Ungrounded Electrical Service

[paulengr]

Paul,. Can you elaborate,. In particular why does pretty much every POCO disagree that delta-wye is the way to go?

Also, is there any reason a wye secondary couldn't be used? You could float the XO.

POCO is effectively three single phase systems except at a power plant. A delta wye feeds back voltage onto a missing phase in a single phase fault on the primary side. Wye wye is also slightly cheaper since primary insulation only has to be rated for line to ground voltage. Juxtapose that against true isolation of ground faults (separately derived system), harmonic mitigation, voltage unbalance mitigation, and elimination of circulating currents. These are all the reasons industrial plants prefer them.

 

[electrofelon]

I thought the discussion was about D-D vs D-Y.

I think every permutation has been on the table....

 

[paulengr]

Keep in mind that the utility is not providing the transformer, so the a grounded-wye primary is not a must, but I would be curious to know why utilities are in favor of it. It's a fine solution if the customer is looking for a grounded-wye on the secondary, but I don't think a grounded-wye primary and ungrounded-wye secondary is a good solution here.

You are grounding the utility and handling any of their faults. This is the inherent problem with wye wye or delta delta...the zero sequence (ground faults current) passes right through in both directions. Your faults are theirs and vice versa.

Plus it’s a multi grounded system which has a lot of issues if it’s own (ground loops) but over long distances as the whole system acts like remote Earth it’s better to provide a direct path back to the source. If you aren’t measuring distances in miles this advantage does not exist.

In the case of say a grounded wye-ungrounded wye the ungrounded side is grounded through the transformer where if it was delta-wye the ground fault path is blocked. As you suggested no reason you can’t just leave the wye ungrounded and use a delta-wye or wye-delta, although in a step down you are more likely to find a replacement with a delta primary.

 

[paulengr]

The usual argument for ungrounded systems is continuity of service but not the logic behind Navy’s Code.

It is my understanding that Navy’s ungrounded requirement has to do with avoiding creating a current path via the ships hull which can greatly increase galvanic corrosion. Imagine if there is a voltage unbalance. For instance with one line longer than another...perhaps someone added an extra shore power cable to one phase, or a higher resistance connection. Current SHOULD flow back via the intentional ground but since it is in parallel with the steel hull, water, and Earth, we get a current through the hull as well.

 

[synchro]

Apparently floating-wye/delta transformers have been commonly used to supply ungrounded systems according to the following article.

I thought the discussion was about D-D vs D-Y.

From post #1:

... is there a best option for transformer configuration to serve the ungrounded system (i.e. wye-delta, delta-delta, etc.) from the primary distribution?

 

[LarryFine]

My bad.