Gen Step Up Trafo EF Protection 2

[bdf5526]

Hi,

I found a drawing of a VT in the Gen step up trafo where the VT is connected parallel to the trafo neutral earth switch (please refer to the attachement)

I know VT can be connected parallel to the power transformer neurtal resistance grounding to detect earth fault.

From the drawing, why do they need the earth switch anyway?. Some help please..

Thanks

 

[RRaghunath]

bdf5526,
The connection vide the attachment makes the 220kV system unearthed. If true, it is strange since 220kV system is generally solidly earthed system in most parts of the world.
Is the 220kV transmission by cables in your case?
Is earth switch normally open or closed, what is the operating philosophy?
Transformer 220kV winding is normally made with graded insulation and cannot be safe if neutral point is unearthed. What is the rating of Surge Arrester in the neutral circuit. Appropriately rated surge arrester may be able to provide protection to the 220kV winding against over voltages.

 

[bdf5526]

Hi Raghum,

Yes, the 220kv of the trafo connected by a XLPE cable to the nearby GIS.

From what I understand, the earth swith is normally closed, and motor operated from remote.

surge arrester name data: type: SB-132/20.4-1
Rated Voltage: 132kv
Count. Voltage: 106kv


Thanks

 

[ScottyUK]

Some places, Kuwait for example, routinely switch the neutrals of their generator transformers on the HV side where the transformer connects to the system in order to limit the earth fault level. A neutral CB and surge arrestor are provided and the winding is fully insulated without any reduction in insuulation level at the neutral end of the winding. Sorry I can't help with the VT.



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

 

[odlanor]

This a philosophe adopted in 345kV South Korean.
345kV system could operated grounding or ungrounding.
At ungrounded condition you have neutral lightning arrester
with open delta protection from TP.

 

[bdf5526]

Could be right, during the visit to the site just now, I saw current limiting reactor on the 220kv hv line, in between GIS and transmission line.

 

[RRaghunath]

bdf5526,
From the neutral switching and the series reactor in the OHL, it appears that the fault level in 220kV system is exceeding the rating of switchgear like CBs etc. in some of the substations. May be you can confirm.
So, when earth switch is closed, VT will be of no significance but when the earth switch is open the voltage on secondary of VT should be providing earth fault alarm to the operator in the control room.
SA rating of 132kV suggests that the transformer neutral insulation is rated to 132kV (if not more).
If earth switch is normally closed, can you find out from your control room operator under what situations, do they open the earth switch.

 

[slavag]

Hi.
It's practic in few places, as Scotty saied, for limit ground fault, but AFAIK, not with GSU transformer , it's usually solid grounded ( except MV industrial application).
About VT, maybe some kind of meas U in case of ungrounded neutral, don't know. Are VT connected to some voltage relay?
Best Regards.
Slava

 

[ScottyUK]

Hi Slava,

The Kuwaitis definitely have the capability to switch out the neutral CB on their GSU transformers as well as those on the transmission system. I was quite surprised to learn this when I worked on a Kuwaiti project but it appears to work satisfactorily in a small nation with a relatively large amount of generation and terrible earthing conditions (hot dry rocks).


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

 

[odlanor]

In Brazil utilities of electric energy accept to receive private energy source in 135kV grounded. Other utilities does requires energy source in 138kV UNgrounded.

 

[bdf5526]

Thanks for the in put guys, just for the info the secondary site of the VT goes to the generator protection. Will let you guys know more when I have found it is used for what protection function.

 

[odlanor]

bdf5526,
I have no doubt! Any ungrounded busbar install TP with open delta secondary to connect 59N.
It is the only protection to detect fault-to-ground on ungrounded system.