What is a Potential Transformer (PT)? Working Principle, Diagram, & Disadvantage

The potential transformer is another type of instrumentation transformer. It is used to measure high ac voltages. The construction of a Potential Transformer (P.T.) is same as that of the normal transformer.

Fig. 1 : Connection of Potential Transformer (PT) to measure voltage.

Connection Diagram of Potential Transformer (PT)

The connection of a potential transformer in a circuit to measure voltage is shown in Fig. 1.

Working Principle of Potential Transformer (PT)

To measure high voltage. the potential transformer which is a step down transformer is connected across high voltage terminals and voltmeter is connected on low voltage side. A potential transformer is an ordinary two winding transformer with high turns ratio, designed for low current carrying capacity. Due to P.T., voltage across voltmeter gets reduced by a factor equal to the turns ratio of PT. The turns ratio is adjusted in such a way that we get a constant secondary voltage irrespective of the primary voltage. The scale of the voltmeter is suitably calibrated by taking into account the turns ratio of P.T. The secondary voltage is given by,

\[{{\text{V}}_{2}}={{\text{V}}_{1}}\times \frac{{{\text{N}}_{2}}}{{{\text{N}}_{1}}}\]

So if the ratio (N₂ / N₁) is known and V₂ is measured then we can obtain the voltage V₁ accurately.

Sources of error in Potential Transformer (PT)

There are two types of errors that are likely to occur in the potential transformers:

1. Ratio error
2. Phase angle error.

Ratio error

To ensure accurate measurement, the turns ratio of the P.T. should be very accurately adjusted. The error introduced due to incorrect ratio is called as the ratio error.

Phase angle error

A little deviation in the phase angle between voltage and current can affect the voltmeter reading considerably. Such an error is called as phase angle error.

Advantages of Potential Transformer (PT)

Some of the merits of using potential transformers arc as follows :

1. It is possible to use the voltmeters of normal voltage range to measure very high voltages.
2. We can use the same voltmeter to measure different voltages by adjusting the turns ratio of P.T.
3. The user, handling the voltmeter, is safe due to the isolation provided by the potential transformer. The P.T. provides isolation between the high voltage to be measured and the user.
4. P.T. can be used to operate indicating and protecting devices such as relays.
5. The same P.T. can be used to provide the measured voltage to several instruments.

Disadvantages  of Potential Transformer (PT)

The only demerit of P.T. is that it can be used to measure only the ac voltage. DC voltage cannot be measured.