Algorithms Of Transient Current Protection For EHV Transmission Lines Based On Improved Recursive Wavelet Transform

Power system in China develops very fast in latest years, and the trend of large capacity, long distance, high voltage is formed, which demands higher transient stability for power system. Clearing the faults rapidly is an effective method not only to increase power transfer but also to improve transient stability of transmission system. So the transient based protection principle with high speed even ultra-high-speed operating performance becomes one of the development directions of power system protective relaying.On the basis of the frequency characteristics of transmission system including transmission lines, line traps and busbars, this paper proposes two new algorithms of protection based on transient components for EHV transmission lines. It is because that a substantial amount of the high frequency components will be attenuated by the busbar capacitance and line traps when a fault occurs outside the protected zone. First, the one algorithm is based on the information of magnitude: improved recursive wavelet transform (IRWT) is applied to capture two desirable frequency components from a fault-generated transient current signal. According to the spectral energies of foregoing two frequency signals, an internal or external fault can be distinguished. Secondly, the algorithm is based on the information of phase: IRWT is adopted to extract phase information of a high desirable frequency component from a fault-generated transient current signal. According to comparison the phase information of the foregoing frequency with that of the mother wavelet of IRWT, a relative distortion coefficient about phase can be calculated, which will be able to used to determine where a fault is inside or outside to the protected zone.Two simulation models of 500kV extremely high voltage transmission systems are established on the platform of PSCAD/EMTDC. A lot of simulations about different fault conditions such as fault location, fault resistance, fault type and fault inception angle are carried out, and the simulation data is treated to validate the algorithms with MATLAB. The simulation results prove that the new algorithms are correct.