Difference between Instrument Transformer and Power Transformer

A transformer is an electrical machine which changes the level of voltage or current to a desired value. A transformer consists of two windings namely, primary winding and secondary winding. These windings are wound on a core and are insulated from the core as well as from each other.

When an alternating power supply is connected to the primary winding, it sets up a time varying magnetic field in the core. This changing magnetic field links with the secondary winding to produce an EMF in it which creates a potential difference across the load terminals.

Depending on the application, there are many types of transformers available such as −

  • Power transformer

  • Distribution transformer

  • Instrument Transformer

In this article, we will discuss the major differences between power transformer and instrument transformer.

What is a Power Transformer?

A power transformer is a transformer with very high kVA rating and mainly installed at generating stations and transmission substations for increasing or decreasing the system voltage.

A power transformer may be a step-up transformer or a step-down transformer. If it is a step-up transformer, it can increase the level of transmission voltage. On the other hand, if it is a step-down transformer, it can reduce the level of transmission voltage.

Power transformers are mainly used in power transmission system, therefore they generally have delta-connected primary and secondary windings.

What is an Instrument Transformer?

An instrument transformer is the type of transformer which is specially designed for protection and measurement applications. Since in an electrical power system, the power flows in the transmission and distribution lines at very voltage and current. Thus, the voltage and current of a transmission and distribution lines cannot be measured directly by using conventional measuring instruments like ammeter, voltmeter, etc.

Therefore, we need to reduce the high voltage and current before suppling it to the measuring instrument so that it does not damage the instrument. For this purpose, there are two instrument transformers namely current transformer and potential transformer.

A current transformer is a transformer which reduces the current of a transmission line to a safer and measurable value. It is a step-up transformer because it increases the voltage in the same ratio in which it reduces the current.

A potential transformer is also a type of instrument transformer that reduces the line voltage to a safer and measurable value. It is a step-down transformer because it decreases the level of line voltage to a desired value.

Difference between Power Transformer and Instrument Transformer

The following table highlights the key differences between a power transformer and an instrument transformer −

Basis of Difference Power Transformer Instrument Transformer
Description A power transformer is the one which has high kVA rating and used for changing the voltage level for power transmission. An instruments transformer is the one that is designed to reduce the level of either voltage or current of a transmission line for measurement and protection purposes.
Purpose Power transformers are primarily used for changing the line voltage levels in a power system. Instrument transformers are primarily used to extend the ranges of measuring devices so that they can measure high voltage, current, power, etc.
Power handling capacity Power transformers can handle large amount of power. Instrument transformers can handle very small power.
Function Power transformers increase or reduce the line voltage of transmission lines. Instrument transformers reduce either voltage or current to a safer and measurable value to supply it to devices like ammeter, voltmeter, protective relays, etc.
Heating As the power transformer handle a huge amount of power, hence the heat produced is more. In instrument transformers, the heat produced is quite less.
Need of cooling The heat produced in the power transformers is more. Therefore, they require extra cooling arrangements. The heating in the instrument transformers is not severe. Thus, natural cooling is sufficient.
Conservator tank The conservator tank is a crucial part of a power transformer. An Instrument transformer does not have the conservator tank.
Load limitation factor In case of the power transformers, the temperature rise is the main limitation factor on the load. In instrument transformers, the accuracy is the primary limitation factor on the load.
Design consideration For power transformers, the cost, efficiency and voltage regulation are the main design considerations. The accuracy and cost are main design considerations for the instrument transformers.
Cost The cost of power transformers is very high. The instrument transformers are quite cheaper than power transformers.
Examples Examples of power transformers are: generating station transformers, receiving station transformers, substation transformers, etc. Current transformer and potential transformer are the two major examples of instrument transformers.


In the above table, we have highlighted all the major differences between a power transformer and an instrument transformer. Both types of the transformers are necessarily used in every power system installation.