A Novel Doubly-fed Induction Wind Generator Control Scheme for Reactive Power
Control and Torque Pulsation Compensation Under Unbalanced Grid Voltage
Conditions
Ted Brekken, Ned Mohan
Dept. of Electrical Engineering,
University of Minnesota, Minneapolis, MN 55455 USA
Abstract-Wind energy is often installed in rural, remote
areas characterized by weak, unbalanced power
transmission grids.
In induction wind generators,
unbalanced three phase stator voltages cause a number of
problems, including overheating and stress on the
mechanical
components from
torque
pulsations.
Therefore, beyond a certain amount of unbalance (for
example 6%), induction wind generators are switched out
of the network.
In doublyfed induction generators,
control of rotor currents allows for adjustable speed
operation and reactive power control. In addition, it is
possible to control the rotor currents to correct for the
problems caused by unbalanced stator voltages. This
paper presents a novel controller design for a doublpfed
induction generator that provides adjustable speed and
reactive power control while greatly reducing torque
pulsations.
pulsations, which can result in acoustic noise at low levels
and at high levels can actually destroy the rotor shaft,
gearbox, or blade assembly [3].
A special type of induction generator called a doubly-fed
induction generator (DFIG), shown in Fig. 1, is becoming the
most popular choice for wind turbines. The ability to control
the rotor currents allows for variable speed operation, so that
a DFIG can operate at maximum efficiency over a wide range
of wind speeds.
stator
w
I. GOALS AND OBJECTIVES
The goal of the research is to develop a control method
for doubly -fed induction wind generators that addresses
issues associated with weak rural grids. The control method
presented improves the robustness and all around
performance of doubly -fed induction wind generators.
A controller is designed and tested in simulation and a
hardware model will be built and tested. Research and
simulation results are presented below.
U
dc-ac
U
ac-dc
Fig. 1. Doubly-fed induction generator.
The goal of the research is to develop a DFIG control method
that exploits the rotor current control capabilities to allow for
adjustable speed, reactive power control and torque pulsation
compensation.
11. INDUCTION
GENERATORUNBALANCE
EFFECTS
Wind energy generation equipment is most often
installed in remote, rural areas. These remote areas usually
have weak grids, often with voltage unbalances and
undedover-voltage conditions [ 1,2]. Under such abnormal
conditions, wind turbines are disconnected from the grid for
their own protection, thus significantly impacting their energy
production and the capacity credit that they are otherwise
due. As an example, the turbines in the southwestern part of
Minnesota are removed from the grid beyond an unbalance of
6% in phase-phase voltages, an under-voltage of lo%, or an
over-voltage of5%.
Induction generators are the predominant type of wind
turbine. When the stator phase voltages, supplied by the grid,
are unbalanced, the torque produced by the induction
generator is not constant. Instead the torque has periodic
0-7803-7754-0/03/$17.00 02003 IEEE
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111. STATORVOLTAGEORIENTED
CONTROL
Fig. 2 [4] shows a steady state space vector diagram for a
doubly -fed induction machine (current and power are defined
as positive going into the machine). In stator voltage oriented
control, the &axis is aligned with the stator voltage space
vector. In steady state operation, the d-axis will rotate
synchronously with the stator space voltage vector.
The rotor power converter controls the rotor current
space vector, (. Due to transformer action, the stator will
c.
draw an equal and opposite current,
The total current
drawn by the stator is equal to the magnetizing component,
,plus the rotor current complement , as shown in (1):
760
cs
speed and torque control. Another view of this is that
csand
the magnetizing flux Bm will be aligned nearly entirely along
I
the negative *axis. Therefore the flux produced by ,i is
orthogonal to the magnetizing flux, thereby producing torque.
In this manner, stator voltage orientated control approximates
stator flux oriented control [5].
d
w.TORQUE PULSATION COMPENSATION
&
The torque equation for doubly-fed induction machine is
given by
P
T , =~-L,
2
Fig. 2. DFIG space vector diagram.
(is,ird- isdirq)
.
(3)
Using the dq equivalent circuits for an induction machine [4],
the dq magnetizing current can be defined as
Therefore, control of the rotor current allows for control of
the stator current.
,,i
i,,
= isd -k ird
(4)
+ i, .
(5)
= is,
The torque equation can now be expressed as
When the dq frame is synchronously rotating, the stator
voltages are balanced, and the generator is in steady state, all
dq quantities are dc. The torque equation can be written as
4is
Since ImsqIrd
and ImsdIrqare constant,
Fig. 3. Relationship between machine power and stator current space vector.
cmwill be constant.
Unbalanced stator excitations will cause perturbations in i,,,
Fig. 3 shows that when the d-axis is aligned with the stator
voltage space vector, V $ , the stator current, <, can be
operated in any of the 4 quadrants and the generator can
produce or absorb real and reactive power. Normal operation
for a DFIG would be with in the left-half plane.
By breaking the rotor current up into its d and q
components, the stator current can be expressed as
and imd at twice the synchronous frequency. The torque
equation for unbalanced excitation can be written as
<
< =i;d + ji:, +t,
= -i,
In (8), T, is a function of the double frequency terms Tms, and
- j i rq
The magnetizing current,
+Tm.
cis, is
(2)
largely dependent only on
the stator voltage. Therefore i, and i, can be used to
control real and reactive power, respectively. Real power
produced by the machine is directly related to torque, which
can be used to control speed. Therefore ird can be used for
ksd. The compensation terms i,&ompand i,o
are added to
the rotor currents to cancel the effects of r,, and im$d,
thus
making T, constant. The compensation terms are calculated
by equating the terms of the balanced torque equation, (7)
with the unbalanced torque equation (8).
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Figs. 4 and 5 show the controller structure block diagrams
with the torque pulsation compensation.
SlW
5
e
7
8
D
9
time (sec)
Fig. 6. Generator torque with 6% unbalance.
Fig. 4. Torque loop.
The tradeoff for the reduction in torque pulsation is an
increase in stator current, as illustrated in Fig. 7. However, as
Fig. 7 shows, this increase in current occurs in the 2 phases
that did not see a drop in voltage. In fact, the current in the
phase in which the voltage has dropped (the A-phase in this
example) decreases once the compensation turns on. The
advantage of this is that if the voltage dip is caused by a fault
in that phase, the current sourced into the fault decreases.
The burden of torque compensation shifts to the phases that
do not have the voltage dip.
I
ti
Fig. 5. Reactive power loop.
When designing the controllers, it is imperative that the
loop bandwidths be much lower than the frequency of the
disturbance (twice the line frequency). For the compensation
to work correctly, the output of the controllers should not be
affected by any loop disturbances near the line frequency.
A significant advantage of this torque pulsation
compensation scheme is that the calculated compensation
currents are not dependent on machine parameter estimation.
The compensation currents are calculated only from other
measured currents and control variables.
1om
om
CphaSe
t
BphaSe -
A-phase -
SIMULATION
RESULTS
V. TORQUEPULSATION
5
A doubly-fed induction generator model was constructed
in Simulink [8]. The simulation results shown below are for
a 750 kW generator, with a 6% voltage unbalance applied to
the stator terminals. Percent unbalance is defined here as the
percent decrease in the rated A-phase voltage. The generator
starts at 0 seconds without the torque pulsation compensation.
At 5 seconds, the line voltage is unbalanced when the Aphase voltage drops from 276 Volts rms to 260 Volts rms. At
8 seconds the torque compensation is turned on. The torque
is shown in Fig. 6. The torque pulsations are decreased by a
factor of 10.
e
7
8
9
I
10
time (sec)
Fig. 7. Stator current (rms) with 6% unbalance.
Fig. 8. shows that the rotor current is of the same form as the
stator current at a lower amplitude. It is not shown but
should be noted that the phase angle relationships between
the stator and phase currents does not change between the
balanced,
unbalanced without compensation,
and
compensated cases. Therefore only the magnitudes of the
rotor and stator phasors change during the simulation. The
phase angles remain approximately the same.
762
e80
-
880
-
8c
820
-
800
-
L
a
780-
uncompensated
8
reo
5
e
7
0
8
0
5
10
15
20
Percent line unbalance
Figs. 9 and 10 show the results of simulations over a
range of unbalance. Fig. 9 shows that the presented torque
compensation method significantly reduces the torque
pulsations over the entire range unbalance situations
simulated. However, Fig. 10 shows that the compensation
causes in an increase in the Gphase stator current that
reaches two times the rated value at 20 percent unbalance.
Therefore with the presented compensation method, the
current limits of the machine may be reached before the
mechanical stress limits. A possible response to this situation
maybe to reduce the speed of the machine, thereby reducing
power and current. This is essentially derating the generator,
which leduces its capacity, but is better than removing the
generator entirely, as would be the case without any torque
compensation.
0
5
10
15
Percent line unbalance
Fig. 9. Torque pulsation vs. unbalance.
20
Fig. 10. Stator current vs. unbalance.
W.HARDWARE
DSP-based Controller
Fig. 1 1 . Lab setup.
To test the presented control scheme, two lab setups are
under consideration. The first, as shown in Fig. 11, consists
of an induction machine coupled to a doubly-fed induction
generator. Both of the machines are connected to 3phase
inverters powered by a common dc link. The induction
machine will supply torque to simulate the wind, and can be
instantaneously controlled by its inverter. The three-phase
inverter powering the doubly -fed induction generator can
supply the necessary phase unbalances. The doubly-fed
induction generator will be 5 kW with 230 Volts at the stator
terminals. The disadvantage of this setup is that it may not be
reasonable to assume that a control scheme implemented on a
5 kW machine will scale up to a 750 kW machine. Dynamics
which may become significant at large sizes, such as shaft
dynamics, may not be accurately modeled by the 5 kW setup.
To address this issue, the second setup under
consideration is to program an FPGA or DSP with a detailed
model of a 750 kW machine, including as much physical and
electrical behavior as possible. This detailed model would
then be connected to another DSP running the control
algorithm.
This would allow real-time testing of the
presented control scheme.
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VII. SUMMARY
WII. REFERENCES
A control scheme for adjustable speed and reactive
power control of a doubly fed induction generator is
presented. The presented control scheme also includes an
algorithm for greatly reducing the torque pulsations produced
by the generator when operating with unbalanced voltages
applied to the stator. Simulation results are presented, as well
as a proposal for hardware testing.
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Turbines on Weak Rural Networks,” Opportunities and Advances m
International Power Generation, 1996, Conference PublicationNo. 419,
pp 164-167.
[2] Allan E. A., “Large Wind Turbines and Weak Rural Electricity
Systems”, Proceedings of the BWEA Conference, Stirling, June 1994.
[3]
E. Muljadi, T. Batan, D. Yildirim, C.P. Butterfield,“LJ”tandingthe
Unbalanced-Voltage Problem in Wind Turbine Generation,” Itxiwhy
Applications Conference, 1999, vol. 2, pp 1359 -1365.
[41 N. Mohan, Electric Drives: An Integrative Approach,Minneapolis,
MNPERE. 2001.
[SI A. Petersson, “Analysis, Modeling, and Control of Doubly-Fed
Induction Generators for Wind Turbines,” Chalmers University of
Technology, Sweden.
[6] Mathworks Company Website: htttx//www.mathworks.com
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