Research On Flexible DC Distribution Systemv Fault Detection And Current Limiting Reactor Optimal Configuration

With the development of power electronics technology,the flexible DC distribution system based on full-control devices and pulse width modulation(PWM)technology has attracted the attention of many scholars because of its advantages of large capacity,high power quality,flexible control,good stability and effective absorption of new energy.Fault protection is one of the key technolo gies of DC distribution system based on voltage source converter(VSC).The fault current of flexible DC system rises very quickly,and the overload capacity of power electronic devices in the system is small,so it is necessary to effectively identify and remove the fault in the shortest possible time.How to realize the fast and effective identification of DC fault in the system in a few milliseconds is a difficult point.At present,the current limiting reactor is often installed to suppress the growth o f fault current,prevent the power electronic devices from being damaged by overcurrent,and relieve the pressure of DC circuit breaker.However,the current limiting reactor will increase the cost of system construction and reduce the co rresponding speed of the system,so it is very important to choose the capacity of the line current-limiting reactor reasonably.Based on the three-terminal DC distribution system with DC line current-limiting reactors,the DC line fault mechanism of the system are analyzed.On this basis,a fast identification method for DC line faults in DC distribution system based on the second derivative of the line-current is proposed in this paper.This scheme can quickly identify the fault side,the fault type and the fault pole,and then the whole line can be protected.Finally,the effectiveness of the proposed fault detection scheme is verified by MATLAB.Secondly,the steady-state power flow calculation of MTDC is realized by Newton-Raphson method,and the node voltage and branch current information of DC power grid during stable operation are obtained.On this basis,the fault current variation law of MTDC system is studied,and the method of DC line fault current calculation for multi-terminal DC distribution system with line current-limiting reactor is proposed with its uniform rules for matrix derivation are given.Comparing the power flow distribution and fault current change trend calculated based on the program with the simulation results,the effectiveness of the power flow calculation and fault current calculation methods is verified,which lays the foundation for the optimal configuration of the line current-limiting reactor.Thirdly,taking the limit reactor capacity range,DC circuit breaker cut-off capacity,DC side voltage fluctuation range at the outlet of converter station as constraints,a mathematical model of optimal configuration of current-limiting reactor in MTDC distribution system is established based on the minimum total inductance value of current-limiting reactor installed in the system and the fault-limiting effect after current-limiting reactor configuration.The optimal solution selection and the processing strategy of external files are given for solving the model by using the MOPSO algorithm,and the basic flow of multi-objective optimal configuration of the line current-limiting reactor in MTDC distribution system is summarized.Finally,the MOPSO algorithm is used to optimize the configuration of the line current-limiting reactors in three-terminal dc distribution system and 11 node dc system.The Pareto front is output and the optimal compromise solution is selected from them,which verifies the effectiveness of the proposed optimal configuration scheme for current-limiting reactors in MTDC distribution system.