Research On Transient Stability And Control Strategy Of A System Involving Wind Power Transimisson By UHVDC/UHVAC

The intermittence,volatility and randomness of wind energy lead to a great risk to the safe and stable operation of power grid when large-scale wind power delivered remotely.Wind power transmitted by ultra-high voltage alternating current(UHVAC)and ultrahigh voltage direct current(UHVDC)with thermal power nearby can meet the requirements of safe and stable operation of the system and improve the utilization rate of transmission lines.Thus,system involving wind power transmission by UHVDC/UHVAC(WIND-UHVDC/AC)has become an effective scheme for large-scale wind farms in China.The installed capacity of wind power and the rated power of UHVDC in a WIND-UHVDC/AC system are very large,and the fault will bring severe challenges to the system.Therefore,it is necessary to research the transient stability characteristics of a WIND-UHVDC/AC system.This paper is constructed as following four aspects: wind power penetration ratio,interaction between the recovery rates of double fed induction generator(DFIG)and UHVDC,calculation algorithm of emergency tripping capacity after UHVDC block and coordinated control strategy for UHVDC commutation failure.Specific work as follows:(1)The model of a WIND-UHVDC/AC system is built in the paper.Based on the transient power characteristics of DFIG and UHVDC,the electrical power equation of the synchronous generator(SG)in the sending-end system is deducted mathematically.Based on the equal-area criterion,the impact of wind power penetration ratio on the rotor angle stability of the sending-end system is analyzed in the cases of short-circuit fault of UHVAC line and UHVDC block,respectively,and the optimal wind power penetration ratio under the constraint of the rotor angle transient stability is determined.Besides,the effects of system parameters,such as the the reactance of UHVAC lines and UHVDC transmission capacity on the optimal wind power penetration ratio are discussed.The analysis of optimal wind power penetration ratio can be referenced for the planning and construction of a WIND-UHVDC/AC system.(2)The impact of the active power recovery rate of DFIG and UHVDC on the rotor angle stability after a short-circuit fault cleared are analyzed respectively,and the impact of the equivalent reactance of the SG on the rotor angle stability is also analyzed.With the consideration of the impact of the reactive power consumed by the rectifier station of UHVDC system on the electrical power of the SG,the impact of the interaction between the active power recovery rates of DFIG and UHVDC on the rotor angle stability is further analyzed.The optimal active power recovery rate of DFIG and UHVDC are determined under different equivalent reactances of the SG to improve the rotor angle stability of the WIND-UHVDC/AC sending-end system.The mechanism analysis can provide theoretical support for the development of the fault recovery strategy of a WINDUHVDC/AC system.(3)The impact of UHVDC block and the operation of filters on the transient voltage characteristics of the sending-end system and UHVAC lines are analyzed.Then the influence of filters operation after UHVDC block and the wind turbines tripping-off on the electrical power of the SG are studied.Based on the above analysis,a calculation algorithm of emergency tripping capacity of wind power and thermal power is proposed with the consideration of filters,it can reduce the total capacity of wind power and thermal power tripping-off required to maintain the transient stability.This calculation algorithm provides a reference for the emergency control after UHVDC block in practical projects.(4)The model of UHVDC transmission system with hierarchical connection mode is built.The domain factors inducing commutation failure of the inverter valves connecting to fault receiving-end system and non-fault receiving-end system are analyzed respectively,and the exchange characteristics of reactive power between inverter valves and its corresponding receiving-end system are researched.Based on the analysis of the influence mechanism of UHVDC continuous commutation failures on the transient stability of the sending-end system,a coordinated suppression strategy of commutation failure in UHVDC system with hierarchical connection mode is proposed to improve the transient stability characteristics of the sending-end system.It can effectively suppress the occurrence of UHVDC continuous commutation failures and reduces the transient over-voltage and unbalanced active power of the sending-end system after the commutation failure.The proposed control strategy can significantly improve the operation stability of WIND-UHVDC/AC sending-end system and wind farms after UHVDC commutation failure.The research on the influence of wind power penetration ratio,interaction between the active power recovery rate of DFIG and UHVDC,UHVDC block and UHVDC commutation failure on the transient stability of the sending-end system provides a theoretical reference for the planning,construction and operation scheduling of a WINDUHVDC/AC system,and provides a new reference for the system control strategy.