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 purpose is to supply current and voltage magnitudes to the relays to carryout protection functions.

The main functions of instrument transformers are:

  • 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.

Why Instrument Transformers Used and Advantages

Voltmeters and Ammeters are used to measure the voltage and current in the circuits. Using instrument transformers ranges of these devices to measure can be increased. A current transformer is bascially a step down transformer hence it steps down the current. When used in conjunction with the low range ammeter, a current transformer (CT) increases the range of the ammeter. Thus a 0-5A ammeter can be used to measure several hundreds or thousand amperes of current. Similarly a potential transformer (PT), which is basically a step down transformer can increase the range of the low voltage voltmeter. Thus a voltmeter designed to measure voltage upto 110V can measure a much higher voltages (several thousnd volts) when used in conjnction with suitable potential transformer

Instrument transformers have many advantages. Some of the advantages are given below:


  • Single range ammeters and voltmeters can measure a wide range of currents and voltages, if used in conjunction with suitable Current Transformers (CTs) and Potential Transformers (PTs)
  • The measuring instruments like ammeter, voltmeter and wattmeters etc are incorporated in the secondary circuit and hence they are totally segregated from the high voltage, thereby ensuring safety for the operator and observer
  • The meter need not be insulated for high voltages which would be the case if they are directly included in a high voltage circuit
  • Using current transformer with suitable split and hinged core, it easy to measure heavy currents in the busbarwithout having to break the conductor carrying current. The core of the Current Transformer (CT) is opened at the hinge, the current carrying conductor is introduced in the center of the core through a opening made and the core is tightly closed again. The conductor itself acts as a single turn primary winding of the current transformer

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