Potential Transformer (P.T)

A potential transformer (P.T.) is used to measure high alternating potential difference (Voltage) in a Power system. The primary of this transformer has many turns while the secondary has few turns as show in the fig. It is clear from the figure that a potential transformer is simply a well designed stepdown transformer. The Stepped down voltage is measure with a low range ac voltmeter. The magnetic core of potential transformer usually has a shell type construction for a better accuracy. It order to provide adequate protection to the operator, one end of the secondary winding is usually grounded.

The primary of the potential transformer is connected across the high voltage line whose voltage is to be measured. A low range (0-110V) a.c voltmeter is connected across the secondary. The line voltage (Vp) and ac voltage (Vs) are related as:

Vp/Vs = Np/Ns

Where Vp/Vs is called P.T ratio

Vp/Vs = P.T ratio

Or Vp = Vs× P.T ratio

Therefore Line voltage (Vp) = A.C voltmeter reading × P.T ratio

For Example

Ac voltmeter is 50V and P.T ratio is 100:1

Then Line Voltage = 50×100/1

= 5,000 V.