Complementary Approach for Reliable High Speed Transmission Line Protection

Line distance protection technology has evolved from sound operating principles using phase comparators used in electromechanical relays to the present day relays using microprocessor technology. Initial microprocessor relays used full cycle Discrete Fourier Transformation to extract the fundamental phasors from sampled analog data and apply them in comparator equations to determine impedance reach. This technique is still being used in most the modern microprocessor relays today. In order to address the effects of dc offset and CCVT transients a compromise is however required between operating speed, reach and security. Also, their performance is relatively independent of Source Impedance Ratio (SIR) once the transients have subsided, usually in two to three cycles. A totally different approach was introduced in 1997 using a time domain algorithm that mimics the operation of the electromechanical phase comparator. This algorithm proved to be one of the fastest and most secure for protection of medium and long lines typical for distance protection applications. The only compromise with this algorithm is the higher operating time for very short lines with high Source Impedance Ratios. This paper discusses the relative performances of each of the above approaches individually and about the advantages of combining the two algorithms to run in parallel.