Zero Sequence Impedance Test in Transformer

Dear all,

1) 2)

Z0 (zero sequence impedance) Z1(positive sequence impedance) and Z2 (negative sequence impedance) are necessary to calculate unbalanced fault currents such as a line to earth fault.

Z0 is an impedance measured with a single phase voltage.

Z1 is an impedance measured with a 3 phase voltage source. Z1 is normally refereed as an impedance of a transformer.

Z2 is an impedance measured with a 3phase but opposite phase rotation such as (a-c-b not normal a-b-c).

In 3 phase transformer Z2 is same as Z1, so there is no need to test Z2 while Z0 is around 90% of Z1.

3)

Test voltage of rated frequency will be applied between the neutral of star and shorted 3 line terminals of a winding. All terminals including neutral of other windings will be shorted.

Adjust the test voltage so that the line current is the rated current of the transformer.

Z0 (Pu) = test voltage applied / rated line to neutral voltage.

or Z0 (ohm) = test voltage applied / measured line current.

= 3 x test voltage applied / measured neutral current.

Regards

Dear rakhesh and akhito

Thank u very much sir.

Now I am clear.

But in some sites,this test is not recomended.whether this test can cause any harm to transformer.

thanks and regards

gova

Hi Gova,

When we work with short-circuits problems can definately occur.

HOWEVER - this is a test in which the voltage applied at the primary must only be increased up to the stage where the secondary delivers full load. If this is done by a competent electrician, there will not be any problems.

Also make sure that the device applying the test voltage is able to deliver the necessary primary current for the test and that the cables on the short can handle the load. The transformer itself is designed for this load.