Research On Improving Voltage Stability Of Grid-connected DFIG Based Wind Farm

Doubly fed induction generator(DFIG) has been the most widely used generator for PQ decoupling control. With increasing of wind power capacity, the problem of.grid voltage stability is more series, and this article focuses on reactive power compensation and voltage regulation to improve voltage stability of DFIG based wind farm.Wind farm equivalence is the fundmental of studying wind power. A new equivalent method was proposed based on auto mutation paricle swarm optimization (AMPSO) for DFIG based wind farms. Above all, taking the pattern of wind turbine as grouping rule. DFIG drived wind turbine equivalent model is obtained by simplying the control strategy. Then, parameters are divided into transient and steady state parameters on basis of the effect to DFIG operational characteristics and caculated by test measurement method and AMPSO respectively. The availability of new equivalent method is verified by simulating, and the application of new equivalent method is benefit of calculating for grid with large-scale wind farms.Reactive power capacity of DFIG drived wind turbines can’t make DFIG based wind farm meet requirements of national standard alone, and this paper proposed two reactive power compensation methods—shunt and series reactive power compensation scheme. Shunt reactive power compensation installs capacitors of some capacity in the port of DFIG-drived wind turbines to coordinate positive and negative reactive power and installs static var compensator (SVC) in low voltage side of step-up transformer. While series reactive power compensation installs static synchronous series compensator (SSSC) in high voltage side of main transformer of DFIG based wind farm, and SSSC operates in current oriented vector control strategy. Finally, the simulation by Matlab/Simulink shows that installing SSSC can improve low-voltage ride through capability of DFIG based wind farm. While SVC and capacitor can’t work effective in voltage drop and may cause voltage overshoot after fault clearing which will lead to twice tripping.On the basis of shunt reactive power compensation, an improved constant voltage control strategy was proposed. Above all, voltage fluctuations can be restrained by introducing equivalent wind speed as feedforward. And three level reactive allocation strategy is introduced in order to improve realibility of DFIG and reduce internal power loss of DFIG based wind farm. It can realize coordinated control of DFIG-drived wind turbines and reactive power compensation equipments. In short-circuit fault, equivalent susceptance of SVC is controlled combining longitudinal difference action of sending line. It can avoid reactive over compensation after fault clearing. Meanwhile, change reactive power allocation to maximize DFIG-drived wind turbines reactive power regulation capability. Finally, the simulation of a practical example shows the method can improve reliability and economy of DFIG based wind farm, reduce voltage drop in fault and prevent twice tripping.For the grid connected with group of DFIG-based wind farms, a coordinated secondary voltage control (CSVC) was introduced. Taking DFIG based wind farm as an unit of auto voltage control (AVC), AMPSO is adopted to determine reference values of constant voltage control of every wind farm. The adaptive function contains voltage violance of connection point, reactive power margin and power losses. The simulation of a practical example shows the method can reduce voltage fluctuations and make wind farms operating with larger var reserves and uniform distribution which could improve fault ride through capacity.