DEVELOPMENTS IN LINE PROTECTION ALGORITHMS

Abstract

In power system, high voltage transmission and distribution lines are vital links from the generating plants to the end users. It is necessary for protecting lines using relay. When fault occurs, relays prevent involving area from growing. Correct action of relay is important in power systems. Line relay tends to over reach in the presence of DC offset component in fault current waveforms. DC offset has to be removed from these waveforms. Developments in Line Protection Algorithm involves the removal of DC offsets in fault current which is done by using different types of algorithms. In the first method, a modified FCDFT (Full Cycle Discrete Fourier Transform) is used to calculate DC offsets. Applying FCDFT requires one cycle plus two samples to calculate and compensate for the DC offsets. HCDFT (Half Cycle Discrete Fourier Transform) only requires half of a cycle plus two or three samples to accomplish the algorithm when input signal has no even order- harmonics. By using this method in distance relays effectively suppresses the DC offsets and quickly decomposes the accurate fundamental frequency components. Modified FCDFT can not be applied for higher order harmonics which requires a low pass filter to remove that harmonics resulting more time to converge the fundamental frequency component. So another method has been proposed which is used to eliminate the disadvantage of the modified FCDFT method. In this method for exact estimation of DC offset in current signals such that DC offsets can be completely eliminated for relaying applications. In this method, it requires one cycle plus two samples to accomplish the algorithm. These two methods are simulated using MATLAB programming and then compared.