CROSS REFERENCE TO RELATED APPLICATION
This application is Divisional of India Patent Application No. 1587/DEL/2004, filed
on 24 August 2004.
FIELD OF THE INVENTION
The invention relates to wind turbine generators. More particularly, the invention
relates to voltage control systems and techniques for use with wind turbine generators
having continuous control of reactive power for at least part of the reactive power
compensation function.
BACKGROUND OF THE INVENTION
Wind power generation is typically provided by a wind “farm” of a large number
(often 100 or more) wind turbine generators. Individual wind turbine generators can
provide important benefits to power system operation. These benefits are related to
mitigation of voltage flicker caused by wind gusts and mitigation of voltage
deviations caused by external events.
In a wind farm setting each wind turbine generator can experience a unique wind
force. Therefore, each wind turbine generator can include a local controller to control
the response to wind gusts and other external events. Prior art wind farm control has
been based on one of two architectures: local control with constant power factor and
farm level control in fast voltage control, or local control in constant voltage control
with no farm level control.
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Both of these prior art control architectures suffer from disadvantages. Local control
with constant power factor and farm level control in fast voltage control requires fast
communications with aggressive action from the farm level to the local level. If the
farm level control is inactive the local control can aggravate voltage flicker. With
constant voltage control on each generator, steady-state operation varies significantly
with small deviations in loading on the transmission grid. This causes the wind
turbine generators to encounter limits in steady-state operation that prevent a response
to disturbances—resulting in a loss of voltage regulation. Because reactive current is
higher than necessary during this mode of operation, overall efficiency of the wind
turbine generator decreases.
SUMMARY OF THE INVENTION
A wind turbine generator control system includes a reactive power regulator to
control reactive power production by the wind turbine generator by adjusting voltage
setpoint to a voltage regulator, the reactive power regulator having a first time
constant and a voltage regulator coupled with the reactive power controller to control
real power production by one or more wind turbine generators, the voltage regulator
having a second time constant. The first time constant is numerically greater than the
second time constant.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is illustrated by way of example, and not by way of limitation, in the
figures of the accompanying drawings in which like reference numerals refer to
similar elements.
FIG. 1 is a block diagram of a wind farm having multiple wind turbine generators
coupled with a transmission grid.
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FIG. 2 is a control diagram of one embodiment of a wind turbine generator control
system.
FIG. 3 is a flow diagram of one embodiment of operation of a wind turbine control
system.
FIG. 4 is an example set of waveforms corresponding to a prior art local control with
constant power factor without wind farm level control.
FIG. 5 is an example set of waveforms corresponding to a prior art local control with
constant power factor and wind farm level control in fast voltage control.
FIG. 6 is an example set of waveforms corresponding to local control of a wind
turbine generator having a controller as described in FIG. 2, without wind farm level
control.
FIG. 7 is an example set of waveforms corresponding to local control in a wind
turbine generator having a controller as described in FIG. 2, with wind farm level
control.
DETAILED DESCRIPTION
A wind turbine generator control system includes relatively fast regulation of voltage
for individual generators with relatively slower overall reactive power regulation at a
substation or wind farm level. The relatively slow reactive power regulator adjusts the
set point of the relatively fast voltage regulator. The fast voltage regulation can be at
the generator terminals or at a synthesized remote point (e.g., between the generator
terminals and the collector bus). Prior art reactive power controllers are designed with
time constants of lower numerical value than those used in voltage regulator design.
That is, in the prior art, the reactive power control loop is inside of the voltage control
loop, which results in a less stable system than described herein.
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FIG. 1 is a block diagram of a wind farm having multiple wind turbine generators
coupled with a transmission grid. FIG. 1illustrates only three wind generators;
however, any number of wind generators can be included in a wind farm.
Each wind turbine generator 110 includes a local controller that is responsive to the
conditions of the wind turbine generator being controlled. In one embodiment, the
controller for each wind turbine generator senses only the terminal voltage and
current (via potential and current transformers). The voltage and current sensed are
used by the local controller to provide an appropriate response to cause the wind
turbine generator to provide the desired reactive power and voltage. A control system
diagram corresponding to one embodiment of a wind turbine generator controller is
described in greater detail below with respect to FIG. 2.
Each wind turbine generator 110 is coupled to collector bus 120 through generator
connection transformers 115 to provide real and reactive power (labeled Pwg and Qwg,
respectively) to collector bus 120. Generator connection transformers and collector
buses are known in the art.
Wind farm 100 provides real and reactive power output (labeled Pwf and Qwf,
respectively) via wind farm main transformer130. Farm level controller 150 senses
the wind farm output as well as the voltage at point of common coupling 140 to
provide a farm level farm level reactive power command (Farm Level Q Cmd) 155.
In one embodiment, farm level farm level controller 150 provides a single reactive
power command to all wind turbine generators of wind farm 100. In alternate
embodiments, farm level controller 150 provides multiple commands for subsets of
wind turbine generators of wind farm 100. The commands to subsets of wind turbine
generators can be based on, for example, additional information related to the
operating conditions of one or more wind turbine generators.
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The control system of FIG. 2 provides an improved control structure involving both
local and farm level farm level control to overcome the disadvantages of the prior art
control architectures described above. The control system of FIG. 2 eliminates the
requirement for fast and aggressive control from the wind farm level. Improved
response is provided if the farm level control is out of service. In addition, efficient
steady-state operation is achieved, while system dynamic response remains well
within the limits set.
FIG. 2 is a control system diagram corresponding to one embodiment of a wind
turbine generator control system. In one embodiment, the control system of a wind
turbine generator generally includes two loops: a voltage regulator loop and a Q
regulator loop. The voltage regulator loop operates relatively fast (e.g., 20 rad/sec) as
compared to the Q regulator loop (e.g., greater than 1 second closed loop time
constant). The Q regulator adjusts the set point of the voltage regulator.
Conceptually, the control system of FIG. 2 provides for wind turbine generator
terminal voltage control by regulating the voltage according to a reference set by a
higher-than-generator-level (e.g., substation or wind farm) controller. Reactive power
is regulated over a longer term (e.g., several seconds) while wind turbine generator
terminal voltage is regulated over a shorter term (e.g., less than several seconds) to
mitigate the effects of fast grid transients.
Operator or farm level Q command 200 is a signal that indicates desired reactive
power at the generator terminals. In farm level operation, the wind turbine generator
Q command 200 is set equal to the output of the farm level control (line 155 inFIG.
1). In local control, the operator command is set manually, either at the wind
generator location or at a remote location. Operator or farm level Q
command 200 can be generated or transmitted by, for example, a computer system
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used to control the wind turbine generator. Operator or farm level Q
command 200 can also come from a utility grid operator or substation.
In one embodiment, operator or farm level Q command 200 is transmitted to
command limiter 220, which operates to maintain reactive power commands within a
predetermined range. Qmax 222 and Qmin 224 indicate the upper and lower bounds
on the reactive power command range.
The specific values used for Qmax and Qmin are based on, for example, generator
reactive capability. In one embodiment the value for Qmax is 800 kVAR and the value
for Qmin is −1200 kVAR for a 1.5 MW wind turbine generator; however, the specific
values are dependent upon the capability of the generators being used.
The signal output by command limiter 220 is Q command 230, which is a command
indicating the target reactive power to be produced. Q command 230 is in the range
between Qmin 224 and Qmax 222. Q command 230 is compared to a signal indicating
measured reactive power 210. The resulting error signal, Q error 235, indicates the
difference between the measured reactive power and the commanded reactive power.
Q error 235 is an input signal to Q regulator 240, which generates V
command 250 that indicates to a generator the reactive power to be provided by the
generator. In one embodiment Q regulator 240 is a proportional integral (PI)
controller that has a closed-loop time constant in the range of 1 to 10 seconds (e.g., 3
seconds, 5 seconds, 5.5 seconds). Other types of controllers can also be used, for
example, proportional derivative (PD) controllers, proportional integral derivative
(PID) controllers, state space controllers, etc. Other time constants can be used for Q
regulator 240 provided that the time constant for Q regulator 240 is numerically
greater than the time constant for voltage regulator 270.
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V command 250 is limited to a predetermined range between Vmax 242 and Vmin 244.
In one embodiment, Vmax 242 and Vmin 244 are defined in terms of percentage of rated
generator output. For example, Vmax 242 can be 105% of rated generator voltage and
Vmin 244 can be 95% of rated generator voltage. Alternate limits can also be used.
V command 250 is compared to a signal indicating measured terminal voltage 255 for
the generator. The difference between V command 250 and measured terminal
voltage 255 is voltage error signal 260. Voltage error signal 260 is the input signal to
voltage regulator 270.
Voltage regulator 270 generates rotor current command 280, which is used to control
generator rotor current. In one embodiment Q regulator 240 is a PI controller that has
a closed-loop time constant of approximately 50 milliseconds. Other types of
controllers can also be used, for example, PD controllers, PID controllers, etc. Other
time constants can be used (e.g., 1 second, 20 milliseconds, 75 milliseconds, 45
milliseconds) for voltage regulator 270 provided that the time constant for voltage
regulator 270 is less than the time constant for Q regulator 240.
In general, there are two components of a rotor current command. They are the real
power component denoted as Irq_Cmd and the reactive power component denoted as
Ird_Cmd. The rotor current command (240) generated as described with respect
to FIG. 2 is the reactive component or Ird_Cmd command. The real component or
Irq_Cmd can be generated in any manner known in the art. Rotor current
command 280 is limited to Irdmax 272 and Irdmin 274. The values for Irdmax 272 and
Irdmin 274 can be based on generator current ratings. For example, Irdmax 272 can be
rated crest current for the generator rotor and Irdmin 274 can be a percentage of rated
crest current for the generator rotor. Alternate limits can also be used.
In one embodiment, all of the limits discussed with respect to FIG. 2 are non-windup
limits; however, in alternate embodiments, a subset of the limits can be non-windup
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limits. The limits have been discussed in terms of fixed parameters; however,
dynamically variable parameters provided by, for example, a lookup table or a
processor or state machine executing a control algorithm can provide the limits. Such
a dynamically variable limit may be based upon a current rating of the generator and
a contemporaneous real power output
FIG. 3 is a flow diagram of one embodiment of operation of a generator control
system. A reactive power command is received, 300. As mentioned above, the
reactive power command can be an Operator command, farm level command, or a
local command.
A voltage setpoint is determined based on the reactive power command, 305. The
voltage setpoint is limited to a range defined by upper and lower limits that are based
on generator terminal voltage. In one embodiment, the limits are defined in terms of
percentage of rated generator output. For example, the upper limit can be 105%,
110%, 102%, 115% of rated generator voltage and the lower limit can be 95%, 98%,
92%, 90%, 97% of rated generator voltage. Alternate limits can also be used.
A rotor current command for the generator is determined based on the voltage
setpoint, 315. The rotor current command is limited, 320, to a range based on, for
example, the current rating of the generator. For example, crest current ratings can be
used for the limits, or percentages of crest current ratings can be used for the limits.
The rotor current command is transmitted to the rotor controller, 325. The rotor
controller causes the commanded current to be provided to the generator rotor. The
generator then provides a reactive power output based on the rotor current
provided, 330.
FIGS. 4 and 5 illustrate typical characteristic wind turbine generator operating
behavior for prior art control systems. These plots show response of the wind turbine
generators and of the total farm with and without fast farm level voltage control. The
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individual wind turbine generators are operated in constant power factor control with
a setpoint to yield overexcited operation (as might be required to support the
transmission system external to the wind farm). In FIGS. 4 and 5, the following
variables are portrayed, from top to bottom (refer to FIG. 1 to see where these are on
the wind farm): Pwg is the real power from an individual wind turbine generator,
Qwg is the reactive power from the generator, Q_Cmd_Farm is the output of farm
level controller (line 155 in FIG. 1), Vwg is the terminal voltage of the generator,
Vpcc is the voltage at point of common coupling (140 on FIG. 1). The goal is
generally to maintain Vpcc at a constant value even when the power fluctuates due to
variations in wind speed.
FIG. 4 is prior art with local control only (i.e., Q_Cmd_Farm is constant). Note that
the signal Vpcc varies considerably with power fluctuations Pwg, which is
undesirable. FIG. 5 is prior art with farm level control activated. While Vpcc is much
more stable than in FIG. 4, the control signal from the farm level varies considerably.
This is because the farm level control must overcome the inherent adverse effects of
the prior art local control.
FIGS. 6 and 7 are comparable to FIGS. 4 and 5, but with the control described
in FIG. 2. The inherent response of the local control is generally relatively good, so
that farm level control provides only trim control. Thus, the objective of allowing
farm level control to be less aggressive and slower are achieved with the new control.
Reference in the specification to “one embodiment” or “an embodiment” means that a
particular feature, structure, or characteristic described in connection with the
embodiment is included in at least one embodiment of the invention. The appearances
of the phrase “in one embodiment” in various places in the specification are not
necessarily all referring to the same embodiment.
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In the foregoing specification, the invention has been described with reference to
specific embodiments thereof. It will, however, be evident that various modifications
and changes can be made thereto without departing from the broader spirit and scope
of the invention. The specification and drawings are, accordingly, to be regarded in
an illustrative rather than a restrictive sense.

We claim:
1. A method comprising:
receiving a reactive power command (200);
determining a voltage set point based on the reactive power command (230), wherein
the voltage set point is limited to a range of upper (222) and lower (224) limits based
on generator terminal voltage;
determining a rotor current command (280) for the wind turbine generator in response
to the voltage setpoint, wherein the rotor current command is limited to a range based
on a current rating of the wind turbine generator and a contemporaneous real power
output;
transmitting the rotor current command to a rotor controller of the wind turbine
generator; and
generating a real and reactive power based on the rotor current command.
2. The method of claim 1 wherein the voltage set point based on the reactive power
command is determined by a reactive power regulator (240).
3. The method of claim 1 wherein a voltage set point based on the reactive power
command comprises generating, with a reactive power regulator (240), a voltage set
point (250) to be transmitted to a voltage regulator (270).
4. The method of claim 3 wherein the time constant of the voltage regulator (270) is
numerically less than the time constant of the reactive power regulator (240).
5. The method of claim 1 wherein receiving the reactive power command (200)
comprises receiving a farm level reactive power command from a farm level
controller that transmits reactive power commands to multiple wind turbine
generators.
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6. The method of claim 1 wherein receiving the reactive power command (200)
comprises receiving a reactive power command locally from a source providing a
reactive power command for a single wind turbine generator.