Modern numerical relays in protection of power transformer

Abstract

This paper presents various protection schemes and their respective modern numerical protection relays to protect the power transformer. Proper protection is needed for the economical and safe operation of electrical power systems. The power transformer protective relay should block the tripping during an external fault or magnetizing inrush and speedily operate the tripping during internal faults. The objective of this paper is to implement protection during in-zone and outzone faults and to operate the relay with proper selective discrimination. The basic approach is to disconnect the power transformer feeder against in-zone faults and prevent disconnection of the power transformer feeder due to other operating conditions. The protection relay was able to discriminate between 2nd harmonic inrush, in-zone fault, and out-zone fault conditions. The protection scheme consists of high-speed protection relays and circuit breakers, which are required for the protection of substation equipment against abnormal conditions. A protective relay senses the fault and determines the location of the fault. Then, the protective relay sends the tripping command to the circuit breaker. Therefore, proper care should be taken in designing and selecting an appropriate relay that is reliable, efficient, and fast in operation. The PT and CT continuously measure the voltage and current of the substation and are connected to the relays to enable them to detect abnormal conditions. This paper describes numerical protection for power transformers, especially when the rating of the system is 50 MVA, 110/33 Kv, and Dyn11 at the substation. The protection relay employed in the transformer feeder is 7SR24 type (Siemens make).