A Guide To Learn About A Voltage Transformer

The potential transformer is used in electrical power system. It helps to lower the system’s voltage to permissible levels that can be input to low ratings relays and meters. The meters and relays that you would find in the market work on a low voltage.

With this general introduction about the potential transformer, let us now learn about the voltage transformer theory.

What is a Voltage Transformer Theory?

To understand this theory, we need to set up a circuit as per the given instructions. Connect the “primary end” of the transformer across the ground and the phase. As the transformer works to lower the voltage, a potential transformer achieves the same objective with the low turns winding at the secondary.

Now apply the system voltage across the primary winding terminals of the potential transformer. Apply a balanced secondary voltage across the secondary winding terminals of the potential transformer.

The secondary voltage of the potential transformer is 110 V. In an ideal voltage transformer, the turn’s ratio is equal to the ratio of the primary voltage and secondary voltage of a transformer.

In actual transformer case, there can be an error in the phase angle between primary voltage and secondary voltage or the voltage ratio. The errors in a voltage transformer are an important part of voltage transformer theory.

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Why error happens in a Voltage Transformer?

The internal impedance causes the primary voltage of the voltage transformer to drop. This voltage is seen across the main winding. It is then altered proportionally to the “turns ratio”, and the secondary winding.

This altered voltage applied across the secondary winding drops due to the impedance of the secondary winding, before it appears across burden terminals. This causes error in the potential transformer.

Error in Voltage Transformer

Before we figure out the error in the voltage transformer, let us understand a few terms.

Is – Secondary current.

Vs – Secondary terminal voltage.

Es – Secondary induced emf.

Rs – Secondary winding resistance.

Ip – Primary current

Xs – Secondary winding reactance.

Vp – Primary terminal voltage.

Ep – Primary induced emf.

Xp – Primary winding reactance

Rp – Primary winding resistance

I0 – Excitation current

KT – Turns ratio = Ratio of primary turns to secondary turns.

Im – Magnetizing component of I0.

Iw – Core loss component of I0.

β – Phase angle error.

Φm – Main flux

Total primary current Ip is equal to the vector addition of “excitation current”. The current is equal to the multiplication of the ratio 1/KT and the reverse of secondary current.

Voltage Error is the difference between the actual value Vs and ideal voltage value Vp/KT. The ′β′ angle between the reversed secondary voltage vectors KT and the main system voltage Vp is the phase error Vs.