Surge arrester on neutral terminal of transformer 2

[lume7006]

Hello guys,

In a project we are participating, there is a transformer wye-wye grounded on both sides, the HV side is grounded through a surge arrester, while the secondary (LV) side is through a resistor.

My question is:
Has anybody dealt with something similar (H0 terminal earthed through a surge arrester)?

I will thank if somebody had some clues or references to understand the application and some possible effects on the protections scheme.

Regards,

 

[elecquest]

Never heard of this but would be glad to get some explanation as well ...

 

[odlanor]

... the HV side is grounded through a surge arrester...

your HV is ungrounded!

When a travelling wave, originated from lightning discharge, is impinged in the HV terminal transformer, surge arresters will protect each winding.With operation of surge arrester refrated wave equal discharge voltage surge arrester goes to the neutral.
If neutral is solidly grounded, refrated wave vanish in earth.
If neutral is ungrounded refrated wave will be refleted to phase winding with double value. Surge arrester is necessary at neutral to avoid this.

 

[magoo2]

I agree. High side neutral is protected by a surge arrester not grounded by one.

 

[electricpete]

Yup, above two are absolutely correct.

On first glance, one may wonder why neutral bushing needs any surge protection at all since it is so deep inside the transformer and surges tend to diminish as they travel through the winding. But it is also a point where travelling waves may do some very weird things. I do recall seeing a severely damaged neutral bushing on a wye winding once which was attributed to surge and I wondered how it could happen. It was subject of long discussion on alt.engineering.electrical in the 1990's. The details have faded from my memory.

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(2B)+(2B)' ?

 

[electricpete]

I found the thread I was thinking of:

http://groups.google.com/group/misc...st&q=electricpete+x0+bushing#457c25f3dc8a0456

The problem is described in the 2nd post at the top.

I should have mentioned the winding of interest (low voltage in our case) was ungrounded wye... and got severe damage to the X0 bushing. Suggests a good reason to include arestor on neutral bushing of ungrounded wye.

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(2B)+(2B)' ?

 

[ScottyUK]

I seem to recall seeing this configuration before - the Middle East I think, maybe Kuwait(?). If I recall correctly the grid there is geographically small and has a heck of a lot of generation on it. The system fault level with all the dYN* GSU transformers is excessive, so on some generating units the neutral is switched out but a surge arrestor is across the neutral switch.

Of course, this could be somewhere else entirely!


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If we learn from our mistakes I'm getting a great education!

 

[m3ntosan]

ScottyUK,

You're right, I can confirm this being used in Eastern Europe as well, to cater for high fault level, usually for GSU you'll have a fast switch to change between neutral grounded through sure arrester (a resistor) and bolted.

For 132/33 or 132/11 kV Yod-11 distribution transformers, it's the same, if you have 3 transformers on the same 132 kV busbar, you'll have one bolted and the other two grounded through a surge arrester (resistor) because of the fault levels, except there's no fast switch required. Not all transformers are OK to be grounded through a surge arrester, depends on level of insulation of the neutral.

May you grow up to be righteous, may you grow up to be true...

 

[odlanor]

I guess 345kV-Y winding transformer of Korean System are connected to ground by surge arrester and disconnect switch in parallel. That means, transformer can operate ungrounded or solidly grounded.

 

[m3ntosan]

I doubt any utility is running 345 kV level transformers with neutral ungrounded, as a system operator you shall switch any transformer out asap, if neutral link is no longer grounded.
Neutral grounded through surge arrestor, doesn't mean it's ungrounded.

May you grow up to be righteous, may you grow up to be true...

 

[davidbeach]

There are no universal utility operating practices. And "grounded" through an arrestor is ungrounded.

 

[m3ntosan]

@davidbeach

Agreed, is likely to find power system nonsense management nowadays

Have you ever seen any surge arrester in service, without a ground terminal?



May you grow up to be righteous, may you grow up to be true...

 

[prc]

Interesting.lume,can you give the ratings(KVA and KV) of the windings and the rating of the neutral arrester employed?How you size the arrester for such an application.

Recently I came across a case from East Europe where for single phase 420 kV GSU they are adopting 245kV voltage class for the star neutral. I could not understand the resaon then.But in India in 420 and 765 Kv systems, GSU neutral class is 38KV AC ( Test voltage).If fault current is the issue, it would have been a problem here also, being a much bigger system.Waht can be the issue?

Last pete, how you store all these old knowledge nuggets?Question from a computer illiterate.

 

[davidbeach]

I have no idea what you might mean by "nonsense management". Different utilities have different practices. Different parts of the world have different practices. There are IEC practices that seem completely alien to me as I am sure that there are IEEE/ANSI practices that seem weird to the IEC world. None of it is necessarily wrong or nonsense.

Just because the arrestor has a ground terminal doesn't mean that it makes for a grounded transformer. At least not an effectively grounded transformer. If each phase to neutral/ground voltage is less than the turn on voltage of the arrestor it is essentially open, no ground reference at all. If I would to connect one that way, I'd use an arrestor rated for phase-phase voltage anyway so that ground faults wouldn't result in conduction until such time as the fault was cleared. That would be a good way to burn out arrestors.

 

[odlanor]

m3ntosan,
I did not find Korean paper about my affirmation.
Attached is a 380kV transformer T5 & T6 ungrounded in the British system.
I guess those transformers are used in the system with multiple grounded with high level of ground fault current. If you ungrounded transformer, you reduce short circuit to ground.

prc,
A Y winding solidly grounded of a transformer shall have insulation gradually reduced from phase terminal up to neutral terminal.
A Y winding ungrounded of a transformer shall have insulation full from phase terminal up to neutral terminal.

 

[ScottyUK]

odlanor,

That diagram is definitely not based on the British grid - our voltage levels are 132kV, 275kV, 400kV, and our system transformers all have a solidly earthed neutral at 132kV and above.



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If we learn from our mistakes I'm getting a great education!

 

[odlanor]

ScottyUK,
it was extract from Network Protection & Automation Guide page chap2-4-15, new version of english eletric protection book.

 

[lume7006]

Hello,

I thank all of your oppinions and very usefull points.

This is a transformers for an industrial customer (it will be only load, no generation inside!).
It is a 115/13.8 kV rated power of 25 MVA, according to the information is YNyn0 and it is a "policiy" of the customer to install the surge arrester to "earth" the H0 terminal.
Up to now, I have no information available about the surge arrester.

As you mentioned, I also think the surge arrester is installed on the H0 terminal only to protect it from some voltage transients that may ocurr on the transmission network.
The result is a transformer protected against these transients and neutral ungrounded, because the surge arrester does not allow an effective connection to earth, isn't it.

Best Regards

 

[jghrist]

There are multiple problems with an ungrounded wye - grounded wye transformer connection. See "The Whys of the Wyes"

http://www.geindustrial.com/publibrary/checkout/GET-3388B?TNR=White Papers|GET-3388B|generic

 

[m3ntosan]

@odlanor,

In the British system is not permitted to operate with transformer neutral ungrounded, at least at the transmission level, needs to be switched out asap, even if you lose demand...

@davidbeach

Please refer to EPRI Copper book and EPRI fault level management, grounded through a surge arrester is grounded, low or high resistance grounded depending on surge arrester characteristics.


May you grow up to be righteous, may you grow up to be true...