Investigation On Overvoltage And Its Protection Of Switching Shunt Reactors In Offshore Wind Farm

With the rapid development of China’s economy,the demand for electricity in the developed coastal areas is increasing year by year,while the coastal areas have abundant offshore wind power resources to be developed.Therefore,the analysis of the operation of offshore wind farms is a hot spot in the research of China’s power system.Unlike offshore wind farms,offshore wind farms usually use submarine cables with far transmission distance and high capacity charging power.For these reason,the shunt reactor and other equipment need to be put into the offshore wind farm.These devices consume inductive reactive power in wind power system and maintain reactive power balance in offshore wind farms to suppress voltage rise for improving system stability.Vacuum circuit breaker is a common switching equipment in offshore wind farms,and has the ability of fast arc extinguishing.Because of small impedance of the submarine cable and large inductance of the reactor,the process of vacuum circuit breaker switching shunt capacitor in offshore wind farms will cause operating overvoltage with high amplitude and steepness and endanger the operation of offshore wind farms.To solve the above problems,we build offshore wind power system based on the high-frequency transient model of key equipment,and study the principle,characteristics and corresponding protection of overvoltage.A high frequency closing model of vacuum circuit breaker with arc characteristics is built in this dissertation.Through the study of its arc characteristics,we precisely set up four stages of closing switch and improve the control conditions to accurately simulate the closing transient effect.We compare the test results of the closing model of vacuum circuit breaker built in this dissertation with the ideal circuit breaker and the 35 kV vacuum circuit breaker.The comparison results verify the practicability and superiority of our model.Meanwhile,by analyzing the electromagnetic transient model of doubly fed fan with high computational complexity,a method of parameter setting for induction generation of doubly fed induction generator is presented in this dissertation.It greatly simplifies the calculation of the model while ensuring the accuracy of the simulation.What’s more,considering the influence of stray capacitance in shunt reactors,we apply multiple equipment model such as cable frequency dependence model and transformer model considering port capacitance in PSCAD component library to build a 35 kV system model of typical offshore wind farms.Our model is applied to analyze operation overvoltage in the system when the shunt reactor is put into operation.Based on the comparison of simulation results between offshore wind farms and land-based wind farms with the same structure,the following conclusions can be drawn: inthe transient of closing operation of the vacuum circuit breaker,the operating overvoltage with high peak and steepness appear in shunt reactor,bus bar and fan end transformer,which is much higher than the operating overvoltage in land-based wind farm.Besides,the transient overvoltage of fans at different locations on the feeder when the reactor is put into the reactor are compared and analyzed.Finally,based on the principle analysis and characteristic research of transient overvoltage in the system,we analyze a variety of protection programs and compare the parameters of the protection equipment.We find that the parallel installation of arresters and RC absorbers is the best way to suppress overvoltage.Our research will provide reference for offshore wind farm design,insulation protection and operation and maintenance of important equipments.