A New Principle Accurate Matching The Transient Model Of Transmission Lines For High Speed Distance Relays

On distance protection, transient overreach can be cause by the transient error of CVT,non-periodic components and high-frequency components in transient process. Thetraditional methods to avoid transient overreach are narrowing down the protective rangeand delaying the protecting in transient state, both of which are detrimental to the safety ofpower system. On recent research，principle methods to prevent transient overreach areadaptive delaying of protection, compensating the secondary voltage of CVT, obtaining theprimary voltage of CVT using the secondary voltage and applying new protectionprinciples. These methods can improve the ranging accuracy to a certain extent, but can’teradicate it radically. Based on the theory of equal transfer process, a new algorithm isproposed to prevent transient overreach from a new perspective.The new algorithm includes three major steps, which are calculating the voltages at faultpoint, virtual digital transferring of CVT and solving the differential equation by the leastsquare algorithm. ATP simulation shows that this algorithm performs better than threeother algorithms including the half-wave Fourier algorithm, the full-wave Fourier algorithmand the traditional differential equation algorithm when various type of fault occur onproximal point off the relay. However, when fault occurs on distal point off the relay, thealgorithm performs badly, even worse than the two Fourier algorithms.The algorithm equates the transmission line as lumped R-L model and neglects thedistributed capacitance, so the ranging error when fault occur in distal point is large. Newalgorithms based on π model, T model or2π model of transmission line are proposed.There are two main steps in all three new algorithms, which is the calculation ofcapacitance’ current and iterative calculation of fault distance. ATP simulation shows thatthe three new algorithms perform better than the R-L model based algorithm, of which theπ model based algorithm performs with the most stable effect and can range the faultlocation accurately when fault occurs250km off the relay.To further improve the performance of the algorithm, the sampling rate, window lengthof the least square algorithm and cutoff frequency of Butterworth Low-pass filter in the π model based algorithm are analyzed. ATP simulation shows that choosing the propersampling rate, window length and cutoff frequency can improve the performance of thenew algorithm.In additional, in the end of the paper, deficiencies of the algorithm are pointed out andthe direction of future research are indicated.