Ground pick up settings-Transformer

[tctctraining]

What's the rule to choose the ground pick up current for transformer protection?
I choose phase pick up current at 166% of ONAN rating. Transformer has ONAF rating too.

 

[dpc]

The ground setting depends on the transformer winding connection and the type of ground sensing. For a wye winding where the transformer is sourcing the ground current, the ground relaying is generally set to coordinate with the downstream ground fault protection on the feeders.

For a delta winding, any ground current is generally a bad sign for the transformer, so settings are more sensitive.

The phase overcurrent pickup setting is generally determined by the maximum forced-cooled rating of the transformer.

David Castor

http://www.cvoes.com

 

[tctctraining]

I don't have any info on downstream feeder ground setting,I only have the upstream breaker ground setting pick up current.
Transformers are delta-wye.

Is there a safe way to set the ground pick up current?
may be set it at transformer FLA?

 

[rbulsara]

Hire a professional who has experience in coordination. For learning see thread238-281559

Rafiq Bulsara

http://www.srengineersct.com

 

[SparkyLarks]

Very much sounds like you neet to hire some professional help.

166% Of a pickup for ground fault sounds very high to me. I have worked on plant where the max ground fault is less than the max onan rating of the transformer.

Ther is no rule as such, and so much depends on the expected Earth fault current.

Another key is how is the ground fault measured, is it a core balance CT, or is the ground fault measure by the residual current from the three phase CT's. If it is the latter you need to allow for magnetising currents as well.

Or are you looking at Residual Earth Fault protection of a CT?

you need to look at the equipment that is being protected, what is it expected load range.
What is the damage curve of the transformer?
Are there cables also protected by this device?
What is their damage curve.
What is the protection of the upstream and downstream devices.

Along with a lot of others

So unless you have expreiece in this type of work you need to get someone who does to help.

 

[tctctraining]

166% pickup current is for phase LTP setting and not ground pick up setting.
My question is more toward setting the ground pick up setting of primary side of a transformer.

Thanks for all inputs

 

[davidbeach]

So, what kinds and magnitudes of ground faults will you see looking into a delta winding? A hint - your delta side ground element will never see a ground fault on the wye side.

 

[mmt019]

Tctctraining. Not to be rude but it sounds like you do not have much experience in power systems studies. 166% pickup is way off the scope in terms of ground fault pickup. The setting depends on the equipment you are trying to protect. Definitely consult someone who has experience in coordination studies. You don't want to learn proper settings by smoking equipment (or worse yet, frying workers)when its too late.

 

[jghrist]

There won't be any zero-sequence current for through faults or load imbalance. The high side ground overcurrent relay should be set as low as possible to detect ground faults in the transformer. The time-overcurrent element should be set at minimum pickup and time dial. The instantaneous element should be set high enough to avoid problems associated with residual flux in the different phase CTs and differences in DC offsets. I normally set the instantaneous at 60% of maximum through fault to cover the case of one phase with full dc offset.

 

[dpc]

jghrist,

Did you mean 160%?

 

[jghrist]

Did you mean 160%?

No, 60%. Remember there is no actual ground through fault.

 

[dpc]

Oh, I thought you were talking about instantaneous phase overcurrent settings. My bad - didn't read it closely enough.

Dave

David Castor

http://www.cvoes.com