Instrument Transformer & its Types

What are Instrument Transformers | Its Types & Advantages

A transformer is an Electrical device used for the transfer of electrical energy between two circuits with increasing or decreasing the voltage & current levels.

 

Instrument Transformer:

It is a type of transformer that is used for its high accuracy and isolation to decrease any voltage or current levels for measurement purposes.

The problem with high voltage measurement is that they are quite hazardous and require more complex and expensive measurement equipment. So the cost-efficient way is to introduce an instrument transformer to decrease the voltages or current before connecting any measurement device.

 There are two types of instrument transformer.

• Current Transformer:

Such type of instrument transformer is used in the measurement of high power current.

In order to measure the line current in any power system, we cannot connect an ammeter directly. Direct measurement of current in such a system is very dangerous because the current is very large.

So CT (current transformer, an instrument transformer) is used to decrease the current. The decrement in the current depends on the turn ratio. A typical ammeter can easily measure this current by connecting it with the CT’s terminals.

The current transformer has only one winding which acts as the secondary winding. The primary winding is a conductor through which the current needs to be measured.

As the number of turns for primary winding (which is a conductor) remains one, the turn ratio of this transformer solely depends on the number of turns N_s in the CT’s winding.

Turn ratio = k = N_s / N_p

K = N_s     since N_p = 1

So the output current of a CT becomes;

I_out = I_in / k = I_in / N_s

Increasing the number of turns in CT decreases the output current but it substantially increases the secondary voltages. That is why its secondary should not be open while in operation. 

Generally, A CT transformer shows the current ratio e.g 200/5A. This implies that a current of 200 A in the conductor will cause 5A to flow through the CT. so the turn ratio becomes;

I_out = I_in / k

k = I_in / I_out

k = 200/5 = 40

• Potential Transformer:

This type of instrument transformer is used in the measurement of high voltages. It is a simple voltage step-down transformer that decreases high voltages but it has very low power ratings.

The output voltage typically resides below 120 VAC, which can be safely applied to any voltmeter.

The voltage decrement depends on the turn ratio of the potential transformer, which is usually described on it.

Advantages of Instrument Transformer:

• The main advantage of these transformers is to measure high voltage & current using low range instruments.
• Reduces the capital required for designing high rating measurement instruments.
• Decrease the hazard of getting electrical shock due to isolation & safe voltage & current ratings.

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• Difference Between Power Transformer and Distribution Transformer
• Transformer & its Operation, Characteristics & Applications
• Types of Electrical Machines