Shuffled frog leaping algorithm‐based static synchronous compensator for transient stability improvement of a grid‐connected wind farm

Dynamic compensation of real and reactive power in wind farms using STATCOM

This paper describes a transformerless static synchronous compensator (STATCOM) system based on cascaded pulse-width-modulation (PWM) H-Bridge converter with delta configuration. It is intend for dynamically compensating reactive power and negative-sequence load current, as well as for improving power quality and reliability of power distribution system. In this paper, attention is paid to reactive-power control, negative-sequence current compensation, along with voltage-balancing control consisting of clustered balancing control and individual balancing control. Clustered balancing control that keeps the dc voltage of each cluster equal to the dc mean value of all the three clusters is achieved by introducing a zero-sequence current flowing through the loop of the three delta-connected clusters in three phases. The aim of this control scheme is to fully extend power-quality- improving capability of STATCOM system. A 100-V 3-kVA downscaled laboratory system is designed, constructed and tested, using large-capacity electrolytic capacitors as dc-voltage sources. Experimental results obtained from the laboratory system verify the viability of this cascaded multilevel PWM STATCOM system.

This paper proposes a coordinated control of the reactive power between the STATCOMs (static synchronous compensators) and the grid-side converters (GSC) of wind farms equipped with PMSGs (permanent-magnet synchronous generators), by which the voltage fluctuations at the PCC (point of common coupling) are mitigated in the steady state. In addition, the level of voltage sags is reduced during grid faults. To do this, the GSC and the STATCOM supply reactive power to the grid coordinately, where the GSCs are fully utilized to provide the reactive power for the grid prior to the STATCOM operation. For this, the GSC capability of delivering active and reactive power under variable wind speed conditions is analyzed in detail. In addition, the PCC voltage regulation of the power systems integrated with large wind farms are analyzed for short-term and long-term operations. With this coordinated control scheme, the low power capacity of STATCOMs can be used to achieve the low-voltage ride-through (LVRT) capability of the wind farms during grid faults. The effectiveness of the proposed strategy has been verified by PSCAD/EMTDC simulation results.

<p>In the recent years, the integration of the wind farms into the electrical grids has increased rapidly. Especially, the wind power plants made up with doubly fed induction generators due to its many advatanges, such as being able to control its reactive power. Hence, some countries have published grid code requirements related to the reactive power that the wind turbines have to satisfy. This paper presents a coordinated reactive power control strategy in which STATCOM and doubly fed induction generators in wind power plants are used in order to bring back the voltage at the point of common coupling in the allowable range. First, reactive power requirements that the wind farms have to fulfill in some European countries are introduced. Second, the reactive power limitations of 2MW doubly fed induction generator are determined. Then, the static synchronous compensator (STATCOM) and the synchronous var compensator (SVC) FACTS (Flexible AC Transmission Systems) devices are presented. Finaly, various reactive power control strategies are applied to 10 MW wind farm, and the simulation results are analysed and compared.</p>

The main causes of wind farms disconnection from the grid is the three-phase grid faults at the point common coupling (PCC) e.g. the voltage dip. The use of a Static Synchronous Compensator (STATCOM) which is from the family of Flexible AC Transmission System (FACTS) devices can be used effectively in a wind park based on FSIG to provide transient voltage and to improve wind system stability. Due to the asynchronous operation nature, system instability of wind farms based on FSIG (Fixed Speed Induction Generator) is largely caused by the reactive power absorption by FSIG because due to the large rotor slip during grid fault. STATCOM contributes to control the grid voltage at PCC and maintain wind farm connection to the grid during some severe conditions of grid faults and used for power flow control and for damping power system oscillations. The evaluation of this control strategy using (STATCOM) is investigated in terms of regulation reactive power and transient stability of the wind farm during grid disturbances.

This paper investigates the use of a static synchronous compensator (STATCOM) to mitigate the voltage fluctuations and improve fault ride-through capability of wind farm with fixed speed induction generators. In order to assure STATCOM of running safely and reliably in both normal and faulty conditions, a novel control strategy which combines negative-sequence voltage feed-forward control with conventional vector control is proposed to make the negative sequence current of STATCOM zero and effectively suppress STATCOM overcurrent. The positive-sequence and negative-sequence components of the voltage at point of common coupling (PCC) are obtained quickly and accurately by the specially designed double synchronous reference frame phase locked loop (DSRF PLL). The simulation results show that the STATCOM not only significantly improves the power quality, but also enhances fault ride-through capability of wind farm. It is also concluded from the simulation results that the proposed control strategy can effectively suppress STATCOM overcurrent and guarantee that STATCOM can operate safely even in abnormal conditions.

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.

The contribution of wind power in conventional generation is increasing rapidly now-a-days. Hence a better understanding of wind power plants is required. But the wind power plants generally containing induction generators exhibit a rather different behavior than the conventional power plants which use synchronous generators. The purpose of this paper is to analyze and study transient stability of a wind farm based on conventional fixed speed Induction generator when it has been integrated with a weak grid. A comparative analysis of the transient stability improvement of a FSIG wind plant has been done, when it is supported with FACTS like S.V.C and STATCOM of equal ratings. Single case of transient condition i.e. a three phase fault has been studied by simulation. With the increase in demand of power and decrease of fossil fuels, mankind has been forced to search alternative sources for the generation of electricity. Wind power in spite of being stochastic in nature has proved itself as a viable solution to this problem. As the wind turbine technology is developing at a good pace, more and more wind power plants are being integrated with the conventional form of generation. With the increase in the ratio of wind generation to conventional generation, several problems related with integration of wind farms have emerged. These problems are due to distinct properties of the generators used with the conventional form (Thermal & Hydro) of generation and wind based generation (1)-(4). In thermal and hydro power based generation synchronous generators are used while in wind based generation mostly induction generators are used. Induction and synchronous generators do not exhibit similar behavior during power system stresses (transients). Moreover, increase in wind power based generation requires a reliable transmission system for power evacuation to the load centers. As the wind farms are mainly concentrated in far flung and rural areas their interconnection with the grid becomes a problem. This problem is mainly due to the existence weak transmission grids in the rural areas (1)-(4). Earlier when the share of wind power generation as compared with the conventional generation was low, during transient conditions; whenever the power system reached on the verge of instability the wind farm was disconnected from the grid and was reconnected when transients passed away. But as the share of wind power in the conventional generation increased, the practice of disconnecting the wind farms from the rest of the grid led to the drop in frequency of the power system. So now-a- days even if a fault occurs in the power system somewhere near the wind farms, they still remain connected with the grid for a critical time period. This phenomenon is called fault ride through (FRT) in wind farms (5). This paper has studied a transient condition (fault) in a grid connected fixed speed induction generator based wind farm and its fault ride through performance. A comparison of the performance of fact devices like static var compensators (svc) and static synchronous compensators (statcom) in improving the transient stability of wind farms during fault is also done.

Small-signal instability and supplementary coordinated damping-control of LCC-HVDC system with STATCOM under weak AC grid conditions

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.

A Static Synchronous Compensator (STATCOM) is one of the Flexible AC Transmission System (FACTS) device which plays an vital role in power system network in alleviation of small and large transient disturbances and better voltage regulation. This paper proposed a self tuning Proportional-Integral (PI) controller for STATCOM which provided a voltage support at point of common coupling (PCC). Self tuning of PI controller was achieved by adapting PI controller gains with system parameters by using Bats Echolocation controller which mimics the Bats Echolocation behavior. In the proposed controller, the system voltage (parameter) at point of common coupling compared with reference voltage and then Bats Echolocation Controller updates the PI controller gains according to the variation obtained between them. This paper also proposed a Cascaded Multilevel Inverter (CMLI) instead of conventional Voltage Source Inverter (VSI) for STATCOM to obtain better voltage regulation at point of common coupling. The voltage imbalance between H-bridge cells in CMLI was rectified by prosing Multicarrier Pulse Width Modulation (PWM) techniques, The proposed controller was validated by comparing the simulation results with fixed gain PI controller and Self tuning PI controller using Neural controller and Particle Swarm Optimization.

A novel image watermark scheme based on the shuffled frog leaping (SFL) algorithm is proposed in the paper. The watermark is embedded to the discrete multi wavelet transform (DMT) coefficients larger than some threshold values, and watermark extraction is efficiently performed with SFL algorithm. Experimental results show both the robustness under attacks and the improvement in watermarked image quality with the proposed SFL algorithm.

As wind power is the fastest growing renewable energy, it is essential to determine problems associated with maintaining a stable power system when it is connected to grid. In this article a simple strategy is described to stabilize a grid-connected wind farm by controlling reactive power and minimizing voltage fluctuation using a simple 6-pulse Static Synchronous Compensator (STATCOM). The STATCOM is controlled by using voltage control loop with PI controller and Pulse Width Modulation with carrier frequency of 9 times fundamental and varying dc voltage. This paper also presents a comparative study of stabilizing a wind farm using this STATCOM or using only a capacitor bank during wind speed change. Simulation results show that the wind farm could be more effectively stabilized by using STATCOM. The choice of this study is justified by simulation in PSCAD/EMTDC program.

This paper presents the performance and comparative analysis of Static Synchronous Compensator (STATCOM) based on 6, 12 and 48-pulse VSC configuration. STATCOM is implemented for regulation of the voltage at the Point of Common Coupling (PCC) bus which has time-variable loads. The dq decoupled current control strategy is used for implementation of STATCOM, where modulation index M and phase angle are varied for achieving voltage regulation at the PCC bus. The 6, 12 and 48-pulse configurations are compared and analyzed on the basis of Total Harmonic Distortion (THD) and time response parameters such as rise time, maximum overshoot and settling time. The simulation of various configurations of STATCOM is carriedoutusing power system block-set in MATLAB/Simulink platform.