What is a Potential Transformer : Construction, Types & Its Applications

Transformers are the electromagnetic passive devices that work on the principle of electromagnetic induction, which transfers electrical energy from one circuit to another circuit magnetically. It consists of two coils, one is primary and another one is secondary coil. Both the windings (coils) are magnetically coupled to each other without any magnetic core and electrically separated. The transformer transmits the electrical energy (voltage/current) from one winding to another winding (coil) by means of mutual induction. There is no change in frequency during the transformation of energy. Transformers are classified into two types based on the core construction like core type transformers and shell-type transformers. Based on the voltage level conversion and winnings, they are step-up transformers and step-down transformers. There are different types of transformers used in the AC circuits, such as power transformers, potential transformer, three-phase transformer, and autotransformer.

What is a Potential Transformer?

Definition: Potential transformers are also known as voltage step-down transformers or voltage transformers or instrument transformer, in which the voltage of a circuit is reduced to a lower voltage for measurement. The electromagnetic device used for the transformation of the higher voltage of the circuit to the lower voltage is called a potential transformer. The output of a low voltage circuit can be measured through voltmeters or wattmeters. These are capable of increasing or decreasing the voltage levels of a circuit, without a change in its frequency and windings. The working principle, construction of a potential transformer is similar to the power transformer and conventional transformer.

Potential Transformer Circuit Diagram

The potential transformer consists of primary winding with more turns and secondary winding with less number of turns. The high input AC voltage is given to the primary winding (or connected to the high voltage circuit to measure). The lower output voltage is taken across the secondary winding by using a voltmeter. The two windings are magnetically coupled to each other without any connection between them.

Construction of a Potential Transformer

Potential transformers are constructed with high quality to operate at low flux density, low magnetic current, and minimized load. When compared to a conventional transformer, it uses large conductors and an iron core. It can be designed in the form of a core type and shell type to ensure the highest accuracy. Usually, core type potential transformers are preferred to transform the high voltage to lower voltage.

It uses co-axial windings to reduce the leakage reactance. As the potential transformers are operated at high voltages, the high voltage primary winding is divided into small sections turns/coils to reduce the insulation cost and damage. The phase shift between an input voltage and output voltage should be monitored carefully to maintain a lower voltage by varying the load. Windings covered with vanish cambric and cotton tape to reduce the insulation cost.

Hard fiber separators are used to separate the coils. Oil-filled bushings are used to connect the high voltage potential transformers (above 7KV) to the main lines. The primary winding of a potential transformer has a large number of turns whereas secondary winding has fewer turns. The multimeter or voltmeter is used to measure the lower output voltage.

Potential Transformer Working

The potential transformer connected to the power circuit whose voltage should be measured is connected between the phase and the ground. That means the primary winding of a potential transformer is connected to the high voltage circuit and the secondary winding of a transformer is connected to a voltmeter. Due to the mutual induction, the two windings are magnetically coupled to each other and work on the principle of electromagnetic induction.

The decreased voltage is measured across the secondary winding with respect to the voltage across the primary winding using multimeter or voltmeter. Due to the high impedance in the potential transformer, the small current flows through the secondary winding and operates similarly to the ordinary transformer with no or low load. Hence these types of transformers operated at a voltage range of 50 to 200VA.

According to the convention transformer, the transformation ratio is

V2 = N1/N2

‘V1’= voltage of the primary winding

‘V2’ = voltage of the secondary winding

‘N1’= number of turns in the primary winding

‘N2’= number of turns in the secondary winding

The high voltage of a circuit can be determined by using the above equation.

Types of Voltage or Potential Transformers

Based on the function of a potential transformer, there are two types,

• Metering voltage transformers
• Protection voltage transformers

These are available in single or three-phase and operate with the highest accuracy. These are used to operate and control measuring devices, relays and other devices. Based on the construction, there are

Electromagnetic Potential Transformers

These are similar to the primary transformer.l where primary and secondary windings are wounded on a magnetic core. It works on a voltage of above or below 130KV. The primary winding is connected to phase and the secondary winding is connected to ground. These are used in metering, relay and high voltage circuits.

Capacitive Potential Transformers

These are also known as capacitive potential dividers or coupling type or bushing type capacitive potential transformers. The series of capacitors are connected to the primary winding or secondary windings. The output voltage across the secondary winding is measured. It is used for power line carrier communication purposes and it is more costly.

Errors in Potential Transformers

In the primary transformer, the output voltage in the secondary winding is exactly proportional to the voltage on the secondary transformer. In potential transformers, the voltage drops due to the reactance and resistance in primary and secondary and also the power factor on secondary causes phase shift errors and voltage errors.

The above phasor diagram explains the errors in potential transformers.

‘Is’ – secondary current

‘Es’ – induced emf in the secondary winding

‘Vs’ – terminal voltage of the secondary winding

‘Rs’ – winding resistance of secondary

‘Xs’ – winding reactance of secondary

‘Ip’ – Primary current

‘Ep’ – induced emf of the primary winding

‘Vp’ – terminal voltage of the primary winding

‘Rp’ – winding resistance of the primary winding

‘Xp’ – winding reactance of primary winding

‘Kt’ – turns ratio

‘Io’ – excitation current

‘Im’ – magnetizing current of Io

‘Iw’ – core loss component of Io

‘Φm’ – magnetic flux

‘Β’- phase angle error

The induced primary voltage EMF is the subtraction of resistance and reactance drops (IpXp, IpRp) from the voltage of primary Vp. The voltage drops due to the reactance and resistance of the primary winding.

The EMF induced in the primary is transformed into secondary by a mutual induction and forms induced EMF in secondary Es. The output voltage across the secondary winding due to the emf drop by the resistance and reactance is Vs. The output voltage on the secondary is obtained by subtraction of reactance and resistance drops (IsXs, IsRs) from the induced EMF in secondary Es.

Let us take the main flux as the reference. The current in primary Ip is obtained from the vector sum of excitation current Io and reverse secondary current Is, which is multiplied by 1/Kt. Vp is the applied primary voltage of the potential transformer.

Ip = (Io + Is)/Kt

Ratio Error

If the normal ratio of potential transformer is differed from the actual ratio of the potential transformer because of resistance and reactance drops, then ratio error occurs.

Voltage Error

If there is a difference between the ideal voltage and actual voltage, then the voltage error occurs. Percentage of voltage error is

[(Vp – Kt Vs)/Vp] x 100

Phase Angle Error

If there is a difference between the phase angle between the primary voltage ‘Vp’ and the reverse secondary voltage, the phase angle error occurs.

Causes of Errors

Due to the internal impedance, the voltage drops in the primary and it is transformed proportional to its turns ratio and secondary winding. Similarly, the same thing happens in the secondary winding.

Reduction of Errors

The errors of potential transformers can be reduced or prevented by improving the accuracy in designing, magnitudes of reactance and resistance of primary and secondary windings, and minimum magnetization of the core.

Applications of Potential Transformers

The applications are

• Used in relay and metering circuits
• Uses in power line carrier communication circuits
• Used in protection systems electrically
• Used for protecting feeders
• Used for the protection of impedance in the generators
• Used in synchronization of generators and feeders.
• Used as protection voltage transformers

FAQs

1). What is the potential transformer?

Potential transformers are also known as voltage step-down transformers or voltage transformers or instrument transformer, in which the voltage of a circuit is reduced to a lower voltage for measurement.

2). What are the types of potential transformer?

Capacitive potential transformers and Electromagnetic potential transformers

3). What are the errors in potential transformers?

Ratio errors, voltage errors, phase angle errors

4). What is the purpose of a potential transformer?

To reduce higher voltage to lower voltage of a Power circuit for measurement.

5). What are the other forms of potential transformers?

Step-down transformer or Instrument transformer

Hence, the working, construction, errors, and applications of potential transformers are discussed above. The purpose of the potential transformer is to convert high voltage to low voltage. Here is a question for you, “what are the advantages and disadvantages of potential transformers?”