Short Circuit Test or Impedance Test of Transformer


     The performance or parameters of a transformer can be well determined by testing the transformer. In order to calculate the efficiency, regulation of the transformer various parameters like equivalent resistance and reactance, losses in the transformer at no-load as well as full-load must be calculated. To determine this the transformer testing must be done. They are,
  • Open-circuit test, and
  • Short-circuit test.

     In the last article, we have determined the iron loss ( core losses ) by doing a no-load test or open-circuit test of the transformer as these losses are constant throughout the operation irrespective of the load. Now let us determine the power loss in the winding resistance, i.e., I2 R loss ( copper loss or variable loss ) with the help of a short-circuit test.



Short-circuit Test of Transformer :


The purpose of this test is to determine the following :
  1. The equivalent resistance and reactance referred to the metering side ( used to find the regulation ).
  2. The copper loss at full load ( used to calculate the efficiency ).

     In the short-circuit test of transformer one winding, mostly low voltage L.V. side ( secondary winding ) is short-circuited by a thick conductor, so that full-load current flows in the H.V. side ( primary winding) or we can connect an ammeter in the short-circuit path, which serves to read the load current at the secondary side as shown in the figure.

Short Circuit Test or Impedance Test of Transformer

     The testing of the transformer is performed on the high voltage H.V. side of the transformer. The supply is given to the transformer through a variac ( autotransformer ), voltmeter, wattmeter, and ammeter as shown in the above connection diagram.

     A low voltage is applied by varying the turn-ratio of the autotransformer. Now the applied voltage is gradually increased by reading out through the voltmeter such that the ammeter reads out the full-load current in the H.V. side.

     The amount of the applied voltage required to circulate full-load current is very small ( mostly up to 5-10% of primary voltage ) such that the amount of flux produced in the core will be also very small. By this, we conclude that the wattmeter reads only full-load copper loss as the flux produced is very small then the iron loss in the core material can be neglected.



Calculation of Short-circuit Test :


     Let Ws, Is, and Vs are the short-circuit test wattmeter, ammeter, and voltmeter readings respectively. Then the full-load copper loss ( I2 R ) which is equal to wattmeter reading would become,

Short Circuit Test or Impedance Test of Transformer

From the wattmeter reading Ws, the equivalent resistance of the transformer referred to the primary side is given as,

Short Circuit Test or Impedance Test of Transformer

The equivalent impedance of the transformer referred to the primary side is,

Short Circuit Test or Impedance Test of Transformer

Therefore, from the equation of resistance and impedance, equivalent reactance referred to the primary side is expressed as,

Short Circuit Test or Impedance Test of Transformer

     These resistance, reactance, and impedance values are referred to the high voltage ( H.V. ) side of the transformer as the test is conducted on the H.V. side of the transformer. By knowing the transformation ratio K of the transformer these parameters can also be determined at the L.V. side.


Calculation of Efficiency :

     Thus, by knowing the iron loss from the open-circuit test and copper-loss from the short-circuit. The losses of the transformer are calculated and the efficiency of the transformer at any output can be determined.

Short Circuit Test or Impedance Test of Transformer


Calculation of Regulation :

     The short-circuit test gives the circuit parameters ( resistance, reactance, and impedance ) of the transformer. From this percentage regulation of the transformer is determined at full-load or at any power factor as,

Short Circuit Test or Impedance Test of Transformer


Why Short Circuit Test is Performed on HV Side of the Transformer ?


     In the short-circuit test all the measuring instruments are also placed on the H.V. side and the test is conducted on the H.V. side only and the L.V. side is shorted.

     The main reason for conducting the short-circuit ( SC ) test on the H.V. side is that the full-load current of the transformer will be low on the H.V. side when compared to the L.V. side. As we know that, the SC test is performed at full-load ( rated ) current, so it is convenient when we take the H.V. side for the SC test ( as the rated current is less ), and also highly rated measuring instruments are not needed.

     As the rated current flows in the H.V. winding, the voltage needed to produce the full-load current on the H.V. side will be less, so we neglect the ( core or iron loss ) and copper loss which are noted have occurred at the full-load current of the transformer will be found by performing SC test on H.V. side.

Also, check out the calculation of iron-losses from the open-circuit test and why the open circuit test is performed on the L.V. side here -


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