Zig-Zag Transformer Zero-Sequence Impedance

[Kencan]

I have never been exposed to grounding transformers until now. Application is to ground an inverter based generator without a neutral so I can connect it to a grounded system. The generator output is only 50kVA at 208V, 3-phase. I am wondering how to calculate:

[ul]
[li]Continuous neutral Current[/li]
[li]Short-time neutral Current[/li]
[li]and the Zero-Sequence Impedance.[/li]
[/ul]

I have done some digging around and found a paper that basically says the Zero Sequence impedance can be estimated by Zo=V(l-l)*sqrt(3)/I(neutral current) which brings me back to, what is the neutral current?

 

[7anoter4]

The zig-zag transformer is required in order to create a neutral for system without neutral point. In a case of a phase-to-ground fault the current is the sum of capacitive currents between phase-to-phase [since the ground will get the fault phase potential].In this case the capacitive reactance phase-to-phase it is the same as it was phase-to-ground reactance. See the attached sketch.

 

[RRaghunath]

I would use an isolation transformer with Dyn vector group to connect inverter based generation to the grounded system. Neutral on star side can be solidly grounded to provide ground fault currents.
This won't be expensive and makes better sense considering the generation is only 50kVA.

 

[xnuke]

I second the isolation transformer suggestion.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[cranky108]

The impedance of a zig-zag transformer can be found on the nameplate, or in the test report. Calculate it out.

Normal neutral current on a balanced system is zero.