wye ground/ wye high resistance ground

[wroggent]

Working on a project where the local utility will supply the transformer for a new facility (13.2kV wye/480V wye, 750kVA). Incoming voltage is 12.47kV. We will be using a high resistance ground on the 480V side to limit SLG faults to 10A. Positive and zero sequence impedances of the utility network from the point of connection are negligible compared to what would be typical of a 10A grounding resistor for 480V systems. Are there any issues to be concerned about specifically regarding the high side winging configuration (grounded wye)? I am more familiar with a delta high side for a setup like this. The only thing I can think of is if there was a loss of the neutral connection on the high side then there would be no path for zero sequence current on the low side (with the exception of through the magnetizing reactance of the core, I guess). I'm not sure if that is likely enough to occur to warrant a concern though. What is your experience with this?

 

[jghrist]

It may not be possible to have a high resistance grounded system with the transformer supplied by the utility. Generally, the utility will use a transformer with the primary and secondary neutrals tied together and solidly grounded. Check with the utility to see if they can supply a delta-wye transformer with an insulated secondary neutral.

 

[dpc]

jghrist - If you can get a wye-wye transformer with the primary and secondary neutrals fully isolated, I don't see why this could not be used with high resistance grounding on the low side. Am I missing something? The delta-wye will also work, of course.

 

[davidbeach]

If the utility is buying large quantities of wye-wye transformers they are likely pad mount transformers with graded insulation on the high side so that the high side must be solidly grounded and then to save a bit more there is a single combined H0X0 bushing. If the utility owns 10s of thousands of such transformers they will be loath to want one odd-ball that can't be interchanged with all the others. That's where the customer owned transformer comes in. If you don't want the expense of ownership and maintenance of a primary service and medium voltage gear, take a regular 480V service and then install your own 480/480 delta-wye transformer and ground it just the way you want. Don't expect favors from the utility.