Grounding Transformer Requirements

[dup18]

I want to develop a 25 ampere resistance grounded system for a delta connected 7.2 kV supply. I want to use three single phase transformers (4160/7200V to 480 volt)to serve as a grounding transformer. The transformers will be connected wye-delta with the grounding resistor connected to the neutral on the wye side. There will be no load connected to the 480 volt delta secondary. The system components must be rated for continuous duty, not short time duty. What is the proper size for the bank of transformers - 100 kVA or 300 kVA?
Dave

 

[waross]

Whatever transformers that you use, 7200V or 4160V, the primary winding will carry 25 Amps. Decide what percentage of overload you are willing to tolerate and take from there.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[burnt2x]

Your primary should have to carry 25/3 amps. But why do you want the fault to persist and not trip the system? (continuous rating of the grounding transformer)

 

[dup18]

It is a requirement that the grounding resistor (and associated equipment) be rated for continuous duty. In reality, we set the relays to trip in a matter of seconds, long enough to allow for downsteam coordination.

 

[odlanor]

The power of three-phase transformer or bank Y / d grounding in kVA is equal to-ground voltage in kV times the neutral current in the ref [IEEE STD.142-2007 item 1.5.3]
Thus the power of the transformer will be:
Nt= 4160 * 25 = 100kVA


some people prefer continuous rating of the grounding transformer , because sometimes during operation ground earth protection trip wrong.
They change for alarm while investigate the problem.
.

 

[burnt2x]

Yep, that's correct.

P = 25/3 * sqrt(3) * 7200 = 104 ~ 100kVA