Analysis On Overvoltage Characteristics Of The DC Short-Circuit Fault In VSC-HVDC Grid

The gradual development of flexible DC transmission technology has made Voltage Source Converter based High Voltage Direct Current(VSC-HVDC)grid a key way to promote the construction of the energy internet.The VSC-HVDC grid can realize the access of renewable clean energy and distributed power sources to the grid.It has the advantages of flexible control,high reliability and high redundancy.It is an effective solution to the uneven distribution of renewable energy and the difficulty in absorption.However,the network structure of the VSC-HVDC grid makes its fault overvoltage characteristics more complicated than previous flexible DC transmission systems.Therefore,it is of great value to research the overvoltage characteristics and influencing factors of different VSC-HVDC grid structures.The paper focused on the overvoltage characteristics and influencing factors of short-circuit faults on the DC side of five types of VSC-HVDC grid structures.based on the MMC electromagnetic transient model,DC circuit breaker model and other key equipment models(converter transformer,current-limiting reactor and overhead line),a four-terminal loop flexible DC grid overvoltage simulation was established on the electromagnetic transient simulation platform Based on this model,pseudo U-shaped,true U-shaped,broken line and diagonal DC power grids were established.Secondly,based on the equivalent circuit of the DC side short-circuit fault of the VSC-HVDC grid,the DC side short-circuit fault current loops of five power grid structures and the mechanism of system overvoltage generation were derived theoretically.Through a large amount of simulation data,the overvoltage characteristics of short-circuit faults on the DC side were statistically analyzed,and the overvoltage levels of key nodes inside and outside the converter station of the five power grid structures were obtained.Finally,the simulation evaluated the influence of the equivalent ground resistance of the fault point,the current-limiting reactor,the system ground electrode resistance and the fault threshold current on the overvoltage of the five flexible DC grid structures.And the sensitivity of the five kinds of grid structures to various influencing factors were evaluated.The paper researched the discharge path and overvoltage generation mechanism of short-circuit faults on the DC side of five VSC-HVDC grid structures,simulated and counted the overvoltage characteristics of five grid structures,and analyzed the influence of system parameters on the overvoltage of five grid structures.It has a reference for the research of overvoltage characteristics and insulation design of flexible DC grid under different grid structures.