Star - Star Transformer

[Cerkit]

Hi,

If I have a star - star transformer with the secondary solidly earthed can I work out the zero sequence impedance of the transformer using the positive sequence value?

Thanks

 

[jghrist]

Yes, as long as the primary is also solidly earthed, Z0 = Z1.

 

[Cerkit]

If the primary is not earthed the can it only be known from factory testing?

 

[jghrist]

If the primary is ungrounded, things get very complicated and Z0 depends on the magnetizing impedance and on the construction (core or shell) of the transformer.

 

[RRaghunath]

If primary neutral is not earthed, there is no way zero sequence currents can circulate in the primary windings and hence, there will be negligible zero sequence currents on secondary as well.
This is unlike Delta on primary, in which case the zero sequence currents will circulate within Delta connected windings.