What is an instrument transformer?

We can see different types of instruments in an electrical system, we use electrical equipment to do the measurement, monitoring, indication, protection, etc. So these instruments should not be connected to the system directly because of the high voltage and current. An example for this will be, we won’t be able to use an ammeter, or voltmeter to measure high current or voltage, so in case of this, we can use an instrument transformer for this. The instrument transformer will reduce the voltage and current and thus it will be suitable for metering. So because of this, an instrument transformer is used with certain instruments such as voltmeter, ammeter, energy meter, and wattmeter.

What are the functions of instrument transformers?

  • These transformers can protect the measuring instruments from high voltage circuits
  • By using an instrument transformer it is possible to measure the high voltage and current by using low range instruments such as voltmeter and ammeter
  • The instrument transformer would also function as a circuit-protecting device
  • Basically, it would convert the high current or high voltage which is connected to its primary windings to low current and low voltage
  • It can do insulation for high voltages
  • It can protect the operator and equipment from high voltages

What are the types of instrument transformers?

There are two types of instrument transformer and they are

  • Current transformer
  • Voltage transformer

Current transformer

This type of transformer is used for the measurement of the current, it is used in a power system if the power in a circuit is really high which can’t be directly applied to the instruments. The current transformer would reduce this high current and this current will be proportional to the current in the circuit. So this reduced current will be suitable for the instruments, in other words, it won’t damage the instruments. The current transformer is called a step-up transformer because during its operation it would step up the voltage and reduce the current.

Construction of the current transformer

A current transformer is constructed in a way that it can reduce the current. The primary windings have fewer turns when compared to the secondary and it will be heavy too. The number of turns of windings in the secondary winding is a lot more and the wire size is very less compared to the primary. Due to the greater number of turns in the secondary winding the primary current is very high compared to the secondary winding. We should not open the secondary circuit if there is a current in the primary, in case we do this then the primary current would become exciting current and thus a huge load will be created on the secondary winding and because of this, the secondary terminals of the current transformer should be shorted if they are not meant to be connected.

Current transformer connection

The primary of the current transformer will be connected to the power system and this connection will be in series. The magnitude of the primary winding will be determined by the high impedance of the power system. The secondary winding in this transformer will be connected to the ammeter.

How to select a current transformer?

  • While selecting a current transformer we must consider the load growth and also according to the increase in the capacity of the installation
  • The secondary current selection is done according to the secondary current of the equipment
  • The CT must be selected by considering the primary current, it must be selected according to the rated current of the installation. The CT should be able to carry the maximum current in the service

Applications of current transformer

  • These transformers would be useful for control circuits
  • It would reduce the current hence instrument protection can be done in case of high current
  • It is used to monitor the current level in a power system if the current reaches a specified level then it would take necessary actions.

Voltage transformer

The voltage transformer is the exact opposite of the current transformer it is a step-down transformer, so during its operation, the voltage will be increased and the current will be reduced. In this transformer, the primary winding has a higher turn and the secondary windings have lower turns. The secondary voltage will be proportional to the primary voltage, this transformer would utilize the electromagnetic phenomena for its operations.

This type of transformer can be used to monitor the single-phase and three-phase power line voltages for the power metering applications. The primary winding in this transformer will be connected in star and this point must be earthed and the secondary winding will be connected in an open delta. So this transformer would lower the voltage of the higher voltage circuit to a lower voltage so that measurements can be easily done. The voltage transformers should be connected parallel to the line which is to be monitored.

Selection of voltage transformer

  • It should be selected by considering the insulation level
  • The voltage level in the primary and the secondary must be considered
  • Frequency should be considered
  • Thermal burden

Applications of the voltage transformer

  • It can be used for the protective relaying purpose
  • Electrical metering systems
  • Electrical potential systems
  • Distance protection of the feeders
  • Generator synchronizing with the grid
  • Impedance protection of the generator

What is the difference between the current transformer and the voltage transformer?

  • The current transformer is connected in series with the power circuit while the potential transformer will be connected in parallel
  • The secondary of the current transformer will be connected to an ammeter while the secondary of the potential transformer is connected to a voltmeter
  • The working of the secondary is in short circuit condition in the case of the current transformer, in the case of potential transformer secondary works in the open circuit condition.
  • In the current transformer, the primary current is dependent on the power circuit current, while in the case of the voltage transformer the primary current is dependent on the secondary burden
  • In a current transformer, the primary current would change according to the variations in the power circuit current, while in the case of the voltage transformer these variations are limited
  • The secondary of the voltage transformer can be used in open circuit conditions while in the case of the current transformer the secondary is never open circuited
  • The current transformers’ primary current and excitation would vary over a wide range. In the voltage transformer, the line voltage is mostly constant and because of this the exciting current and flux density would vary over a short-range

What are the factors that could affect the accuracy of the instrument transformer?

  • Design of the instrument transformer
  • Accuracy is also affected by the circuit conditions such as voltage, frequency, and current
  • The burden would also affect the accuracy of the instrument transformer, burden is the total impedance of the loads which is connected to the instrument transformer. The burden which we can see in an instrument transformer will be the interconnecting leads, the secondary winding of the instrument transformer, and the connected devices

What are the advantages of instrument transformers?

  • We can measure the high voltage and current of the power system by low range measurement devices with the help of instrument transformers
  • The instruments which are used for the measurements are standardized and thus there will be a decrease in cost for the measuring instruments
  • The requirement for the insulation of the instrument is reduced
  • We can connect many instruments to the electrical system by using one transformer
  • The power consumption is reduced because of the low voltage and current level in the circuits


post-graduate in Electronics & communication.

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