Analysis Of Transient Overvoltage Of Switching Off Operation In The Off-shore Wind Farm

With the improvement of offshore wind farm technology in the recent years,both the quantities of wind turbines and the capacity of per wind turbines continue to increase.The impact of the integration of large-scale offshore wind power into the power grid on the power system has aroused increasing attention.The variation of offshore wind energy is very random.The output power and voltage of direct-drive permanent magnet wind turbines are affected by the variation,which end up with the switching of the vacuum circuit breakers(VCB)at both ends of the turbine-side transformer.The action of the circuit breaker will cause overvoltage on the transmission line.The switch operating overvoltage in offshore wind farm system has the characteristics of fast rise speed,large amplitude and long oscillation time.In long term,it has harm to electrical insulation of the equipment and seriously affects the safe and reliable of the equipment.Hence,this article will analyze the transient overvoltage of the offshore wind farm's while the VCB is disconnecting the charge,and study the effective measures to mitigate overvoltage.First of all,this paper analyzes the formation mechanism and transient characteristics of the overvoltage of the offshore wind farm at disconnecting operation.Based on the reignition characteristics when the circuit breaker switches off,the overvoltage of the distributed parameter equivalent model is calculated.Then use the electromagnetic transient simulation software PSCAD/EMTDC to customize models of key offshore wind farm equipment,and model direct-drive permanent magnet synchronous generator according to the wind turbine module,converter module,control module and filter module;Parameter correction of the characteristic submarine cable model,modeling of the circuit breaker considering the high-frequency reignition overvoltage characteristics;parameter setting of the unified magnetic equivalent circuit(UMEC)transformer model to make it have high-frequency response characteristics.Secondly,through the custom equipment model,build an overall offshore wind farm system model,analyze the power system of a single wind turbine,and explore the impact of direct-drive permanent magnet wind turbines and AC filters on the transient overvoltage of opening operation,then analyze the overall offshore wind farm,study the influence of feeder length and different wind farm capacity mode on the terminal of the feeder.Finally,based on the principle of overvoltage protection equipment,different equipment models for limiting overvoltage were built,and the suppression effect of overvoltage equipment on line overvoltage was verified,and suggestions were made on overvoltage methods of wind farms.