Dynamic modeling of doubly fed induction generator wind turbines

Abstract

It is now recognized that many large wind farms will employ doubly fed induction generator (DFIG) variable speed wind turbines. A number of such wind farms are already in operation and more are planned or under construction. With the rising penetration of wind power into electricity networks, increasingly comprehensive studies are required to identify the interaction between the wind farm(s) and the power system. These require accurate models of doubly fed induction generator wind turbines and their associated control and protection circuits. A dynamic model has been derived, which can be used to simulate the DFIG wind turbine using a single-cage and double-cage representation of the generator rotor, as well as a representation of its control and protection circuits. The model is suitable for use in transient stability programs that can be used to investigate large power systems. The behavior of a wind farm and the network under various system disturbances was studied using this dynamic model. The influence of the DFIG control on the stability of the wind farm was also investigated by considering different control gains and by applying network voltage control through both stator side and rotor side converters.

Full text

IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 18, NO. 2, MAY 2003 803
Dynamic Modeling of Doubly Fed Induction
Generator Wind Turbines
Janaka B. Ekanayake, Senior Member, IEEE, Lee Holdsworth, XueGuang Wu, and
Nicholas Jenkins, Senior Member, IEEE
Abstract—It is now recognized that many large wind farms
will employ doubly fed induction generator (DFIG) variable
speed wind turbines. A number of such wind farms are already
in operation and more are planned or under construction. With
the rising penetration of wind power into electricity networks,
increasingly comprehensive studies are required to identify the
interaction betweenthe wind farm(s) and the power system. These
require accurate models of doubly fed induction generator wind
turbines and their associated control and protection circuits. A
dynamic model has been derived, which can be used to simulate
the DFIG wind turbine using a single-cage and double-cage
representation of the generator rotor, as well as arepresentation of
its control and protection circuits. The model is suitable for use in
transient stability programs that can be used to investigate large
power systems. The behavior of a wind farm and the network
under various system disturbances was studied using thisdynamic
model. The influence of the DFIG control on the stability of the
wind farm was also investigated by considering different control
gains and byapplying network voltage control throughboth stator
side and rotor side converters.
Index Terms—Doubly fed induction generators, power system
dynamic stability, power system modeling.
NOMENCLATURE
Stator voltage.
Rotor voltage.
, Stator and rotor current.
, , Stator, rotor,and double-cage machine
resistance.
, Synchronous and rotor angular frequency.
Flux linkage.
Magnetizing inductance.
Mutualinductancebetweentworotorcoils.
, , Stator, rotor, and double-cage leakage
inductance.
, , Stator, rotor, and double-cage self induc-
tance.
Rotor slip.
ManuscriptreceivedOctober23,2002.Thisworkwassupportedinpartbythe
Association of Commonwealth Universities, EPSRC. and in part by the Tyndall
Centre for Climatic Change Research.
J. B. Ekanayake is with the Department of Electrical and Electronics En-
gineering and the University of Peradeniya, Sri Lanka, 20400, and the Tyn-
dall Centre for Climate Change Research, Norwich, NR4 7TJ, U.K. (e-mail:
jbe@ee.pdn.ac.lk)
L. Holdsworth, X.G. Wu, and N. Jenkins are with The Manchester Centre
for Electrical Energy, University of Machester Institute and Technology,
U.K. and the Tyndall Centre for Climate Change Research, Norwich, NR4
7TJ, U.K. (e-mail: l.holdsworth@umist.ac.uk; w.xueguang@umist.ac.uk;
n.jenkins@umist.ac.uk).
Digital Object Identifier 10.1109/TPWRS.2003.811178
Moment of inertia of entire wind turbine.
, , Mechanical, electromagnetic, set point
torque.
Optimal torque.
Optimal torque/speed constant of the wind
turbine.
Superscript indicates a per unit quantity.
First subscript indicates direct and quadra-
ture axes quantities.
Secondsubscriptindicatesstator,rotor, and
double-cage.
I. I
NTRODUCTION
M
ANY countries have now recognized the wind as a sus-
tainable source of energy and the installed capacity of
wind generation worldwide now exceeds 25 GW. Both for rea-
sons of network compatibility and to reduce mechanical loads,
many large wind turbines (installed either offshore or onshore)
will operate at variable speed and use doubly fed induction gen-
erators (DFIGs). [1]
In the past, most national network design codes and standards
did not require wind farms to support the power system duringa
disturbance. For example during a network fault or sudden drop
in frequency wind turbines were tripped off the system. How-
ever, with the increased use of wind energy, wind farms will
have to continue to operate during system disturbances and sup-
port the network voltage and frequency. Network design codes
are now being revised to reflect this new requirement. There-
fore,it is necessary to carry out simulation studies to understand
the impact of system disturbances on wind turbines and conse-
quently on the power system itself. These studies require accu-
rate steady state and dynamic models of wind turbines and their
associated control and protection.
Reduced order models of DFIG wind turbines for dynamic
studies have been published [2]–[4]. These models are based on
a single-cage representation of the rotor. For correct representa-
tion of the DFIG wind turbine, it is important to model the con-
trol system used. In [2] and [3], it was assumed that the d-axis
coincides with the maximum of the stator flux (this assumption
leads to difficulties when initializing the dynamic model from
a power system load flow) and the papers give only limited de-
tails of the control system used. In [4], no information of the
controller used is given.
It has long been recognized that in order to represent an in-
duction machine under system disturbances such as a fault, it is
0885-8950/03$17.00 © 2003 IEEE
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804 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 18, NO. 2, MAY 2003
Fig. 1. Basic configuration of a DFIG wind turbine.
desirable to use a double-cage model, which represents the tran-
sient and subtransient behavior of the machine more accurately
[5]. This paper presents a model that can be used for single-cage
and double-cage representation of the DFIG and its control and
protection circuits.
II. M
ODELING OF THE DFIG
DFIG wind turbines utilize a wound rotor induction gener-
ator, where the rotor winding is fed through back-to-back vari-
able frequency, voltage source, converters [1], [2]. A typical
configuration of a DFIG–based wind turbine is shown schemat-
ically in Fig. 1. The machine and converters are protected by
voltage limits and an over-current “crowbar” circuit. The con-
verter system enables variable speed operation of the wind tur-
bine by decoupling the power system electrical frequency and
the rotor mechanical frequency. A more detailed description of
the DFIG system together with its control and protection cir-
cuits can be found in [6].
A. Machine Modeling
Thegeneralized reduced ordermachinemodel was developed
based on the following conditions and assumptions.
a) The stator current was assumed positive when flowing to-
ward the machine.
b) The equations were derived in the synchronous reference
frame using direct (
) and quadrature ( ) axis representa-
tion [7], [8].
c) The
-axis was assumed to be 90 ahead of the -axis in
the direction of rotation.
d) The
component of the stator voltage used within the
model is chosen to be equal to the real part of the gener-
ator busbar voltage obtained from the load flow solution
that is used to initialize the model.
e) The dc component of the stator transient current was ig-
nored, permitting representation of only fundamental fre-
quency components.
f) The higher order harmonic components in the rotor in-
jected voltages are neglected.
The reduced order machine model in per unit was obtained as
(1)
(2)
(3)
where
(4)
and
(5)
In (4) and (5)
and
From (4) and (5), the stator current can be derived in the per
unit form as
(6)
(7)
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EKANAYAKE et al.: DYNAMIC MODELING OF DOUBLY FED INDUCTION GENERATOR WIND TURBINES 805
Using (2) to (5), the rotor currents can be derived in the fol-
lowing per unit form. See (7) at the bottom of the previous page
where
and
The per unit electromagnetic torque (positive for a motor) is
calculated using
(8)
Finally, if
is the mechanical torque, which depends upon
wind speed, the machine swing equation is given by
(9)
Equation(9),ofcourse,representsthewindturbineasasingle
lumped inertia. It has been shown by others [9] that for some
studies, it is important to represent the dynamics of the wind
turbine mechanical drive train using a multiple mass model. If
this is required, (9) may be expanded, as shown in [9], to in-
clude the masses of the generator, gearbox, and blades with the
associated torsional stiffnesses and damping.
Equations (6) – (9) were used to obtain the reduced order
dynamic model of the DFIG.
B. Modeling of the DFIG Converters and Control System
For the model, it was assumed that the converters are ideal
and the dc link voltage between the converters is constant. This
decouples converter C2 from C1. Converter C1 was modeled as
a voltage source whereas converterC2 wasmodeled as a current
source.
The rotor speed was controlled by
which is the -com-
ponent of the injected voltage, through converter C1. The con-
trol scheme used for speed control is shown in Fig. 2. The op-
timum torque – speed curve shown in the figure was used as the
reference for generator torque demand. This curve was mainly
characterized by three sections namely: (a) an optimal charac-
teristic curve given by
(where is the mea-
sured rotor speed) in between the cut-in wind speed and speed
limit, (b) a constant speed characteristic up to the rated torque,
and (c) a constant power characteristic beyond the speed limit
followedbybladepitch control actionfor high wind speeds.The
set point torque corresponding to the speed of the machine was
translated into
using the block “ ” where the function is
given by the following equation:
(10)
Fig. 2. Speed control scheme of the DFIG.
Fig. 3. No load PFC and VC through C1 for the DFIG.
The current error (the difference between the desired and
achieved
) together with a PI controller was used to obtain
.
The
-component of the converter voltage was used
for compensation for the generator magnetizing reactive power
(No load PFC) as shown in Fig. 3. An outer loop (shown
in dotted lines) was introduced for the voltage control (VC)
through converter
. The VC through converter was
introduced by comparing
with a reference voltage and
the error was regulated through a PI controller to obtain the
current
required to be added to the generator output current
as shown in Fig. 1.
Analternativeapproachwould be touse a currentcontrolcon-
verterfor C1, thus the rotor current can be tracked directly. How-
ever simulating this control technique would require significant
modifications to the machine model given in (7).
C. Modeling of the DFIG Protection
The controller model of the DFIG system included rotor
voltage and current limits. The limits were selected depending
on the megawatt capacity of the generator and the rating of the
converters. Converter C1 was protected against over-current on
the rotor circuits by a “single-shot crowbar,” as shown in Fig. 1.
The operation of the crowbar was modeled by deactivating the
converters upon the detection of rotor current magnitude above
the current protection limit and short-circuiting the generator
rotor.
III. D
OUBLE FED INDUCTION MACHINE UNDER FAULTS
A. Fault Current Contribution
Consider a fixed speed induction generator (FSIG) in which,
immediately after a fault occurs, the stator voltage and flux re-
duces toward zero. The voltage drop depends, of course, on the
location of the fault. The rotor current then increases to attempt
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806 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 18, NO. 2, MAY 2003
to maintain the flux linkage within the rotor windings constant
[10]. However, for a DFIG the increase in the rotor current im-
mediately after a fault will be determined by two factors. The
first is the change in the stator flux and the second is the change
in the rotor injected voltage.
According to Figs. 2 and 3 and (10), for a given mechanical
torque and speed
and .
Therefore, as a fault occurs
decreases and
increases. These changes will reflect on the controllers, thus
changing
and . The magnitude of the change acts
directly through the proportional gain of the controller. The
change in the rotor voltage components has a direct impact on
the rate of change of the rotor current components due to the
inductive nature of the rotor circuit (see (7)). In Figs. 2 and
3, the operation of the controllers results in a change in the
injected voltages that oppose the sudden increase in the rotor
current.
The following two operating conditions were simulated, by
changing the controller gains.
1) The proportional gain of the controller was set to a low
value. The increase in the rotor current triggered the
crowbar, thus interrupting the operation of the DFIG. In
practice, this would result in the turbine circuit breaker
being opened and the wind turbine being braked to a
standstill.
2) The proportional gain of the controller was set to a high
value. The rotor current was then less than the current
limit, thus ensuring continuous operation of the DFIG
during the fault.
If the crowbar is not triggered at the inception of the fault and
the wind turbine continues to operate then during the fault,
remains low butnearly constant. Therefore, and
are also nearly constant and depending on the speed of response
and gains of the controller, both
and start to follow
their respective reference inputs.
While the flux within the machine and injected rotor voltages
play a key role, the imbalance between the mechanical power
and electrical power also contributes to the machine operation
under a fault as in the case of a FSIG.
B. Behavior at Fault Clearance
During the fault, the stator voltage and rotor flux have been
reduced, the injected rotor voltage has been changed and the
rotor speed has been increased. Immediately the fault is cleared
the stator voltage is restored, and the demagnetized stator and
rotor oppose this change in flux thus leading to an increase in
the rotor and stator currents. However, restoration of the stator
voltage changes
and , and immediately the fault
is cleared
and . This leads to sudden
changes in
and . With a high proportional gain, the
change in the rotor injected voltage maintains the rotor current
below its current limit, thus ensuring continuous operation of
the DFIG.
Thus, the operation of the crowbar is mainly determined by
the rotor current at the inception and clearance of the fault,
which depends onthe proportional gainof the controller. Hence,
Fig. 4. Network for the fault studies (add ).
the integral gain component of the controller does not have a
material effect on the stability of the DFIG.
IV. S
IMULATION RESULTS
The DFIG was simulated using its single-cage and double-
cage representation. Appendix A gives the parameters of the
2-MW, 690-V wind generator used for the study.
To investigate the performance of DFIG under system fault
conditions, the two-bus double circuit power network shown in
Fig. 4 was modeled. At the point of connection of the DFIG
(
), a short circuit level of 40 MVA with to ratio of 5
was used to represent the network connection. The connection
transformer was rated at 2.5 MVA and leakage reactance was
chosen as 5.9%.
A. Fault Current Contribution and Post-Fault Behavior
Forfaultstudies presentedin Figs.5 and 6, a three-phase fault
was introduced at
s, with a clearance time of 150 ms.
Further, it was assumed that the mechanical input to the turbine
was 0.6 p.u.
The fault was introduced at the mid-point of one of the lines
(pointA)withtwovaluesofcontrollerproportionalgain settings
(
of 0.3 and 1.0). The stator current, electromagnetic torque
and speed of the machine during the fault and after the fault
was cleared is shown in Fig. 5 for
and Fig. 6 for
. With of 0.3, the crowbar was triggered by
the high rotor current when the fault was cleared. However with
of 1.0 normal operation of the machine was maintained
once the fault was cleared.
Simulations were also carried out with mechanical inputs of
0.4 and 1 p.u. Similar results were obtained.
Fig. 5(a) shows the magnitude of the stator current with both
a single cage and double cage rotor representation. When the
fault is applied, the double cage model shows a higher initial
current peak but a more rapid decay due to the smaller subtran-
sient time constant. The subtransient time constant is approxi-
mately 1 ms with the transient time constant around 110 ms. A
similar effect can be seen when the fault clears and at this point
the over-current protection operates the crowbar circuit. From
the simulations, it was found that the rotor current in per unit
during the transient is very similar to the stator current.
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EKANAYAKE et al.: DYNAMIC MODELING OF DOUBLY FED INDUCTION GENERATOR WIND TURBINES 807
(a)
(b)
(c)
Fig. 5. For a three-phase fault at the point A of network shown in Fig. 4 with
and .
Fig. 5(b) shows the electromagnetic torque, , during the
transient.
is negligible compared to and so, during the
transient
mainly depends on , and thus, on [see (8)].
The variation of
and hence are determined by the change
in rotor speed due to the speed control shown in Fig. 2, and in
rotor flux. The variation in rotor flux depend on the single cage
and double cage representation.
Fig. 5(c) shows how the operation of the crowbar forces the
speed of the machine to near 1 pu (fixed speed operation) al-
though in practice the main generator circuit breaker would be
opened once the crowbar operates. The different response of the
speed of the machine between 49.85 s and 50 s is due to the dif-
ferent
obtained from the single cage and double cage model.
(a)
(b)
(c)
Fig. 6. For a three-phase fault at the point A of network shown in Fig. 4 with
and .
As discussed, will act immediately to vary through the
effect of the controller.
Fig. 6(a) shows the stator current with the same fault applied
but with a higher proportional gain in the controller. The double
cage model shows higher sub-transient peak currents but oth-
erwise the response from the two models is very similar. This
is because the controller limits the peak currents in both cases.
Fig. 6(b) shows the electromagnetic torque,
, during the tran-
sient. As the change in the rotor speed is very small,
mainly
depends on the current transients at the inception and clearance
of the fault. Fig. 6(c) shows that, during the fault, the speed of
thegenerator ismaintained close toits prefault value and returns
to normal operation.
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808 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 18, NO. 2, MAY 2003
Fig. 7. Network behavior under power factor control (PFC) and VC on
different converters.
These simulation results demonstrate the importance of the
control system in limiting the generator currents during a fault.
Although a double cage model is useful for completeness quite
similar results are obtained with the single cage representation.
A more important assumption that has been made is that both
converters continue to operate normally during and after the
fault.
B. Power Factor Control and Voltage Control
The behavior of the wind farm and stability of the associated
networkdependson the control actionsofconverters
and .
In order to investigate the effect of various control approaches
to the behavior of the machine and the network after the fault
was cleared, the following three cases were simulated using the
DFIG double-cage model. In all three cases the proportional
gain of the controller was set low (
).
a) Compensation for the generator magnetising reactive
power (No load PFC) was applied through C1.
b) Compensation for the generator magnetising reactive
power (No load PFC) and terminal VC were applied
through
.
c) Compensation for the generator magnetising reactive
power (no load PFC) was applied on
and terminal
VC on
.
The wind farm terminal voltage at the point of connection
(
) and the generator speed wasobtained for all three cases
and shown in Fig. 7. In the case of no-load reactive power com-
pensation only, the generator goes unstable following the oper-
ation of the crowbar circuit. However, with active voltage con-
trolimplemented either through C1or C2, thegeneratorremains
stable. There is small difference in terminal voltage, and hence,
generator speed depending on whether voltage control is imple-
mented through C1 or C2. This is caused by the differentcontrol
actions of the two converters (C1 is a voltage source while C2
acts as a current source).
These simulations demonstrate that, as with any synchronous
generator, the reactive power control scheme has a significant
impact on stability.
V. C
ONCLUSIONS
Withincreasing wind penetrationin power systems, many na-
tional grid codes will demand complete models and simulation
studies under different system conditions in order to ensure that
the connection of a wind farm will not have a detrimental im-
pact on the network to which it will be connected.
Hence a dynamic model with reduced-order double cage rep-
resentation for the DFIG and its associated control and protec-
tion circuits has been developed. It was then used to simulate
the response of wind turbine to network faults on a simple two
busbar system.
It was demonstrated that by properly selecting the propor-
tional gain of the speed andpowerfactorcontroller, it is possible
to enhance significantly the stability of the DFIG.
Stability was improved using the following techniques:
(a) a high proportional gain in the rotor converter limited the
rotor current during the fault to a level below the trip setting of
the crowbar circuit and (b) fast-acting reactive power control
(applied through either converter) improves the stability of the
generator.
Voltage control using the rotor side converter (C1) is likely
to be preferred to using the network side converter for this task.
This is mainly because of the reduction in the converter rating
requirement as reactivepower injection through the rotor circuit
is effectively amplified by a factor of 1/slip.
The models that have been developed are suitable for in-
cludingin large powersystemtransient stability programs.They
include some representation of the practical limitations of the
converters (i.e., voltage and current limits) but the representa-
tion assumes that the dc link voltage remains constant. This as-
sumption is only valid if the dc link capacitor and converters
are designed to enable continued operation of the DFIG with
low generator busbar voltages caused by close-up faults.
A
PPENDIX
A. 2-MW Induction Wind Turbine Model Parameters (Star
Equivalent Circuit)
V, MW, ,
Hz
Stator resistance (
): 0.004 88 p.u.
Stator leakage reactance (
): 0.092 41 p.u.
Rotor resistance (
): 0.005 49 p.u.
Rotor leakage reactance (
): 0.099 55 p.u.
Double-cage resistance (
): 0.2696 p.u.
Double-cage reactance (
): 0.0453 p.u.
Magnetizing reactance (
): 3.952 79 p.u.
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EKANAYAKE et al.: DYNAMIC MODELING OF DOUBLY FED INDUCTION GENERATOR WIND TURBINES 809
Rotor to double-cage mutual reactance ( ): 0.02 p.u.
Lumped inertia constant (
): 3.5 s
B. Control Model Parameters
Cut-in speed
r/min, Speed limit r/min,
Shutdown Speed
r/min.
, ,
ACKNOWLEDGMENT
The authors wouldalso like to acknowledge the contributions
of Dr. A. E. Efthymiadis of IPSA Power Ltd.
R
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Janaka B. Ekanayake (M’95–SM’02) was born
in Matale, Sri Lanka, on October 9, 1964. He
received the B.Sc degree in electrical and electronic
engineering from the University of Peradeniya,
Sri Lanka, in 1990, and the Ph.D. degree in electrical
engineering from the University of Manchester
Institute of Science and Technology (UMIST), U.K.,
in 1995.
Currently, he is a Commonwealth and Tyndall Re-
search Fellow with theUniversityof Machester Insti-
tute of Science and Technology, U.K. He is a Senior
Lecturer in the Department of Electrical and Elec-
tronics, University of Peradeniya. His research interests include power elec-
tronics, FACTS devices, and renewable energy sources such as wind and small
hydro schemes.
Lee Holdsworth was born in England in 1971. He received the B.Eng. (Hons.)
and Ph.D. degrees in electrical and electronic engineering from the University
of Northumbria at Newcastle, U.K., in 1996 and 2001, respectively.
His industrial experience includes periods with BP Chemicals and BP Oil.
Currently, he is a Research Associate with the Manchester Centre for Elec-
trical Energy based at the University of Manchester Institute of Science and
Technology, Machester, U.K. His research interests include renewable energy,
with particular focus upon wind energy and power electronics applied to power
systems.
XueGuang Wu was born in Yunnan, P.R. China,
on August 16, 1966. He received the B.Sc degree
in electrical engineering from the Northeast Electric
Power Institute, China, in 1988, the M.Sc. degree
in electrical engineering from Electric Power
Research Institute (EPRI) of China in 1996, and the
Ph.D. degree in electrical engineering from Wuhan
University, China, in 2000.
Currently, he is a Research Associate at MCEE,
University of Manchester Institute of Science
and Technology, Manchester, U.K., working on
integrating renewables and CHP into the U.K. electricity network. His
research activities involve electrical power system modeling, system analysis,
system control, renewable energy, and embedded generation. He was an
Academic Visitor at MCEE, University of Manchester Institute of Science and
Technology, working on new and renewable energy technology in 2000.
Nicholas Jenkins (SM’97) received the B.Sc.
degree from Southampton University, U.K., the
M.Sc. degree from Reading University, U.K., and
the Ph.D. degree from Imperial College, London,
U.K., in 1974, 1975, and 1986, respectively.
His industrial experience includes periods with
Eastern Electricity, U.K., Ewbank Preece Consulting
Engineering, U.K., and BP Solar and Wind Energy
Group, U.K. He joined the University of Manchester
Institute of Science and Technology (UMIST) in
1992 where he is now a Professor and leader of the
Electrical Energy and Power Systems Group. His research interests include
renewable energy, embedded generation, and FACTS.
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