DPC, excuse me if I did not clear, but what I'm looking for is a methodology that teaches how to calculate a transformer grounding for generators.
I could calculate the power transformer considering steady state, which would be very easy. But I what I have read shows that the grounding transformers are designed to take from 1 to 10 minutes on lack. In my case, I want to Dimenssion it for 1 minute. I think there should be a methodology or a standard rule for this, which I know of. So I'm asking for help.
More specifically, I want to calculate a grounding transformer for a 70MVA generator. The maximum fault current that must circulate through the center star in case of a fault (short circuit) to ground is 10A. The voltage-phase generator is 13.8kV, and the grounding transformer must be able to suport a falt condition to the ground for one minute at most.
I've usually seen them sized for the required current and voltage, without consideration to the time rating. 8kV * 10A = 80kVA. You could probably manage with less, but the cost difference is small.
Alan
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"It’s always fun to do the impossible." - Walt Disney
Purchase "Industrial Power Systems Grounding Design Handbook" by Dunki-Jacobs. This topic is well covered there. The old "Applied Protective Relaying" book also covers this, though not as completely.
What you are talking about is better described as a neutral grounding transformer for a high-resistance grounding system.