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Instrument Transformer | Current Transformer | Potential Transformer

What is the Instrument Transformer?

The transformer is a static device that is used to either boost voltage level or bucks the voltage level of the circuit without a change in frequency, the terminal voltage of the transformer depends on the number of the turns of the transformer coil. 

this principle is used to measure the high voltage and which transformer is used to measure high voltage and high current are called Instrument Transformer.there are two types of instrument transformer


  1. (a)Current transformer a
  2.  (b) Potential Transformer

Current Transformer



A current transformer is used to measure current high tension transmission line.it is a step-up transformer.it has a primary winding of few turns or even a single turn to carry the current to be measure and is connected in series with the main circuit.

the secondary winding consists of more turns of thinner wire connected across the ammeter terminal. the circuit diagram given below shows the connection of the current transformer.


Principle of working of Current Transformer

When the primary of the current transformer is connected in series with the high voltage transmission line then a very high current start flowing through the primary of current transformer and this high current is changes in a low current by means of transformer action in the secondary of the current transformer which is connected across the terminal of an ammeter and this measured current is then multiplied by the transformation ratio.

The ratio of Current Transformer

There are three transformer ratio of the current transformer which are defined as follows

Transformation Ratio (R)= Primary Winding current/Secondary Winding Current

Nominal Ratio (K)=Rated Primary Current /Rated Secondary Current

Turn Ratio (P)= Number of turns in secondary/Number of turns in the primary

Potential Transformer



The potential transformer is used for measuring the voltage of high tension and extra high tension lines. It is a step-down transformer.

it has a primary winding of a large number of turns of fine wire connected to the line whose voltage is to be measured. The secondary winding consists of few turns of thick wire and is connected across the low range of the voltmeter. 

For safety purposes secondary winding of the transformer is completely insulated from the primary and is additionally grounded for providing protection to the technician. The circuit diagram given below describes the connection of a potential transformer.

Working Principle of Potential Transformer

The working principle of a potential transformer is the same as that of the working principle of the current transformer but here it is used to measure high voltage instead of high current.

When the potential transformer is connected across the high voltage transmission line this voltage is converted into a low voltage by means of transformer action across the secondary terminal and this secondary terminal voltage is measured by using a low range transformer.

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