Research On Pilot Protection And Fault Location Principles Of DC Line For Multi-terminal Flexible DC Grid

Based on modular multilevel converter(MMC),multi-terminal flexible direct current(MTDC)system is one of the important development directions of smart grid in the future.MTDC grid interconnects multiple DC networks,and its fault transient characteristic is complex.The short-circuit current amplitude of MTDC grid is large and rises rapidly.At the same time,problems such as long distance,trans-regional transmission and seriously affected by climate of DC lines,which lead to line faults occur frequently and difficulties in finding fault points.Therefore,it is of great significance to explore the DC line protection and fault location scheme of MTDC grid with high performance for the safe and reliable operation of DC system.The existing flexible DC line protection is affected by factors such as transition resistance,noise,communication,and DC line boundary.The protection sensitivity and reliability need to be improved.At present,the traveling wave method is mostly used for fault location research in DC projects,but the calibration of fault traveling wave head and the influence of traveling wave velocity on the location results will still affect the final location accuracy.In this paper,the following research has been done in view of the existing problems of DC line protection and fault location in MTDC grid.(1)Research on pilot protection for the DC line of MTDC grid based on fitting characteristics of Measured impedanceFirstly,the amplitude-frequency characteristics of the measured impedance at the MTDC system under different fault types are derived.And,using generalized S transform to extract the fault characteristic in frequency domain.The theoretical value of the measured impedance at the protection is fitted with the actual value by the least square method(LSM),and the slope of the fitting curve on both sides is used to realize the fault area identification.Then,based on the fault difference between positive and negative pole lines,the low frequency power ratio of positive and negative pole lines is constructed to realize fault pole selection.Finally,a simulation model is built.The simulation results show that the method can sensitively,reliably and quickly identify different types of faults,has strong fault transition resistances and anti-noise abilities and applies to the circumstances when the system parameters vary.(2)Research on fault location scheme for the DC line of MTDC grid based on VMD-SVDFirstly,the variation characteristics of fault voltage traveling wave at fault point and line port of DC line in MTDC grid are deduced.Then,the fault voltage traveling wave is effectively decomposed by variational mode decomposition(VMD),and the traveling wave head is accurately calibrated by singular value decomposition(SVD).On this basis,a new fault location formula is used to complete the fault location.Finally,the simulation results demonstrate that the fault location algorithm in this paper is not affected by the communication delay at both sides.In the meanwhile,it eliminates the range error caused by the uncertainty of traveling wave velocity,and is able to accurately calibrate the wave head with little interference from the transition resistance and having strong anti-noise capability,and the high precision ranging results can be obtained in different fault locations.