Functions of Instrument Transformers (CTs and PTs)

AC type protective relays are actuated by the current and voltage supplied by the current and potential (voltage) transformers which are generally classified as instrument transformers. Generally instrument transformers are used for mainly two purposes. For metering purpose which steps down and displays voltage and current levels from kV to (0-110 volts in case of PT) and few kilo amps to (0-5 amps in case of CT) . Second purose is to supply current and voltage magnitudes to the relays to carryout protection functions

The main functions of instrument transformers are:

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• Instrument transformers (current and potential transformers) provide insulation against the high voltages of the power circuit and to protect the apparatus and the operating personnel from contact with the high voltages of the power circuits
• Instrument transformers (CTs and PTs) supply protective relays with current and voltages of magnitude proportional to those of the the power circuits. These current and voltage magnitudes supply by the instrument transformers are sufficiently reduced such that the relays can be made relatively small and inexpensive
• Instrument transformers helps in attaining different types of secondary connections to obtain the required current and voltages

For proper applications of CTs and PTs required considerations are:

Mechanical construction , type of insulation (dry or liquid), ratio in terms of primary and secondary currents or voltages, continuous thermal rating, short time thermal and mechanical ratings, insulation class, impulse level, service conditions, accuracy and connections

For the safety purpose, the secondaries of the current and potential transformers (CTs and PTs) are grounded