Study Of Security&Stability Feature And Cascading Failure Defense Strategy About Wind Farms

With the discussion about the status of grid’s cascading failures study, analysis the model and the control mechanism of DFIG;Based on NARI’s software FASTEST and jiuquan wind farms,studies the security factors about voltage and frequency,summarize the main modes of cascading failure.Finally, design the corresponding defense system,propose the defense strategy and demonstrate the effectiveness of the policy through examples.1)The mathematical model、the active&reactive power decoupling control mechanism and LVRT mechanism of DFIG are studied,so as to the wind farms modeling; In the flow calculation, when the wind farm chooses constant power factor control, the node is processed as a PQ node; Or the node will be as a PV node (Q finite value).2)A detailed analysis was made to learn how the wind power level,the reactive characteristics of DFIG(including crowbar protection) and reactive power compensation device influence the collection bus’voltage stability.The following are conclusions:(1)High active output of wind farm will result in a low bus voltage (excluding reactive devices);(2)When grid failures,DFIG will absorb reactive power after flux decays, which will increase the extent of the voltage drop and may cause the adjacent wind farms LVRT failure or crowbar protection chain action;(3)Capacitor and SVC can improve the grid’s voltage level, but may result in the bus voltage higher on the instant after the fault.3)The influence factors about grid’s frequency stability of wind farms are also been studied from the ratio of wind active power output and the frequency protection value of DFlG.It has found that the ratio is greater,the frequency stability will be worse;And improper protection value will worse the power imbalances, triggering a chain security control actions.4)Then,the major models of wind farms’cascading failures were summarized. The voltage、frequency and overload can all lead to wind farms’cascading failures.5)Based on the hierarchical and partitioning control idea,a defense system with four-tier structure to prevent the wind farms’cascading failure was designed and the defence strategies combined online/offline intergrated with prevention control, emergency control, correction control were proposed,to solve wind farms’cascading failure caused by voltage、 frequency、overload and other problems. Through the simulation of offline strategy example has demonstrated the effectiveness of the defense system.