Potential Transformer (PT)


Potential Transformer (PT)

The potential transformer (PT) may be defined as an instrument transformer used for the transformation of voltage from a higher value to the lower value. This transformer step down the voltage to a safe limit value which can be easily measured by the ordinary low voltage instrument like a voltmeter, wattmeter and watt-hour meters, etc

Potential transformer (voltage transformer) is used for protection and measurement. Actually they are protective type voltage transformer. They may be single phase or three phases. Voltage transformer is necessary for directional, voltage, distance protection. The primary of voltage transformer is connected to power circuit between phase and ground.

The volt-ampere rating of potential transformer is less than power transformer. Hence potential transformer is that type transformer which protect from over voltage and also in earth fault protection.

It works on law mutual induction as in simple transformer. It’s also used in calibration of measuring instrument.
Potential transformer is step down type transformer which steps down the voltage from high voltage to low voltage.

The basic principle of operation and construction of this transformer is similar to the standard power transformer. In common, the potential transformers are abbreviated as PT.

Construction of Potential Transformer

  • The potential transformer is made with high-quality core operating at low flux density so that the magnetising current is small.
  • The terminal of the transformer should be designed so that the variation of the voltage ratio with load is minimum and the phase shift between the input and output voltage is also minimum.
  • The primary winding consists of a large number of turns which is connected across the high voltage side or the line in which measurements have to be taken or to be protected.
  • connection of potential transformer
  • The secondary winding has lesser number of turns which is connected to the voltmeters, or potential coils of wattmeter and energy meters, relays and other control devices.
  • voltmeters and potential coils of other meters have high impedance, a small current flows through the secondary of PT. Therefore, PT behaves as an ordinary two winding transformer operating on no load.
  • Due to this low load on the PT, the VA ratings of PTs are low and in the range,
  • of 50 to 200 VA
  • These can be single phase or three phase potential transformers. Irrespective of the primary voltage rating, these are designed to have the secondary output voltage of 110 V.

Error in Potential Transformer (PT)

phasor diagram of error in PT

Phasor diagram of potential transformer at lagging power factor


1. Phase Angle Error in Potential Transformer :

  • The phase angle error δ is usually given in minutes and indicates by what angle the secondary voltage flow from the applied primary voltage.
  • The increase in the number of measuring instruments and relays connected to the secondary of the PT increases the burden there by increasing the ratio and phase angle error.
  • The angle ′β′ between the primary system voltage Vp and the reversed secondary voltage vectors KT.Vsis the phase error.

2. Ratio Error in Potential Transformer :- 

  • 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,

voltage error in potential transformer



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