Reactive Power Optimization Strategy For New Energy Cluster Connecting Into VSC-HVDC Through Isolated Network System

In recent years,the problems of global energy shortage and environmental pollution are becoming more and more serious,so it is urgent to change the energy structure.Under this background,the new energy represented by wind power and solar energy has been developed rapidly.China’s new energy resources are mainly concentrated in the western and northern areas,far from the load center,and the grid structure of access areas is relatively weak,it is difficult to obtain effective reactive voltage support in grid connected power generation.HVDC system based on voltage source converter(VSC-HVDC)has the potential to adapt to the transmission of new energy through isolated network system.Due to the inherent intermittent,random and fluctuating nature of the new energy power generation,the voltage stability of the grid connected system will be affected to a certain extent,and the new energy set will be studied.This thesis establishs the energy cluster connecting to the VSC-HVDC system model in DIgSILENT/PowerFactory simulation platform,and validates the effectiveness of the control mode of the converter station for the VSC-HVDC system adapted to the new energy cluster passive system.Based on this model,it considers the power fluctuation characteristics of wind power and photovoltaic power respectively and analyzes the superiority of the VSC-HVDC system in maintaining system voltage stability when it is applied to the energy cluster through isolated network.Based on the equivalent system of the new energy cluster connecting to single-ended the VSC-HVDC,considering the constraints of the VSC-HVDC system of power regulation capability,the effects of the output voltage mudulation ratio and the converter station capacity on the PV curve under different power factors were studied respectively,and the maximum power of the stable transmission of the system under different operating conditions.The constraints of the steady-state operation of the sending-end converter station are studied.The influence of converter transformer tap position,voltage reference value,the equivalent distance and impedance characteristics of the new energy cluster and the VSC-HVDC converter station on the steady-state operation interval is analyzed.It provides a feasible method for obtaining the steady state operation range of the converter station,which is the safe range of new energy clusters that can meet the constraints.For the VSC-HVDC system with new energy clusters,considering the power safety margin of the sending-end converter station and the safe operation of new energy clusters,a reactive voltage optimization control strategy for AVC main station of the new energy clusters is proposed.The minimum network loss in the system and the macimum reactive power margin of the converter station are taken as the optimal control objective function.The simulation results show that the control strategy can effectively maintain the voltage stability of the central node of the new energy clusters,improve the economic efficiency and the power safety margin of the converter station of the VSC-HVDC system.