Research On Dc Short Circuit Fault And Configuration Scheme Of Current Limiting Device In Flexible Dc Grid

At present,the flexible dc power grid has become an important direction for the development of modern power grids due to its advantages of distributed energy and energy storage equipment access and flexible operation methods.Among them,the modular multilevel converter(modular multilevel converter,MMC)has the characteristics of good power quality,small switching loss,and easy expansion,and has been widely used in flexible dc projects.However,the flexible dc grid itself has low damping characteristics,rapid fault development,and wide range.It is necessary to adopt a reasonable and effective fault current suppression scheme to limit the fault current level and ensure the safety and reliability of the dc grid.At this stage,how to choose the installation quantity,installation location and impedance parameters of the current limiting device lacks a reasonable and effective theoretical method.There is little research on the cooperation between various current-limiting devices.Therefore,there is an urgent need to analyze the dc short-circuit fault of the flexible dc power grid,and on this basis,a reasonable and effective configuration of current-limiting device is proposed.In order to solve these problems,the timing matching method of the resistance-type and inductance-type fault current limiters applied in flexible dc power grid are studied.The equivalent circuit model of the flexible dc grid after the fault is established,and the state equations of the dc grid network after the resistance,inductance and metal oxide surge arrester are switched are corrected.Using MATLAB to solve the network state equation to obtain the numerical solution of the fault current considering the action of the resistance-type and inductance-type fault current limiters.The numerical solution of the fault current,and the Zhangbei dc grid was built on the PSCAD/EMTDC simulation platform.After comparing the calculated value with the simulation value,the results show that the proposed method for calculating the fault current of the flexible dc grid considering the action of the current limiting device has a higher calculation precision.Secondly,a multi-objective optimal configuration scheme considering the cooperation of the fault current limiter and the current limiting reactor is proposed,aiming to ensure the safe and reliable operation of the healthy part of the flexible dc grid under the fault.Taking the system global current limiting effect as the best,the minimum total inductance value of the configured reactor and the minimum total impedance value of the installed fault current limiter as the goals,consider the fault current breaking capacity of the dc circuit breaker,the upper limit of the overcurrent protection of the converter valve,constraints such as the boundary of the protection area of the dc line,the dynamic performance of the dc system and the dc voltage at the outlet of the MMC,a multi-objective optimal configuration model considering the cooperative cooperation of the current limiting reactor and the fault current limiter was established,and a multi-objective hybrid frog was adopted.The multi-objective shuffled frogleaping algorithm(MOSFLA)solves the multi-objective optimization model to obtain the final Pareto optimal solution set,and uses the fuzzy membership function to calculate the standard satisfaction of each optimization configuration scheme to find a good compromise.Finally,based on the PSCAD/EMTDC simulation platform,Zhangbei dc grid was built to verify the proposed optimal scheme,and the simulation results verified the effectiveness of the proposed configuration scheme.Finally,with the goal of the system’s global current limiting effect being the best and the total inductance value of the current limiting reactor being the smallest,a multi-objective optimization configuration model of cooperative cooperation of the buck control command and the reactor parameters was established,and MOSFLA was used to perform solve to get the final Pareto optimal solution set.Based on the PSCAD/EMTDC simulation platform,Zhangbei dc Grid was built to verify the proposed optimal scheme.The simulation results show that the reactor parameter selection scheme proposed in this paper can further restrain the fault current of dc line and reduce the breaking capacity of dc circuit breaker on the basis of passive current limitation of reactor,and will not cause the fault current stimulation of ac valve side and MMC blocking.