Measurement of power using instrument transformer


To measure the power using current transformer and potential transformer.


Electrical power transformer is a static device which transforms electrical energy from one circuit to another without any direct electrical connection and with the help of mutual induction between two windings.

A transformer is a device which consists of two winding called primary and transfers energy from one side to another with suitable changes in in level of current or voltage.

if there is one winding which is supplied by an alternating electrical source, the alternating current through the winding produces a continually changes flux or alternating flux that surrounds the winding. If any other winding is brought nearer to the previous one, some portion of this flux links with the second. As this flux linkage in the second winding or coil. According to faraday laws of electromagnetic induction, and emf is induced in the second. If the circuit of the later winding is closed, there is a current flowing through it. This is the simplest from of electrical power transform which is the basic working principal of transformer.

Current transform:

a current transformer(ct) is used for measurement of alternating electric current. Current transformer, together with voltage (or potential) transformers (vt or pt), are known as instrument transformers, when current is too high to apply directly to measuring instrument of alternating electric current in the circuit, which can be conveniently connected to measuring and recording instrument. A current transformer isolates the measuring instrument from what may be very high voltage in the monitored circuit current transformer isolates the measuring instrument from what may be very high voltage in the monitored circuit. Current trans former are commonly used in metering and relays.

a current transformer basically has a primary coil of one or more turns of heavy cross-sectional area. This is connected to the coil of one or more turns of heavy cross-sectional area. This is connected to the coil of normal range ammeter. The circuit diagram of current transformer is shown in fig.

Circuit diagram:

  • Phasor diagram of current transformer:

This figure shows the phasor diagram of current transformer. I0 is the exciting current im is its magnetizing component is is its loss component

Potential transformer or voltage transformer has application in electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings meters and relays. Commercially available relays and meters used for protection and metering, are designed for low voltage.

A voltage transformer theory or potential transformer theory is similar to step down transformer. Primary of this transformer is connected across the phase and ground. Just like the transformer used for stepping down purpose, potential transformer i.e. pt has lower turns winding at its secondary. The system is applied across the terminals of primary winding of the transformer , and then proportionate secondary voltage appears across the secondary terminals of the p.t.

The circuit diagram of pt transformer  is shown in fig . The primary winding of the transformer is connected across the lines.the voltage to be measured and the voltmeter etc are connected across the secondary. Since secondary winding are required to operate instruments or relays, their ratings are usually 40 to 1000w. The normal secondary voltage ratings is 110v.

  • Circuit diagram of potential transformer :

Phasor diagram:

The phasor diagram of a potential transformer is shown in the figure

In contrast to the transformer it is here that we are interested in the ratio of primary of the secondary terminal voltage. Vectors AB,BC,CD,DE represent small voltage drops  due to resistance and resistance of the transformer winding. Vector AB represents total resistive drops as referred to primary and vector BC represents total resistive drops s referred to primary.

Errors introduced by transformers:

When current is measured by current transformers, the transformation ratio of transformer should be fixed and known accurately so that no error gets introduced. But it is no so as it depends up on the magnetizing  and energy components, the secondary winding load current and its power factor.


  1. Set the primary voltage by variable voltage source and with the help of variable transformer.
  2. Then set the primary current of current transformer.
  3. Calculate the power factor manually and observe the power factor from digital panel meter.
  4. Calculate the secondary voltage by taking the turns ratio as 2:1 of potential transformer for all the readings.
  5. Calculate the secondary current by taking the turns ratio as50:5 or 10:1 for all the readings.
  6. Power is calculated by using p=VIcosɸ where v= voltage and I =current and cosɸ=power factor.
  7. Calculate error.



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