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vyral2311
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Join Date: Mar 2013
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Earth Fault Protection Coordination of Star-Star Transformer

01/08/2014 4:29 AM

In one of my application Grid Power is imported through a transformer of rating 40/50MVA, 66kV/34.5kV, YNyn0. Both side neutral is solidly grounded. Down the line 33kV network is little large and earth fault anywhere in this network gets reflected in 66kV side. Relay settings in the Grid substation are sensitive enough to trip immediately (within 200msec) for 33kV side earth faults and authorities there are refusing to reconsider their settings. And due to this I am suffering power loss quite frequently. I am thinking of keeping the neutral of 66kV in open condition so that earth faults may not get reflected.

Is it feasible to do so or is there any other alternative?
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Pathfinder Tags: Earth fault coordination star-star transformer
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TonyS
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#1

Re: Earth fault protection coordination of star-star transformer

01/08/2014 6:44 AM

You can't run the system with no earth. I can't believe you've even considered this, it's suicidal!

From the scant information you give a NER springs to mind coupled to REF and inverse time EF.

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vyral2311
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#2
In reply to #1

Re: Earth fault protection coordination of star-star transformer

01/08/2014 7:08 AM

opening of 66kV side neutral will not make system unearthed since it is further supplied from 220kV/66kV YNyn0 where 66kV is earthed.
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TonyS
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#5
In reply to #2

Re: Earth fault protection coordination of star-star transformer

01/08/2014 10:54 PM

Sorry but you weren't clear that you wanted the 66KV Y→E link removed. I assumed it was the 33KV you wanted to remove.

It is entirely at the discretion of the supply authority. But I'll be honest I can't see a reason for them to do so. As PWS has said, you need to sort your system out.

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PWSlack
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#3

Re: Earth Fault Protection Coordination of Star-Star Transformer

01/08/2014 9:02 AM

The best alternative is to remove the source(s) of the earth fault(s).

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vyral2311
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#7
In reply to #3

Re: Earth Fault Protection Coordination of Star-Star Transformer

01/10/2014 2:26 AM

PWSlack

Are you trying to tell that if vehicle is punctured then I should fly it!
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adee2007
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#4

Re: Earth Fault Protection Coordination of Star-Star Transformer

01/08/2014 10:54 AM

Rather than grounding or opening the neutral, why don't you go for differential protection?
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vyral2311
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#6
In reply to #4

Re: Earth Fault Protection Coordination of Star-Star Transformer

01/10/2014 2:23 AM

transformer differential is already in place, but what if earth fault takes place in out zone as entire plant distribution is on 33kV!
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adee2007
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#9
In reply to #6

Re: Earth Fault Protection Coordination of Star-Star Transformer

01/11/2014 10:52 AM

I agree with your thought, but, if one of the side of the transformer is grounded then it is already preventing the flow of zero sequence currents....
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raghun
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#8

Re: Earth Fault Protection Coordination of Star-Star Transformer

01/10/2014 4:12 AM

I agree that opening of 66kV neutral will make the grid side earth fault protection not see the faults in 33kV system.

But, since the 66kV winding will be without any earth reference, the neutral point that is floating may assume undesirable potential that can harm the insulation.

I am not sure if the winding with graded insulation (IEC requires only transformers up to 33kV to be uniformly insulated) can handle that well!

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adee2007 (2); PWSlack (1); raghun (1); TonyS (2); vyral2311 (3)

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