Zero sequence model of ungrounded auto transformer with closed delta winding

[dfdt]

We are using 5 mva 22/11 kv auto transformer with delta wdg in ungrounded mode.Can some one show me zero sequence network for such connection?

 

[prc]

Why such ungrounded operation? To my knowledge such operation always results in problems like neutral inversion (L F Blume) In zero sequence impedance diagram, the T branch Z is the net of Neutral grounding impedance and tertiary. When you are not grounding the neutral, Z of T branch will increase as the N branch will remain open,( AIEE paper Auto Transformers for Power system-O T Farry, December 1954)

 

[dfdt]

Thanks prc- this auto is just converting voltage from 22kV to 11kV to feed a rural feeder. The primary side(22kV) side of Auto is connected to grounded star winding of star/star 110/22kv transformer.The reason for running ungrounded is perhaps to limit ground fault current.

 

[dfdt]

Prc- do you have a pdf copy of OT Farry’s paper by any chance?

 

[stevenal]

From the description I'm imagining a single delta connected winding with the corners at 22 kV and 11 kV taps coming off between the corners. There would be no neutral connection available for such a transformer, the neutral being carried straight through to feed line to neutral loads. Am I understanding correctly?

 

[aussiejohn2]

Stevenal,
Might not even be a neutral if loads are all line-line or balanced.
John.

 

[dfdt]

Yes there are no line to neutral load on this feeder.The autotransformer is merely to step dpwn the voltage from 22 to 11kV.

 

[stevenal]

No answer yet, so I'll try. The zero sequence network would consist of a single impedance extending left to right. No open circuit, and no short to the reference bus.

 

[HamburgerHelper]

I believe it is an open connection. If you take a wye-wye-delta transformer and open all the parts related to ground impedance, zero sequence can't flow through it.

Slide 139

http://sites.ieee.org/houston/files/2016/01/2015-09-29-Basic-Power-System-Comp-Calculations_Rev0.pdf

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If you can't explain it to a six year old, you don't understand it yourself.

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[stevenal]

OP didn't say it was wye wye delta. He said it was a delta connected autotransformer.

With thanks to Cuky

 

[HamburgerHelper]

He didn't specify but given how few autos aren't wye wye with a delta tertiary, that isn't a wild assumption. I would bet a quarter that the delta winding his is referencing is the tertiarry.

Even if it is a delta connected auto, its zero sequence network is still just an open circut.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[stevenal]

To me, an autotransformer connected wye-wye is a contradiction. Autos have a single winding with a possible tertiary.

Please explain your open circuit statement. If the source is grounded it looks to me like there's a path, but I admit I am unsure how to just how to analyse Cuky's diagram.

 

[prc]

I took bholas statement as a Y connected auto with a stabilized tertiary winding butt neutral of Y auto not grounded - He said an auto transformer with delta winding and NOT delta connected auto transformer. So in the first case there will be a neutral connection ( as evident in slide 139 where ZL is always connected to No irrespective of the neutral connection of other windings)In the discussion part of above IEEE paper that I mentioned it is well explained by the GE veteran Mr B A Cogbill. Unfortunately due to copy right reasons I not able to upload IEEE paper. In the delta connected auto that stevenal showed I believe there will be no connection to N in the zero sequence circuit. Thanks Hamburgerhelper for the links- I never knew that Hard to find Information manual by Jim Burke had been revised. I had only a 2002 version. How fast time flies away !

 

[prc]

I have to correct myself. Even though L will be connected to N, when auto neutral is floating no path for current flow in to zero sequence circuit both from HV and MV sides.

 

[waross]

I see a secondary ground fault circulating back to the source ground.
The fault current will be limited in part by the impedance of the secondary part of the wye winding of the faulted phase.

Bill
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