6 - Instrument transformers

Abstract

This chapter presents instrument transformers. The voltage transformers and current transformers continuously measure the voltage and current of an electrical system and are responsible to give feedback signals to the relays to enable them to detect abnormal conditions. The values of actual currents in modern distribution systems vary from a few amperes in households, small industrial/commercial houses, etc. to thousands of amperes in power-intensive plants, national grids, which also depend on the operating voltages. Similarly the voltages in electrical systems vary from a few hundreds of volts to many kilo volts. The transformer is one of the high efficient devices in electrical distribution systems that are used to convert the generated voltages to convenient voltages for the purpose of transmission and consumption. A transformer comprises two windings—primary and secondary—coupled through a common magnetic core. Electromagnetic voltage transformers may be connected interphase or between a phase and the earth. However capacitor voltage transformers can only be connected phase-to-earth. Voltage transformers are commonly used in three-phase groups, generally in the star–star configuration. All current transformers used in protection are basically similar in construction to standard transformers in that they consist of magnetically coupled primary and secondary windings, wound on a common iron core, the primary winding being connected in series with the network unlike in voltage transformers. The chapter explains these two types of current transformers—wound primary type, and the bar primary type.