Electrical Power and Energy Systems 49 (2013) 8–18
Contents lists available at SciVerse ScienceDirect
Electrical Power and Energy Systems
journal homepage: www.elsevier.com/locate/ijepes
A comprehensive review on the grid integration of doubly fed induction generator
H.T. Jadhav, Ranjit Roy ⇑
Dept. of Electrical Engineering, S.V. National Institute of Technology, Surat, India
a r t i c l e
i n f o
Article history:
Received 4 January 2011
Received in revised form 12 November 2012
Accepted 16 November 2012
Available online 30 January 2013
Keywords:
Doubly fed induction generator
Frequency regulation
Low voltage ride through capability
Maximum power point
Reactive power ancillary
Transient stability
a b s t r a c t
As a result of the growing demand for electricity and environmental constraints, the generation of electrical energy from renewable sources of energy has increased recently. The renewable energy sources,
especially wind power plants are integrated to power networks all around the world. The rising share
of wind turbine energy, in the existing power system, has created new opportunities and challenges.
For wind turbine energy generation doubly fed induction generators are most suitable due to their various advantages over fixed speed wind turbine systems. These generators have ability to improve stability
and power quality of the existing power systems. Therefore more attention has been paid by many
researchers recently to address various challenges of grid connection of DFIG. A comprehensive survey,
of different issues associated with integration of DFIG based system into the grid is presented in this
paper.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
The development in the field of wind power conversion technology has been seen since 1970 [1]. The countries like Denmark,
Portugal, Spain and Germany have the share of power provided
by wind sources is 21%, 18%, 16% and 9%, respectively [2]. In the
past, majority of wind power generation was based on fixed speed
wind turbine system. This system consists of a standard squirrelcage induction generator coupled with multi-stage gearbox driven
by aerodynamically controlled wind turbine. The stator of induction generator is connected to the grid directly or through power
electronic block. On account of increase in share of wind power
in power system and the stringent technical requirements for grid
connection, the technology has developed toward variable speed
wind energy conversion systems (WECSs). The use of variable
speed wind power conversion systems is becoming more popular
due to development in the field of power electronics. These machines can be controlled so as to meet the requirements of grid
as they can provide reactive power and frequency support in addition to active power. There are basically two types of variable
speed generator systems that are commonly used for wind power
application. The first type consists of a multi-stage gearbox and a
doubly fed induction generator with small size back-to-back power
converter in the rotor circuit.
The second type consists of a direct-drive low-speed synchronous machine with a fully rated power electronic converter. Because of uncertain nature of input power, the functional
⇑ Corresponding author. Tel.: +91 9904402937; fax: +91 261 2227334.
E-mail address: rr@eed.svnit.ac.in (R. Roy).
0142-0615/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ijepes.2012.11.020
characteristics of wind turbine generators are fundamentally different from conventional power plants. This fact has led to the
establishment of codes for the connection of the wind farm into
power grid. These grid codes are usually issued by the system operators of grid and are typically for large-scale wind farms connected
to transmission networks. The most common requirements of different countries for wind farm connection to grid include low voltage ride-through capability, frequency regulation ability and
reactive power support. The grid code requirements for wind farm
connection have been already discussed in several papers in the
past [3]. The use of DFIG is becoming more and more common
for power generation as it is suitable for the implementation of advanced features required for grid integration. This study, therefore,
deal with some of the aspects of grid integration of DFIG based
wind energy conversion system. The different areas in which the
researchers have contributed can be categorized in several ways.
However the major concern of most of the work deals with topics
such as, DFIG modeling for stability analysis, small signal stability
analysis, maximum power point tracking algorithms (MPPTs), low
voltage/fault ride through capability (LVRT/FRT) enhancement,
DFIG operation under network unbalance, frequency regulation
and DFIG as reactive power ancillary service.
2. Overview of doubly fed induction generator
As shown in Fig. 1, the doubly fed induction generator normally
consists of back-to-back converter in rotor circuit sharing a common dc link. The stator winding of DFIG is directly connected to
the grid, whereas the rotor winding is connected to the grid
through slip rings and power electronic converters. The generator
H.T. Jadhav, R. Roy / Electrical Power and Energy Systems 49 (2013) 8–18
9
Fig. 1. Vector control simulation of DFIG [18].
Fig. 2. Typical power coefficient versus tip-speed-ratio curve.
can be controlled to deliver power to the grid at sub-synchronous
as less at super-synchronous speed. The power converters connected between the rotor circuit and grid are generally rated at
25–30% of the nominal rating of machine. For DFIG based WECSs,
gearboxes are still required since the design of multi-pole lowspeed DFIG is technically not feasible. The power electronic converters of DFIG can be controlled to regulate both the active and
reactive power delivered to the grid. In addition to this the converters in rotor circuit can eliminate the need of soft-starter during
grid connection. There are two subsystems namely electrical and
mechanical control systems that represent the overall control
scheme of DFIG. These control systems are characterized by different objectives but the main aim is to control the power injected
into the grid. The active power supplied to grid is generally controlled by rotor side convertor (RSC) whereas the reactive power
injection is controlled by both converters. The electrical system is
also configured to provide overload protection. Normally a vector
control technique is employed for the control of rotor circuit
parameters to regulate the power.
The mechanical subsystem is designed to limit the mechanical
power output of wind turbine by means of pitch adjustment. The
function of electrical control system can be divided into three different categories. The first category includes the basic functions
that ensure the proper functioning of the power converters, by
maintaining proper voltages and currents on the machine side, in
the common DC link, and on the grid side. The second category includes specific functions to enable optimum extraction and the
limitation of the power. The third category includes extra functions
such as improving the power quality and contributing to voltage/
frequency stability of grid. This may be accomplished by responding to a supervisory command from transmission system operator
to take benefit of these added functions whenever required.
value of TSR. This implies that the turbine rotational speed should
be changed for different value of wind speed for capturing the maximum mechanical power from the wind. Below rated wind speed
the pitch angle is usually fixed to low value and speed of generator
is controlled electrically by means of vector or direct torque control
to extract maximum power. But, at high wind speeds as the generator power exceeds, the pitch control is enabled to limit the power
output to rated value in order to protect the generator from
mechanical damage.
The pitch control is used to mechanically control the speed of
wind turbine system to limit the power above rated power. The
mechanical power output of a turbine in steady state is given by:
1
qAV 3 C P
2
ð1Þ
where q is the air density (kg/m3), A is the swept area of wind turbine in m2, V the wind speed (m/s) and CP is the aerodynamic efficiency (power coefficient), which depends on pitch angle (b) and tip
speed ratio (k). The power coefficient CP is given by:
k¼
xR
V
Fig. 1 shows a typical vector control simulation for controlling a
DFIG system. It is a based on a Transformation of three phase variables to the synchronous frame variables in d–q reference frame.
The reference frame can be aligned to the stator or rotor flux of
the machine rotating at synchronous speed. However, a reference
frame aligned to the stator voltage space vector is a feasible choice
because the machine works as a generator fed from constant stator
voltage in grid connected operation. The vector control makes it
possible to independently control the flow of the active and reactive between the grid and generator. This method requires precise
information about the resistance and inductance of stator and rotor
winding. For the control scheme shown in Fig. 1, the rotor-position
angle is obtained by rotary position sensor or encoder. Sensorless
control of DFIG is another alternative for accurate and reliable control. In this control scheme the rotor position angle is obtained
from measured stator voltage and currents in combination with
open-loop estimators or closed-loop observers. The description of
vector control scheme of DFIG based wind energy conversion system is given in [4,5].
2.3. Direct torque control (DTC) of DFIG
2.1. Power control strategies of DFIG
W¼
2.2. Vector control strategies of DFIG
ð2Þ
where x and R represents the rotational speed and radius of the
turbine propeller, respectively. Fig. 2 shows a typical relationship
between the power coefficient CP and the tip-speed-ratio (TSR). It
can be noted that the value of CP becomes maximum for particular
Direct torque control (DTC) of the asynchronous motor drives
has been developed in mid 1980s [6]. The DTC method requires
the information of stator flux and torque to control the machine
by selecting appropriate voltage vectors. The stator flux is determined by integrating the stator voltage. The disadvantage of basic
DTC scheme is that its performance is inferior at start-up and at
very low speed [7]. In order to apply all the available voltage vectors in proper sequence and to address this problem, several approaches have been proposed such as using a dither signal [8],
predictive techniques [9] or a modified switching table [10].
3. Modeling of wind turbine with DFIG for stability analysis
With the growing share of wind energy the dynamic behavior of
the power systems will change considerably. The effective inertia
of the system will reduce if the contribution of power delivered
by DFIG based wind farms becomes comparable. This has an
10
H.T. Jadhav, R. Roy / Electrical Power and Energy Systems 49 (2013) 8–18
impact on system reliability. Therefore suitable models of wind
turbine and wind farm should be developed for power system stability studies. The simulation studies of the DFIG based wind turbine system has been reported in many literatures, most of the
research work, however, is based on use of electromagnetic models
to develop appropriate control strategies for power converters. The
fundamental frequency models for stability studies taking into account the integration of DFIG based wind farms into large-scale
power systems has been explored several literatures [11,12]. The
model of power converter in these literatures is still involved as
it consists of the power controller and back to back converter with
common DC link. But, in most of the cases, the internal dynamics of
power electronic converter are of little importance for power system stability analysis. Also as the simulation time step required by
the current controller is small; such models are computationally
expensive and not suitable for integration with existing power system simulation software packages [13]. To examine the impact of
the presence of DFIG based wind farm the power system dynamics,
simplified models have been considered wherein the effect of the
dc component of current and the stator transient currents are ignored [14,15]. The control of power devices in rotor circuit of DFIG
is generally accomplished by robust PI controllers. However, in order to properly adjust the gains of PI controllers the knowledge of
machine parameters and dynamics of power system is required.
For controlling the rotor side converter, a fuzzy controller and nonlinear controller have been proposed in [16,17], respectively and
the performance is compared with conventional linear control
schemes. These control schemes have been found to improve the
dynamic behavior of power system during grid disturbances. The
impact of FACTS controllers on the dynamic performance of the
power system connected to a DFIG based wind farm is reported
in [18]. Apart from transient stability studies, the small signal stability analysis of grid-connected DFIG type of wind farms have
been reported in many literatures. These studies mainly consider
deterministic approach for stability analysis. As the wind power
is stochastic in nature, the deterministic stability analysis can only
lead to solutions for specific operating conditions and the findings
may be too conservative or optimistic. This motivated the
researchers to use probabilistic approach for stability analysis
wherein the uncertain nature of wind can be represented by suitable statistical function such as weibull probability distribution
function. Two methods for probabilistic stability analysis method
have been presented in most of literatures namely point estimate
method [19–21] and Monte Carlo simulation [22–26]. In [26] a
multi-machine power system connected to DFIG based wind farms
considering different locations is analyzed by considering stochastic fluctuations modeled by weibull probability distribution function. With increasing level of wind penetration, the probability of
instability is more even though the same system is stable deterministically. While [27] proposes a control scheme optimized by
using PSO algorithm to coordinates damping controller of conventional synchronous generators and DFIG based wind turbine system in multi-machine power systems. It is shown that the
scheme improves network damping at light or rated wind speeds.
In general most of approaches reported for small signal stability
studies propose a suitable controller or power system stabilizer
(PSS). A critical analysis of transient and small signal stability studies of power system with DFIG based wind energy conversion systems is presented in Table 1.
4. Maximum power point tracking methods
As seen in Section 2.1, the maximum extractable power from
wind stream depends on the operating point of the wind turbine
system. Therefore, the maximum power point tracking (MPPT) is
of great significance in wind turbine systems for economic benefits. There are basically four control techniques employed for
extracting maximum power from wind. These are tip speed ratio
(TSR) control, optimal torque (OT) control, Power signal feedback
(PSF) control and perturbation and observation (P&O) control
[60–71]. The description of these techniques is given in [72,73].
However, for the sake of completeness and for the convenience
of the reader, these techniques are briefly explained below.
4.1. Power signal feedback (PSF) control
The PSF control scheme uses the reference optimum power
curve of the wind turbine obtained by simulation and field tests
and is recorded in a lookup table [74,75]. The MPPT is implemented without speed measurement which makes the scheme stable. However this technique requires accurate field date which is
difficult to obtain from field tests.
4.2. P&O method
The perturbation and observation (P&O), or hill-climb searching
(HCS) method normally needs the information about rotor speed
and the turbine power variation for extracting maximum power.
The advantage of the method is that it does not require knowledge
of wind turbine characteristic curve or generator parameters
[76,77]. It is robust to parameter variations. However, this method
is not suitable for large-inertia wind turbine systems since it fails
to reach the maximum power points if the wind speed variation
is very rapid. Moreover, it requires proper selection of appropriate
step size for faster response minimum oscillations around the peak
point.
4.3. TSR control
In this method the rotational speed of wind turbine is changed
in accordance with wind speed variations to achieve the optimal
tip-speed ratio [78,79]. Even though this method seems to be simple and flexible, an accurate measurement of wind speed and wind
turbine shaft speed is continuously required which is impossible in
actual practice. The wind speed measurement is generally done by
anemometer while the generator speed is measured by tachometers or absolute position encoders.
4.4. Optimal torque control
In this method, the generator torque is controlled to its optimal
value to obtain maximum value of power coefficient [80,81]. This
method also suffers from slow response time since the speed information is not available directly. It means the changes in wind
speed do not reflect instantaneously. The performance of different
MPPT control methods is presented in Table 2.
5. Low voltage/fault ride through capability
To allow large-scale dissemination of the wind energy into
power grid without stability problem, the turbines should contribute to the network in the event of fault. The grid code introduced
by certain countries define the operational limit of a wind turbine
connected to the network as regards frequency range, voltage tolerance, power factor, and fault ride through capability [3]. Among
these technical requirements, fault ride through is main challenge
to the wind-turbine manufacturers. In accordance with the regulations of the German Transmission and distribution utility, a wind
turbine should remain connected for a period of 625 ms during
the malfunction, while the voltage at the point of the connection
H.T. Jadhav, R. Roy / Electrical Power and Energy Systems 49 (2013) 8–18
11
Table 1
Transient and small signal stability analysis: A critical review.
Refs.
Control scheme and methodology proposed
Special findings
[25]
Deterministic and probabilistic approach for small signal stability analysis is
presented
[28–
32]
The reduced-order model is proposed which includes the flux dynamics, For
stability studies, reduced order models have been used by many researchers
by neglecting flux dynamics
Controller is proposed to control rotor circuit converter to enable DFIG to
exchange active and reactive power with grid to mimic STATCOM behavior
For the triggering of crowbar circuit, consideration of the oscillating
component in the rotor current is necessary. The ‘‘normal’’ reduced order
model is not suitable for crowbar consideration. Paper proposes enhanced
reduced order model of DFIG to enable triggering of crowbar protection
circuit in the event of fault
Third order and fifth order models are proposed to study impact of short
circuit and voltage dips on system stability and crowbar circuit functioning.
The test system containing DFIG based wind farm becomes unstable with
increasing of wind farm real power output for certain wind speed wind speed
distribution
The simulation results for same voltage sag produces similar results both
with the full- and the reduced-order model of the system. So the full order
model may be avoided to reduce computational burden
It is proved by simulation that the short-term voltage stability limit of the
system can be improved with proper control of wind farm connected to grid
Simulation results agree with field measurements results for practical system
during significant voltage drop
[33]
[34]
[35]
[36]
[37–
44]
[45,46]
[47]
[48,49]
[50]
[51–
53]
[54]
[55]
[56]
[57]
[58]
[59]
A nonlinear controller is proposed for DFIG connected to grid using
differential geometry theory
Power factor and voltage control mode is implemented for DFIG connected to
interconnected system for small signal stability analysis
An eigenvalue analysis and dynamic simulation is carried to study the effect
of induction generator (IG) and torsional interaction in series compensated
power system for different compensation levels
The control scheme to examine the IG effect of DFIG in series compensated
power system is proposed. The impact of wind speeds, series compensation
levels and control scheme on IGE phenomena is also studied using eigenvalue
analysis
A small signal stability analysis to analyze sub-synchronous resonance
between a DFIG based wind farm and a series compensated power line is
presented
A supplementary controller is added in the grid side converter control loop to
suppress sub synchronous resonance in series compensated power system
integrated with DFIG based wind farm. The controller is designed to
modulate reactive power output of DFIG wind farm
Small signal stability model of system is developed and then an eigenvaluebased objective function is solved by using particle swarm optimization
algorithm (PSO) [51] Bacterial foraging [52] differential evolution (DE) and to
tune the parameters of DFIG controller. DFIG with tuned controller
parameters is tested on SMIB system and the multi-machine system [53]
An analysis of system containing large number of synchronous generator
connected to DFIG based wind farm of comparable size is presented. The
impact of increased penetration of wind power on transient and small signal
stability of a power system is analysed
A control scheme to regulate current in RSC is proposed to damp inter-area
oscillations. An active and reactive power modulation is carried out to
investigate their effect on inter-area oscillations
An analysis of active and reactive power modulation of DFIG based system for
their effectiveness in damping and their interaction with wind turbine shaft
is presented
A damping controller for HVDC link designed by using modal control theory
is designed for DFIG based offshore wind farm (OWF) system
A detailed system model, considering two-mass drive train, pitch control and
other components is presented. An eigen-value analysis is carried out to
study controller parameters which induces the oscillatory instability, when
wind speed is lower than rated value of DFIG system
A small signal stability analysis considering squirrel cage, permanent magnet
synchronous generator and DFIG base wind energy conversion system is
presented. The effect of these wind generators on local, inter-area, torsional
and control modes of the system, with respect to different wind penetration
levels is investigated
drops to 15% of the nominal value (Fig. 3). Only if the supply voltage drops below the curve, the turbine is allowed to disconnect
from the network. When the voltage is in ‘‘no trip’’ area, the turbine should also provide reactive power to the grid to help grid
to restore normal supply voltage.
At the time of a fault in the power supply network, a grid connected DFIG experiences serious rotor over currents [102], which
could damage the rotor side voltage source inverter (VSI). A lot
of literature describes a popular method to protect the power elec-
(1) Model simplification does not affect DFIG transient response.
(2) Very low crowbar resistance results in instability & false restart of
RSC, whereas very high value leads to over speeding of DFIG due to
reduced electrical torque.
System damping and the fault ride-through capability of DFIG is improves
DFIG contributes to increase electromechanical oscillation damping or that
DFIGs can even provide good damping performance into a weak grid
For higher compensation level and low wind speed, the network resonant
frequency tends to increase which results in poor damping; while the
damping of the network resonant mode improves at higher wind speed and
compensation level. At high wind speed shaft mode becomes dominant
At higher wind speed the damping of the network mode is better however it
reduces as compensation level rises
The sub-synchronous interaction between DFIG wind turbine-generator
system and series compensated power systems is due to controller
interactions
The supplemental controller is shown to be effective in damping SSR and
torsional interaction at a high level of compensation
The dynamic performance of wind turbine system improves and provides the
fault ride through capability to DFIG
The increased penetration of DFIG have both beneficial and detrimental
impact on inter-area oscillation damping
The active power modulation is better than reactive power modulation to
damp inter-area oscillations
The damping of the shaft mode reduces due to active power modulation
while reactive power modulation is free from such risk
The proposed controller for line-commutated HVDC provides adequate
damping to DFIG-based OWF under various wind speeds and disturbance due
to fault
The Hopf bifurcation boundaries for critical parameters of controller are
determined to ensure stable operation of DFIG based wind power system
For higher penetration level the control modes more affected than local and
inter-area oscillating frequencies in case of DFIG, whereas the effect of PMSG
is more on inter-area oscillating frequencies. SCIG wind farms increase the
damping of the system for limited wind power penetration
tronics during such situation is the use of a crowbar circuit, which
short-circuits the rotor when a fault occurs [103–106]. After fault
clearance, the rotor side converter is reconnected. However, control of the real and reactive power output of the DFIG is only possible as long as the rotor side VSI remains connected to the rotor.
Several research studies have been conducted in the literature to
determine effect of voltage sag on electrical quantities of DFIG.
These studies, however, have conflicting opinion about the cause
of over currents in the rotor circuit of DFIG. The simulation study
12
H.T. Jadhav, R. Roy / Electrical Power and Energy Systems 49 (2013) 8–18
Table 2
Maximum power point tracking algorithms: a critical review.
Refs.
Control algorithm used
Contribution and control scheme
Special findings
[82]
TSR
The method reduces drive-train fatigue by dampening drivetrain torque fluctuations while tracking maximum power
[83]
TSR
[84,85]
OT
[86]
OT
[87]
OT
[88]
TSR
[89]
OT
[90]
OT
[91]
P & O/hill-climbing
method
[92,93]
TSR
[94]
P&O
[95]
TSR
[96]
TSR
A model-based predictive control strategy for the fieldoriented control of a doubly fed induction generator is
presented
An alternative to the conventional back-propagation
approach in neural network design called radial basis
function network is used to characterize the aerodynamics of
wind turbine system and hence to estimate wind speed
(sensor less approach). The power losses and the dynamics of
the WTG shaft system is also included to estimate turbine
shaft power
A sliding mode controller is proposed for tracking optimum
power from DFIG wind turbine system [84]
The rotor position phase lock loop observer which requires
information of stator and rotor currents, and stator voltages
to estimate generator speed is proposed
The nonlinear predictive current regulator which predicts the
rotor currents to control the DFIG power is proposed in place
of conventional PI current regulator
A feedback/feedforward nonlinear controller is proposed
which is capable of tracking maximum power and
responding to system operators request for active and
reactive power (power factor)
An improved Elman Neural Network-Based Control
Algorithm (IENN) is proposed for adjusting pitch angle of
DFIG based system to limit power below rated value. The
learning rates of IENN are optimized by modified PSO
algorithm. The performance is compared with convection PI
and Fuzzy logic based controllers
A MIMO control strategy based on model predictive control
theory is proposed to control speed and pitch of DFIG based
system
A nonlinear rotor side controller is developed for a DFIG
based on nonlinear quadratic Gaussian (LQG) control theory
with aim to Transfer the maximum power from the turbine
to the generator.
A vector control scheme to control the RSC to independently
control the active and reactive power and to track the
maximum wind power point is proposed. A neuro-fuzzy gain
tuner is proposed to control the DFIG The gains of PI
controllers are tuned by neuro-fuzzy logic schedulers
A controller consisting of two fuzzy logic based subcontrolers namely main fuzzy MPPT which uses hill climb
search algorithm (HCS), and other fuzzy controller which
adapts to HCS is proposed
A direct AC–AC matrix converter system in place of
conventional AC–DC–AC converter in rotor circuit of the DFIG
is developed. The system control is assisted by flywheel
Fuzzy logic controller based on Takagi–Sugeno (TS) fuzzy
model is proposed
[97]
TSR
[98]
OT
[99]
TSR
[100]
TSR
[101]
Modified OT
The mechanical and the electromagnetic subsystem of the
DFIG are represented in terms of port-controlled Hamiltonian
system. A novel nonlinear energy-based excitation
controlling strategy is developed to control DFIG wind
turbine system
Second-order sliding mode to control the wind turbine DFIG
The optimal rotor currents used to control speed to achieve
optimum TSR are obtained by using a detailed model with
stator resistance, changing stator flux linkage, and core loss
(neglected in most of research paper)
The modeling and control approach using instantaneous real
and reactive power instead of dq components of currents in
case of vector control scheme is proposed
To control the power an additional proportional controller is
added to reduce the effect of the moment of inertia and the
damping coefficient of the wind turbine generator system
in [12], for example, assumes that the rotor over current is due to
sudden change in the stator flux. On other hand, the results presented in [107] are based on the fact that the cause of rotor over
currents is rise in the stator current which is due to voltage dip.
The speed controller provide effective damping to lowfrequency torsional oscillations of the wind turbine generator
system during maximum power point tracking
Controller tracks optimum power with reduced torque ripple
As the speed estimation does not require knowledge of
parameters of machine components the response is free from
Parameter variations
The controller works even under network unbalance and
eliminate torque and active power oscillations
Controller provides seamless control between maximum
power tracking mode to power regulation mode as per
requirement
The technique is able to perform better than conventional PI
and fuzzy logic controller and remains stable even with
parameter uncertainties
Performance is better than PI controller in terms of
robustness towards parameter variations and drive train
torsional torque overshoots
The controller continuously seeks to maximize the power
absorbed by the wind turbine under steady state as well as
transient state
The controller proposed, provides faster system response
with minimum overshoot, small settling time, and minimum
steady state error.
The overall adaptive controller provides better performance
compared to the traditional power-tracking loop
The flywheel minimizes the fluctuations in wind
The controller is robust against parametric uncertainties, and
wind disturbance and power conversion efficiency achieved
is 48%
The performance when compared with the PI controller is
better during the turbulent wind speed condition
The response is fast and chattering-free and robust to grid
disturbances and unmodeled dynamics
The method is more efficient and fast to track maximum
power when compared with exhaustive search method
The implementation scheme is simple and performance is
robust compared with vector control due to absence of
reference frame concept and variable transformation
The technique improves robustness of systems to the
parameter variation of the DFIG as the torque feedback
control is absent, The dynamic performance is better than
conventional OTC, the mechanical stresses on the wind
turbines are reduced
Earlier research work suggests simple solutions to improve faultride-through capability to DFIG by introducing bypass resistors in
the rotor circuit without disconnecting rotor and grid side convertors. The converters remains connected to grid continue to supply
H.T. Jadhav, R. Roy / Electrical Power and Energy Systems 49 (2013) 8–18
13
Fig. 3. E.ON Netz requirement for fault ride-through capability of WT.
Table 3
Low voltage ride through enhancement schemes: A critical review.
Refs.
Control scheme proposed
[113]
[125]
[126]
[127]
[128,129]
[130]
GA-based optimal controller is proposed to control RSC
RSC generates rotor current in phase opposition with stator over current during stator voltage dip without using any external crowbar circuit
A converter with transformer is placed in series with stator winding which minimizes voltage drop at stator terminals during grid fault
A dynamic resistor is connected in series with the rotor winding that limits the rotor over current during fault
A superconducting fault current limiter is connected in stator circuit during voltage dip
Crowbar with optimum resistance is introduced for short duration during transient fault and then disengaged to enable DFIG to continue to feed active
power
Additional feed-forward current control loop is introduced to compensate transient rotor over currents with minimum crowbar application
[132]
reactive power to grid [108]. This bypass external resistance can
improve the torque characteristic contributing to dynamic stability
of the machine during grid faults. But the introduction of crowbar
circuit converts the DFIG to a squirrel cage mode which continues
to consume the reactive power during the fault. Another approach
in which the crowbar circuit with optimal resistance is introduced
during fault and both rotor converters are operated in parallel is
proposed in [109,110]. The crowbar resistance is gradually removed to minimize reactive power consumption by stator winding. Another configuration to minimize stator inrush current and
electrical torque oscillations during grid fault is suggested in
[111,112]. It consists of passive crowbar connected in series with
stator winding with active LVRT compensator controlled through
rotor voltage control. The passive crowbar (resistance) is activated
during grid fault only. The control of DFIG converters is usually
done by proportion-integral current controllers. However, the performance is dictated by suitable selection of controller gains. It has
been shown in [113] that the dynamic performance of DFIG during
grid fault is better if the PI controller gains are optimized by genetic algorithm. Though the gains of current controllers are tuned,
these controllers have very limited control bandwidth. This requires a best compromising solution between system stability over
wide operation range and desired response under transient conditions. To address this issue the study in [114] proposes two control
strategies integrated with a supervisory control unit. Under normal
conditions, traditional PI current controller is used while during a
vector-based hysteresis current control system is activated during
grid fault which controls RSC for LVRT reason. The other studies includes design of suitable rating of the rotor-side converter which
can withstand the over currents in rotor circuit during grid faults
to achieve fault ride-through. For instance, an analytical expression
to size the rotor-side to enhance the ride-through capability during
three phase voltage dip is presented in [115] and the study in [116]
deals with sizing RSC to sustain the unsymmetrical sag. Moreover
certain schemes [117] are proposed to keep DC link voltage of rotor
convertor constant during voltage dip during grid fault in order to
have the ride-through capability. During grid fault the voltage at
stator terminal reduces drastically and the stator flux which is
function of stator voltage drops very rapidly. The flux cannot disappear instantaneously and therefore induces large electromotive
forces in rotor winding causing high rotor currents. The study in
[118–120] proposes a strategy to control rotor-side converter in
which a component of current in rotor current opposes the components in the stator-flux linkage responsible for rotor over current. It
can be inferred from above that most of the studies deals with
development of suitable controller to improve rotor dynamics during voltage dip or grid fault. Some studies have demonstrated that
there is requirement of suitable control strategies even after clearance of fault as post fault transient may lead to unstable situation
for DFIG under certain operating condition and can be addressed
by suitable nonlinear controller [121]. The large voltage dip at
point of common connection can demand reactive power which
may not be possible to be supplied by converters of DFIG. Under
this situation STATCOM can be another option to reduce voltage
sags and the temporary inequality between the electrical power
generated and the mechanical supplied [122,123,131]. For optimizing reactive power from STATCOM and rotor converters in order to reduce cost and size heuristic dynamic programming is
proposed in [124]. Table 3 presents methods to improve stability
margin of power system and impart LVRT capability to DFIG by
deploying deferent control schemes.
6. DFIG operation under network unbalance
Wind farms are usually located in remote areas and connected
to grid through weak and inherently unbalanced power lines. The
stator current could also be unbalanced even for small imbalance
in stator voltage, if the control system of DFIG does not consider
this unbalance in the voltage. The unbalanced currents create unequal heating on the stator winding as well as torque and power
pulsation in the generator. Induction generators are switched out
of the network if the amount of unbalance is beyond limit. This
can further weaken the grid performance. Therefore, a control
scheme that can reduce the unbalance in the stator currents to
14
H.T. Jadhav, R. Roy / Electrical Power and Energy Systems 49 (2013) 8–18
eliminate torque and reactive power pulsations is required for
proper operation of DFIG. The methods proposed in literatures
are mostly based on the symmetrical component theory. The control schemes and DFIG are modeled in positive and negative synchronous reference frames. Some studies show that the rating of
converters increases due to negative sequence currents [133].
The study reported in [134] shows that the supplementary feedback controllers in rotor circuit with very high gain at the known
disturbance frequency can compensate the torque and reactive
power pulsations for unbalanced stator voltage. A voltage unbalance detector and auxiliary controllers are proposed in [135] to detect and operate DFIG under unbalanced condition. The generated
current references are used by main and the auxiliary controllers to
generate the required rotor and grid control voltages. Direct power
control strategy is suggested in [136]. The active and reactive
power is controlled without decomposition of the machine variables. A proportional integral controller and a resonant compensator in proposed in [137,143]. Under unbalanced condition, it
controls RSC to eliminate the torque pulsation. The oscillations of
the stator output active power are controlled by GSC. The study
in [138] presents a scheme in which either the grid side or the rotor side converter is enabled to inject negative sequence current
into the AC system to mitigate unbalance in grid voltage. A dual sequence – positive and negative sequences control scheme is proposed in [139] to suppress the torque pulsation and dc voltage
ripple. A series grid-side converter is proposed in [140]. This converter is activated once the voltage sag is detected. A direct power
control techniques to remove torque oscillations is proposed in
[141]. In this scheme the active and reactive power references
are generated for RSC and GSC to eliminate stator current unbalance. Another approach similar to [141] is proposed in [142]. In
this work the output power of DFIG and GSC are regulated without
using positive and negative sequence decomposition. An improvement in performance of fixed speed induction generators by seeking help of closely paced DFIG under network unbalance is
proposed in [144]. The control strategy suggested is to operate
RSC and GSC respectively to compensate torque ripples and voltage
unbalance. An application of sliding mode control scheme with
space vector modulation is proposed in [145]. The robustness of
controller against parametric variations is demonstrated in this research work. In [146] the rotor current control loops are replaced
with main and auxiliary controllers. The main controller controls
the positive sequence stator voltages with positive sequence rotor
voltage, and the auxiliary controller controls negative sequence
stator voltages directly with negative sequence rotor voltage. A
sliding-mode-based direct power control scheme is proposed in
[147]. It is designed to perform tasks such as producing sinusoidal
and symmetrical grid current, filtering reactive power ripples and
removing active power oscillations. A direct torque control scheme
that extracts both positive and negative sequence components and
independently controls the output torque and stator reactive
power is proposed in [148].
in penetration level of variable speed wind turbine system. To account for this issue recently the system operators of many countries have introduced grid codes regarding frequency and active
power support from wind farms [3,149]. Usually the objective
wind farm operator is to employ control scheme to have maximum
power production for economic reasons. Therefore, this strategy
does not have provision to preserve the power margin that can
help during frequency excursion. In order to have short duration
frequency support from wind turbines, suitable mechanism for active power control is a necessary. The control schemes that impart
frequency response capability to variable speed wind turbine system have been proposed recently in some of the literatures. These
approaches mainly deal with inertial control and power reserve
control [150–157]. The proposed control schemes works like speed
governor and adjusts power output automatically during frequency excursion. The efficiency of these schemes however depends on available generating margin. The operation of DFIG
under deloaded optimum power extraction curve is reported in
[158–161]. In this the active power output of each DFIG tends to
increases or decreases in accordance with frequency changes. The
control scheme is designed to regulate the power from DFIG by
controlling both static converters and pitch control system as per
deloaded optimum power extraction curve. The pitch control
scheme is designed to limit the mechanical power from wind turbine during very high wind speeds. Ref. [160] shows that the sudden wind gusts and wind ramps increases the drop in rotor speed
due to the supplementary control loop used for frequency regulation but the rotor speed is stabilized eventually. Ref. [161] presents
simulation studies by considering local and central control strategies. The effect of wrong set point of reference active power for frequency regulation on frequency response following disturbance is
reported. A scheme for supervisory control of wind farms is proposed in [162]. The scheme makes use of either an external energy
storage device or a reserved power due to part-loading of turbines
in wind farm. The capability of DFIG based wind farm to provide a
short-term frequency support in a hydro dominated system is presented in [163]. The wind reserve allocation on the basis of available wind speed in DFIG based wind farm to provide primary
reserve for frequency control is presented in [164]. The higher
the wind velocity, the more the deloaded margin for that wind turbine. It is possible that the fast response capability of electronically-controlled WECS, allows the kinetic energy stored by
rotational masses to be partly and transiently released in order
to provide earlier frequency support [165,166]. A power system
stabilizer (PSS) installed in wind farm of DFIG type to provide additional damping to the electromechanical modes of oscillation is
proposed in [167]. A moving average method is suggested to preserve a certain amount of wind power reserve for frequency support is suggested in [168].
7. Contribution of DFIG to frequency control of grid
As more and more wind turbines replaces the traditional synchronous machine based plants the system inertia will reduce significantly. This is because the variable speed modern wind turbines
are equipped with power electronics block which effectively
decouples the inertia of the wind turbine from the power network.
The reduced inertial of system will require large restoring forces to
bring back the disturbed synchronous machines to equilibrium.
Moreover, the drastic drop in frequency following a disturbance
will result in transient stability problems. Thus the power system
operators will face challenge of frequency regulation due to rise
Fig. 4. DFIG capability limits.
H.T. Jadhav, R. Roy / Electrical Power and Energy Systems 49 (2013) 8–18
8. DFIG as reactive power ancillary service
The fundamental task of transmission system operator (TSO) is
to balance the reactive power in the grid. Since wind power portion
is increasing the reactive power generation during steady state and
as well as transient condition the wind turbines should contribute
to reactive power generation. During the low speed operation the
active power output of DFIG WT is far less than it’s rated power.
For the improvement of voltage profile reactive power can be supported to the grid by DFIG. It may be necessary in future for wind
farm operators to supply reactive power to grid during normal as
well as transient conditions. When the wind speed is low the active
power output of DFIG WT is far less than its rated power, it is possible that the DFIG can provide reactive power support to grid for
improving voltage profile [169–171]. The reactive Power capability
limit and steady state limits of DFIG are shown in capability curve
in Fig. 4.
The control of RSC and GSC in coordinated manner to supply
reactive power is proposed in [172]. The detailed information
about controller tuning controller is not given. The improper operation of converters may lead the absorption of reactive power.
DFIG is used as reactive power ancillary service and is presented
in [173]. The cost components as well as reactive power cost model
are derived in [174] for wind farm. The authors have demonstrated
that the reactive power support from DFIG improves profiles of
post-fault voltage by damping oscillations and preventing overshoots. The utilization of extended reactive limits in voltage control may prevent system collapse. A dynamic reactive power
regulator for DFIG based wind farm is proposed in [175], which
provides optimum reactive power for voltage regulation of distribution system. Linear control technique is proposed to determine
the gain of reactive power controller. For optimal reactive power
support of an offshore wind farm a predictor–corrector primal–
dual interior point method is used which is presented in [176].
The Constraints of these optimization problems are maximum
allowable stator, rotor currents and the steady-state stability limit
of the generator. The results of the optimization problem for different wind speeds and parameter variation have been considered for
study.
9. Conclusions and future trends
In this paper different aspects for integration of doubly fed
induction generator (DFIG) based wind generator are reviewed as
proposed in recent research literature. There are several issues presented by researchers for improving the overall performance of
power system which has substantial contribution from DFIG based
wind turbines. However, most primarily the focus of most of the
research work deals with areas like control algorithms for maximum power extraction from wind, enhancing the stability of system when DFIG based wind farm is connected to grid, also low
voltage ride through capability and frequency control support by
wind turbines. Moreover the focus of some of the research is devising schemes for operation of DFIG under network unbalance and
reactive power support to the grid.
The survey shows that the researchers have extensively employed the potential of power electronics and advanced control
theories to optimize the performance of DFIG based power system.
The foregoing study shows that the potential of nature inspired
optimization algorithms is not fully exploited for optimum performance of power system.
The share of DFIG based wind turbine in increasing in market
due to its the various advantages over other type of wind energy
converter system. This is mainly because of requirement of low
cost and small size power electronics system. As a result of growth
15
in power electronic technology the use of variable-speed wind turbines with a full-scale power converters may be another alternative DFIG. Furthermore, the use of flexible AC Transmission
devices to further support the grid performance can be another
possible alternative for better power generation and control
systems.
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