With reference to the Alstom Protective relays guide. application
With Star/Star transformers Zo = Infinite if any of the neutral points are unearthed.
If both are earthed then you can take Z0 = Z1 as in Dyn transformers.
HTH Euclid
Hi All
>
>Main Tx: 88/3.3kV Yd1 2MVA 5.71%
>
>Earthing Tx: Zy 3.3/0.4kV 100kVA 6.55%
> Zo = 6.58 Ohm
>
>The earthing transformer secondary is used to provide auxillary power. I
>understand the 6.58 Ohm to refer to the earthing impedance for the 3.3kV
>system and the 6.55% to refer to the positive sequence impedance of the >transformer
>
>We all know that the zero sequence impedance of a Dy transformer can be
>considered equal to the positive sequence impedance. What is the zero
>sequence impedance of this vector group? Am I correct in assuming that the
>2 nameplate values refer to separate issues, namely the earthing impedance
>for the 3.3kV system and the positivesequence impedance of the 100kVA
>transformer by the normal method?
>
>I have no problem with the 3.3kV earth fault level but I'm trying to
>calculate the earth fault level on the 400V side?
>
>Thanks
>
>Andrew
>