Potential transformer

Potential transformer – phasor diagram and more

In AC circuits measurement of voltage, current, energy, and power are measured by instrument transformer. An instrument transformer is either a current transformer or a potential transformer (PT) also known as a voltage transformer.

High voltage lines also require measurement of voltage, power, etc. so to measure it the high voltage of voltage lines is stepped down before applying to the measuring instruments.

Now you guys are wondering why voltage is being stepped down before measurement?

The answer is simple and necessary, voltage is stepped down because it is essential to ensure the safety of the operator, reduce the size of the instrument, and save on insulation costs.

What is a potential transformer?

The potential transformer or voltage transformer is defined as the instrument transformer which is used an electrical system to step down the voltage which is to be measured. This transformer steps down the voltage to be measured to a safe limit which can be later easily measured by the ordinary low voltage instrument such as voltmeter, wattmeter, etc.

Circuit diagram

It is very similar to the conventional transformer. It has a primary winding having more number of turns and a secondary winding having less number of turns.

The primary winding is connected across the high voltage line whose voltage is to be measured and the measuring instruments like voltmeter, wattmeter, etc. are connected to the secondary winding. The secondary winding voltage must be a definite fraction of the primary voltage.

The circuit diagram of PT is shown below.

Diagram of the potential transformer.
Primary winding having more turns is connected to the load.
A voltmeter is connected to the secondary winding of the potential transformer.


  • Core

The core is the most important part of the transformer. The core may be shell type or core type of construction. The shell-type core is used in a low voltage transformer.

  • Winding

Primary and secondary windings of the transformer are coaxial to reduce the leakage reactance to a minimum. The low voltage secondary winding is placed nearer to the core, by doing this the insulation requirement in the transformer is reduced.

In a low voltage transformer, the primary winding can be made by a single coil but for a high voltage transformer, the primary winding must be divided into several short coils which helps in reducing insulation difficulties between layers.

  • Insulation

Cotton tape and varnish are the most common insulations used for PT. during coil construction these are used over the windings. At low voltage, the transformer is usually filled with the compound and at high voltage (above 7kV), the transformers are oil-immersed.

  • Bushing

Generally, an oil-filled potential transformer uses oil-filled bushing as it helps in reducing the overall height of the transformer.

Phasor diagram of Potential transformer

The figure shown below plots the phasor diagram of the potential transformer under operating conditions.

Phasor diagram of potential transformer

Vp = primary supply voltage

Vs = secondary terminal voltage

Ep = primary induced emf

Es = secondary induced emf

Ip & Is = primary and secondary current

I0 = no-load current

Rp & Rs = resistance of primary and secondary windings

Xp & Xs = reactance of primary and secondary windings

IM = magnetizing component of current

IC = core loss component of current

Φm = main flux

Types of potential transformer

The potential transformer is classified into two types that are listed below:

Conventional wound type

This type of transformer is expensive to build due to its insulation requirements.

Capacitor type

It is also known by a few other names such as capacitive potential dividers or coupling type or bushing type transformer. Its diagram is shown below:

This consists of a capacitance potential divider with a conventional auxiliary transformer. The capacitance divider consists of two sections of capacitors C1 and C2 and the auxiliary transformer consists of inductance L.

The capacitance potential divider steps down the voltage are to be measured and its output voltage is further stepped down by the transformer to desired secondary voltage.

Read more: Different varieties of voltage transformers.

Errors in potential transformer

When we talk about the ideal transformer, errors are as much as a number of bad movies directed by Christopher Nolan (i.e zero). Btw there is no such thing as ideal. Every machine has some losses in it.

But in reality, practical potential transformers have losses that lead to some error in it.

Ratio error

The ratio error is expressed by the formula given below


Where Kn = nominal ratio

              R = actual ratio

Phase angle error

In an ideal potential transformer, there is no phase difference between the primary windings and secondary windings reversed.

On the other hand in a practical transformer, there will be a phase difference between the secondary winding and primary winding.

As the secondary voltage terminal is in exact phase opposition with the primary voltage terminal. The error between them is known as phase angle error.

Causes of errors

Due to internal impedance, the voltage in primary winding reduces, and it is further transmitted proportionally to its turn ratio, to the secondary winding. And the transmitted voltage on the secondary side is further reduced by the internal impedance of the secondary.

How to reduce errors in PT

  • Reduction of magnetizing and loss component

Reducing the loss component and magnetizing component i.e. no-load component of primary current and such a reduction can be achieved by reducing the length of magnetic path in the core, using good quality core material, designing with the appropriate value of flux densities, and adopting suitable precautions in the assembly and interleaving of core laminations.

  • Reduction of resistance

Winding resistance can be reduced by using thick conductors and by adopting to reduce the length of mean turn of winding.

  • Reduce leakage reactance

Winding leakage flux and hence leakage reactance can be minimized by keeping the primary and secondary winding as close as possible. And the spacing between the two windings should be compatible with insulation requirements.

  • Turns compensation

At no load, the transformation ratio exceeds the turns ratio by (ICrp+IMXp)/VS. with increased loading this difference between the transformation ratio and turns ratio increases due to further voltage drop in winding resistance and reactance.

This problem can be solved by making the turns ratio equal to the nominal ratio.

And this can be achieved by either reducing the no. of turns in primary winding or increasing the number of turns of the secondary winding.

Application of potential transformer

  • Used in protection device electrically
  • Used for protection feeders
  • Used in metering and relay circuits
  • Used as protection voltage transformer

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