To Study a Design of Current Transformer and Propose Improvement for Metering and Release Power Up

Abstract

Air Circuit Breaker is a switching device. Capable of making and carrying current under normal circuit conditions and breaking currents under specified abnormal system conditions such as short circuit faults, ground faults etc. Current Transformer (CT) is an integral part of an air circuit breaker. It is uses for measurement and protection purpose. The CT construction consists of two type of coils: Iron CT and Rogowski coil. Iron CT is used for generating the required power for Electronic release and Rogowski is used for measurement of the current owing through the breaker. CT senses the current and continuously gives the input to Electronic re- lease. During normal operations, current owing through the breaker will be displayed on the Electronic release. Under abnormal conditions, the trip command is given by the electronic release, when it gets the signal from the CT. There is a market requirement of 1 percent accuracy in metering characteristics of Rogowski coil for which existing design needs to be upgraded. Also, release powerup at 20 percent of single phase and 10 percent of three phase is required which will be achieved through design modification of iron CT. The main problem associated with the CT is the accuracy for the lower rating of Rogowski and the size reduction in the Iron core. The CTs have the ratings of 400/630/800A, 1000/1250/1600A and 2000/2500A. The Breaker of 1000A having 1000/1250/1600A CT cannot set protection setting at current lower than 1000A. So a need is felt to develop a new CT with rating 1000/800/630A. This will help the customer to set protection setting at lower current for 1000A breaker (800 or 630A). Similarly, a new CT design for 2000/1600/1250A needs to be developed on the same line.