U.S. patent application number 10/672075 was filed with the patent office on 2005-03-31 for method and apparatus for controlling power drawn from an energy converter.
Invention is credited to Cutler, Henry H..
Application Number | 20050068012 10/672075 |
Document ID | / |
Family ID | 34376267 |
Filed Date | 2005-03-31 |
United States Patent
Application |
20050068012 |
Kind Code |
A1 |
Cutler, Henry H. |
March 31, 2005 |
Method and apparatus for controlling power drawn from an energy
converter
Abstract
Methods, apparatus, media and signals for controlling power
drawn from an energy converter to supply a load, where the energy
converter is operable to convert energy from a physical source into
electrical energy. Power drawn from the energy converter is changed
when a supply voltage of the energy converter meets a criterion.
The criterion and the change in the amount of power drawn from the
energy converter are dependent upon a present amount of power
supplied to the load. The methods, apparatus, media and signals
described herein may provide improvements to DC to AC maximum power
point tracking in an energy conversion system such as a
photovoltaic power generation system.
Inventors: |
Cutler, Henry H.; (Palm
Harbor, FL) |
Correspondence
Address: |
CHRISTENSEN, O'CONNOR, JOHNSON, KINDNESS, PLLC
1420 FIFTH AVENUE
SUITE 2800
SEATTLE
WA
98101-2347
US
|
Family ID: |
34376267 |
Appl. No.: |
10/672075 |
Filed: |
September 29, 2003 |
Current U.S.
Class: |
323/234 |
Current CPC
Class: |
Y02E 10/563 20130101;
H02J 3/385 20130101; H02J 3/381 20130101; Y02E 10/56 20130101; Y02E
10/58 20130101; H02J 2300/26 20200101; H02J 2300/24 20200101; H02J
3/383 20130101 |
Class at
Publication: |
323/234 |
International
Class: |
G05F 001/10 |
Claims
What is claimed is:
1. A method of controlling power drawn from an energy converter to
supply a load, where the energy converter is operable to convert
energy from a physical source into electrical energy, the method
comprising changing the amount of power drawn from the energy
converter when a supply voltage of the energy converter meets a
criterion, said criterion and a change in the amount of power drawn
from the energy converter being dependent upon a present amount of
power supplied to the load.
2. The method of claim 1 further comprising measuring said supply
voltage.
3. The method of claim 1 wherein changing said power drawn from the
energy converter comprises decreasing said power drawn from the
energy converter by an amount corresponding to a change in said
power supplied to the load in a time interval.
4. The method of claim 1 wherein changing said power drawn from the
energy converter comprises increasing said power drawn from the
energy converter by an amount associated with a range of power
supplied to the load.
5. The method of claim 1 further comprising deeming said supply
voltage satisfies said criterion when said supply voltage is within
a first range of voltages relative to a reference voltage.
6. The method of claim 5 wherein said reference voltage corresponds
to a maximum power point of the energy conversion device.
7. The method of claim 5 wherein said first range includes voltages
greater than said reference voltage.
8. The method of claim 5 wherein said first range includes voltages
less than said reference voltage.
9. The method of claim 5 wherein said first range includes voltages
less than said reference voltage and voltages greater than said
reference voltage.
10. The method of claim 5 wherein said first range excludes a range
of voltages within a limit of said reference voltage.
11. The method of claim 5 wherein said first range is dependent
upon a trend in measured voltage.
12. The method of claim 11 wherein said first range is dependent
upon a change in voltage occurring after an increase in power.
13. The method of claim 12 wherein said first range is bounded
between minimum and maximum limits.
12. The method of claim 1 further comprising performing said method
periodically.
13. The method of claim 12 further comprising defining a period for
performing said method periodically.
14. The method of claim 13 wherein defining said period comprises
defining said period as a function of said power supplied to the
load.
15. The method of claim 14 further comprising increasing said
period when said power supplied to the load is relatively low and
decreasing said period when said power supplied to the load is
relatively high.
16. The method of claim 5 further comprising adjusting said
reference voltage periodically.
17. The method of claim 5 further comprising increasing said
reference voltage when a change in power drawn from the energy
converter results in a change in supply voltage within a second
range.
18. The method of claim 17 wherein said second range is dependent
upon the amount of power being supplied to the load.
19. The method of claim 18 wherein said second range is relatively
small when a relatively large amount of power is supplied to the
load and wherein said second range is relatively large when a
relatively small amount of power is supplied to the load.
20. The method of claim 17 wherein an amount by which said
reference voltage is decreased is dependent upon the amount of
power supplied to the load.
21. The method of claim 20 wherein the amount by which said
reference voltage is decreased is relatively large when the amount
of power supplied to the load is relatively low and wherein the
amount by which said reference voltage is decreased is relatively
low when the amount of power supplied to the load is relatively
high.
22. An apparatus for controlling an energy transfer device operable
to draw electrical energy from an energy converter operable to
convert energy from a physical source into electrical energy, and
supply said electrical energy to a load, the apparatus comprising:
a load power sensor operable to measure power supplied to the load
by the energy transfer device; a voltage sensor operable to measure
a supply voltage the energy converter; and a processor, in
communication with said voltage sensor, said load power sensor and
the energy transfer device, said processor being configured to
cause the energy transfer device to change the amount of power
drawn from the energy converter when the supply voltage of the
energy converter meets a criterion, said criterion and the change
in power drawn from the energy converter being dependent upon a
present amount of power being supplied to the load.
23. The apparatus of claim 22 wherein said processor is configured
to decrease said power drawn from the energy converter by an amount
corresponding to a change in said power drawn from the energy
converter in a time interval.
24. The apparatus of claim 22 wherein changing said processor is
configured to increase said power drawn from the energy converter
by an amount associated with a range of power supplied to the
load.
25. The apparatus of claim 22 wherein said processor is configured
to deem said supply voltage satisfies said criterion when said
supply voltage is within a first range of voltages relative to a
reference voltage.
26. The apparatus of claim 25 wherein said reference voltage
corresponds to a maximum power point of the energy conversion
device.
27. The apparatus of claim 25 wherein said first range includes
voltages greater than said reference voltage.
28. The apparatus of claim 25 wherein said first range includes
voltages less than said reference voltage.
29. The apparatus of claim 25 wherein said first range includes
voltages less than said reference voltage and voltages greater than
said reference voltage.
30. The apparatus of claim 25 wherein said first range excludes a
range of voltages within a limit of said reference voltage.
31. The apparatus of claim 25 wherein said first range is dependent
upon a trend in measured voltage.
32. The apparatus of claim 31 wherein said first range is dependent
upon a change in voltage occurring after an increase in power.
33. The apparatus of claim 32 wherein said first range is bounded
between minimum and maximum limits.
34. The apparatus of claim 22 wherein said processor is configured
to periodically measure said supply voltage and change said supply
power drawn from the energy converter accordingly.
35. The apparatus of claim 34 wherein said processor is configured
to define a period for measuring said supply voltage.
36. The apparatus of claim 35 wherein said processor is configured
to define said period as a function of said power supplied to the
load.
37. The apparatus of claim 36 wherein said processor is configured
to increase said period when said power supplied to the load is
relatively low and decrease said period when said power supplied to
the load is relatively high.
38. The apparatus of claim 25 wherein said processor is configured
to adjust said reference voltage periodically.
39. The apparatus of claim 25 wherein said processor is configured
to increase said reference voltage when a change in power drawn
from the energy converter results in a change in supply voltage
within a second range.
40. The apparatus of claim 39 wherein said second range is
dependent upon the amount of power being drawn from the energy
converter.
41. The apparatus of claim 40 wherein said second range is
relatively small when a relatively large amount of power is being
drawn from the energy converter and wherein said second range is
relatively large when relatively small amounts of power are being
drawn from the energy converter.
42. The apparatus of claim 39 wherein said processor is configured
to decrease said reference voltage by an amount dependent upon the
amount of power supplied to the load.
43. The apparatus of claim 42 wherein said processor is configured
to decrease said reference voltage by a relatively large amount
when the power supplied to the load is relatively low and to
decrease said reference voltage by a relatively small amount when
the power supplied to the load is relatively high.
44. The apparatus of claim 22 wherein said processor includes an
output operable to provide a power command signal to said energy
transfer device, and wherein said processor is configured to
produce said power command signal to represent said change in power
to be drawn from the energy conversion device.
45. A system comprising the apparatus of claim 1 and further
comprising said energy transfer device.
46. The system of claim 45 wherein said energy transfer device
includes a DC to DC converter connected between said energy
converter and said load.
47. The system of claim 46 wherein said energy transfer device
includes a Dc to AC inverter connected between said DC to DC
converter and said load.
48. The system of claim 45 further comprising said load.
49. The system of claim 48 wherein said load includes an AC power
grid.
50. The system of claim 45 wherein said processor includes an
output operable to provide a power command signal to said energy
transfer device, and wherein said processor is configured to
produce said power command signal to represent said change in power
to be drawn from the energy conversion device.
51. An apparatus for controlling an energy transfer device operable
to draw electrical power from an energy converter operable to
convert energy from a physical source into electrical energy, and
supply said electrical energy to a load, the apparatus comprising:
means for measuring power supplied to the load by the power
transfer device; means for measuring a supply voltage of the energy
converter; and means, in communication with said means for
measuring power, said means for measuring voltage and the energy
transfer device, for changing the amount of power drawn from the
energy converter by the energy transfer device when a supply
voltage of the energy converter meets a criterion, said criterion
and a change in the amount of power drawn from the energy converter
being dependent upon a present amount of power being supplied to
the load.
52. A computer readable medium encoded with codes for directing a
processor to control an energy transfer device operable to draw
power from an energy converter operable to convert energy from a
physical source into electrical energy, and supply said energy to a
load, the codes directing the processor to cause the energy
transfer device to change the amount of power drawn from the energy
converter when a supply voltage of the energy converter meets a
criterion, said criterion and a change in the amount of power drawn
from the energy converter being dependent upon a present amount of
power supplied to the load.
53. A computer readable signal encoded with codes for directing a
processor to control an energy transfer device operable to draw
power from an energy converter operable to convert energy from a
physical source into electrical energy, and supply said energy to a
load, the codes directing the processor to cause the energy
transfer device to change the amount of power drawn from the energy
converter when a supply voltage of the energy converter meets a
criterion, said criterion and a change in the amount of power drawn
from the energy converter being dependent upon a present amount of
power supplied to the load.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of Invention
[0002] This invention relates to energy conversion and more
particularly to methods and apparatus for controlling power drawn
from an energy converter operable to convert energy from a physical
source into electrical energy.
[0003] 2. Description of Related Art
[0004] Energy conversion devices such as photovoltaic arrays are
commonly used to provide power to electrical loads. Often these
loads are direct current (DC) loads such as batteries, for example.
Recently, efficiencies in power conversion devices are giving rise
to solar power systems that supply power to an alternating current
(AC) load such as an AC power grid such as may be operated by a
public utility company. Such power systems may employ a
photovoltaic array and an interface for converting power in a form
received from the photovoltaic array into a form operable to be
received by the AC power grid. Such an interface may involve a DC
to AC inverter.
[0005] Interfaces of the type described above often seek to cause
maximum power to be provided to the AC power grid. The maximum
power available to be provided to the AC power grid depends upon
the conditions under which the energy conversion device is operated
and in the case of a photovoltaic array, these conditions include
the amount of insolation and the temperature of the array, for
example. A maximum power point, or voltage at which maximum power
may be extracted from the array, is a desirable point at which to
operate the array and conventional systems seek to find this point.
The maximum power point changes however, due to changes in
insolation and due to changes in temperature of the array and thus
control systems are employed to constantly seek this point.
[0006] One way of seeking the maximum power point is to
periodically perturb and observe the power output of the array and
then adjust the power demanded from the array accordingly to cause
the voltage of the array to be as close as possible to the maximum
power point. Typically, such perturb and observe methodologies
involve perturbing the present power supplied to the load by a
fixed amount such as 4 watts, for example and then observing the
effect on power supplied by the array and the voltage measured at
the array. Perturbing involves temporarily increasing the power
supplied to the load by a fixed amount such as 4 watts, for
example. If the change in power is negative and voltage measured at
the array drops by a significant amount, too much power is being
extracted from the array and the power demand on the array must be
reduced, in which case the power supplied by the array is usually
reduced by some fixed incremental value, such as 4 watts, for
example. If the voltage does not change by a significant amount
when the power is perturbed, perhaps not enough power is being
extracted from the array and the present power drawn from the array
must be increased in which case the power demanded from the array
is usually increased by a fixed amount, such as 4 watts.
[0007] The above described perturb and observe methodology is
typically conducted at the switching speed of a switching mode
power supply connected to the array, e.g., 100 kHz, and results in
a dithering of power drawn from the array, in fixed amounts. Where
the incremental amount is 4 watts for example, as described above,
there will be a constant dithering of power demanded from the
array, in the amount of 4 watts about a common mode value which may
be approximately equal to the maximum power output of the array.
When the load is an AC power grid, the load effectively fluctuates
at the line frequency of the grid, which in North America is
typically 60 Hz. Consequently, the 100 kHz perturb and observe
frequency of most switching mode power supplies used to supply DC
loads is too fast for applications where the load is an AC power
grid. Thus, the perturb and observe frequency must be
decreased.
[0008] However, decreasing the perturb and observe frequency can
waste power, especially when changes in insolation occur.
[0009] Changes in insolation can change the maximum power available
from the array from say 200 watts to 2000 watts in a matter of
seconds. This situation may occur when a cloud, for example, moves
or dissipates from a position blocking sunlight shining on the
array to a position in which full sun is received on the array.
With 4 watt power increments, and a perturb and observe period of
50 mSec, the time to change the power drawn from the array from 200
watts to 2000 watts would be about 22 seconds. During this period
the full available power is not being drawn from the array
resulting in inefficient operation.
SUMMARY OF THE INVENTION
[0010] In accordance with one aspect of the invention there is
provided a method of controlling power drawn from an energy
converter to supply a load, where the energy converter is operable
to convert energy from a physical source into electrical energy.
The method involves changing the amount of power drawn from the
energy converter when a supply voltage of the energy converter
meets a criterion, said criterion and a change in the amount of
power drawn from the energy converter being dependent upon a
present amount of power supplied to the load.
[0011] The method may involve measuring the supply voltage.
[0012] Changing the power drawn from the energy converter may
include decreasing the power drawn from the energy converter by an
amount corresponding to a change in the power supplied to the load
in a time interval, or it may include increasing the power drawn
from the energy converter by an amount associated with a range of
power supplied to the load.
[0013] The method may involve deeming the supply voltage satisfies
the criterion when the supply voltage is within a first range of
voltages relative to a reference voltage. The reference voltage may
correspond to a maximum power point of the energy conversion
device. The first range may include voltages greater than the
reference voltage, or the first range may include voltages less
than the reference voltage. Alternatively, the first range may
include voltages less than the reference voltage and voltages
greater than the reference voltage. The first range may exclude a
range of voltages within a limit of the reference voltage, and the
first range may be dependent upon a trend in measured voltage. The
first range may be dependent upon a change in voltage occurring
after an increase in power. Moreover, the first range may be
bounded between minimum and maximum limits.
[0014] The method may further involve performing the method
periodically, and defining a period for performing the method
periodically. Defining the period may include defining the period
as a function of the power supplied to the load. The method may
involve increasing the period when the power supplied to the load
is relatively low and decreasing the period when the power supplied
to the load is relatively high.
[0015] The method may further involve adjusting the reference
voltage periodically, or may involve increasing the reference
voltage when a change in power drawn from the energy converter
results in a change in supply voltage within a second range. The
second range may be dependent upon the amount of power being
supplied to the load. The second range may be relatively small when
a relatively large amount of power is supplied to the load and the
second range may be relatively large when a relatively small amount
of power is supplied to the load. The amount by which the reference
voltage is decreased may be dependent upon the amount of power
supplied to the load. The amount by which the reference voltage is
decreased may be relatively large when the amount of power supplied
to the load is relatively low and the amount by which the reference
voltage is decreased may be relatively low when the amount of power
supplied to the load is relatively high.
[0016] In accordance with another aspect of the invention there is
provided an apparatus for controlling an energy transfer device
operable to draw electrical energy from an energy converter
operable to convert energy from a physical source into electrical
energy and supply the electrical energy to a load. The apparatus
includes a load power sensor operable to measure power supplied to
the load by the energy transfer device, a voltage sensor operable
to measure a supply voltage the energy converter, and a processor,
in communication with the voltage sensor, the load power sensor and
the energy transfer device. The processor is configured to cause
the energy transfer device to change the amount of power drawn from
the energy converter when the supply voltage of the energy
converter meets a criterion, wherein said criterion and the change
in power drawn from the energy converter is dependent upon a
present amount of power being supplied to the load.
[0017] The processor may be configured to decrease the power drawn
from the energy converter by an amount corresponding to a change in
the power supplied to the load in a time interval. Alternatively,
the processor may be configured to increase the power drawn from
the energy converter by an amount associated with a range of power
supplied to the load.
[0018] The processor may be configured to deem that the supply
voltage satisfies the criterion when the supply voltage is within a
first range of voltages relative to a reference voltage. The
reference voltage may correspond to a maximum power point of the
energy conversion device.
[0019] The first range may include voltages greater than the
reference voltage, it may includes voltages less than the reference
voltage, or it may include voltages less than the reference voltage
and voltages greater than the reference voltage. The first range
may exclude a range of voltages within a limit of the reference
voltage, and may be dependent upon a trend in measured voltage. The
first range may further be dependent upon a change in voltage
occurring after an increase in power, and may be bounded between
minimum and maximum limits.
[0020] The processor may be configured to periodically measure the
supply voltage and change the power drawn from the energy converter
accordingly, and may be further configured to define a period for
measuring the supply voltage, and the period may be defined as a
function of the power supplied to the load. The processor may be
configured to increase the period when the power supplied to the
load is relatively low and decrease the period when the power
supplied to the load is relatively high.
[0021] The processor may be configured to adjust the reference
voltage periodically. The processor may be configured to increase
the reference voltage when an increase in power drawn from the
energy converter results in a change in supply voltage within a
second range. The second range may be dependent upon the amount of
power being drawn from the energy converter. The second range may
be relatively small when relatively large amounts of power are
being drawn from the energy converter and the second range may be
relatively large when relatively small amounts of power are being
drawn from the energy converter. The processor may be further
configured to decrease the reference voltage by an amount dependent
upon the amount of power supplied to the load. In particular, the
processor may be configured to decrease the reference voltage by a
relatively large amount when the power supplied to the load is
relatively low and to decrease the reference voltage by a
relatively small amount when the power supplied to the load is
relatively high. The apparatus may include an output operable to
provide a power command signal to the energy transfer device, and
the processor may be configured to produce the power command signal
to represent the change in power to be drawn from the energy
conversion device.
[0022] In accordance with another aspect of the invention there is
provided a system including the foregoing apparatus and further
including the energy transfer device. The energy transfer device
may include a DC to DC converter connected between the energy
converter and the load, and may also include a DC to AC inverter
connected between the DC to DC converter and the load. The system
may further include the load, and the load may include an AC power
grid. The processor may include an output operable to provide a
power command signal to the energy transfer device, and the
processor may be configured to produce the power command signal to
represent the change in power to be drawn from the energy
conversion device.
[0023] In accordance with another aspect of the invention there is
provided an apparatus for controlling an energy transfer device
operable to draw electrical power from an energy converter operable
to convert energy from a physical source into electrical energy,
and supply the electrical energy to a load. The apparatus includes
provisions for measuring power supplied to the load by the power
transfer device, provisions for measuring a supply voltage of the
energy converter, and provisions, in communication with the
provisions for measuring power, the provisions for measuring
voltage and the energy transfer device, for changing the amount of
power drawn from the energy converter by the energy transfer device
when a supply voltage of the energy converter meets a criterion,
wherein said criterion and a change in the amount of power drawn
from the energy converter are dependent upon a present amount of
power being supplied to the load.
[0024] In accordance with another aspect of the invention there is
provided a computer readable medium encoded with codes for
directing a processor circuit to control an energy transfer device
operable to draw power from an energy converter operable to convert
energy from a physical source into electrical energy, and supply
the energy to a load, the codes directing the processor circuit to
cause the energy transfer device to change the amount of power
drawn from the energy converter when a supply voltage of the energy
converter meets a criterion, said criterion and a change in the
amount of power drawn from the energy converter is dependent upon a
present amount of power supplied to the load.
[0025] In accordance with another aspect of the invention there is
provided a computer readable signal encoded with codes for
directing a processor circuit to control an energy transfer device
operable to draw power from an energy converter operable to convert
energy from a physical source into electrical energy, and supply
the energy to a load, the codes directing the processor circuit to
cause the energy transfer device to change the amount of power
drawn from the energy converter when a supply voltage of the energy
converter meets a criterion, said criterion and a change in the
amount of power drawn from the energy converter being dependent
upon a present amount of power supplied to the load.
[0026] Other aspects and features of the present invention will
become apparent to those ordinarily skilled in the art upon review
of the following description of specific embodiments of the
invention in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] In drawings which illustrate embodiments of the
invention,
[0028] FIG. 1 is a block diagram of an energy conversion system
according to a first embodiment of the invention.
[0029] FIG. 2 is a graph of power-voltage characteristics of a
photovoltaic cell array for various values of insolation S at a
temperature of 25 degrees Celsius.
[0030] FIG. 3 is a block diagram of an energy transfer device
according to an embodiment of the invention.
[0031] FIG. 4 is a block diagram of a processor circuit of the load
interface shown in FIG. 3.
[0032] FIG. 5 is a flow chart of a main routine executed by the
processor circuit shown in FIG. 4.
[0033] FIG. 6 is a flow chart of a regulation window calculation
subroutine called by the main routine shown in FIG. 5.
[0034] FIG. 7 is a schematic representation of regulation zones
associated with the photovoltaic array shown in FIG. 2.
[0035] FIG. 8 is a flow chart of a regulate routine called by the
main routine shown in FIG. 5.
[0036] FIG. 9A is a flow chart of a more power routine called by
the regulate routine shown in FIG. 8.
[0037] FIG. 9B is a table for determining a change in power
according to a current AC power being provided by the system shown
in FIG. 3.
[0038] FIG. 10A is a flow chart of a less power routine called by
the regulate routine shown in FIG. 8.
[0039] FIG. 10B is a Table relating present AC power to a DC offset
value, an MPPT increase value and a back-off value used by the
second portion of the less power routine shown in FIG. 10A.
[0040] FIG. 11A is a flow chart of a find MPPT subroutine called by
the main routine shown in FIG. 5.
[0041] FIG. 11B is a Table relating current AC power to an MPPT
limit used by the find MPPT subroutine shown in FIG. 11A.
DETAILED DESCRIPTION
[0042] Referring to FIG. 1 an energy supply system according to a
first embodiment of the invention is shown generally at 10. The
system includes an energy converter 12 and an energy transfer
device 14 which together cooperate to supply energy to a load
16.
[0043] The energy converter 12 is of a general class of energy
conversion devices that are able to supply electrical power in
response to a supply of physical energy. Such devices are able to
be operated under conditions where the supply voltage and supply
current produced by the device are optimized such that for a given
physical power input a maximum electrical power, i.e. a maximum
working power is produced. The supply current and supply voltage
conditions under which maximum working power can be extracted from
the energy conversion device change depending upon the physical
power available and operating conditions of the device.
[0044] For example, the energy converter 12 may include a
photovoltaic array and the energy transfer device 14 may include a
DC to AC converter for supplying electrical energy to an AC load
such as an AC power grid.
[0045] Where the energy converter 12 includes a photovoltaic array,
physical energy in the form of light energy is converted by the
photovoltaic array into electrical energy. The maximum working
power that can be drawn from the photovoltaic array depends upon
the physical power available, i.e. the amount of light insolating
the array and the temperature of the array. For every insolation
and temperature combination there is a maximum power point at which
the supply voltage and supply current produced by the array are
optimized to cause maximum energy conversion efficiency, or in
other words to allow the most working power possible to be drawn
from the array.
[0046] Changes in voltage at the array are effected by changes in
the amount of current drawn from the array. In general, the greater
the current draw, the less the voltage. Since the power drawn from
the array may be calculated as the product of the current and
voltage at the array, the power output of the array may be plotted
relative to voltage as shown in FIG. 2, for various levels of
insolation. From FIG. 2 it can be seen that the power output of the
photovoltaic array increases to a point and then decreases with
increasing array voltage. The point at which the power is the
greatest is the maximum power point. The embodiment described
herein seeks to find this maximum power point and regulate the
output voltage of the array to it.
[0047] In the embodiment shown, referring back to FIG. 1, in
accordance with one aspect of the invention, the energy transfer
device 14 controls the power drawn from the energy converter 12 by
measuring a supply voltage of the energy converter and when the
measured supply voltage satisfies a first criterion dependent upon
the power supplied to the load, the amount of power drawn from the
energy converter is, in one mode of operation, changed by an amount
dependent upon the amount of power being supplied to the load 16.
Changing the amount of power being drawn from the energy converter
may involve decreasing the power by an amount corresponding to a
change in the power supplied to the load during a time interval or
increasing the source power by an amount associated with a range of
output power.
[0048] Referring to FIG. 3, to appreciate how the energy transfer
device 14 can produce this effect, the following example is
provided in which the energy converter 12 is a photovoltaic array
18 and the energy transfer device 14 includes a Xantrex Suntie.RTM.
utility grid interactive inverter 20. The inverter 20 employs two
conversion stages including a DC to DC converter 22 operable to
convert input power from the array 18 at a nominal supply voltage
of 48 volts to stored power at a voltage of about 380 volts. This
converter is a closed loop device and is operable to provide power
at a constant DC voltage of 380 volts. The inverter 20 further
includes a DC to AC inverter 24 operable to convert the stored
power at 380V into AC power at 240 RMS volts. The load 16 is an AC
power grid operated by a public utility company, for example.
[0049] The power inverter 20 has a processor 26 operable to control
the DC to DC converter 22 and DC to AC converter 24 to change the
amount of working power drawn from the array 18 to correspondingly
change the amount of working power supplied to the AC load 16. To
do this the inverter 20 includes a DC current sensor 28 for sensing
the current supplied by the array 18, a DC voltage sensor 30 for
sensing the supply voltage at the array 18, and an AC power sensor
32 for sensing AC power supplied to the AC load 16. These sensors
28, 30 and 32 are in communication with the processor 26 and the
controller is able to read and interpret signals therefrom as array
current (lk), array voltage (Vk) and AC power (ACP) respectively.
The sensors 28, 30 and 32 may respectively provide a current
measurement resolution of about 62.5 mA, a voltage resolution of
about 0.125V, and an AC power resolution of about 1 W, for
example.
[0050] Referring to FIG. 4, the processor 26 may include a
microchip PIC 16F876A, for example, having a CPU 40, a program
memory 42, random access memory 44 and an I/O interface 46. Signal
lines 48, 50 and 52 operable to receive a signal from the DC
current sensor 28, a signal from the DC voltage sensor 30 and a
signal from the AC power sensor 32, respectively, are connected to
the I/O interface. The I/O interface 46 also provides an AC power
command signal to the DC to AC inverter 24, specifying a desired AC
power to be supplied to the AC power grid 16. In general, in
response to the DC current signal, the DC voltage signal and the AC
power signal, an appropriate AC load power command signal is
produced by the processor 26 to control the DC to AC inverter 24
such that maximum power is extracted from the array 18.
[0051] The processor 26 may be the same processor used to control
switching of transistors in the DC to DC converter 22 and the DC to
AC converter 24, for example, and programs for controlling the DC
to DC converter 22 and DC to AC converter 24 may be stored in the
program memory 42. In addition, the program memory 42 may be
programmed with codes for directing the processor 26 to carry out
methods according to various embodiments and aspects of the
embodiment of the invention as described herein. In particular,
these codes may cause the processor 26 to implement control
routines described by way of the flowcharts, tables and graphs
shown in FIGS. 5-11B, to effect the functionality of the methods
according to this embodiment of the invention.
[0052] Referring to FIG. 5, a main routine according to the first
embodiment of the invention is shown generally at 50. This routine
is run every 16.66 milliseconds. This 16.66 millisecond period is
chosen because it is the period of the line frequency (60 hertz) of
the AC power supplied to the grid. Thus, the main routine is
invoked once for every cycle of the AC waveform provided to the AC
power grid.
[0053] The main routine begins by causing the processor 26 to
execute any DC to DC and DC to AC control modules, as shown at 52.
As part of these modules, a measurement of the photovoltaic array
voltage Vk is taken, a measurement of the current Ik produced by
the photovoltaic array is taken and an AC power measurement ACP is
taken. Also within these modules, the array voltage Vk and array
current Ik measurement values are multiplied together to produce a
power value Pk associated with the current pass through the
routine. A power value calculated from one or more previous passes
through the routine may be stored to enable a change in power value
to be calculated within these modules. A representation of a change
in power dP from one pass through the routine to the next is
required in subsequent routines described herein. Similarly, a
change in voltage dV from one pass to next is calculated for use in
subsequent routines.
[0054] After completing the DC to DC and DC to AC control modules
52, block 54 directs the processor 26 to a "calculate regulation
window" routine.
[0055] Referring to FIG. 6, the "calculate regulation window"
routine is shown at 56 and begins with a first block 58 that causes
the processor 26 to determine whether the change in power since the
last pass is greater than zero and whether the change in voltage
since the last pass is less than or equal to zero and whether or
not a state variable labeled "action" is equal to a regulate
increase state. (The way in which the action state variable is set
will be described below).
[0056] Assuming the above conditions are met, block 60 directs the
processor 26 to set a variable referred to as dv_mp equal to the
change in voltage since the last increase in power caused by a
"more power" routine described below, and is thus dependent upon
the trend in measured array voltage. Block 62 then directs the
processor 26 to determine whether this dv_mp value is greater than
a pre-set value, in this instance 2.0 volts, and if so, block 64
directs the processor 26 to set the dv_mp value to 2.0. Block 66
directs the processor 26 to determine whether the dv_mp value is
less than another predetermined value, in this case 0.25 volts, and
if so, block 68 directs the processor 26 to set the dv_mp value
equal to 0.25 volts. In effect, blocks 62 through 68 cause the
processor 26 to set the dv_mp value to the average change in
voltage over the last two power increases between a maximum value
of 2.0, and a minimum value of 0.25.
[0057] Referring to FIG. 7, the dv_mp value is used to define a
boundary between a regulate increase zone shown generally at 70 and
a no-action zone shown generally at 72 among the possible range of
array voltages V.sub.k. The effect of changes in dv_mp are to
adjust up or down, the boundary between the regulate increase zone
70 and the no-regulate zone 72 indicated by line 74. The no-action
zone 72 is defined between this boundary 74 and a line 76
determined by a maximum power point tracking reference voltage
(MPPT_ref) which is initially set at about 84% of the open circuit
voltage of the array in this embodiment. Below this line 76 a
regulate decrease zone 77 is established. The regulate increase
zone 70 acts as a first range of voltages relative to a reference
voltage corresponding to a maximum power point of the array, and
includes voltages greater than the reference voltage (MPPT_ref).
The regulate decrease zone includes a range of voltages less than
the reference voltage (MPPT_ref). A first range of voltages for
which criteria for changing the amount of power drawn from the
array are considered to be met thus includes voltages in the
regulate increase and regulate decrease zones 70 and 77 and
excludes a range of voltages within a limit of the reference
voltage, i.e., the no-action zone 72. Thus the criteria for
changing the amount of power drawn from the energy converter are
whether or not the array voltage is within the regulate increase
zone 70 or the regulate decrease zone 77.
[0058] When the change in voltage on successive passes through the
routine shown in FIG. 6 is low, the no-action zone 72 is relatively
small and the regulate increase zone 70 is relatively large.
Conversely, when the dv_mp value is large, the no-action zone 72 is
large and the regulate increase zone 70 is relatively small. The
0.25 and 2.0 lower and upper limits effectively bound the first
range of voltages for which the criteria for changing the amount of
power drawn from the array are met within minimum and maximum
limits.
[0059] Referring back to FIG. 6, effectively the calculate
regulation window value routine sets the dv_mp value to establish
the boundary 74 shown in FIG. 7 between the regulate increase zone
70 and the no-action zone 72 thus defining the width of the
no-regulate zone. The establishment of the variable-sized no-action
zone 72 eliminates dithering and allows the processor circuit to
change its sensitivity to changes in voltage, depending upon the
trend in voltage increases and decreases.
[0060] Referring back to FIG. 5, after calculating the regulation
window, block 80 directs the processor 26 to determine whether a
loop time out value has occurred. Later in the main routine 50, the
loop value is set as shown at block 82 as will be described below.
An initial loop value of 50 milliseconds, for example, may be set
such that for example, on every third pass through the main routine
shown at 50, the loop time out value will expire, and as shown at
block 84, control of the processor 26 will pass to a regulate
routine.
[0061] Referring to FIG. 8, the regulate routine is shown in
greater detail at 85. The regulate routine begins with a first
block 86 that directs the processor 26 to determine whether the
present voltage of the array is greater than the sum of the
MPPT-ref voltage and the dv_mp value. In other words, this block
determines whether or not the array voltage V.sub.k is in the
regulate increase zone 70 shown in FIG. 7. If so, block 88 directs
the processor 26 to set the action state variable to "regulate
increase" and then a "more power" routine is called as shown at 90
to increase the power drawn from the array. If, however, at block
86, the array voltage V.sub.k is not in the regulate increase zone
70, block 92 directs the processor 26 to determine whether the
array voltageV.sub.k is in the regulate decrease zone 77. If so,
block 94 directs the processor 26 to set the action state value to
"regulate decrease" and block 96 directs the processor 26 to call a
"less power" routine to reduce the power demanded from the array.
If the array voltage V.sub.k is in neither the regulate increase
zone 70 or the regulate decrease zone 77 as determined by blocks 86
and 92, the regulate routine 85 is ended and the processor 26 is
returned to the remaining portion of FIG. 5 (block 150). Referring
to FIG. 7, when the voltage of the array is in the no-action zone
72, no action is taken to increase or decrease the power demanded
from the array.
[0062] Referring back to FIG. 8, when the processor 26 calls the
more power routine as shown at block 90, the more power routine
shown at 100 in FIG. 9A is executed. Generally, the more power
routine 100 begins with a first block 102 that causes the processor
26 to determine whether or not the array voltage V.sub.k is greater
than the sum of the MPPT_ref voltage and a predefined value, for
example, 2.0 volts. When the array voltage V.sub.k is more than 2.0
volts above the MPPT_ref voltage, block 104 directs the processor
26 to set a power step variable according to Table 2 shown in FIG.
9B. Use of this table involves using the presently measured AC load
power value as an index to the table to determine which of a
plurality of power ranges, the present AC load power value falls
into. If the AC load power value is between zero and 40 volts, for
example, the power step value is set to 4 watts. If the AC load
power is between 800 watts and the maximum power available, the
power step value is set to 24 watts, for example. In general,
progressively larger AC load power ranges are associated with
progressively larger power step values.
[0063] Once the power step value is known, referring back to FIG.
9A, block 106 causes the power command signal to be set according
to the power step value to increase the power demanded from the
array by the power step value, subject to unit limits. Referring
back to FIG. 9B, it will appreciated that as the AC load power
increases, the power step value increases and thus the change in
power in the power command is larger, at larger AC load power
values.
[0064] Referring back to FIG. 9A, if the array voltage V.sub.k is
not greater than (MPPT_ref+2.0 ), block 108 sets the power step
value to a fixed value, in this case 4 watts, and block 106 causes
the power command to be increased to request 4 more watts from the
array. In effect, the more power routine 100 provides for larger
increases in power demanded from the array when the array voltage
V.sub.k is relatively high and the supplied AC load power is high.
Similarly, when the array voltage V.sub.k is closer to the MPPT_ref
value, a smaller, fixed change in power is used, since it is
assumed that the array is operating closer to the maximum power
point. The effect of varying the change in power according to the
present AC power being supplied to the load and subject to the
array voltage meeting the indicated condition, provides for wide
increases in power at higher AC load power levels, thus allowing
the maximum available power from the array to be supplied to the AC
grid quicker, than if a fixed, relatively small step size such as 4
watts were used. This allows the system to achieve its optimal
operating point very quickly, allowing it to change its output from
200 W to 2000 W in about four seconds, for example.
[0065] Referring back to FIG. 8, when the array voltage V.sub.k is
less than the MPPT_ref value, the less power routine is called at
block 96.
[0066] The less power routine is shown in FIG. 10A, with further
reference to FIG. 10B. A first part of the less power routine is
shown in FIG. 10A at 110 and begins with a first block 112 that
causes the processor 26 to determine whether the change in power
since the last pass through the main routine shown in FIG. 5 is
negative. If so, block 114 directs the processor 26 to issue a
power command to the DC to AC converter to cause a decrease in the
power demanded from the array by an amount equal to the difference
in power demanded since the last pass through the main routine. On
the other hand, if the change in power is not negative, i.e., zero
or positive, block 116 directs the processor 26 to issue a power
command that decreases the power demanded from the array by a fixed
amount such as 4 watts, for example. In effect, block 114 decreases
the power demanded from the array by an amount depending on the
change of power, and block 116 decreases the power demanded from
the array by a fixed amount.
[0067] After either block 114 or 116 has been executed, a second
part of the less power routine as shown at 118 is executed. This
second part 118 of the less power routine includes a first block
120 that causes the processor 26 to determine whether or not a
backoff timer has timed out. If not, the less power routine is
ended. If so, block 122 directs the processor 26 to determine
whether or not the action state variable has been set to "regulate
decrease". If not, then the less power routine is ended. If so,
however, block 124 directs the processor 26 to use Table B of FIG.
10B to determine a DC offset value shown in column 126 associated
with an AC load power range shown in column 128 in which the
current AC load power falls. Then, block 130 directs the processor
26 to determine whether or not the present array voltage V.sub.k is
less than the current MPPT_ref value less the DC offset value found
in block 124 and if not, the less power routine is ended. If the
array voltage V.sub.k is less than the MPPT_ref less the DC offset,
i.e., it is within a second range, block 132 increases the MPPT_ref
value by the amount indicated in Column 134 of Table B in FIG. 10B
associated with the power range in which the current AC load power
falls. Thus, the second range is dependent upon the amount of power
being supplied to the load. The second range is relatively small
when a relatively large amount of power is supplied to the load and
is relatively large when a relatively small amount of power is
supplied to the load. Any increases in MPPT_ref may be limited to
ensure MPPT-ref is no greater than the open circuit voltage of the
array less some guard value such as 3 volts, for example. (The open
circuit voltage of the array may be measured periodically to allow
for changes in the open circuit voltage to be monitored.)
[0068] Similarly, block 136 directs the processor 26 to set a
back-off timer value selected from column 138 in Table B of FIG.
10B associated with the AC power range in which the current AC
power falls.
[0069] Referring back to FIG. 10A, block 140 then directs the
processor 26 to determine whether or not the current adjusted
MPPT_ref value is less than 85% of the open circuit voltage of the
array. (The open circuit voltage is previously known from initial
measurements). If the MPPT_ref value is not less than 85% of the
open circuit voltage of the array, the less power routine is ended,
leaving the back-off value at that which was selected from Table B
in FIG. 10C. Otherwise, if the MPPT_ref value is less than 85% of
the open circuit voltage of the array, block 142 directs the
processor circuit to set the back-off value to a minimum value
which, in this embodiment may be 240, for example. The effect of
the second part of the less power subroutine is to prevent
constantly changing conditions from excessively adjusting the
MPPT_ref value too often. Effectively, the MPPT_ref value is
increased only when the regulation routine described above is not
able to keep the array voltage V.sub.k above a threshold value.
[0070] Referring back to FIG. 5, after the call regulate routine
has been executed, or if the loop timeout value has not yet been
reached, the processor 26 is directed to block 150 of FIG. 5.
[0071] Block 150 directs the processor 26 to determine whether it
is time to find a new MPPT_ref value. To do this, a timer may be
preprogrammed with an appropriate value to cause the timer to time
out every 10 seconds, for example. Thus, every 10 seconds, it is
time to find a new MPPT_ref value. Provisions may be included for
ensuring the source power is above some minimum value such as 20
Watts, for example, before enabling the 10 second timer. Initially,
the MPPT_ref value is set to about 84% of the open circuit voltage
of the array. This is typically about 0% higher than the expected
MMPT_ref value for most photovoltaic arrays, but it allows the
circuit to more readily adapt to different arrays. This also
eliminates the need to sweep the array.
[0072] When the timer times out, block 152 directs the processor
circuit to call a find MPPT subroutine as shown at 153 in FIG. 11A,
with reference to FIG. 11B. Referring to FIG. 11A, the find MPPT
routine 153 begins with a first block 156 that directs the
processor 26 to determine whether or not this is the first pass
through the routine. Detection of whether or not the present pass
through the routine is the first pass may be achieved by detecting
whether or not a first pass flag (not shown) has been set and if
not set, setting it and directing the processor to block 158. If
the first pass flag has been set, then the present pass through the
MPPT routine is not the first pass and the processor is directed to
block 168. When the present pass through the routine is the first
pass, block 158 directs the processor 26 to determine whether the
array voltage V.sub.k is less than the MPPT_ref voltage, i.e.,
whether the array voltage is within the regulate decrease zone
shown in FIG. 7. If so, block 160 directs the processor 26 to set
the action state variable to "sweep decrease" and then block 162
directs the processor 26 to call the less power routine described
in FIG. 10A. Block 163 then directs the processor 26 to reset the
MPPT loop timeout value so that another ten seconds will pass
before the find MPPT routine is run again. The routine is ended
after block 163.
[0073] If at block 158 the array voltage V_k is not less than the
MPPT_ref value, block 164 directs the processor circuit to set the
action state variable to "sweep increase" and block 166 directs the
processor 26 to call the more power routine shown in FIG. 9B.
Referring back to FIG. 11A, after the more power routine has been
called, the MPPT routine is ended.
[0074] Referring back to FIG. 11A, if at block 156 it is determined
that the current pass is not the first pass through this routine,
the processor 26 is directed to block 168 which causes it to
determine whether or not the change in power since the last pass
through this routine, is greater than or equal to zero. If not,
block 170 directs the processor 26 to set the action state variable
to "sweep decrease" and block 172 directs the processor 26 to call
the less power routine shown in FIGS. 10A. As described above, the
less power routine includes blocks that employ table B in FIG. 10B
to increase MPPT_ref. The effect of the table is to use the current
AC load power to determine an array voltage limit below which no
increase in MPPT_ref occurs and above which a specific increase
dependent upon current AC power is effected. This also makes the
circuit sensitive to trends in power, rather than to instantaneous
power.
[0075] Referring back to FIG. 11A, if at block 168, the change in
power is less than zero, the processor 26 is directed to block 174
which causes it to determine whether or not the change in voltage
since the last pass through this routine is less than or equal to a
threshold value of, in this embodiment, 2 volts for example. Block
176 then directs the processor 26 to set the action state variable
to "sweep increase" and block 178 directs the processor 26 to call
the more power routine shown in FIG. 9A.
[0076] Referring to FIG. 9A, the more power routine includes a
block 179 shown in broken outline which determines whether or not
the action state variable is equal to "sweep increase" as set by
block 176 in FIG. 11A. If so, the processor 26 is directed to block
102 of FIG. 9A wherein the power step value is set to 4 watts such
that block 106 causes a power command to be issued to request 4
more watts from the system.
[0077] Referring back to FIG. 11A, block 182 directs the processor
26 to determine whether or not the present power being provided to
the AC grid by the system is within pre-specified limits. If not,
the find MPPT routine is ended. If so, block 184 directs the
processor 26 to determine whether a total decrease of the MPPT_ref
value as of this pass through the routine is less than an MPPT
limit value established according to Table C in FIG. 11B in
response to the present AC power provided by the system. If so,
block 186 is permitted to reduce the MPPT_ref value by 1/8.sup.th
volts unless the MPPT_ref value is less than 40 volts, which in
this case is a minimum voltage bound. The 1/8 volt reduction is
equivalent to the resolution of the voltage sensor 30. The bounds
checking at blocks 184 and 186 avoids excessive changes in MPPT_ref
and instead makes the circuit sensitive to power trends rather than
instantaneous power values. In general, the amount by which
MPPT_ref is decreased is dependent upon the amount of power
supplied to the load and is relatively large when the power
supplied to the load is small and is relatively small when the
power supplied to the load is large. After block 186, the find MPPT
routine is ended but the MPPT loop timer is not reset, therefore on
the next pass through the main routine shown in FIG. 5, it will
still be time to find MPPT and another pass through the MPPT
routine will be initiated.
[0078] After the call less power routine is invoked at block 172,
or the change in voltage is not less than or equal to Vth at block
174, or the AC Power is not less than the Powerlimit-Pth at block
182 or the total decrease of MPPT_ref is not less than MPPT_limit
at block 184, block 187 directs the processor 26 to reset variables
and reset the MPPT loop timer to cause the processor to wait
another ten seconds before executing the find MPPT routine
again.
[0079] The effect of the MPPT routine is--while the change in power
is greater than or equal to zero and while the change in voltage is
less than or equal to the threshold voltage--to reduce the MPPT_ref
value by 1/8.sup.th volts on each pass through the routine until a
maximum reduction amount is achieved, the maximum reduction amount
being determined by the present AC load power being provided to the
AC grid. When the present AC load power is low, the MPPT limit is
high, whereas when the present AC power is high, the MPPT limit is
low. The MPPT routine thus acts as a modified perturb and observe
routine that decreases MPPT_ref while the less power routine serves
to increase MPPT_ref. Both of these routines adjust the MPPT_ref
value on the basis of the present amount of power being supplied to
the load. Consequently, the apparatus tracks the maximum power
point of the energy converter more accurately. Since the MPPT_ref
value is dependent upon the present power being supplied to the
load and since the MPP_ref value establishes the boundaries shown
in FIG. 7. Effectively the criteria to be met by the array voltage
to cause a change in the amount of power drawn from the array are
dependent upon the present power being supplied to the load.
[0080] After completion of the MPPT routine shown in FIG. 11A, the
processor 26 is directed back to block 82 of FIG. 5. Block 82
directs the processor 26 to set the loop value for the loop timeout
test at block 80 as a function of he power supplied to the load. In
this embodiment, setting of the loop value is done according to the
formula (2560/(AC_power+1) subject to upper and lower bounds which
in this embodiment are 60 and 3 respectively. When the dynamic loop
value is 60 for example, the loop time out and hence the regulate
routine will be run every second and when the dynamic loop value is
3, for example, the loop time out and hence the regulate routine
will occur approximately every 50 milliseconds, or 20 times per
second. The loop value is dependent upon the AC power and as the AC
power increases, the loop value decreases causing the loop time out
to occur more frequently. Similarly, as AC power decreases, loop
time out occurs less frequently. When the amount of power supplied
to the AC load is low, capacitors in the DC to DC converter and in
the DC to AC converter are the source of power for any increases in
power and thus any increase in load measured at the array will be
delayed. Consequently, it is desirable to cause the loop timeout to
occur more frequently so that the processor can react more quickly
to increases in the AC load. When operating at high power levels,
the capacitors are being drained more quickly and thus, changes in
load are more readily seen by the processor and therefore more
frequent loop timeouts serve no useful purpose. Thus, at high power
levels the loop timeout value can be high resulting in less
frequent monitoring by the processor circuit. The specific formula
for calculating the loop value is appropriate for the Suntie.RTM.
inverter and it will be appreciated that in other systems employing
different capacitors, the formula may be different with the general
goal of enabling the processor circuit to respond less frequently
at low AC power levels and more frequently at high power
levels.
[0081] Effectively the method and apparatus described herein cause
power to be extracted from an energy converter in a manner in which
maximum power is drawn from the energy converter. This is achieved
by operating the energy converter such that current is drawn at a
level that maintains the supply voltage of the energy converter as
close as possible to a maximum power point tracking voltage of the
energy converter. Since this maximum power point tracking voltage
changes with operating conditions of the energy converter, one part
of the control method described herein updates this maximum power
point tracking voltage and another part adjusts the amount of power
drawn from the energy converter to cause the energy converter
voltage to track as close as possible to the maximum power point
tracking voltage.
[0082] The methods and apparatus described herein effectively load
the energy converter until the power extracted from the energy
converter starts decreasing and the voltage of the energy converter
is also decreasing. This condition signifies that the energy
converter is operating past its peak power point. The energy
converter voltage at this point is considered a reference voltage
(MPPT_ref) and subsequently, the level of current drawn from the
energy converter is generally maintained such that the voltage of
the energy converter is held as close as possible to this reference
value, at least until it is updated.
[0083] In general where switching power supplies are used in
conjunction with an energy converter, such devices have little
tolerance for being on the negative side of the MPPT_ref point and
are subject to collapse. Therefore the control methods and
apparatus described herein attempt to keep the energy converter
voltage on the positive side of the MPPT_ref point. Furthermore, in
the specific application described herein DC to DC switching power
supplies driving DC to AC inverters generally do not act in a
linear manner to changes in power imposed by the DC to AC
converter, especially due to power storage in each device. Thus,
the methods and apparatus described herein attempt to observe
trends in power and voltages to ensure more reliable operation and
set changes in the amount of power drawn from the energy converter
on the basis of power supplied to the load rather than power drawn
from the energy converter to enable these control methods to be
used in DC to AC energy conversion applications.
[0084] While specific embodiments of the invention have been
described and illustrated, such embodiments should be considered
illustrative of the invention only and not as limiting the
invention as construed in accordance with the accompanying
claims.
* * * * *