Download Automatic generation control of a wind farm with variable speed

IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 17, NO. 2, JUNE 2002
279
Automatic Generation Control of a Wind Farm With
Variable Speed Wind Turbines
José Luis Rodríguez-Amenedo, Member, IEEE, Santiago Arnalte, and Juan Carlos Burgos, Member, IEEE
Abstract—Wind farms are considered to be negative loads from
the point of view of a utility manager. Modern variable-speed
wind turbines offer the possibility for controlling active and
reactive power separately. This paper presents a new integrated
control system of a wind farm according to the utility manager
requirements. This control system is based on two control levels:
A supervisory system controls active and reactive power of the
whole wind farm by sending out set points to all wind turbines,
and a machine control system ensures that set points at the
wind turbine level are reached. The system has been validated
by numerical simulation using data from a wind farm with 37
variable-speed wind turbine situated in the North of Spain. An
automatic generation control of these characteristics promises
improved performance of the system and a better grid integration
of the wind energy without significant extra costs.
Index Terms—Reactive power control, variable speed drives,
wind power generation.
I. INTRODUCTION
S
PAIN is the third country on the world to utilize wind
power, just after Germany and the United States [1].
Prospects for 2010 point to 8000-MW installed power from
wind energy, which will represent a significant percentage of
the total capacity in the Spanish electrical system. Nevertheless,
in spite of this large installed wind power, reduced effort has
been made to appropriately control these energy production
systems.
Most of actual wind farms in Spain are made up of several
wind turbines installed in one site operating almost independently of each other. Newer wind turbines are variable-speed
units that use power electronic converters, which allow decoupled control of torque and power factor of the generator [2]. This
technology can be used to support wind farm integration in the
electrical system. More than 50% of all wind turbines installed
in Spain use this advanced technology. Most of them include
doubly fed induction generators.
The aim of this paper is to present a new integrated control
system of the total active and reactive power generated by the
wind farm. The control system reduces the number of wind turbine shutdowns produced by over/under voltage variations beyond limits and thus increases the number of operation hours.
Besides that, reactive power control makes it possible to hold
unit power factor at the point where the reactive meter is located
or even to produce reactive power, which allows the wind farm
owner not to be penalized for reactive power consumption.
Manuscript received March 16, 2001; revised December 5, 2001.
The authors are with the Electrical Engineering Department, Carlos III University, Leganés, Madrid, Spain.
Publisher Item Identifier S 0885-8969(02)05417-7.
Fig. 1.
Yerga wind farm layout.
II. WIND FARM DESCRIPTION
Yerga Wind Farm is located at La Rioja, Spain. It is made up
of 37 G47/660 variable-speed wind turbines, which use a doubly
fed induction generator, with a total rated power of 24.42 MW.
Fig. 1 shows the electrical layout of the wind turbines distributed on four underground medium-voltage (MV) circuits at
20 kV. MV circuits meet at Yerga substation (ST). Voltage is
boosted up to 66 kV (HV) by means a step-up transformer 35
MVA, 66/20 kV. ST connects the wind farm with the point of
common coupling (PCC), which is located at Quel substation
through a 16.38-Km long feeder.
Main parameters of the wind farm are given in the Appendix.
III. GENERATOR CONTROL
Fig. 2 shows the main components of the variable-speed
pitch-controlled G47/660 wind turbine used at Yerga wind
farm. The electrical system consists of a doubly fed induction
generator (DFIG), whose stator winding is directly connected
to the secondary of a three winding transformer. Rotor winding
0885-8969/02$17.00 © 2002 IEEE
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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 17, NO. 2, JUNE 2002
Fig. 2. G47/660 wind turbine.
is connected at the tertiary of the transformer through a bidirectional frequency converter made up of two back-to-back IGBT
bridges (which are referred to as the supply-side and rotor-side
converters).
The rotor-side converter (RSC) controls rotor current in the
stator flux frame reference [3]. Direct and quadrature current
components allow decoupled control of torque and reactive
power. Direct rotor current can be used in the same way as field
current in a synchronous generator. Quadrature rotor current
is used to control the generator torque to achieve the desired
rotational speed in the variable-speed system. The supply side
converter (SSC) is current controlled to deliver rotor power to
the grid at supersynchronous speed (or to draw rotor power
from the grid at subsynchronous speed). Vector control of SSC
also allows reactive power compensation. Harmonic distortion
of grid-injected currents is maintained into admissible limits by
means of a suitable PWM technique and the three-phase choke
linking SSC and tertiary transformer winding.
IV. WIND TURBINE CONTROL SYSTEM
The control system consists of two separated control loops:
one for machine active power control (MAPC) and the other for
reactive power control (MRPC).
The control objectives of the active power control system
without grid operator command reference are based on the following strategies for a variable-speed wind turbine with variable
pitch.
—
Power optimization below rated wind speed: The energy capture is maximized by making the turbine work
at maximum power coefficient [4]. The pitch angle
is kept constant at its optimal value, whereas the tip
speed ratio is driven to its optimal value by varying
the rotational speed . Reference rotational speed is
assured by a speed controller (SC) acting on the generator torque.
—
Power limitation above rated wind speed: The power
controller has to assure rated output of the wind turbine
by acting on the pitch angle. The speed controller keeps
rotational speed constant at its rated value.
When considering a better integration into the grid, the wind
turbine can no longer be considered a negative load, but some
kind of power control has to be achieved [5]. Under this new
assumption, another control objective is considered.
—
Power limitation below rated wind speed: The power
controller assures reference power
is achieved by
acting on the pitch angle.
The block diagram of the proposed control system is shown
in Fig. 3. The control system consists of two control loops:
—
a speed control loop, which controls generator speed
by acting on the generator torque;
—
a power control loop, which controls wind turbine
output by acting on the pitch angle.
When no power reference is given from the supervisory
control system, power reference is set up at its rated value.
Bellow rated wind speed, a negative power error is achieved,
and the controller output (reference pitch angle ) decreases
until the controller reaches its lower saturation (optimal pitch).
Therefore, the turbine works at the optimal pitch. On the other
hand, the optimal speed is obtained as a reference for the speed
controller from the characteristic that represents the wind
turbine output as a function of the optimal speed: the so-called
maximum power tracking strategy (MPT). This characteristic
is truncated at rated speed; therefore, no speed reference over
rated speed is provided. Above rated wind speed, a positive
power error is achieved, and the pitch angle is driven to positive
values until rated power is achieved. On the other hand, for
this level of power, rated rotational speed reference is obtained,
and the speed controller assures this reference over this range
of operation. Note that in steady state, as the power controller
assures rated output, and the speed controller assures rated
speed; the generator torque is also rated.
When a power reference is given from the supervisory control
system, there could be two situations. First, wind velocity is high
enough. Then, the power controller will drive the blade pitch
angle to the adequate value, but on the other hand, a rotational
speed reference is given to the speed controller that it is not
optimal (in fact, smaller than optimal). Therefore, tip speed ratio
is smaller than optimal and power coefficient results that are
also decreased by this variable, i.e., the speed control loop helps
the pitch drive to reduce power resulting in a less demanding
action on this system. If wind is not high enough, the power
controller will reach saturation at optimal pitch angle, and an
optimal rotational speed reference will be provided to the speed
controller, obtaining maximum power for such wind speed.
The reactive power control system is based on a reactive
power controller and a subordinated voltage control loop. The
subordinated control loop assures that the voltage limits are not
violated when trying to reach the reactive power reference. The
principle of the reactive power control is as follows. First, a
RODRÍGUEZ-AMENEDO et al.: AUTOMATIC GENERATION CONTROL OF A WIND FARM
Fig. 3.
281
Control system block diagram.
reactive power reference
is set up by a supervisory control
system. The reactive power controller computes the reactive
. In the
power error and sets up a voltage level reference
following, the machine voltage controller (MVC) will compute
the voltage error and set up an excitation current reference
for the machine excitation current controller.
The reactive power reference cannot be reached when the
controller reaches saturation at the maximum or minimum
voltage limit. In such a case, the control system will control
the voltage level so that the limit is not violated, assuring the
availability of each individual machine, i.e., the control system
prevents the voltage protection to shoot and disconnect the
machine.
V. SUPERVISORY CONTROL SYSTEM
The purpose of supervisory control system is to control the
active and reactive power injected by the whole wind farm into
and
). The system consists of two control
the grid (
loops for active and reactive power control respectively.
The active control loop is based on a wind farm active power
controller WFAPC. Under grid operation, the controller will refrom the grid operator. The conceive a power reference
troller then computes the active power error and sets up a power
for each machine active power controller.
reference
Note that if power reference is increased when one or more
machines are working at maximum power, the rest of the machines will automatically assume the load, as the active power
controller will increase computed reference power due to the
power error.
The reactive control loop is based on a wind farm reactive
power controller (WFRPC) and a subordinated voltage control
loop. The subordinated control loop assures that the voltage
Fig. 4. Electrical generator model.
limits are not violated when trying to reach the reactive power
reference. The principle of the reactive power control is as
follows. First, a reactive power reference is set up, and usually,
unity power factor is desired. The reactive power controller
computes the reactive power error and sets up a voltage level
at the wind farm substation. In the following,
reference
the wind farm voltage controller (WFVC) will compute the
for
voltage error and set up a reactive power reference
each machine-reactive power controller.
The reactive power reference cannot be reached when the
controller reaches saturation at the maximum or minimum
voltage limit. In such a case, the control system will control
the voltage level so that the limit is not violated, assuring the
availability of the wind farm, i.e., the control system prevents
the voltage protection to shoot and disconnect the wind farm.
VI. SIMULATION PROCESS
In order to validate the proposed control systems, a simulation process has been carried out. Each simulation starts at a
steady-state where a wind velocity, voltage level at the PCC,
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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 17, NO. 2, JUNE 2002
Fig. 7. Case 1: Active and reactive power delivered.
Fig. 5. Wind farm electrical network.
Fig. 6.
Wind speed at wind turbines number 3, 9, 19, and 33.
and desired wind farm active and reactive power are given. From
these inputs, each machine voltage, active power, and reactive
power is obtained. The dynamic simulation starts when one or
more inputs change. The simulation process is as follows.
1) The dynamic model of the supervisory control system
will produce a change in the active and reactive power
references of each machine control system.
2) For each machine, the dynamic model of the machine
control system will produce a change in the current references of the electrical generator current control loops
and/or a change in the blade pitch control system.
3) The dynamic model of the drive train will provide each
machine rotational speed. For this purpose, turbine and
generator torque first have to be evaluated. A simple static
nonlinear turbine model is considered. The model computes turbine torque considering wind velocity, rotational
speed, and blade pitch as inputs. The generator torque reference provided by the speed control loop is taken as the
actual generator torque, i.e., no dynamic is considered as
the generator torque control loop is much faster than any
of the considered dynamic models.
4) If each generator torque and speed is known, the static
model of the Fig. 4 is used to compute the current injected
by each generator . The model uses as inputs the stator
, the current injected by the grid-side power
voltage
converter , and the rotor current . First, rotor current is obtained from the current control loops. Note that
these control loops provide rotor current components in
a reference frame aligned along the stator flux ( – components) so that rotor current vector has to be projected
on a reference frame aligned along the stator voltage.
Next, stator current and rotor voltage are calculated.
Then, rotor active power can be obtained, and taking into
account that the grid-side power converter objective is to
transfer the rotor power to the grid, the current injected
by the power converter can be obtained. Note that under
vector control, this power converter can transfer active
power to the grid with a desired power factor. Here, unity
power factor is taken. Finally, the total current injected
into the grid is obtained.
5) If the voltage at the PCC and each generator current is
known (see Fig. 5), the voltages at each machine terminals
and the current injected at the PCC are calculated considering the wind farm network admittance matrix. From
here, each machine and the wind farm active and reactive
power can be obtained, providing the feedback to the dynamic models of the control systems.
The process is repeated each time step. The simulation uses a
variable step size with a maximum of 0.1 s. A new wind speed
is read from the input file when the simulation time step corresponds to the step of the wind data. Different wind data are
used for each WASP-programmed machine that uses a single
anemometer measure processed to consider the wake effect and
turbulence intensity in each machine, taking into account topography data of the wind farm and layout of machines.
VII. SIMULATION RESULTS
Three different cases have been run. Fig. 6 shows the wind
velocity used in the all the cases (only wind in four turbines is
shown). In the first case, the reactive power reference is set at
zero, whereas the active power shows that the system achieves
the three control objectives for machine active power control.
RODRÍGUEZ-AMENEDO et al.: AUTOMATIC GENERATION CONTROL OF A WIND FARM
Fig. 8.
Case 1: Rotational speed at wind turbines number 3, 9, 19, and 33.
283
Fig. 10. Case 1: Maximum and minimum voltage at buses 1 to 37.
Fig. 9. Case 1: Pitch angle at wind turbines number 3, 9, 19, and 33.
In Figs. 7–9 from 0 to 400 s, no external active power reference
has been specified. Then, below-rated wind speed 14 m/s energy capture is maximized through speed variation. Above-rated
wind velocity, rotational speed, and power are limited to rated
values through pitch variation. From 400 s, 12.5-MW wind farm
power reference has been specified, and Fig. 7 shows that specified reference is achieved. Finally, Fig. 10 shows the voltage
profile at each turbine during simulation by indicating maximum and minimum rms voltage values.
The second case is the same as before, except that a reactive power step is applied at 200 s. Fig. 11 shows that the commanded reactive power is achieved by even share among machines. As a result, Fig. 12 shows the voltages at the wind farm
substation and in four of the machines.
The last case is the same as the first one, but a high PCC
voltage (1.12 p.u.) is considered. Therefore, each machine supplies a different reactive power (see Fig. 13), and wind farm recannot be achieved. As a conactive power reference
sequence, the upper limit in the machine voltages (1.09 p.u.) is
reached; see Fig. 14. Nevertheless, voltages (in the wind farm
substation and in each machine) can be controlled, and none of
the machines is disconnected.
VIII. CONCLUSIONS
An automatic generation control system for wind farms
has been designed and tested by simulation. Real data from
a Spanish wind farm has been used. The proposed system is
Fig. 11.
Case 2: Active and reactive power delivered.
Fig. 12.
Case 2: Voltages at HV bus and at buses number 3, 9, 19, and 33.
based on a hierarchical architecture with a supervisory control
system controlling the active and reactive power at the wind
farm substation and a machine control system controlling the
active and reactive power at each particular wind turbine. The
active power control loop allows either following a power
reference or maximizing energy capture when no reference
is supplied. The reactive power control loop makes possible
voltage control and then achieves a high availability of wind
turbines.
Simulation results show good performance of the control
system during a highly variable wind, large electrical disturbance and commanded active and reactive power steps.
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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 17, NO. 2, JUNE 2002
Wind Turbine
Rotor diameter
Mechanical time constant
Pitch system time constant
Speed range
m
s
s
r/min.
Electrical system (Fig. 4)
Stator rated power 660 kVA
m ,
m ,
m ,
m ,
Power factor range 0.,94 lead and lag at full load
Feeder
Fig. 13.
Length
Impedance
Shunt admittance
Short circuit power at PCC
Case 3: Active and reactive power delivered.
Km
Km
S/Km
MVA.
ACKNOWLEDGMENT
The authors would like to thank E. de la Rioja S.A. (Yerga’s
wind farm owner) and I. Ingeniería Consultoría S.A. for data
provided.
REFERENCES
Fig. 14.
[1] Papoutsis et al., “Wind energy. The facts,” Eur. Wind Energy Assoc.,
ISBN92-828-4571-0.
[2] Z. Chen and E. Spooner, “Grid interface options for variable-speed, permanent magnet generators,” Proc. Inst. Elect. Eng. Elect. Power Appl.,
vol. 145, no. 4, July 1998.
[3] R. Pena, J. C. Clare, and G. M. Asher, “Doubly fed induction generator
using back to back PWM converters and its application to variable-speed
wind energy generation,” Proc. Inst. Elect. Eng. Elect. Power Appl., vol.
143, no. 3, May 1996.
[4] S. Heier, Grid Integration of Wind Energy Conversion Systems. Chichester, U.K.: Wiley, 1998.
[5] E. N. Hinrichsen, “Controls for variable pitch wind turbine generators,”
IEEE Trans. Power App. Syst., vol. PAS-103, Apr. 1984.
Case 3: Voltages at HV bus and at buses number 3, 9, 19, and 33.
APPENDIX
LV/MV transformers: 775 kVA, 20 kV/690-300 V,
%
MV/HV transformer: 35 MVA, 66 kV/20 kV,
Underground cable
%
Type
Section mm
Km
Km
F/Km
At circuit 1 and 2 underground cables are type 1.
Circuit 3: from ST to WT14 cables are type 3
from WT14 to WT17 cables are type 2
from WT17 to WT26 cables are type 1
Circuit 4: from ST to WT27 cables are type 3
from WT27 to WT29 cables are type 2
from WT29 to WT37 cables are type 1
Mean distance between wind turbines in the wind farm is
102 m.
José Luis Rodríguez-Amenedo (M’01) received the B.S. degree in energy engineering from the Universidad Politécnica de Madrid, madrid, Spain, in 1993
and the Ph.D. degree in electrical engineering from Universidad Carlos III de
Madrid in 2000.
His current interests are control drives and wind energy systems. He is now
Assistant Professor at Universidad Carlos III de Madrid.
Santiago Arnalte received the B.S. and Ph.D. degrees in electrical engineering
from Universidad Politécnica de Madrid, Madrid, Spain, in 1989 and 1993, respectively.
His current interests are control drives and wind energy systems. He is now
an Associate Professor at Universidad Carlos III de Madrid.
Juan Carlos Burgos (M’01) received the B. S. and Ph.D. degrees in electrical
engineering from Universidad Politécnica de Madrid, Madrid, Spain, in 1981
and 1987, respectively.
His current interests are control drives and wind energy systems. He is now
an Associate Professor at Universidad Carlos III de Madrid.