Application of STATCOM-supercapacitor for low-voltage ride-through capability in DFIG-based wind farm

The wind farm based on Doubly Fed Induction Generator (DFIG) is built. Not only it can be as active power source to provide active power to network, but also as reactive power source to stabilize voltage or supply Var compensation (reactive power absorbed or generated). Therefore, Var coordinated control strategy is proposed, instead of expensive equipments, such as Static Var Compensator(SVC) or Static Synchronous Compensator (STATCOM), in order to provide sufficient reactive power to maintain system stability. DFIG comprehensive model is built based on correlation models in Dig SILENT/Power Factory. The eigen value analysis method is employed to verify its impact on low-frequency oscillation modes and small signal stability of power system integrated with wind farm. The results from eigen value analysis are elucidated the effectiveness of Var coordinated control in DFIG system.

Enhancing voltage stability and LVRT capability of a wind-integrated power system using a fuzzy-based SVC

Since the doubly fed induction generator (DFIG) converters are designed by the slip power (a fraction 25-30% of the total power) to achieve full control of the generator, there is not enough capacity to provide reactive power when the voltage dips below the tolerable grid voltage level at the point of common coupling (PCC). We propose in this paper to use the static synchronous compensator (STATCOM) for compensating the grid voltage dip to improve the power quality in a weak grid or asymmetrical load. The simplified active and reactive power control strategies of a 6 MVA wind farm with DFIG are described. The maximal power point tracking (MPPT) algorithm is implemented. The simulation results using the platform between Matlab/Simulink and PLECS demonstrate the good performance of the wind farm with DFIG and STATCOM, validating the analytical conclusions.

According to the grid codes, on wind farm for reactive power compensation and voltage stability requirements, this paper analyzes the extent of a static synchronous compensator (STATCOM) to voltage stability of doubly fed induction generator (DFIG) wind farm, compared to a static VAR compensator (SVC), and proposes a new scheme for coordinated control of reactive power between DFIG and STATCOM, to do this, DFIG stator, grid-side converter (GSC) and STATCOM supply reactive power to the grid coordinately, where the advantages and disadvantages of rotor-side converter (RSC) and GSC are taken into more consideration, and the reactive power control ability of DFIG is fully utilized. Based on test system, research results show that, STATCOM can provide more reactive power than SVC for voltage stability under the same capacity, especially during the serious voltage sag; The coordinated control scheme can not only improve the transient voltage stability, but also help reducing the capacity of STATCOM so that save the cost of investments in wind farms.

This paper deals with the simultaneous tuning of proportional integral controller of static synchronous compensator (STATCOM) associated with lead-lag controller (LL) in order to improve the stability performance of combined wind farm (CWF). This type of farms has been developed to summarize the main benefits of Squirrel-Cage Induction Generator (SCIG) and doubly fed induction generators (DFIG). The performance of CWF with the tuned STATCOM is studied during line-to-line and three-phase faults. Multi-objective Lightning Attachment Procedure Optimization Algorithm (LAPO) is used as an intelligent optimization technique to determine the control parameters which can achieve the specified objective functions; minimizing the error signal and maximizing the output signal. The multi-objective LAPO is used to obtain the optimized values of AC voltage regulator, DC voltage regulator, current regulator, gains and time constant of LL. The ability of STATCOM tuned by multi-objective LAPO is examined by studying the performance of three wind farms. The first one is CWF and two other wind farms; one of them is based only on SCIG (SCIG-WF) and the other based only on DFIG (DFIG-WF) when they associated with the tuned STATCOM LAPO during grid fault. The results show that CWF associated with the tuned STATCOM LAPO has the best performance during grid faults compared with the other wind farms.

The analysis of reactive power for offshore and onshore wind farms connected to the grid through high-voltage alternating-current transmission systems is considered in this paper. The considered wind farm is made up with doubly fed induction generators (DFIGs). Modeling and improved analysis of the effective reactive power capability of DFIGs are provided. Particularly, the optimal power-tracking constraints and other operational variables are considered in the modeling and analysis of the DFIG reactive power capability. Reactive power requirements for both overhead and cable transmission systems are modeled and compared with each other as well as with the reactive power capability of the wind farms. Possibility of unity power factor operation suggested by the German Electricity Association (VDEW) is investigated for both types of installations. Aggregate reactive power demands on both wind farms are assessed such that the bus voltages remain within an acceptable bandwidth considering various operational limits. The reactive power settings for both types of wind farm installations are determined. In addition, the minimum capacity and reactive power settings for reactive power compensation required for cable-based installations are determined. Several numerical examples are given to illustrate the reactive power characteristics and capability of DFIGs, performance of transmission lines and reactive power analysis for DFIG-based grid-connected wind farms. A summary of the main outcomes of the work presented in this paper is provided in the conclusions section. Copyright © 2012 John Wiley & Sons, Ltd.

Using the FACTS controllers, such as static synchronous compensator (STATCOM), as it provides continuous reactive power, in the grid including wind turbine (WT) equipped with doubly fed induction generator, for improving voltage profile (under normal circumstances) and providing a transition ability from inductor generator transition state has been proposed. In this paper, in order to control the controllers and explained goals, nonlinear controller, as a substitute for the traditional controller, is presented. Replacing STATCOM controller in wind farm, which is equipped with doubly fed induction generator (DFIG) using input-output feedback linearization controller, the needed reactive power in order to stable wind farm equipped with DFIG is considered when error is occurred. The proposed control method has been simulated for IEEE-9 Bus, bus No 5, and the achievability to the desired targets in STATCOM efficiency for its reactive power has been investigated. The reasons of using these controllers in bus 5 are voltage dropping and reducing reactive power in this bus. It can be seen by compensating for voltage and reactive power in this bus that these two parameters have improved in other buses. The results show that with the proposed controller, STATCOM has done its duty well and the network bus voltage and reactive power to sustain the wind farm equipped with DFIG in transient mode is provided.

Fault ride through capability improvement of DFIG based wind farm by fuzzy logic controlled parallel resonance fault current limiter

Due to the increased renewable power penetration level renewable power plants have to provide low-voltage ride-through (LVRT) capability with simultaneous dynamic voltage support, to ensure the grid stability during grid faults. Concerning doubly fed induction generator (DFIG) based wind energy conversion systems (WECS) large electromotive forces and rotor currents, which may damage the rotor-side converter, or adversely affect the DFIG's controllability are induced into the rotor circuit in case of voltage dips. To handle and limit the rotor currents in case of asymmetrical voltage dips without crowbar triggering, a virtual resistance control approach based on the standard dq-control in the synchronous reference frame is discussed. The theoretical results are compared with those of more demanding virtual inductance control. The LVRT capability is verified with measurement results, recorded during a certification campaign at a 2.1 MW WECS concerning the Indian grid code.

In this paper, the doubly-fed induction generator (DFIG) for transient stability evaluations of power systems was systematically discussed, and the impacts of grid-connected large DFIG based wind farms on power system transient stability were then studied in detail. In the modeling of DFIG, an equivalent generator model was employed to reflect the combined behavior of the DFIG and its rotor converter. Furthermore, the control strategy of rotor-side converter, which was for the decoupled controls of the stator active and reactive power, was considered. The corresponding numerical simulations were carried out to explore and exploit the influences and interactions between synchronous generators and wind farm respectively on power system transient stability. In particular, different synchronous generator dynamic models and load models are considered during the simulation comparisons. Finally, some meaningful conclusions and comments are given in this paper.

Although the wind farms based on squirrel cage induction generators (SCIG) is cheaper than the wind farms based on doubly fed induction generators (DFIG), it is always in desperate need for reactive power compensation. Nevertheless, the wind farms based on DFIG are expensive compared with the SCIG wind farm, it features by its ability to control the active power independent of reactive power. However, combined wind farm (CWF) has been developed to collect the benefits of SCIG and DFIG wind turbines in the same wind farm. In this article, artificial neural network (ANN) is used to evaluate gain parameters of static synchronous compensator (STATCOM) in order to improve the stability performance of CWF. The impact of tuned STATCOM on the performance of CWF during gust wind speed and during three-phase fault is comprehensively investigated. The performance of CWF with STATCOM tuned by ANN is compared with its performance when the STATCOM tuned by the multiobjective genetic algorithm (MOGA) and whale optimization algorithm (WOA). The results show that the performance of CWF can be enhanced using STATCOM tuned by ANN more than MOGA and WOA.

As installed capacity of wind power increases significantly around the world, the power from wind energy starts to replace conventional generation and the active power control of wind farms has become a major concern to the stability and security of the entire power system. It is expected that the wind farm should operate upon operator's request and control its active power according to the dispatch command, which means the wind farm should behavior in some ways more similar to the conventional power plants. This paper mainly focuses on the control system and control capability of the double fed induction generator (DFIG)-based wind farm as well as the wind turbine. Besides, the requirements and different control modes of active power in wind farm are also presented. The control performance demonstrated by the simulation result validates the feasibility of active power control methods and control system presented in this paper.

The focus of documented research in this paper is the Low Voltage Ride Through (LVRT) capability of electric grid connected Wind Energy Conversion System (WECS) using Takagie-Sugeon (TS) type fuzzy controller. The increased deployment of Wind Turbine technology in modern power systems makes the LVRT capability of WECS one of the important research topics. Consequently, to avoid power network instability, power utilities defined technical specifications for modern WECS. These specifications are known as Grid Codes and are further categorized as static and dynamic requirements. The LVRT capability falls under the category of dynamic requirements of Gird Codes. The most frequently and extensively used generator for wind turbines is the Doubly Fed Induction Generator (DFIG) due to its lower cost, less power loss, variable speed, and four-quadrant operation. However, DFIG based WECS are highly sensitive to electric network disturbances because the stator of the DFIG is directly connected to the power system. Therefore, in this research study, the main attention is drawn towards the control of power electronic interface i.e., grid side converter of DFIG based WECS. The simulation results of symmetrical and asymmetrical faults, with different fault injection locations, are analyzed in order to provide a comprehensive understanding of the impact of electric grid disturbances on WECS. The results of the TS-type fuzzy controller are compared with the PI controller to illustrate the effectiveness of the fuzzy controlled DFIG based WECS. The aforementioned comparison of simulation results demonstrated that the transient stability of WECS is considerably improved by using TS type fuzzy controller. Thus, it is concluded that the TS-type fuzzy controller can be an efficient and promising solution for modern DFIG based WECS.

The ESS is considered as an effective tool for enhancing the flexibility and controllability of a wind farm, and the optimal control scheme of a wind farm with distributed ESSs is vital to the stable operation of wind power generation. In this paper, a coordinated active and reactive power control strategy based on model predictive control (MPC) is proposed for doubly fed induction generator (DFIG)-based wind farm (WF) with distributed energy storage systems (ESSs). The proposed control scheme coordinates the active and reactive power output among DFIG wind turbines (WTs), grid-side converters (GSCs), and distributed ESSs inside the WF, and the aim is to decrease fatigue loads of WTs, make the WT terminal voltage inside the extent practicable, and take the WF economic operation into consideration. Moreover, the best reactive power references of DFIG stator and GSC are produced independently based on their dynamics. At last, the control scheme generates optimal power references for all ESS to make the SOC of each ESS converge to their average state. With the distributed ESSs, the WF controller regulates the WTs inside WF more flexibly. A WF composed of 10 DFIG WTs was utilized to verify the control performance of the proposed coordinated active and reactive power control strategy.

In order to meet the grid codes and ensure safe operation of doubly-fed induction generator (DFIG) based wind energy conversion system (WECS) during asymmetrical grid faults, firstly, this paper designs low voltage ride-through (LVRT) control schemes for the back-to-back converters of DFIG-based WECS. Then, due to the converter capacity constraint, the controllable regions of DFIG during asymmetrical grid faults are analyzed by optimal method while the asymmetrical LVRT limitation of DFIG is given, which points out that DFIG-based WECS can achieve LVRT only under mild asymmetrical fault situations. At the last, a typical simulated system of a 9 MW DFIG-based wind farm connected into power system is used for case study. The simulation results validate the accuracy of the analysis for the asymmetrical LVRT capability of DFIG-based WECS.