Zigzag transformer modeling

[cfloriano]

Hi,

In the company I work for, zigzag transformers were used only as grounding transformes.
In a new substation we will install a two winding 69/0.38 kV zigzag - earthed wye transformer. The wye winding will be used as a source to the substation auxiliaries.
I inserted this transformer in the database of the short circuit program we use in my company and its zero sequence impedance between the 69 kV and 0.38 kV buses was modeled as an open circuit by the program.
Can anyone explain why this kind of transformer has a low zero sequence impedance from 69 kV bus to ground but is an open circuit between 69 kV and 0.38 kV buses.

Thanks in advance

 

[scrammblur]

Typically the classic short circuit model doesn't include the copper losses. This is usually ok because the parallel line and fault impedance are significantly smaller than than the transformer model. Basically the positive and negative sequence models assumes infinite impedance on the sides of the pi model (node a to ground and node b to ground) so it just looks like just one simple impedance from node a to b. Without the model of the copper losses there is no positive / negative sequence connection between the phases. But the the zig zag configuration will create a zero sequence connection of the bus to ground. Look at any wye-wye transformer on your system i would bet those transformers don't have any phase to phase positive and negative sequence impeadance.