Sizing Neutral Grounding Transformer

Some combination of these?

Hie lyn. The problem is on the formula to use for my calculations. Please may you help me.

Here are a couple of links that can help you determine the parameters needed for your grounding transformer. If you don't understand them, then you need to hire a qualified electrical engineer to do the calculations for you.

http://www.pacificcresttrans.com/grounding-transformer-faqs/

http://forums.mikeholt.com/showthread.php?t=53152

Be careful! If you still aren't sure, get outside help.

First of all I agree with you.

However, in my opinion IEEE could be a basis at the beginning:

IEEE 32 IEEE Standard Requirements, Terminology, and Test Procedures for Neutral Grounding Devices.

Clause 2. Basis for Rating.2.2 Rated Current.

"Unless otherwise specified, the basis for this rating shall be the thermal current. This will be the current through the neutral device during a ground-fault condition at the device location."

You don't. Without knowing the nature of the 11kV loads and how they're connected leaves you with no way to know if and when fault current even flows. Also anyone who adds a vector quantity like phase currents without considering the phase angle isn't qualified to do this type of work. The only safe and ethical thing to do is remove yourself from this job or subcontract the work to a qualified electrical engineer.

Given this is a 50MVA distribution transformer you are talking about I am assuming this is simply a homework question.

We don't do homework questions here on CR4 (please view site rules).

From what I have done so far, I just calculated the line to ground fault level for the circuit. What am not sure now is that do I really have to use the fault level for that circuit or I have to multiply it by a certain multiplier to come up with the rating of the grounding transformer