Transformer Differential Ground Relay

[Dumbo2929]

Can anyone point me to the calculations need to set a transformer 87G differential ground relay? I have a customer with a nuisance trip and we believe it is set too low. I'm looking for a good explaination of the calculation. It's a delta-wye substation transformer 69kv-13.8kv, 18MVA.

 

[slavag]

Hi Dumbo2929.
What is a type of relay:
1. Low impedance or high impedance?
2. What is a CT's?
3. Are you check CT performance?
4. Are you check stability. polirity?
5. What is a setting?
And last: What is relay, type, mnf?
Maybe was external ground fault?

To your Q, actualy...no calculation, set is low as possible.
Best Regards.
Slava

 

[alehman]

Take a loot at some of the protective relaying references in faq238-1287

 

[ScottyUK]

87G is a restricted earth scheme? I'm more used to seeing REF coded as a 64 function but I can't think what else 87G might be. Check for a reversed neutral CT.


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

 

[dpc]

Scotty,

87G is, or at least was common terminology in the US for what we call ground differential.

 

[jghrist]

I would start with the relay instruction manual. Generic calculations may not cover all of the nuances of the particular relay.

 

[slavag]

Scotty.
I stop understand those symbols.
for me 87G is generator differntial
and 87N is ground differential, but 64N/G is REF too.
Now we start learn IEC61850 symbols PDIS, PTOC, etc.
Good Luck to us.
Best Regards.
Slava

 

[ScottyUK]

ANSI publish a great code number system and we then make up the letters to suit ourselves...

I haven't seen the actual ANSI spec: does it assign letters to specific functions, or is that left to the discretion of the protection engineer? G and N seem to get confused far too often in relation to earth fault relaying. I agree with Slava that G signifies a generator function, but I am bound to say that with my background.

What is a 'ground differential' fucntion - a core balance or residually connected relay? We continue to be separated by a common language.

Good luck... and may the Force be with us!


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

 

[rcw retired EE]

Scotty - 87G in this application is very similar to a restricted earth fault (REF) protection on a solidly grounded wye winding. One input to the 87G relay is the neutral (residual connection) of the three phase CT's and the other is the neutral bushing CT. Ratio matching CT's are used where necessary. It is common on transformers and smaller generators. (Reference Figs 196 & 209 of "Buff Book" -ANSI/IEEE Standard 242 - "Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems".)

Confusion reigns with the letter designations. Sometimes G is Generator and other times it means Ground. I recommend using a legend on the one line diagram.

 

[ScottyUK]

Thanks.


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