Potential Transformer – Classification, How it Works, Errors, Applications

Potential Transformer which is a Voltage Transformer steps down the voltages of high transmission lines. This article will discuss what is Potential Transformer (a.k.a. Voltage Transformer), its classification, how it works, errors, applications, advantages and disadvantages.

What is Potential Transformer

Potential Transformer is the Instrument Transformer which is used for measuring and transforming the voltage which is higher in value to lower voltage. As the voltage increases, Potential Transformer comes into play, where in they bring the voltage value to a safe limit. This can be measured with instruments such as Voltmeter or Watt Hour Meters and so on.

Fig. 1 – Introduction to Potential Transformer

Fig.2 below shows the circuit diagram of Potential Transformer. This type of Transformer is always connected in parallel to the transmission line and the Secondary Winding is lower with respect to the Primary Winding. Their performance, equivalent circuit and Phasor diagrams are similar to that of a Power Transformer.

Fig. 2 – Circuit Diagram of Potential Transformer

Classification of Potential Transformers

Potential Transformers can be classified based on two aspects:

• Function of the Transformer
• Construction of the Transformer

Classification Based on Function of Potential Transformer

On the basis of its function, a Potential transformer can be classified in two types. They are:

• Metering Voltage Transformer
• Protection Voltage Transformer

Metering Voltage Transformer

Metering Voltage Transformer is used to measure the voltage using measuring devices and these are the Transformers which operate with most accuracy and are found in Three Phase or Single Phase Transformer.

Protection Voltage Transformer

Protection Voltage Transformer protects the line from over voltage and they also operate with accuracy and are found in Three Phase or Single Phase Transformer.

Fig. 3 – Types of Potential Transformer

Classification Based on Construction of Potential Transformer

On the basis of its construction, a Potential transformer can be classified in two types. They are:

• Electromagnetic Potential Transformer
• Capacitive Potential Transformer

Electromagnetic Potential Transformer

These Transformers have similarities to the Primary Transformers where both the winding, Primary and Secondary are wound to the magnetic core. These Transformers are usually used in relay, meter and high voltage circuits. In this type of Transformer, the Primary Winding is connected to the Phase and Secondary Winding is in contact with the ground.

Capacitive Potential Transformer

These Transformers are also known as Coupling type or Potential Dividers or the Bushing types. The Capacitors are wound to either the Primary or Secondary Winding with the voltage measured across the Secondary Winding. This Transformer is expensive and used for the Power line carrier communication.

How Does Potential Transformer Work

This type of Transformer helps in maintaining the voltage frequency from a high voltage to the required frequency and providing the reading for the measurement required to the instruments. This helps in distributing required amount of voltage across the circuit and maintaining a balance.

The working principle of Transformer is based on electrical energy being transferred efficiently by Magnetic Induction from one circuit to another. When one winding of a Transformer is supplied with alternating current (AC) source, alternating magnetic field is established in the Transformer core.

Fig. 4 – Schematic Representation of Working of Potential Transformer

As the Second Winding is around the same Core, voltage is induced by the magnetic field. This results in the current flow in the circuit which is connected to the terminals of Secondary Winding. Hence, the coils are said to be magnetically coupled but are electrically isolated from each other.

We can calculate Voltage across Primary Winding from Turns Ratio equation given by:

Where;

N_P = Number of Turns in Primary

N_S = Number of Turns in Secondary

V_P = Voltage across the Primary

V_S = Voltage across the Secondary

Errors of Potential Transformer

In this type of Transformer, there is constant drop in the primary and secondary resistance. There is also the factor of power burden on the secondary. This is the reason for the cause of errors in Potential Transformers. The two types of errors are:

• Ratio Error
• Phase Angle Error

Ratio Error

The variation of actual ratio of transformation from the nominal ratio is said to be the ratio error. This is explained with the formula below,

Phase Angle Error

Phase Angle Error occurs when there exists a Phase difference between the Vp and the Vs reversed. It is also known as the phase difference between the reversed Secondary voltage and Primary voltage.

Fig. 5 – Approximate Method of Expressing Phase Angle Error

To improve the efficiency and accuracy, the reduction of these errors has to be designed in such a way that the transformer winding have the appropriate magnitude of reacting capability and resistance. Also, the requirement of the Core should have the minimum magnetizing and component of the core loss of the existing current.

Phase Angle Error is considered positive if the Reversed Current Phasor leads the Primary Current Phasor else it is considered negative. Phase Angle Error is given by the equation:

Applications of Potential Transformers

The applications include:

• They are used in Electrical Metering System.
• They are widely used in Electrical Protection System as circuit breakers.
• They are used for Synchronization of generators with grids.
• They are also used in operating protective relays.

Advantages of Potential Transformers

The advantages are listed below:

• Voltmeter and Ammeter can be connected to measure high voltage and current respectively.
• Irrespective of the measurement of voltage or current, the rating of the low range meter can be fixed with the help of Potential Transformer.
• The working of these transformers is utilized for operating many types of protecting devices such as the pilot lights and relays.
• A single transformer can be utilized to monitor several instruments.

Disadvantages of Potential Transformers

Due to the efficiency of the instrument there are not many drawbacks. The only drawback that it holds is that this instrument is limited to only AC circuits and cannot be applied to the DC circuits.

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