- 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 used in an 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 matches what voltage transformers supply.
Voltage Transformer or Potential Transformer Theory
A voltage transformer theory or potential transformer theory is the same as the theory of a general purpose step down transformer. The primary of this transformer is connected across the phase and ground. Like the transformer used for stepping down, a potential transformer (PT) has the lower-turns winding on its secondary.
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 is connected across the secondary, the ratio of primary and secondary voltages equals the turns ratio and the two terminal voltages are in precise phase opposition. In an actual transformer there is error in the voltage ratio and in the phase angle between primary and secondary voltages.
Errors in voltage transformers can be shown with phasor diagrams.
Error in PT or Potential Transformer or VT or 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.
As with a 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, voltage drop due to resistance and reactance of the primary winding caused by primary current Ip appears. After subtracting that drop from Vp, Ep appears across the primary terminals. This Ep is the primary induced emf. That primary emf is transformed to the secondary winding by mutual induction and the transformed emf is Es. Es then drops across secondary winding resistance and reactance, and the result that appears across the burden terminals is Vs.
So if system voltage is Vp, ideally Vp/KT should be the secondary voltage of the PT, but the actual secondary voltage of the 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 difference 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. It then appears across the primary winding and is transformed, in proportion to the turns ratio, to the secondary winding. That transformed voltage across the secondary winding drops again due to the internal impedance of the secondary, before appearing across the burden terminals. That is the cause of errors in a potential transformer.
