Research On AC Fault And Small Signal Stability Of Offshore Wind Power System Based On MMC-HVDC

Offshore wind power generation has received great attention in recent years.Offshore wind power system based on Modular Multilevel Converter(MMC)has great potential in deeper ocean,as it can be used in long-distance transmission with low power lost.The AC voltage of the offshore wind power system transmitted by the MMC-HVDC is established by the MMC.When the offshore AC system short-circuit fault occurs,the fault characteristics are very different compared with the offshore wind power system transmitted by the AC transmission line.However,there are few relevant studies at present.In this paper,based on the operation characteristics and control scheme of MMC and offshore wind farm,the fault characteristics on offshore AC transmission line when symmetrical short circuit and asymmetric short circuit occur respectively are studied,the corresponding optimization schemes are proposed,and the stability of the asymmetry control scheme is analyzed.Firstly,the basic topology of the offshore wind power system transmitted by MMCHVDC is introduced in this paper and the equivalent mathematical model of MMC is established.The basic control strategy and modulation strategy is also analyzed briefly.In addition,the operation principle and control method of the wind turbines are analyzed.Secondly,the short-circuit fault characteristics of the offshore AC system are analysed when three-phase short-circuit fault occurs in transmission lines.Theoretical analysis and mathematical derivation are made about the power reverse flow and frequency fluctuation during the fault,as well as the AC overvoltage problems during the fault recovery stage.Based on this,the control strategy of the offshore MMC is improved.Then,fault characteristics of the offshore AC system are studied when the single-phase grounding short circuit,two-phase short circuit and two-phase ground short circuit fault occur.The phase sequence network of the offshore wind system based on MMC-HVDC under various asymmetric faults are established,the mathematical models of the system overvoltage and overcurrent are deduced,and the reasons causing these are analysed.On this basis,an asymmetric short circuit controller for offshore MMC is proposed.According to the mathematical relationship between the electrical quantities of different phase sequences,the control instructions of MMC are modified to effectively eliminate the problems of overvoltage and overcurrent during the asymmetric short circuit.Finally,in this paper the small signal stability of offshore wind power system based on MMC-HVDC is analyzed.The small signal model of the system is established considering the effects of positive and negative sequence separation in asymmetry control scheme,the phase lock loop in wind farms and the circulation suppression control.The small disturbance stability of the system under different operating conditions and control parameters by using this model.