Research On Longitudinal Differential Protection And Fault Location Principles For Hvdc Transmission Lines

HVDC(High voltage direct current)has become an important method for long-distance and large-capacity transmission and regional grid interconnection.However,the large span of HVDC transmission space and harsh operating conditions along the way make HVDC line faults the most common fault in the system.Therefore,researching reliable HVDC line protection and fault location schemes is of great significance for the safe and stable operation of HVDC transmission systems.The pilot protection of HVDC transmission lines that has been put into operation is susceptible to interference from factors such as fault distance,transition resistance,noise,etc.,and there are common problems in the calculation of the fault criterion threshold setting and the lack of theoretical basis for setting.At present,the actual line fault location devices of HVDC transmission projects all adopt the traveling wave method,but the problems of the arrival time of the initial traveling wave head of the fault and the accurate calibration of the traveling wave instantaneous frequency have not been effectively solved.Aiming at the current related problems of HVDC system pilot protection and fault location,the protection principle is simple and easy to set.It can realize fast and selective action of HVDC transmission line pilot protection under various fault conditions.The scheme and the fault initial traveling wave head are effectively calibrated and the fault traveling wave velocity is determined at that moment.An organic and unified HVDC line fault traveling wave location algorithm.The main research contents are as follows:(1)Pilot protection method of HVDC transmission line based on single loop theoremAiming at the problem that the protection easily refuses to operate when the high-resistance grounding fault occurs in the HVDC line,a pilot protection method based on the single-loop theorem is proposed.First,the current mutations on both sides of the HVDC line are used as state variables to construct a singular value equivalent matrix,and spectrum analysis is performed according to the single-loop theorem to realize internal and external fault identification;then,the generalized S transform is used to extract the transient energy of the two-pole current mutation in a specific frequency band To select the fault pole based on the ratio feature of sum and to select the fault pole;finally,the pilot protection scheme is given.Simulation verification shows that the proposed protection principle is simple,easy to set,and can achieve fast and selective actions under various fault conditions.(2)Traveling wave fault location method of HVDC transmission line based on VMD-SMMGHilbert transformVMD(Variational Mode Decomposition)is a completely non-recursive modal variational method,which can effectively decompose the fault current traveling wave;Multi-resolution Morphological Gradient SMMG(Serial Multi-resolution Morphological Gradient)algorithm can realize the successive transformation of the weak signal,accumulate and amplify the sudden change characteristics of the traveling wave signal,and then accurately calibrate the arrival time of the initial traveling wave head of the fault;at the same time,the Hilbert transform is performed on the high frequency component of the traveling wave of the fault current to determine the first arrival.The frequency of the high-frequency component of the fault current traveling wave at the measuring points at both ends of the rectifier and inverter sides is used to determine the corresponding fault current traveling wave speed at the moment.The improved practical method for fault location of HVDC transmission lines can realize the effective calibration of the initial traveling wave head of the fault and determine the organic uniformity of the traveling wave speed of the fault at that moment,and it has high reliability when special faults such as high resistance grounding occur at the end of the line.