Design and Development of Digital Relays for Power System Protection

Abstract

An electrical power system consists of generators, transformers, transmission lines, etc. Short circuits and other abnormal conditions often occur on a power sys- tem. The heavy current associated with short circuits is likely to cause damage to equipment if suitable protective relays and circuit breakers are not provided for the protection of each section of the power system. Short circuits are usually called as faults. If a fault occurs in an element of a power system, an automatic protective device is needed to isolate the faulty element as quickly as possible to keep the healthy section of the system in normal operation. The protective scheme includes circuit breakers and protective relays to isolate the faulty section of the system from the healthy sections. The function of the protective relay is to detect and locate a fault and issue a com- mand to the circuit breaker to disconnect the faulty element. Protective relay is a device which senses abnormal conditions on a power system by constantly moni- toring electrical quantities of the system, which di er under normal and abnormal conditions. The basic electrical quantities which are likely to change during abnor- mal conditions are current, voltage and frequency. Protective relays utilize one or more of these quantities to detect abnormal conditions on a power system. Pro- tective relays have evolved from electromagnetic to static to digital relays. Digital relays are the latest development in this area. A power system is divided into various zones for protection and there is a suitable protective scheme for each zone. If a fault occurs in a particular zone, it is the duty of the primary relay of that zone to isolate the faulty element. If due to any reason, the primary relay fails to operate then there is a back-up protective scheme to clear the fault.