Star Star Tx - Zero Seq Impedance

[jbond]

I was just modelling a Wye-Wye Tx on SKM PowerTools and noticed that it defaults the zero sequence impedance to be 5 times that of the positive sequence impedance for a standard core type transformer.

Is there any technical basis for this?


Also, the Tx in reality has a delta tertiary winding for auxiliary power, does that have implications on the other wye windings in terms of the zero sequence impedances?

 

[Marmite]

Yes there is. The zero sequence impedance will be very high if no delta winding is present. The actual value depends on whether there is a low reluctance return path for third harmonic flux. For three limb designs without a delta the return path for the flux is through the air, the tank and the core support framework. If a delta winding exists the third harmonic flux creates a circulating current in the delta, and the zero sequence impedance is determined by the leakage path between the star and delta windings. The presence of the delta tertiary auxiliary winding reduces the zero sequence impedance of the main windings.
Regards
Marmite

 

[saladhawks]

You definitely need to use the 3-winding transformer component if a delta tertiary exists due to the change in zero sequence impedance noted by Marmite above.

In addition, SKM has a pretty useful FAQ named "How to fill-in the data required for 3-Winding Transformer." Give SKM tech support and call and have them e-mail it to you.