P.T Potential transformer.

P.T Potential transformer

We know that measuring high voltage in transmission and distribution systems is not very easy. Instrument transformers P.T (potential transformers) are used to measure high voltage.

Details about Instrument Transformer

Current transformers are used to measure the current in the transmission and distribution lines, and potential or voltage transformers are used to measure the voltage.

Read more about current transformers C.T

“Potential transformers are also called voltage transformers. It is a transformer that converts a large amount of voltage to a low range. It is used to measure high voltage in a circuit with a low range meter. "

We know that in order to measure voltage, the meter has to be connected in parallel with the load. The basic principle and function of a potential transformer are very similar to that of a standard power transformer. Potential transformers are called short P.T.

Function and operation of the potential transformer.

From the figure above, the primary side is basically the high voltage side and the secondary side is the low voltage side. The number of punches in the primary winding of a potential transformer is higher than that of the secondary winding. So this is a step-down transformer. There is a voltmeter attached to the secondary winding with which I can measure the voltage.

The secondary voltage of PT is usually designed to be 110 volts. Since the potential coils of PT voltmeters and other meters have high impedance, there will be a small current flow in PT secondary. This is why PT behaves like a two-winding transformer in no-load conditions. PT has a very low VA rating due to its low load. The secondary side is grounded with one edge for safety convenience.

PT transformation ratio like normal transformer.

V1 / V2 = N1 / N2

The high voltage side can be measured if the readings of the voltmeter and the transformation ratio are known from the above equations.

However, this is true in the case of the ideal voltage transformer. But not entirely correct in the case of the actual PT. This is because the voltage drops in the primary and secondary windings of the PT. This is usually due to the power factor, resistance. Therefore, the ratio of PT and phase angle error can be seen.

Potential Transformer Error:

Is - Secondary current.

Es - Secondary induced emf.

Vs - Secondary terminal voltage.

Rs - Secondary winding resistance.

Xs - Secondary winding reactance.

Ip - Primary current.

Ep - Primary induced emf.

Vp - Primary terminal voltage.

Rp - Primary winding resistance.

Xp - Primary winding reactance.

KT - Turns ratio = Numbers of primary turns/number of secondary turns.

I0 - Excitation current.

Im - Magnetizing component of I0.

Iw - Core loss component of I0.

Φm - Main flux.

β - Phase angle error.

In the case of a current transformer, the primary current Ip is the vector sum of the excitation current and this current is equal to the inverse of the secondary current which is also a 1 / Kt ratio.

Then, Ip = (Io + Is) / Kt

If Vp system voltage is supplied to the primary of the PT then the voltage will drop due to resistance in the primary winding where the primary current Ip will also come. Ep can be found after this voltage drop Vp is omitted. This Ep is equal to the primary induced E, M, F.

This primary E, M, F will be converted to secondary winding by mutual induction. Again this Es voltage will drop due to the presence of resistance and reactance in the secondary winding. As a result, it can be found in the Burden terminal which is expressed by Vs.

If you do not know Burden, note the text below

Ideally, if the system voltage is Vp, then the secondary voltage of the PT will be: Vp / Kt, but in reality, the actual secondary voltage of the PT is Vs.

So the difference between the ideal value Vp / Kt and the actual value Vs is voltage error or ratio error. It is expressed by the following equation:

Application of Potential Transformer:

• In electric metering system
• In the electric protection system
• In the synchronizing generator with the grid.
• In the impedance protection of the generator.

PT Burden:

Burden basically refers to the operant power which is related to the potential transformer. The Burden rating of the transformer provides the highest value of the operating power that the transformer typically performs under certain conditions. The actual burden is the value of the actual operant power with the output load of the transformer.