Transient Analysis And Protection Method Of Single-pole Grounding Fault In Flexible DC Distribution System

With the development of DC power distribution systems in the future,with the gradual increase in voltage levels,output power and power supply radius requirements,the length of the line will become longer,and the distributed capacitance of the line can no longer be ignored.When a single-pole grounding fault occurs in the MMC(Modular Multilevel Converter,MMC)flexible DC distribution network using small current grounding methods,the short-circuit current is small(mainly distributed capacitance current),and there is no steady-state fault current(or the steady-state fault current is very high).Small),the system can run for a period of time with faults,and the reliability of power supply is high,but at the same time there is a problem of difficulty in fault identification.Considering that there are abundant transient processes at the moment of a single-pole ground fault,the transient currents of the faulted line and the non-faulted line have obvious differences.This feature provides a new idea for the detection of single-pole ground faults.There is a lack of detailed studies on this transient process,and there are relatively few studies on the protection of unipolar ground faults in flexible straight systems.Therefore,studying the transient characteristics of unipolar ground faults and the corresponding protection methods have important theoretical significance and engineering application value for the safe and stable operation of flexible direct distribution networks.Aiming at the MMC-type multi-terminal flexible DC power distribution system with low-current grounding,this article first introduces the phase-to-mode transformation matrix suitable for DC systems,derives the impedance calculation formula of the DC line in the mode network,and analyzes the effects of converters and other components.The equivalent model in the model network is analyzed through phase-to-mode transformation.The zero-mode network and the one-mode network of the unipolar ground fault on the DC side are analyzed.The zeromode network and the one-mode network are reasonably simplified and equivalent,based on the boundary conditions of the unipolar ground fault.,The equivalent circuit of the composite model for transient analysis of unipolar ground faults is further established.Secondly,based on the equivalent circuit of the single-pole ground fault transient analysis compound model,the transient characteristics of the fault point current under different transition resistances are analyzed,and the analytical expression of the fault point transient current is given.At the same time,the unipolar ground fault is analyzed.The magnitude of the non-fault pole overvoltage caused by the oscillation during the ground fault transient process.Based on the single-pole ground fault zero-mode network,according to the impedance characteristics of the zero-mode network,the distribution characteristics of the zero-mode current of each line are analyzed,and the characteristic differences between the zero-mode current of the faulted line and the nonfaulted line are summarized,and the theoretical analysis is verified by simulation.Finally,based on the characteristic difference of the transient zero-mode current between the faulted line and the non-faulted line in the characteristic frequency band,two suitable single-pole grounding fault line selection methods are proposed.They are the transient direction longitudinal line selection method and the transient zero-mode current selection method.Line selection method of differential current amplitude and polarity.The paper explains in detail the transient feature extraction method,the principle and implementation scheme of the protection method,and evaluates the performance of the protection method.Finally,the proposed protection method is verified on the PSCAD/EMTDC platform.The simulation results show that the implementation process of the protection method proposed in this paper is simple and effective,does not rely on the boundary elements of the line,can reliably identify the faulty line,and is expected to be applied to the actual project on site.This paper provides a theoretical reference for the transient characteristics analysis of unipolar ground faults in MMC flexible DC distribution systems using small current grounding,and enriches the ground fault protection technology of flexible DC distribution systems.