Potential Transformer: How it Works & Its Importance

Since the origin of the cardinal constant-potential transformer in 1885, transformers have become indispensable for the transmission, dispensation and utilization of electric power. Among many types of transformers, potential transformers are high-class accuracy electric devices used for the isolation or transformation of voltage. If you want to learn more about the working principle of potential transformers, this article will help you.

What is a Potential Transformer?

The potential transformer is a device that reduces high voltages to a much lower and usable voltage level where it can be used to supply electrical equipment or measurement devices. It is also called a voltage transformer.

A potential transformer has three key functions:

It adapts the value of the voltage on the primary to the characteristics of metering or protection devices by supplying a secondary voltage that is proportional and lower.
It isolates power circuits from the metering and/or protection circuit.
It senses abnormalities in voltage and gives voltage signals to protective relays to isolate the defective system.

How Does a Potential Transformer Work?

The potential transformer is capable of decreasing the voltage levels of its supply without modifying its frequency. The voltage being measured connect the primary winding which has a large number of turns and is connected across the circuit. The secondary winding which has a much smaller number of turns is coupled magnetically through the magnetic circuit to the primary winding.

The potential transformer reduces the voltage to a reasonable operating value. Primary winding consists of large numbers of turns while secondary has less number of turns. The primary is connected across the high voltage line while the secondary is connected to the low-range measurement device coil. The potential transformer is always a step-down transformer.

Potential transformer ratio

The difference in voltage between the primary and the secondary windings is achieved by changing the number of turns in the primary winding compared to the number of turns on the secondary winding. As the transformer is a linear device, a ratio now exists between the number of turns of the primary coil divided by the number of turns of the secondary coil. This ratio is called the ratio of transformation, more commonly known as a transformer “turns ratio”. This turns ratio value dictates the operation of the transformer and the corresponding voltage available on the secondary winding.

It is necessary to know the ratio of the number of turns of wire on the primary winding compared to the secondary winding. The turns ratio, which has no units, compares the two windings in order and is written with a colon, such as 3:1 (3-to-1). This means that if there are 3 volts on the primary winding there will be 1 volt on the secondary winding.

The formula of the potential transformer turns ratio:

a – Turns ratio

V1 – Primary Voltage

V2 – Secondary Voltage

N1 – Number of Primary Windings

N2 – Number of Secondary Windings

Operating characteristics

The operation of a potential transformer is more simple than that of a current transformer because the secondary voltage is virtually independent of the load, due to it being connected through a high impedance (virtually used in an open circuit). Therefore, the secondary must not be short-circuited. Under these conditions, an excessively high current will damage the transformer.

Composition

These comprise a primary winding, a magnetic core and one or several secondary windings with everything encapsulated in an insulating resin.

Types

Potential transformers have two types according to how they are connected:

phase/phase: Primary connected between two phases.
phase/earth: Primary connected between the phase and the earth.

Most potential transformers are designed for 120 VAC operation at the secondary terminals when the high voltage is applied at the primary winding. Because the load supplied by the potential transformer is small, VA ratings are small, in the order of 50 to 200 VA. PTs are typically rated for 120 to 460 VAC and 50 VA. If PTs are to make measurements on three-phase systems, sets of two or three transformers are installed. There are, however, higher-rated, oil-filled potential transformers, rated up to 500 kV, for use outdoors in substation switching yards.

Why potential transformer is connected in parallel?

The potential transformer should be connected in parallel like a voltmeter. This is necessary because objects in parallel experience the same potential difference.

Why secondary of the potential transformer is grounded?

The secondary of the potential transformer is grounded to prevent secondary circuits from reaching dangerous potential. Grounding shall be made at only one point on a voltage transformer secondary circuit or galvanically interconnected circuits.