YNyn zero sequence of neutral earthing

[UHEngineer]

Hey all,

currently working on a 145/40 transformer that is earthed (neutral) directly to the ground on the upside and through a 500 Ohm resistor on the downside (i.e. the neutral).
My questions:
- how to model the zero-sequence impedance correctly on the downside, with regards to the 500 Ohm resistor? Model will be used for protection calculations later on.
- the most recent test on the transformer shows that the zero-sequence impedance on the downside is measured to be roughly 30 Ohm's - does this take into account the grounding resistor of 500 Ohm's?

I appreciate any hints or links that lead me to more knowledge. Google doesn't find anything useful on this particular configuration.
UHengineer

 

[ScottyUK]

All you ever wanted to know: "The whys of the wyes" by GE.

 

[UHEngineer]

Thanks for the link ScottyUK.

But I am looking more for a description of how this can be applied to fault calculations specifically.
Consider a phase-to-earth fault on the secondary side - how would you model the zero-sequence impedance for that case (with this particular transformer)?
Would the zero-sequence impedance of the network be transformed to the secondary side, or will it be "eliminated" due to directly-earthed upside?

Big thanks for the help!

 

[ukmike8]

What are the voltage levels ?

 

[dpc]

You should be able to find zero sequence network diagrams for typical transformer connections by doing a Google search. For this particular transformer, the zero sequence impedance seen from the secondary side is the sum of the primary and secondary impedances plus three times the secondary grounding resistor.

With any transformer, if current (other than excitation current) is flowing in one winding, there must be an equivalent current flowing in the other winding. With the wye-wye configuration, the zero sequence current must flow through the earth on both sides, so the two leakage reactances are essentially in series, similar to normal positive sequence current. The grounding resistor's impedance will dominate but you still need to include the transformer impedance.