Voltage Transformer or Potential Transformer Theory

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Key learnings:
  • Voltage Transformer Definition: A voltage transformer, also known as a potential transformer, steps down high system voltages to safer, lower levels suitable for standard meters and relays.
  • Basic Operation: These transformers connect their primary windings across a phase and the ground, operating like other step-down transformers but specifically for voltage management.
  • Standard Secondary Voltage: The secondary voltage output of a typical voltage transformer is generally 110 V.
  • Common Errors: Errors in voltage transformers include deviations in voltage ratios and phase alignments, impacting accuracy.
  • Cause of Errors: Errors are mainly caused by voltage drops across the transformer’s internal resistance and reactance, affecting both the primary and secondary windings.

Potential Transformer Definition

A voltage transformer, also known as a potential transformer, is utilized in electrical power system to reduce system voltage to a safe level suitable for low-rated meters and relays. Relays and meters available commercially are designed for low voltage, which aligns with the function of voltage transformers.

Voltage Transformer or Potential Transformer Theory

A voltage transformer theory or potential transformer theory is just like a theory of general purpose step down transformer. Primary of this transformer is connected across the phase and ground. Just like the transformer used for stepping down purpose, potential transformer i.e. PT has lower turns winding at its secondary.

potential transformer voltage transformer

When system voltage is applied to the primary winding of a voltage transformer, a corresponding secondary voltage is produced at the secondary terminals.

The secondary voltage of the PT is generally 110 V. In an ideal potential transformer or voltage transformer, when rated burden gets connected across the secondary; the ratio of primary and secondary voltages of transformer is equal to the turns ratio and furthermore, the two terminal voltages are in precise phase opposite to each other. But in actual transformer, there must be an error in the voltage ratio as well as in the phase angle between primary and secondary voltages.
Errors in voltage transformers, crucial to understanding their function, can be effectively illustrated using phasor diagrams.

Error in PT or Potential Transformer or VT or Voltage Transformer

phasor diagram of Voltage Transformer


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.

Similar to current transformer, the total primary current (Ip) in a voltage transformer is the vector sum of the excitation current and the reversed secondary current multiplied by the ratio 1/KT.

If Vp is the system voltage applied to the primary of the PT, then voltage drops due to resistance and reactance of primary winding due to primary current Ip will come into picture. After subtracting this voltage drop from Vp, Ep will appear across the primary terminals. This Ep is equal to primary induced emf. This primary emf will transform to the secondary winding by mutual induction and transformed emf is Es. Again this Es will be dropped by secondary winding resistance and reactance, and resultant will actually appear across the burden terminals and it is denoted as Vs.
So, if system voltage is Vp, ideally Vp/KT should be the secondary voltage of PT, but in reality; actual secondary voltage of PT is Vs.

Voltage Error or Ratio Error in Potential Transformer (PT) or Voltage Transformer (VT)

In a voltage transformer, the voltage or ratio error is the discrepancy between the ideal value (Vp/KT) and the actual output (Vs).

Phase Error or Phase Angle Error in Potential or Voltage Transformer

The angle ′β′ between the primary system voltage Vp and the reversed secondary voltage vectors KT.Vs is the phase error.

Cause of Error in Potential Transformer

The voltage applied to the primary of the potential transformer first drops due to the internal impedance of the primary. Then it appears across the primary winding and then transformed proportionally to its turns ratio, to the secondary winding. This transformed voltage across the secondary winding will again drop due to the internal impedance of the secondary, before appearing across burden terminals. This is the reason of errors in potential transformer.

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