All about Current Transformer(CT) and Potential Transformer(PT)

Current Transformer(CT) and Potential Transformer(PT) are very useful and essential instrument transformers. In this post, we are going to learn What is CT, What is PT, what are the uses of CT and PT, the symbols of CT and PT and also many important things about CT and PT especially why secondary of CT should not be open-circuited.

Why we need the Current Transformer(CT) and Potential Transformer(PT)?

Actually, when we want to measure high voltage and high current there is a problem that we need a voltmeter and ammeter having a very high rating. But very high-rating voltmeters and ammeters are not available or it is very difficult to make the very high-rating ammeter and voltmeter. So, to overcome this problem we must use the Potential transformer(PT) and Current transformer(CT).

On the other hand, Relays which are used for protection purposes also have low current and voltage ratings. So for the operation of relays, we used the Current transformer and the potential transformer.

What is the Current Transformer(CT)?

You already know that CT means current transformer. The current transformer is an instrument transformer that is used to measure high current. It is a Step up transformer. As it is a step-up transformer, it has less secondary current than its primary current.

Some of the current transformers have very few turns in their primary winding and it is connected to the supply and load for measurement of current.

Some of the current transformers have no primary winding only have secondary winding. this type of current transformer uses a conductor whose current is to be measured as the primary winding. See the below figure for a better understanding.

As you see in the above figure when a current flows through the conductor it creates magnetic flux and it cuts the secondary winding an EMF is induced in the secondary winding. When an ammeter is connected to its secondary winding a current flows through the ammeter and it shows the reading. The current ratio depends upon the no of turns ratio. As it decreases current we can use a low rating ammeter for measurement of high current.

What is the Potential Transformer(PT)?

A potential Transformer is a Step-down transformer. It is connected in parallel with the supply whose voltage is to be measured. It has more turns in its primary winding than its secondary winding in a specific ratio. It steps down the voltage from the high voltage to the low voltage. So we can connect a low-rating voltmeter to its secondary and can measure high voltage. The potential transformer is also known as a voltage transformer.

Symbol of Current Transformer(CT) and Potential Transformer(PT):

Why Secondary of Current Transformer(CT) should not be an open circuit?

As the Current Transformer is connected in series with the supply and load so its primary current depends upon the load current. Its primary current does not depend on the secondary current. In a power transformer primary current depends on the secondary current but in the case of CT secondary current depends on the primary current when it is short-circuited.

At normal conditions, the flux of the primary winding which is produced by the primary current is opposed by the flux of the secondary which is produced by the secondary current. So there is a very low emf that will be induced in the secondary winding. But when the secondary of the CT is open-circuited there is no current flowing through the secondary winding and also no flux will be produced. But the primary current which is a very large amount is the same as the previous condition. So a huge amount of flux will be produced as there is no opposition flux and it will cut the secondary winding. As secondary winding has a large number of turns, it will produce a large voltage across the secondary winding which is very dangerous.