Potential transformer

 Potential transformer is a voltage step-down transformer that reduces the voltage of a high voltage circuit to a lower level for the purpose of measurement. These are connected across or parallel to the line which is to be monitored.

The basic principle of operation and construction of this transformer is similar to the standard power transformer. In common, the potential transformers are abbreviated as PT.

The primary winding consists of a large number of turns which is connected across the high voltage side or the line in which measurements have to be taken or to be protected. The secondary winding has a lesser number of turns which is connected to the voltmeters, or potential coils of wattmeter and energy meters, relays, and other control devices. These can be single-phase or three-phase potential transformers. Irrespective of the primary voltage rating, these are designed to have the secondary output voltage of 110 V.

Since the voltmeters and potential coils of other meters have high impedance, a small current flows through the secondary of PT. Therefore, PT behaves as an ordinary two winding transformer operating on no load. Due to this low load (or burden) on the PT, the VA ratings of PTs are low and in the range of 50 to 200 VA. Compared to the conventional transformer, potential transformers or PTs use larger conductor sizes and core. PTs designed for ensuring greater accuracy and hence, at the time of designing economy of the material is not considered as main aspect.