Research On Electromagnetic Transient Characteristics Of Uhv LCC-MTDC System

The development of UHV Multi-terminal HVDC technology is of great significance for the interconnection of regional power grids and the cooperation of power and energy in neighboring countries.Compared with the UHVDC transmission system,the interconnection topology of multiple converter stations will lead to changes in the electromagnetic transient characteristics of multi-terminal systems.In order to ensure the safe and stable operation of the system,it is necessary to simulate the electromagnetic transient process and overvoltage characteristics of the Multi-terminal HVDC transmission system.This thesis takes UHV four-terminal HVDC system as the research object,and studies the control strategy of Multiterminal HVDC system,electromagnetic transient characteristics and the effect of fault faster interruption on the overvoltage of the system.Based on the theory of current margin control and the open loop off angle control theory,a hierarchical control model for electromagnetic transient simulation of UHV four-terminal HVDC with coordinated control layer,polar control layer and valve control layer is established in this thesis.The model realizes coordinated control of starting and stopping of four terminal converter station and steady process switching.Then the accuracy of the pole control model is verified by the comparison with the commissioning waveforms of the JinbeiNanjing DC project,which lays a foundation for the study of the overvoltage characteristics of the system.The simulation of the overvoltage level of the key nodes and the action characteristics of the MOA arrester for the UHV four-terminal HVDC system under the typical operating conditions of the converter station failure and the DC line fault,the overvoltage level of the main equipment is basically the same as that of the two systems.The distribution of over voltage along the four-terminal ring-type branch line leads to the most serious overvoltage fault point in the middle point of the DC overhead line connecting two rectifying stations.The comparison with the two-terminal systems shows that the maximum overvoltage amplitude of the four-terminal system is higher than that of the two-terminal when the same line is grounded.The influence of the Hybrid DC circuit breaker excision circuit fault on the electromagnetic transient characteristics of the UHV four-terminal DC transmission system is studied,and the maximum slow wave overvoltage level of the circuit breaker with different action time is obtained.The simulation results show that when the current limiting inductance is added to the terminal impedance of the converter station,the circuit breaker can remove the fault in a specific time and effectively suppress the overvoltage.The research work in this thesis can provide reference for overvoltage level suppression and insulation coordination design of UHV multi terminal HVDC transmission system.