Delta vs Wye Transformer

[kenaslan]

Is there an advantage to using a solidly grounded no NGR wye 2.5 mVA transformer over a delta transformer 4160V to 480V? There are only 3ph 480V motor loads and no 277 loads.

 

[jim dungar]

In general, my advice is to stay away from delta distribution systems, especially ungrounded ones.
Ungrounded deltas are subject to overvoltages and transients - definitely not for the faint of heart.
Corner grounded deltas seem to be confusing to many electricians that usually live in the world where circuits 'always have neutrals'.
I do recommend deltas on some 'single machine' applications.

 

[templdl]

The line to ground voltage is alos overlooked. A "wye" results in a 277v L-G voltage which also allows for 277/480v breakers to be used. With a 480v grounded 'B' phase delta the L-G voltage will be 480v making it a necessity to use a 480v breaker which often times cost more.

 

[kenaslan]

thanks

breaker cost is not an issue.

Most of the transformers I deal with are delta tri-delta with phase shifted secondary that feed a single VFD and 1500hp+ motor.

 

[templdl]

thanks

breaker cost is not an issue.

Most of the transformers I deal with are delta tri-delta with phase shifted secondary that feed a single VFD and 1500hp+ motor.

Great. But may one consider the 480v or 277v to ground issue? Motor failure, when a winding fails and goes to ground, what voltage would you prefer should there be an arcing fault? What voltage would be kinder on the phase to ground insultation resistance? Which one may do less damage should there be a winding failure?
If you did have a choice which direction would be the best?
The 1500hp more may not likely be 480v but 2300v are possibly 4160. But the phase shifted secondary may not even be an issue unless it relates to another transformer secondary voltage.
Now that you brought up having a delta consisting of (3) 1ph transformers it may have an advantage of being able to continue to supply the load should 1 transformer in the back goes south. But the remaining (2) transformers must be of a suitable size to suppy the load. Otherwise a singe 3ph transformer with a delta secondary does have much to bring to the party over a transformer with a wye secondary in my opinion.

 

[kenaslan]

The motors are 4160V then jump up to 13.8KV at over 1500hp the 9 power leads then feed three DC Rect, to a DC bus then to the VFD.

But we digress from my orig post.

 

[kingpb]

As stated there are inherent issues with ungrounded systems and historical dat on why they are not utilized much, but there are applications where there are advantages of using a delta-delta connection for isolation purposes.

 

[JoeStillman]

I always use a grounded wye secondary, whether there is a neutral load or not. Delta-delta transformers can have circulating currents when the voltage ratios are unequal between phases. Circulating currents cause higher losses and can cause transformer overload. If you use delta-delta, you should also have ground detection because the first ground fault doesn't cause any current to flow until the second one happens.

Just because the transformer has a grounded neutral point doen's mean the MCC or switchgear has to have a neutral bus or a grounded service conductor. You are permitted to bond the neutral point to a grounding electrode system at the transformer itself. You will need ground fault protection on the secondary main.