Reverse feeding transformer 75kw 208v to 480 v

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rodney2525

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I'm trying to reverse feed a transformer. The manufactory eaton has approved my 75kva primary 480 delta secondary 208 secondary.

Eaton has informed me not to corner ground delta to the egc.

All other sources online recommend corner grounding 480 delta.

What should I do?
 
I think some information has gotten lost in translation.

The manufacturer gets to decide what is required to reverse feed their hardware. If they say you can't corner ground the 480V secondary (originally primary) then you either need to have an ungrounded 480V system or you will need additional hardware to create a wye neutral that you can ground.

Going ungrounded means following all the rules for ungrounded systems, including ground fault detection. Additionally during a fault an ungrounded system becomes inadvertently corner grounded. If Eaton says you can't corner ground but doesn't say anything about ungrounded systems,... that doesn't sound right.

Is it possible that they are saying that you can't use the primary EGC to corner ground your system? That would make sense. Corner grounding an SDS means following the appropriate rules for grounding electrode conductors, connecting to building steel, etc. You can't simply use the primary EGC. You might check back with Eaton to confirm exactly what they were saying.

To create a wye neutral requires the addition of a 'grounding transformer'. This is an autotransformer which gets connected to the delta secondary and creates a neutral point which can be grounded. If you go this route you will get the equivalent of a standard neutral grounded 480/277V system, but requires significant additional hardware.

IMHO the _best_ approach you can take is to get a proper 208V delta: 480/277V wye transformer.
Next best is corner grounding, if you can get clearance from Eaton.
Next best is wye grounding with a grounding transformer
As a last resort go ungrounded. If you need the particular benefits of an ungrounded system, look at impedance grounded systems.

-Jon
 
Ok I chose your IMHO option two and called back eaton. I talked with a different representative and he assured me I can corner ground the delta 480v and use as secondary no problems.

And now I'm left with one last question. Do I run a GEC from my newly seperatley derived system transformer to "building steel" grounding electrode?
 
One point that I did not mention ( but @tom baker did) is that some _loads_ don't like being supplied from a corner grounded system.

In particular if you are feeding any VFD systems, make sure you review their manual for permissible supply systems. Some won't work with corner grounded supplies, so require modification. These sort of loads also don't like ungrounded sources, which will limit your options to getting the correct transformer or using a grounding transformer.

-Jon
 
winnie said:
One point that I did not mention ( but @tom baker did) is that some _loads_ don't like being supplied from a corner grounded system.

In particular if you are feeding any VFD systems, make sure you review their manual for permissible supply systems. Some won't work with corner grounded supplies, so require modification. These sort of loads also don't like ungrounded sources, which will limit your options to getting the correct transformer or using a grounding transformer.

-Jon
Click to expand...
Based on some bad experiences, I highly support this post and urge you to check with your VFD manufacturer/supplier.
 
Also, it won't be an issue if you are using a fused disco, but If you are feeding distribution equipment such as a panel board, you will need more expensive non slash rated breakers. For example a Siemens BQD would be their regular 277/480V breaker but you couldn't use that with 480 corner grounded. Keep in mind the machine might have integral slash rated breakers.
 
winnie said:
One point that I did not mention ( but @tom baker did) is that some _loads_ don't like being supplied from a corner grounded system.

In particular if you are feeding any VFD systems, make sure you review their manual for permissible supply systems. Some won't work with corner grounded supplies, so require modification. These sort of loads also don't like ungrounded sources, which will limit your options to getting the correct transformer or using a grounding transformer.

-Jon
Click to expand...
Ok so I did take your advice and checked with the vfd manufacturer. Turns out it specifically says in the mauel that it will not except corner grounded delta systems. The manufacturer technician informed me that it will degrade the insulation on the motherboards overtime and fry. Bad harmonics.

I am now in the proccess of taking the _best_ approach of Winnies HU.

I owe everyone on this feed a steak dinner . Thanks
 
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