RECURSIVE FILTERS AND POLYPHASE DISTANCE RELAYS
Nanuan, Kulwant Singh
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Distance relays, alone and in conjuction with communication channels, are used to protect modern EHV transmission lines. These relays check the electrical distance from a relay location to the fault and decide whether the fault is inside or outside the relay's assigned protected zone. Presently, distance relays use electromechanical devices or electronic (solid-state, analog) circuits as relay structures. The cost of both the electromechanical and solid-state analog systems has been increasing rapidly during the last fifteen years. On the other hand, the cost of digital processors has been rapidly decreasing. It seems that the use of digital processors for control and protection systems is at the verge of becoming economically viable. This thesis reviews the developments in the area of digital relaying and polyphase distance relays. Procedures for designing digital lowpass and bandpass recursive filters are outlined. Feasibility of using the designed recursive filters in digital distance relays is then examined. The studies indicate that using recursive filters for processing fault data does not necessarily permit relaying decisions to be made in shorter times. Lowpass recursive filters do, however, stabilize the calculated impedances and would, therefore, be useful in determining the locations of faults. Possibility of using the general exponential smoothing technique for processing power system voltages and currents is also investigated. A generic polyphase distance relay suitable for implementation on a microprocessor is designed. Also included is a brief description of the relay software and logic. The performance of the relay was tested in the real time-mode; some test results are also included.