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. References [1] Cheng KWE, Lin JK, Bao YJ, Xue XD. 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