Voltage Transformer or Potential Transformer Theory

Potential Transformer Definition

Potential Transformer or Voltage Transformer are used in electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings meters and relays. Commercially available relays and meters used for protection and metering, are designed for low voltage. This is a simplest form of Potential Transformer Definition

Voltage Transformer or Potential Transformer Theory

A Voltage Transformer theory or Potential Transformer theory is just like theory of general purpose step down transformer. Primary of this transformer is connected across the phases or and ground depending upon the requirement. Just like the transformer, used for stepping down purpose, potential transformer i.e. PT has lowers turns winding at its secondary. The system voltage is applied across the terminals of primary winding of that transformer, and then proportionate secondary voltage appears across the secondary terminals of the PT.

potential transformer voltage transformer

The secondary voltage of the PT is generally 110V. In an ideal Potential Transformer or Voltage Transformer when rated burden 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.

The errors in Potential Transformer or Voltage Transformer can best be explained by phasor diagram, and this is the main part of Potential Transformer theory

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

Io - Excitation Current

Im - magnetizing component of Io

Iw - core loss component of Io

Φm - main flux

β - phase angle error

As in the case of Current Transformer and other purpose Electrical Power Transformer, total primary current Ip is the vector sum of excitation current and the electric current equal to reversal of secondary current multiplied by the ratio 1/KT

Hence,Ip = Io + Is/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 comes 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)

The difference between the ideal value Vp/KT and actual value Vs is the voltage error or ratio error in a potential transformer, it can be expressed as ,

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

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