Research Of FRT Strategy Of Wind Power Transmitted By HVDC System

With the increasingly shortage of fossil energy,wind power,as a clean energy with wide reserves,gradually plays an important role in the power supply.Due to the uneven distribution of wind energy reserves,wind power needs to be transmitted by high voltage direct current(HVDC)transmission system.The wind turbines will be confronted with severe fault ride through problem under HVDC blocking contingencies when AC/DC system fault occurs.And the safe and stable operation of power system will also be threatened.Therefore,the transient characteristics of wind power transmitted by HVDC system under different types of faults are studied in this paper.Firstly,model characteristics of doubly-fed induction generator(DFIG)are introduced.The transient process of electromagnetic variables under symmetric/asymmetric faults is also studied.The main variables and controllable variables under transient conditions are obtained.It lays a theoretical foundation for the following control strategy.Secondly,wind farm will be confronted with severe high voltage ride through(HVRT)problem under HVDC blocking when large-scale wind power is transmitted by HVDC system.The power assignation principle and controllability for grid-side converter(GSC)of DFIG considering its constraint condition was discussed firstly.Then the sequence of events such as control module,putting into static var compensators(SVC)and cutting off the sending AC filters was coordinated on the basis of the different response time.An effective HVRT coordinated control strategy was put forward at last.Then,the control method is adopted to suppress the double frequency fluctuation of DC bus voltage of DFIG under asymmetrical conditions when the wind turbines are confronted with fault ride through problem under HVDC blocking contingencies because of asymmetrical short-circuit faults of the sending bus.In order to prevent the voltage dipping and swelling from hitting the generators too much,the operations such as putting into SVC and cutting off the sending AC filters was coordinated at the same time.It can realize the unsymmetrical fault ride through of DFIG.Finally,the simulated results indicated that the scheme could both ensure that the generators did not run off the grid under different types of faults and also accelerate the recovery of fault network by providing reactive power support to it.