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:
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:
Advantages:
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