Fault Analysis Of Flexible DC Grid Based On MMC And Optimal Configuration Of Current Limiting Equipment

The flexible DC power grid of modular multilevel converter is an important technical means to realize the smooth access,reliable transmission and flexible consumption of renewable energy such as wind power generation and photovoltaic power generation.However,compared with AC power grid,DC power grid presents the characteristics of "low damping" and "low inertia",which leads to faster development of fault current and wider range of fault influence after fault.Therefore,in order to ensure the safe and reliable operation of DC power grid,it is very important to take necessary current limiting measures to curb the rapid growth of fault current.In addition,the current research on the impedance parameters of current limiting equipment and the coordination of various current limiting equipment is not mature.Therefore,it is particularly important to put forward a reasonable optimal configuration and coordination scheme of current limiting equipment based on the fault analysis of DC power grid.Firstly,the equivalent model of MMC DC side short-circuit fault is analyzed,and then a variety of fault current-limiting devices are introduced according to the current-limiting position and current-limiting principle of the DC grid fault current-limiting device,and they are subdivided into source-side step-down,source side resistance increase,grid side resistance increase,grid side buck.In addition,the grid-side current-limiting equipment is analyzed,and its current-limiting mechanism is theoretically analyzed by taking the grid-side resistance-increasing and currentlimiting equipment as an example,and the coordination sequence of the grid-side resistanceincreasing and current-limiting equipment is given.Secondly,considering the influence of the refraction and reflection process of traveling wave on the fault transient response,a time domain analysis method of fault transient response taking into account the refraction and reflection of traveling wave is proposed.The basic principle of traveling wave is introduced and the process of traveling wave on the line after short circuit between poles is analyzed.The frequency domain and time domain expressions of the fault traveling wave are deduced,and the equivalent model of each time sequence is established according to Peterson’s rule to obtain the fault current.Finally,a three-terminal DC power grid model is built in PSCAD/EMTDC simulation software for simulation comparison.The results verify the accuracy of the fault current calculation method considering traveling wave refraction and reflection.Finally,the voltage stress at both ends of the three fault current limiters is analyzed,and the parameters of the impedance parallel fault current limiter are optimized.Based on the above fault current calculation method,taking the minimum breaking current of the DC circuit breaker and the minimum voltage stress at both ends of the fault current limiter as the optimization goals,the current limiting impedance parameters of the fault current limiter in the DC power grid were optimized by using the particle swarm optimization algorithm.configuration,and get its Pareto optimal solution set.Finally,the optimized current-limiting impedance parameters are brought into the PSCAD/EMTDC three-terminal DC grid simulation model for verification,and the simulation results verify the effectiveness of the proposed optimal configuration method.