DIGITAL PROTECTIVE RELAYING FOR OVERHEAD EQUIPMENT OF 25 KV, SINGLE-PHASE TRACTION SYSTEM

Abstract

The electromechanical relays, presently used for the protection of the overhead equipment of 25 KV single-phase 50 Hz traction system lack good selectivity and fast operation. Unnecessary interruptions and lack of definite indication of the type of fault result from the poor selectivity and unavoidable delays in clearing faults _fkom slow relay operation. The author has made an attempt to improve the performance of protective relaying of OHE by modifying the relay characteristics suitably and has developed a microprocessor-based relaying system with the modified relay characteristics. It is shown that a quadrilateral characteristic in place of the present admittance characteristic for earth fault distance relay can ensure the desire selectivity of the relay to various operating conditions. It is further observed that for the wrong phase coupling protection, non-offset admittance characteristic should be discarded because of its unwanted operation and the offset admittance characteristic, wherever in use, may be replaced by another quadrilateral characteristic in the second quadrant of the R-X plane. For a faster back-up protection, definite timelag overcurrent relay may be preferred to the existing inverse definite minimum time overcurrent relay. iii lv AllofM these relays, as also an instantaneous overcurrent relay for clearing closeup earth faults, have been implemented on a system based around the 8-bit, Intel 8085A microprocessor and tested for their steady state and dynamic erformances.