Instrument Transformer: Definition, Applications, Types, and Advantages

Instrument Transformer_ Definition, Applications, Types, and Advantages

In a power system network, the voltage level is different for all stages of the network. And as we know, to reduce the loss and make the system cost-effective, we use high voltage and current.

In this condition, it is very difficult to measure the different quantities of the power system like; voltage, current, frequency, power factor, etc. And it is very costly to design the measuring instruments to measure the quantity at this high level.

Hence, to avoid this critical condition, a transformer is used to down the voltage and current level of the power system network. And also, it is used with the protective relays to protect the power system network.

In this article; we will briefly understand the transformer used for the special function of measuring quantities of a power system.

Application of Instrument Transformer in Power System Network

  • Instrument transformers used in a power system network for measuring different quantities like; current, voltage, power, frequency, power factor, etc.
  • It makes a standard ratio for measurement. And due to that, the cost of the instrument reduced. For example, the current transformer rated 5A and the voltage transformer is rated 110V.
  • The instrument transformers also used in SMPS (Switched Mode Power Supplies) and power electronics circuits for measuring purposes.
  • It also used to test surge protection devices for lightning simulation.
  • In the motor controller circuit and variable speed drives, the current transformer is used in capacitor discharge surge testing.
  • The instrument transformer used to measure harmonics in main power cables in electromagnetic compatibility (EMC) testing.

What is Instrument Transformer?

A transformer used to measure the electrical quantities of the AC system known as the instrument transformer.

The instrument transformer reduces the voltage and current level at a predefined rated level. Hence, it will make a standard measuring system for an entire power system network.

A current transformer designed to operate at 5A and the voltage transformer is designed to operate at 110V.

The primary winding of the instrument transformer connected to the high voltage or high current network. The measuring devices connected to the secondary winding.


The instrument transformers are mainly classified into two types;

  • Current transformer (CT)
  • Potential transformer (PT) or Voltage Transformer

Current Transformer

Current transformers known as the short name CT. In a power system network, the current transformer used in the metering element to measure the system current. Therefore, it connected in series with the line or the part of a system.

It designed to have an accurate current ratio and phase relationship. So, the metering element measures accurate reading.

The primary winding of CT connected with the line and the measuring element or meters and protection devices connected with the secondary winding.

CT used to step down the current for safety and reduce the cost of metering and protection equipment.

Current Transformer
Current Transformer CT

Current Ratio

The current transformer described by the current ratio. The standard secondary current of CT is 5A or 1A. But, in most of the cases, 5A CT is used. Hence, it is available with a current ratio like; 100/5, 1000/5, etc.

For example, if you have CT with a current ratio of 100/5. It means, when 100A current will flow through the primary winding, the output will be 5A in the secondary winding.

Potential Transformer

Potential transformers also known as in the short name of PT. It also known as a voltage transformer. Generally, the potential transformers used to measure the voltage of the line or part of the network.

It designed to have an accurate voltage ratio and phase relationship. Hence, the metering element measures an accurate voltage.

There are three types of potential transformers available;

  • Electromagnetic
  • Capacitor
  • Optical
Potential Transformer PT
Potential Transformer PT

Voltage Ratio

Typically, the potential transformer designed with the standard secondary voltage 120V and 70V. But in most of the cases, the secondary voltage will be 120V. Hence, standard ratios like; 600/120V, 480/120V, etc.

If you consider the voltage ratio of 600/120V. It means, if the primary voltage is 600V, the output in secondary winding will be 120V. And standard voltage measuring equipment designed with the 120V.


There are several applications in the power system network of instrument transformer, because of the many advantages. The advantages of the instrument transformers listed below.

  • It will make a standard measuring ratio for voltage and current. It will help to design measuring equipment of the same ratio. Hence, it will reduce the cost of measuring instrument and can be replaced easily.
  • It will provide electrical isolation between the high voltage high current system with the voltage and current sensitive measuring equipment. This will make a safe measuring and protecting operation.
  • Multiple pieces of equipment can be connected with the secondary of the instrument transformer.
  • The power consumption of the measuring instrument will be less due to operation in low voltage and low current.

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