CT- Current transformer PT- Potential transformer

What are CT- Current transformer PT- Potential transformer ?

CT or Current Transformer & PT or Potential Transformer are measuring devices in AC system. These are also called as instrument transformers. As AC system deals with very high power hence we require Ammeter & voltmeter of humongous sizes to measure such high power which is impractical & expensive too.

A CT has following properties:-

1. It is a step up transformer.
2. It enhances the voltage & thus the current gets reduced for a fixed power, since P= V×I×cosΦ. Thus for a fixed amount of power, current can easily be measured.
3. Primary winding of a CT is always connected in series with the load & secondary winding is connected with the ammeter.

A PT has following properties:-

1. It is step down transformer.
2. It reduces the voltage for a fixed power & thus the voltage can easily be measured, since P= V×I×cosΦ
3. The primary winding of the PT is always connected in parallel to the load & secondary winding is connected to the voltmeter.

CT- Current transformer

PT- Potential transformer

CT and PT form the sub parts of instrument transformer .They are extensively used in power system for metering and protection purpose .These 2 form the most important part of any substation .Since in actual electrical power system we deal with high voltage and high current and it is not feasible and economical to manufacture devices to measure such high values .So to measure high values in power system use CT and PT.The primary of of these transformer is placed in main line and secondary is placed in any meter or relay depending on the application.The turns of primary and secondary are adjusted in such a manner that the current in secondary is small and easy to measure for CT and voltage in case of PT .Also they provide isolation between primary and secondary which is often required in electrical.