Answer Posted / electrical in detail

(1) Core Balance or Magnetic Balance of a transformer is well needed. This test is carried out to determine inter turn faults or magnetic imbalance in transformer.

(2) It is very well known that each turn of the winding of a transformer is isolated from other turn through appropriate insulation. A properly designed transformer of 11 KV / 415 V means applying 11 KV on one side will produce 415 V on other side and vice versa. Now if insulation between turns of a winding loses their property, then turns of the winding are short circuited and this situation is called inter-turn fault.

(3) When an inter-turn fault occurs, magnetic property of transformer is imbalanced and this imbalance is conveyed to secondary side of the transformer as the two windings are connected magnetically. It means voltage that is produced by mutual induction will also get affected. To identify this imbalance, magnetic or core balance (in fact, imbalance) test is carried out.

(4) This test is generally performed on STAR side of the transformer. Forgetting the international nomenclature, let us name three leads of STAR side as R1, Y1 and B1 while that of other winding as R2, Y2 and B2.
B1 is kept opened. A supply of say 440 V is applied between R1 and B1. Next step is to measure various voltages between different phases:

V (R1-Y1) = V (R1-B1) + V(Y1-B1)
For example,
440 V = 250 V + 190 V

(5) As both the windings are connected magnetically, applying voltage to star winding will induce voltage on other winding. Induced voltage in other winding shall be in same proportion to applied voltage at star side. But if, any inter turn shorting is there, the voltage induced on the secondary winding will not be in proportion to the applied voltage.

This is an indicative test only. Accurate results can be obtained in conjunction with other tests.

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