What is Instrument Transformer? Where and why is it used? (CT / PT)

What is instrument transformer? Many of us think of transformers as voltage ups and downs by keeping only the power fixed. But, what actually is an instrument transformer?

In fact, the transformer can not only increase or decrease the voltage but also measure the current and voltage. Again, those transformers that can measure voltage-current are what is Instruments Transformer.

Current transformer 

What is Instrument Transformer- A transformer
A transformer

The transformers that are used to measure high current are called current transformers.

Potential transformer 

Transformers that are used to measure high voltage are called potential transformers. 

Now, the question is natural, you have heard all your life that in order to measure voltage-current there is the use of an ammeter and a voltmeter. But, now you are saying that measuring of voltage-current can also take place with a transformer. Let’s try to find out the answer to your question. 

What is Instrument Transformer- ammeter-voltmeter
Ammeter-Voltmeter

Suppose, you are able to lift heavy objects up to a maximum of 10 kg. Now, if I put a weight of 30 kg on you it will be very painful for you and you can fall to the ground. 

Similarly, the voltage and current of the generating/grid/distribution substation are of very high quality. For instance, ordinary ammeters and voltmeters cannot measure it as you could not weigh 30 kg. 

Now, how will the current transformer and the potential transformer measure the current and voltage?

  1. Similar to power transformers, CT and PT have primary and secondary windings. Again, the insulation of the windings is on both sides from each other. 
  2. The current transformer has a single turn in the primary winding and several turns in the secondary winding.  
  3. The secondary coil of the CT has a measuring device that measures the ratio of current. 
  4. Similarly, since the number of turns in the primary coil of a potential transformer (PT) is higher and the number of turns in the secondary coil is less, the induction of voltage in the secondary coil is less. This measurement is very easy to do. 
  5. In order to measure the current through a current transformer, it is important to keep the second part of the current transformer short. Simply, there is no way to keep it open.

Why the secondary part of the current transformer can’t be kept open?

  • Now, from the basic concept of an open circuit, we know that the current flow in an open circuit is close to zero. 
  • Therefore, if the second part of the current transformer is open, the current will approximately zero. But, the whole voltage induced by the flux of the primary winding will be absorbed by the secondary part. Again, there is a possibility of burning the coil. 
  • As a result, the secondary side of the CT has to be kept short.

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Courtesy: Iqbal Mahmood

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