Home Education Everything about Instrument Transformer and it’s specifications

Everything about Instrument Transformer and it’s specifications


Instrument transformers are combination of Current Transformer CT and Potential Transformer PT. In this we will learn about their specifications like Burden, ratio and phase error, etc

Instrument transformers are such type of transformers which are used to measure the quantities like current, voltages, power factor, frequency, etc

They are combination of Current Transformer and Potential Transformer. They are basically a transformer with high accuracy class used for metering and protection purposes.

What’s it in for me

  1. Instrument Transformer
  2. Current Transformer
  3. Potential Transformer
  4. Accuracy Test on CT
  5. Accuracy Test on PT
  6. Conclusion
  7. FAQ

let’s start…

1. Instrument Transformer

The instrument transformers are high accuracy class electrical devices. The instrument transformers are mainly used for metering or protection purposes.

Instrument transformers are such type of transformers which are used to measure the quantities, like current and voltages etc.

1. Current transformer CT (to measure the current) are always installed in series

2. potential transformer PT (to measure voltages) are always installed in parallel.

Types of transformer

The transformer are mainly divided into three types:

  1. power transformer 
  2. Distribution transformer
  3. Instrument transformer

Types of instrument transformer

There are two types of instrument transformer: 1. voltage transformer also known as potential transformer 2. current transformer.

Life span of Instrument transformer

When an instrument transformer is operated under ANSI / IEEE basic loading conditions (ANSI C57.96), its normal life expectancy is about 20 years

Uses of instrument transformer

Instrument Transformers are used in AC systems for measurement of electrical quantities i.e. voltage, current, power, energy, power factor, frequency.

Instrument transformers are also used with protective relays for protection of power systems.

The commercial ammeter or voltmeter would not be able to measure high value of current or voltages.

Hence they must be stepped down to a low values proportional to the actual values of current and voltages.

The transformer which steps down current or steps up the voltage is called the current Transformer. 

And the transformer which steps down the actual high voltage in primary to a proportional low voltage is called a voltage or Potential Transformer.

2. Current Transformer

What is the current transformer?


image source : Wikipedia and Wikimedia

The current transformer is designed to connect in series with the line to transform the line current to the standard 5 amperes suitable for the meter or relay.

The voltage transformer is designed to connect in parallel with the line to transform the line voltage to 115 or 120 volts suitable for the meter or relay.

To keep the voltage at the meters and relays at a safe value, the secondary circuit must be grounded.

A current transformer is basically a device which can step up or step down current means it can increase or decrease the output current in accordance to as per change in the input current.

Outdoor type oil cooled Current transformer upto 66kV

Uses of Current Transformer

In very high power lines high current are present and our ammeter and protecting relay circuit cannot work with such high current values.

So in order to measure high current and to use our protective relays they are required.

  1. To measure high current.
  2. For supplying current to the relays.
  3. To provide isolation between high and low current circuit.

Types of Current Transformer

Generally designs three types of CT they are:

  1. Wound type
  2. Window type
  3. Bar type

1. Wound type or toroidal type

Both windings are present  and the primary winding is connected in series with the line carrying the actual current.

They have better accuracy and Better Burden capabilities. They are specially designed for low current ratio requirements.

2. Window type

No primary winding is present instead the wire which is carrying the actual current acts as Primary winding. The core is made up of magnetic grade steel concentrically wound upon itself.

3. Bar type

Bar-type Current Transformer – This type of current transformer uses the actual cable or bus-bar of the main circuit as the primary winding, which is equivalent to a single turn. They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device.

The Specification of CT

1. Burden

Permissible load specified on the secondary side of CT at which the value of errors do not exceed specified value. Burden on a CT means anything on secondary I.e. connecting wire or relay/Ammeter.

The typical burden ratings used in the market are generally 1.5 VA, 3 VA, 5 VA, 10 VA, 15 VA, 20 VA. 

2. Ratio Error 

The turns ratio is given by:

For ideal CT current ratio = turns ratio, but in practical it is not so due to various factors P.F, magnetization and core loss, etc 

T.R ratios does not remain constant and %Ratio error =  ((Nominal ratio – Actual ratio) / Actual ratio) * 100

3. Phase angle error

For accurate measurements in CT it is must that secondary current should be exactly displaced with an angle of 180 degree from angle of primary current but again due to the factor like P.F, the angle displacement of both current is not same. So error occurred in measurement which is called as phase angle error in CT.

4. Rated primary current 

 The current that is normally going to flow through CT

5. SCR or Short current rating 

For eg :- 40 kA/3 sec means, The following CT can withstand 40 KA short   circuit current for 3 sec.

6. ISF for metering core

If ISF<=5 for 10 sec means that the CT will be safe if it has 5 times more current than the rated primary current for 10 sec.

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3. Potential Transformer

PT is defined as an instrument transformer used for the transformation of voltage from a higher value to the lower value.

The transformer step down the voltage to a safe limit value by which can be easily measured by the order low voltage instrument like voltmeter, wattmeter and watt hour meter, etc

The PT has Two windings one is High voltage HV winding and other is Low voltage LV winding. For reducing the leakage reactance the CO – axial winding are preferred.

The PT have following ratings 11kV/110V, 22kV/110V, 33kv/110V, etc

The primary winding is done with the wire having a thinner cross section than LV winding.

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Uses of PT

  1. Used for metering purpose.
  2. For protection of feeders.
  3. For protecting the impedance of the generator.
  4. Used for synchronising the generator and feeder.

The following figure represent the construction of Potential Transformer.

Specification of PT 

1. Ratio error

The primary and secondary voltages are not exactly proportional to each  other due to voltage drop, this results in volage ratio error.

2. Phase error 

Ideally the phase shift should be 180 degree between primary and secondary voltages, but it is not so in reality due to some voltage drop, thus results in phase angle error it is usually small hence measured in minutes.

3. Burden on PT 

The total external Volt-Amp load. The PT should not cross rated burden because it will ruin it’s accuracy. The load should be less than the rated burden.

You can read more on potential transformer burdens and errors on Wikipedia

4. Accuracy or  RE – PE test of Current Transformer

Let’s see the example of an accuracy test carried out for CT of ratio 100-200-400/5-5-5 A with 4 primary turns.


1) Firstly determine the class, burden and rating of a CT

2) First set the connection of the CT accuracy test panel according to the ratio or ratings of CT.

3) Connect the primary winding terminals P1 and P2 of CT with the supply channel of 100 AMP current

4) Connect secondary terminal S1 and S2 with the secondary terminals.

5) Connect the secondary load of appropriate KVA, here 15 KVA 

6) Move the variac and check the percentage of errors of the test CT with respect to the standard CT in Ratio and phase at full load current.

7) If it is of class 0.5 then it must have an Accuracy limit of 0.5% at 100% of current i.e full load current.

8) Read the accuracy data in digital bridge.

The connection diagram for a RE-PE test is as follows

5. Accuracy Test or RE – PE test of Potential transformer


1) First determine the composite error, Accuracy limit factor such as 5P10, 5P15, 10P15, etc.

2) Also determine its rating such as 11kV/110V, 22kV/110V, 33kV/110V, etc

3) Make the connection of Standard PT and Test PT as shown in figure with the panel.

4) Set the appropriate VA or Burden of each phase.

5) Connect the P1 of primary winding of each PT with the desired primary voltage.

6) If the reading shown by the bridge meter is same as the rated one, then the PT has passed the test and is ready to use.

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Hence from this detailed study of the instrument transformer, Current Transformer (CT) and Potential Transformer (PT), we can realize the importance of these transformers, without the measuring and protection, the power System could not work.

And the society will not be able to get the power supply into their houses or for industrial uses.

These transformers play an important role by being as a part of the Generation, Transmission and distribution process.

Their construction requires a lot of consideration, every parameter should meet the requirements even the smallest errors can lead to the failure of these transformers leading to failure of power supply.

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Frequently asked questions

Why is the secondary side of the CT kept short?

There are mainly two reasons 

1. Core Saturation
If the CT secondary is kept open, then no current will flow in secondary windings. As a result, there will be no secondary ampere turns or zero MMF (magnetomotive force) in the secondary side to counter the primary heavy ampere turns or primary MMF and  this results in core saturation.

2. Shock hazard or insulation failure
There are always more turns in secondary windings as compared to primary in Current Transformer. Therefore, with secondary open a large Voltage will appear across secondary, leading to insulation failure or to a shock hazard

What is the difference between Measuring CT and Protection CT?

Protection CT
This CT is used to protect the system from the faults. Hence this CT’s must not saturate due to the fault current.
This CT produces the output according to fault current which could be many times higher then the full load current.
Hence their accuracy  does not matter much, so its accuracy is low and they are designed much below the saturation point. 

Measuring CT
This is used to produce most accurate output according to the input current. So, it is designed near saturation point.
They will measure up to the full load or rated current only. After that the CT will be Saturated.

What are the types of CT based on construction?

Basically there are three types of CT based on the construction they are Bar type, Window type and Wound type



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