Study Of High Voltage Off-grid Problem Of Wind Power System Based On DFIG

The survivability of grid-connected wind farm under fault condition has aroused wide attention and profound thoughts, since wind farm off-grid accidents frequently occurred in recent years. In fact, the accident rate in wind farms completed LVRT capability transformation has already been greatly reduced. Meanwhile, the research of wind farm HVRT is insufficient and the high voltage off-grid accidents are still happening. So the high voltage off-grid is the main problem facing wind farm survivability. Therefore, the research on wind farm high voltage off-grid problem is necessary and significant, in order to improve the fault ride through ability. This paper tries to solve the above problem in two aspects, improving the HVRT ability of wind generator and optimize the control strategy of reactive power compensation equipments.The basic structure and operational principles of DFIG are introduced and the math model of DFIG under d-q coordinates is built in this paper. Then the control system of grid side converter and rotor side converter are analyzed. The conclusion that DFIG can achieve four-quadrant operation is got, which means that the reactive power adjustment ability of DFIG should be considered, to provide voltage support to the grid during high voltage fault.To improve the HVRT ability of DFIG, a new control strategy is proposed. This strategy is adaptive to the grid voltage level and considers the reactive power support ability of DFIG, which can be quantified. DFIG achieves HVRT by reactive power adjustment when grid voltage swell is mild, and by turning on hardware protection circuits when grid voltage swell is deep. The validity of the strategy is proved in PSCAD/EMTDC. This strategy can improve the survivability of DFIG during grid voltage swell and provide reactive power support to the grid, under current hardware condition.In view of the phenomenon of high voltage off-grid intensified by the worse dynamic response ability of reactive power compensation device, the typical accident chain reappearances and the evolution mechanism of reactive power-voltage problem is deeply shown. Based on the above, a positive voltage control strategy of STACOM based on wind farm AVC system is proposed, by changing the control method and introducing a reverse reactive power compensation branch with a delay locked signal. Its validity is also be proved by simulation on improving the dynamic reactive power adjustment ability and suppressing the high voltage of wind farm.