Research On New Pilot Protection Scheme For EHV Transmission Line

With the rapid development of power system, the voltage grades oftransmission lines become higher and higher, and the transmission capacity growsbigger and bigger, EHV transmission lines are playing a more and more importantrole in the power system, which elevates the requirement of the operatingcharacteristic of relaying protection. However, the protection schemes based onfundamental frequency rarameters can not meet the requirement of high rapidity.Pilot protection scheme and faulty phase identification method based on transienthigh-frequency componts for EHV transmission line are researched in this paper.The fault transient process of EHV transmission lines is studied, and theinfluence of line trap, busbar stray capacitance, series compensation capacitance andunloaded switching to transient components are analyzed. The differences of faulttransient signal’s frequency characteristic and energy in internal and external arealso researched.A new protection scheme based on wavelet transient energy extracted from highfrequency component of the fault signals is presented. First, the current signals atboth ends of the transmission line are decomposed by discrete wavelet multi-scaledecomposition, and the wavelet detail coefficients of a selected frequency band areacquired. After that, the difference and the average of the wavelet detail coefficientsat both terminals of the transmission line is computed, and their transient energywithin a sliding window is also calculated, which is respectively used for theoperating quantity and the restraining one to form the transient energy differentialprotection criterions. The proposed scheme provides high sensitivity and fast timeresponse for internal fault, and high reliability for external fault. Meanwhile, thescheme is immunized to fault type, fault location, fault resistance, fault time, faultinception angle and power swings. Simulation with Matlab/Simulink verifies that theproposed scheme can meet the requirements of EHV transmission line relaying,which has availability and practicality.A new fault phase selection method based on wavelet energy entropy for EHVtransmission line is presented. First, the current signals at both ends of thetransmission line and the grounded phase are decomposed by discrete wavelet multi-scale decomposition, and the wavelet detail coefficients of multi-frequencybands are acquired, and then the difference and the average of which is computed.The wavelet energy entropy of each phase current is computed using the sum ofwavelet energy at different levels within a sliding window, which forms the faultphase selection criterions. The proposed method is verified and analyzed bysimulation with Matlab/Simulink. The result shows that it can identify the faultphases quickly and accurately, and it has high rapidity and high reliability.Meanwhile, the scheme is immunized to fault resistance, fault location, and faultinception angle, which can meet the requirements of high-speed fault phase selectionfor EHV transmission line.