Research On Protection For AC Transmission Line Of The Offshore Wind Power Transmission System Based On MMC-HVDC

In the context of the national development strategy of "carbon peaking and carbon neutrality",the large-scale development and utilization of offshore wind power has a broad application prospect in the construction of new power system.The flexible HVDC system has the advantages of flexible operation regulation and support for passive network,etc.Centralized transmission and grid integration of offshore wind power via MMC-HVDC system becomes the preferred solution for offshore wind power transmission.The two ends of the AC offshore transmission line are connected to the offshore wind farm and the offshore converter station respectively.The fault characteristics of the AC transmission line are significantly different from the traditional synchronous generator due to the influence of the control strategy of the converters at both ends.The fault evolution mechanism of the AC transmission line is still unclear,and the conventional protection is faced with an overall challenge,which seriously affects the reliable identification of the fault line.This paper takes AC offshore transmission line as the core,fully considers the influence of converter control strategy and the protection requirements of AC transmission line,and carries out a three-part study on fault characteristics analysis of AC transmission line,construction of primary and backup protection schemes.The main contents are as follows:At first,based on the introduction of the basic working principle and model building of offshore wind power via MMC-HVDC system,the control strategy of offshore wind power,offshore converter station and onshore converter station under normal operation and fault condition are deeply analyzed.When the fault occurs in the AC offshore transmission line,the offshore wind farm adopts the negative sequence suppression strategy,and the offshore converter station adopts the control strategy combining negative sequence suppression and low voltage link.According to different fault types,a fault equivalent sequence network is established,which comprehensively considered the control strategies of offshore converter station and offshore wind farm.The fault characteristics of AC transmission line are studied.It is found that the fault current can still form a flow path through distributed capacitors when the negative sequence suppression strategy was adopted for the converters at both ends.The simulation results show that the fault characteristic analysis is correct.Secondly,when asymmetrical faults occur on the AC transmission line,according to the feature that the fault point will form a negative sequence current loop with the distributed capacitance of the line,The distribution of negative sequence current under internal and external faults is analyzed.The amplitude differences of the negative sequence current between the faulty line and non-fault lines connected to the same bus are analyzed.Then,the amplitude difference is emphasized by the integral of the negative sequence current.Protect associated information is built.Based on the ratio of the transient negative sequence current integral,a protection scheme is proposed.Simulation results show that the proposed main protection scheme can identify faults quickly and accurately,and ensure the safe and reliable operation of the centralized AC transmission system.Finally,by taking advantage of the flexible controllability of offshore converter station to inject negative sequence current for a period of time after the fault,the change of the amplitude of negative sequence current at the protection installation before and after the active injection control of offshore converter station is analyzed.It is found that when faults occur on the AC transmission line,The negative sequence current at the protection of the side of the offshore converter station increases obviously after active injection of the offshore converter station,the negative sequence current at the protection of the side of the offshore booster station does not change clearly after active injection of the offshore converter station.However,the negative sequence current fault has opposite characteristics when the outside-zone fault occurs on the AC transmission line.The negative sequence current injection amplitude is determined by considering the overvoltage level of the suppressed line.Then,a protection criterion based on the change of negative sequence current is constructed,and a backup protection scheme of AC transmission line based on active injection is proposed.Simulation results show that the proposed backup protection scheme is feasible and effective.