Small Signal Stability Analysis Of Wind Farm Integration And Discussion On Trip-off Of Wind Turbine

For the recent wind turbine cascading trip-off, the paper starts from theunderlying mechanism of DFIG (Doubly-fed Induction Generator) to study thetrip-off causes and seek improvement measures. In order to analyze the characteristicsof wind farm, firstly, the detailed simulation models of DFIG are established inDIgSILENT/PowerFactory (Digital SImuLation and Electrical NeTwork). Secondly,additional control strategy is proposed for DFIG converter, to inhibit weak dampingof power system integrated with wind farm. Finally, Var compensation device and Varcoordinated control strategy is proposed, to improve Small Signal Stability of powersystem. The main work of this paper is as follows:(1) A Small Signal dynamic model of DFIG is established. And then furtherdiscussion on the tightness of integration, the output of wind farm, and the connectionschemes of wind farm impact on the Small Signal Stability of power system arecarried out to reveal the nature of cascading trip-off from grid. Through singlemachine infinite system and New England39system, the incentives of wind farmtrip-off from grid are analyzed.(2) A Supplementary Damping Control System (SDCS) is proposed, and is addedinto the DFIG converter. The SDCS is similar to the PSS (Power System Stabilizer)of conventional generator, which can improve the system damping by convertercontroller, without any other additional device. Simulations based on Four-machineTwo-area and Ten-machine Four-area systems are carried out, and the results revealthat the proposed control strategy can improve the power system damping and thenimprove the Small Signal Stability of power system.(3) The actual analysis of trip-off phenomenon shows that wind turbine requires alarge amount of reactive power during the development of trip-off. In order to preventfurther fluctuations of voltage during trip-off, due to the surge of reactive powerdemand, STATCOM (Static Synchronous Compensator) is proposed to stabilize thesystem voltage. According to the reactive power characteristics of wind farm, Varcompensation device is installed at PCC (Point of Common Coupling). By analyzingthe eigenvalues of Four-machine Two-area integrated with wind farm, the STATCOMcan improve the stability of power system and inhibit cascading trip-off. (4) Var coordinated control strategy is proposed, instead of expensive reactivecompensation devices, to improve the voltage of PCC and reduce the absorption ofreactive power. The proposed converter controller can improve the dynamiccharacteristics of wind turbine, to improve the stability of power system. Single DFIGand large scale wind farm are analyzed, to verify the proposed Var coordinatedcontrol strategy.