of the power transformers to yield system
ratings in accordance with the following table.
1 Power Flow Value Duration MVA delivered, leading or 3.0
Continuously lagging MVA delivered, leading or 7.5 1-2 seconds in
event of lagging, overload condition transmission or distribution
fault detection Average MW delivered to utility 2.0 0.4 seconds in
event of (for an exemplary D-SMES transmission or distribution
fault module). detection
[0021] Each inverter unit 46 includes three parallel inverter
modules (not shown). Because inverter units 46 are modular in form,
a degree of versatility is provided to accommodate other system
ratings with standard, field-proven inverter modules. A level of
fault tolerance is also possible with this modular approach,
although system capability may be reduced. Each inverter module 46
is equipped with a local slave controller (not shown) that manages
local functions, such as device protection, current regulation,
thermal protection, power balance among modules, and diagnostics,
among others. The inverter units and modules are mounted in racks
with integral power distribution and cooling systems.
[0022] Inverter 44 is coupled to distribution line 20 through one
or more step-down power transformers 50 and one or more switchgear
units 52 (see also FIG. 1). Each power transformer 50 is a 24.9
kV/480 V three-phase oil-filled pad mount transformer having a
nominal impedance of 5.75% on its own base rating. The power
transformers are mounted outdoors adjacent to the system enclosure
with power cabling protected within an enclosed conduit (not
shown). As is shown in FIG. 1, a fuse 53 is connected between
step-down power transformer 50 and distribution line 20.
[0023] Referring back to FIG. 2, each switchgear unit 52 provides
over-current protection between power transformers 50 and inverter
units 46. Each of the four main inverter outputs feeds a circuit
breaker rated at 480 V, 900 A RMS continuous per phase with 45 kA
interruption capacity. Switchgear units 52 also serve as the
primary disconnect means for safety and maintenance purposes. The
switchgear units are generally mounted adjacent to the inverter
unit enclosures.
[0024] Referring again to FIG. 1, system controller 60 is a
multiprocessor-driven system, which utilizes adaptive control
processes. System controller 60 operates as a multi-state machine
for processing inputs from distribution line 20 via line 42 and
inverter units 46 of inverter system 44. System controller 60, as a
function of unit inputs and predetermined internal control rules,
dynamically determines the phase and magnitude of inverter units 46
as well as the real power output of D-SMES module 30. System
controller 60, in operation, passes real-time voltage and current
waveform data to the data acquisition system for processing and
transmission to monitoring sites. System controller 60 also
supports local user interfaces and safety interlocks. Controller 60
necessarily has a response time sufficient to ensure that the
transfer of power to or from energy storage unit 32 occurs at a
speed to address a fault or contingency on the utility system.
[0025] With reference to FIGS. 3 and 4, the operation of controller
60 and inverter 44 is described in conjunction with an exemplary
contingency occurring on the utility power network. At the outset,
the nominal voltage of the utility power network is monitored. For
example, the nominal voltage on distribution line 20 is sensed
either directly or from a remote device. When the nominal voltage
has dropped below a predetermined threshold value (e.g., 90%), a
request is made to operate inverter 44 in overload mode (200) and,
in response, controller 60 transmits a trigger signal to cause
inverter 44 to increase its output current above its steady-state
rating (202). This steady-state rating is referred to in the
figures as InvtrlRefMax/I.sub.max.
[0026] Referring to FIG. 4, inverter system 44 is activated to
provide capacitive reactive power and real power from energy
storage unit 32. In the example depicted in FIG. 4, the energy
storage unit delivers 3 MWatts of real power and about 6.8 MVARs of
capacitive reactive power. After inverter 44 is activated, the real
power is decreased in a period 70, here the decrease is linear, to
about 2 MWatts as the magnet discharges. During period 70 (e.g.,
600 milliseconds) in which the real power is decreased, the
capacitive reactive power is increased from 6.8 MVARs to about 7.2
MVARs. When energy storage unit 32 reaches its cut-off current
level, controller 60 provides a signal to inverter 44 to stop
delivery of real power. The cut-off current level of the energy
storage unit 32 represents a power level of the energy storage unit
that should be maintained for reasons relating to the reliability
of the energy storage unit. That is, the energy storage unit 32 is
generally not allowed to drop below this cut-off current level. At
this point (point 72 of FIG. 4), capacitive reactive power is
increased to comprise the entire maximum overload value for a
period 74 (e.g., 400 milliseconds).
[0027] Referring to FIG. 5, the thermal heat capacity of inverter
44 is shown as a function of time. The inverter's ability to
dissipate energy is referred to in the figures as
InvtrCapacityLimit, which, if exceeded, will lead to destruction of
the inverter. At point 80 (FIG. 4), controller 60 controls inverter
44 to begin decreasing its output current, since the inverter has
reached its maximum thermal heat capacity.
[0028] As can be seen from FIGS. 4 and 5, although the output
current of the inverter rises sharply from the non-overloaded,
steady-state mode to the overload mode, the heat energy rises
gradually over a period 76 (FIG. 5). Thus, this period of time can
be used to provide a substantially greater amount of power to the
utility power network than is normally available in the
steady-state mode. During this time period, controller 60 controls
inverter 44 so that the thermal limit of the inverter is not
exceeded.
[0029] Referring again to FIG. 4, to ensure precise control of
inverter 44, upon increasing the output current level of inverter
44, controller 60 begins to compute the accumulation of energy
being dissipated in inverter 44 (204). This calculation is
performed once every line cycle. To calculate the accumulation of
energy dissipation (i.e., power dissipation per unit time) in the
inverter, it is recognized that dominant loss mechanisms are
proportional to I.sup.2 (inverter current squared). To obtain the
accumulated energy, the power being dissipated over time is
integrated over all samples. The sampled data equivalent of a
continuous time system is a summation of samples of the power
quantity, which is multiplied by the sample time interval as
follows: 1 1 f s n I n I n ,
[0030] where 1/f,=t.sub.s, the sample period and I.sub.n is the
sampled instantaneous inverter current.
[0031] To obtain a value that is proportional to the energy that is
dissipated above the rated, steady-state dissipation capability of
the inverter (i.e., a value related to the transient thermal
capacity limit), a ratio of the instantaneous inverter current
(I.sub.n) to the steady state limit (I.sub.max=InvtrlRefMax) is
obtained as follows: 2 1 f s n ( I n I max ) ( I n I max ) = 1 f s
n ( I n I max ) 2 ( 1 )
[0032] This expression represents the accumulated thermal energy of
the inverter, a static variable that is updated every AC line
cycle. Calculation of the accumulation of energy continues, as
shown by the dotted line of FIG. 3.
[0033] Referring again to FIG. 4, once the period of time 74 has
expired, the capacitive reactive power is decreased in ramp-like
fashion--here, linearly--to a steady-state value (e.g., 3 MVARs).
The capacitive reactive power is decreased in this manner to avoid
an abrupt, step-like change in the reactive power transfer to the
utility power network. A step-like abrupt change in the inverter
current from, for example, 2.5 times the steady-state maximum to
the steady-state maximum, can generate undesirable transients
(e.g., ringing oscillations) on the utility power network, which
can cause false switching and possible damage to equipment on the
utility power network. Thus, the current is steadily decreased in
accordance with the ramp-like profile (210). The ramp-down process
is initiated at a time that ensures that when the inverter current
reaches the steady-state maximum value (InvtrlRefMax), the thermal
capacity limit of the inverter is exhausted. Selecting the
ramp-down profile in this manner provides maximum power delivery to
the load and reduces the probability of line voltage collapse,
while also guarding against initiating undesirable transients on
the network.
[0034] The ramp-down profile is typically a function of the
characteristic impedance of the utility network to which it is
connected. However, the characteristic impedance of a network
changes unpredictably over time. In one approach, a suitable
characteristic impedance value of the network can be derived from
knowledge of the types of loads, conductors, reactive devices and
transformers connected to the network. Alternatively, the
characteristic impedance of the network can be determined by
periodically applying a stimulus (e.g., a step function load) to
the network and measuring the response of the network. In
particular, inverter 44 can be used to apply the step function
load, while controller 60 measures the response. Of course, the
step function load would be of sufficiently low magnitude to
prevent stimulation of undesirable oscillations. The characteristic
impedance is then used to determine the ramp-down profile.
[0035] The summation of each cycle of inverter heat energy being
accumulated must be calculated for each AC line cycle beginning
with the initiation of the overload current above the steady-state
maximum value. This summation is mathematically simple. But, the
accumulation must also be dynamically estimated for each remaining
cycle of the ramp-down process in order to be able to determine
when to initiate the processes, as well as to ensure that ramp-down
is proceeding such that the inverter's heat capacity limit will not
be exceeded. Because the value of inverter current is controlled
and predictable for each cycle of the process, a conceptually
straightforward summation of each of the heat contributions during
each of these cycles can be performed, but not without significant
mathematical overhead, in practice. However, this mathematically
intensive calculation can be simplified dramatically using the
closed form approach described below. Simplifying this calculation
permits the use of a less costly controller and/or significantly
conserves the controller's bandwidth for other tasks.
[0036] Referring to FIG. 6, parabolic curve 100 represents the
I.sup.2 value of inverter 44, as a function of time. The area under
parabolic curve 100 bounded by points abcga represents the energy
dissipated as the inverter current ramps from the value of I to
zero along ramp profile 102. However, of interest is the area
bounded by points abega, which represents the energy dissipated as
the inverter current decreases from the value of I to I.sub.max. To
obtain the area bounded by points abega, the area bounded by points
gecg is first obtained by recognizing that this area is exactly 1/3
the area of rectangle bounded by the points gecfg. The area bounded
by the points abega is then obtained by subtracting the area
bounded by points gecg from the area bounded by the points abcga.
The closed form expression is represented as: 3 1 3 1 f s 1
AmpsPerCycle ( I 3 InvtrIRefMax 3 - InvtrIRefMax ) ( 2 )
[0037] where I is the inverter current, InvtrIRefMax (=I.sub.max)
is as defined above, and AmpsPerCycle is the slope of the ramp-down
of the current. The foregoing expression represents the thermal
capacity predictor for determining when the inverter must begin or
continue the ramp-down of overcurrent toward the maximum
steady-state value.
[0038] The final expression for limiting the overcurrent period of
inverter 44 is the sum of equations (1) and (2), as follows: 4 1 f
s [ n [ ( I n InvtrIReMax ) 2 ] ] + 1 3 f s ( I - InvtrIRefMax )
AmpsPerCycle I 2 InvtrIRefMax 2
[0039] Note that the slope of the thermal energy content (heat
content) of the inverter gradually declines during the ramp-down
period in which the capacitive reactance from inverter 44 is
reduced, and the slope becomes negative only after the inverter
current reaches its maximum steady-state rating.
[0040] At this point, the process has computed the accumulation of
energy being dissipated in the inverter through regions 74 and 82.
Region 74 refers to that part that has actually accumulated, while
region 82 represents the estimated accumulation that will occur
from the current sample until the inverter current reaches the
steady-state level. Samples are accumulated once per cycle for both
regions 74 and 82, although the accumulation in region 82 is for
estimation purposes. Moreover, inverter 44 generally cannot
dissipate its heat at the same rate that the power delivered to the
utility network is reduced. Thus, controller 60 must have
sufficient intelligence to recognize that, in the event of a
subsequent contingency, the thermal energy content of the inverter
may not have decreased back to a level corresponding to the
steady-state current level. When the inverter current declines to
the InvtrIRefMax level (212) (FIG. 3), the inverter will begin to
cool. To reflect the cooling process, the accumulation procedure
must be modified. In particular, although accumulation of heat
energy is still computed, what is accumulated is a recovered
capacity rather than an extended capacity. To do this, controller
60 begins the process by selecting (214) an incrementally higher
value of estimated inverter current than the level of InvtrIRefMax
(the maximum steady-state value) and using this value as if it were
the actual inverter current. By using this value in the heat
accumulation estimation process described above, controller 60 can
verify whether the estimated current can be successfully reduced to
InvtrlRefMax quickly enough so as not to exceed the thermal
capacity limit of the inverter (in the event that a subsequent
request for an over-current is required). In particular, controller
60 determines whether the inverter thermal capacity limit will be
exceeded if the ramp-down process were to be initiated at the
incrementally-larger estimated current level previously mentioned.
If it is not exceeded, a constant value is subtracted from the
accumulation of heat energy (216) and the value of the current is
incremented by the value depicting the slope of the ramp-down
process, called AmpsPerCycle. The estimate is again performed at
the next sample period. The constant value represents the
inverter's thermal recovery increment, a value that essentially
gauges the state of recovery of the inverter from the overload. If
the estimated current results in a prediction that exceeds the
inverter's heat capacity limit, the thermal recovery increment is
still decremented by the constant value, provided that the inverter
current is actually at or below InvtrIRefMax, but the inverter
current estimate remains unchanged, as it is used to constrain the
peak current if a new overload current is requested. The process
continues and, eventually, the full overload thermal capability of
the inverter is restored and the overload current reaches its limit
of N times the steady-state rating.
[0041] Thus, controller 60 controls inverter 44 to provide a
maximum amount of inverter current should another contingency
occur. Controller 60 does so without exceeding the capability the
inverter and by providing a ramping-down to the steady state
InvtrIRefMax level, while ensuring that the thermal capacity of the
inverter is not exceeded by the time that the current declines to
the InvtrIRefMax level.
[0042] For example, as shown in FIG. 4, a second follow-on
contingency (point 86) may occur while the thermal capacity is
still elevated. In this case, when inverter 44 is controlled to
provide additional reactive power to the utility power network, the
inverter current cannot be increased to the previous 750 MVAR level
because the pre-established slope of the ramp down would result in
the thermal capacity of the inverter being exceeded before reaching
InvtrlRefMax. Thus, the inverter current is limited to, in this
example, approximately 600 MVARs (point 86). At this peak inverter
current, the inverter current can still decrease at the ramp down
rate to InvtrlRefMax without, as shown in FIG. 5, exceeding the
thermal limit of the inverter.
[0043] Other embodiments not explicitly described herein are also
within the scope of the claims. For example, in the embodiment
described above in conjunction with FIG. 1, an energy storage unit
32 was used to provide real power during period 70. However, in
certain applications, inverter 44 may be used without an energy
storage unit in order to solely provide reactive power
compensation.
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