These can be classified in to two parts. They are, Current Transformers and Potential Transformers. These transformers are generally used to measure the high voltages and high currents. In power stations and substations, we can see these types of transformers. Also these transformers are used as protecting equipment.
Current Transformers (CT)
A current transformer is a measurement device designed to provide a current in its secondary coil proportional to the current flowing in its primary. Current transformers are commonly used in metering and protective relaying, where they facilitate the safe measurement of large currents. The current transformer isolates measurement and control circuitry from the high voltages typically present on the circuit being measured. It is not possible to use line current directly for measurement and relaying purpose due to its high value and insulation problem. CURRENT TRANSFORMER which is widely known as ‘CT’.
Current transformers are classified into two groups according to their applications,
- Protective CT, used in association with relay, trip coils pilot wires etc.
- Measuring CT, used in conduction with ammeter, Wattmeter
Potential Transformer (PT)
Potential transformers are used for measuring and protection. A voltage transformer is use to steps down voltage from a high value to a low value. There are two type of Potential transformers are in available.
- Measuring type Potential Transformer
- Protective type Potential Transformer
They may be either single phase or three phases. VT’s are also used for instrumentation. When a VT is connected to a line, a conductor makes a separate connection from the line to the VT. A potential transformer has all the features of a power transformer but has a capacity not exceeding a few hundred volt-amperes. The out put voltage of the potential transformer has been standardized. It is set at normally 110 V.