wye input transformers

[winnie]

What are the issues with using wye-wye transformers?

I am looking at an application where we will be using an isolation transformer to provide a wye secondary for high resistance grounding.

However the input might be 208V or 480V.

I was wondering if it would be reasonable to try to use a transformer with primary taps that span from 208 to 277, and connect the primary in wye for 480V use.

Or are there transformers with 'dual voltage' primaries that have the taps to handle 208-240 which can be reconnected 480?

Thanks

Jon

 

[mivey]

As for issues

"3rd harmonic currents circulate freely and can cause telephone interference
Ungrounded primary can cause dangerous 3rd harmonic voltages"

This is a note I have in a banking spreadsheet I wrote. I can look for more info but the reference is not handy right this minute. Hope this is what you were asking.

 

[jim dungar]

What are the issues with using wye-wye transformers?

I am looking at an application where we will be using an isolation transformer to provide a wye secondary for high resistance grounding.

However the input might be 208V or 480V.

I was wondering if it would be reasonable to try to use a transformer with primary taps that span from 208 to 277, and connect the primary in wye for 480V use.

Or are there transformers with 'dual voltage' primaries that have the taps to handle 208-240 which can be reconnected 480?
Jon

It is all but impossible to purchase an off-the-shelve dry typr wye-wye transfromer. This is one reason the this topic is rarely discussed.

I would simply purchase a "drive isolation" transformer which are normally available as 460-460Y/277 or a standard step up 208-480Y/277.

There is absolutely no good engineering reason to use a dry type wye-wye for feeding an HRG.

 

[templdl]

The construction of a true Y-Y transformer if more expensive because they must have a 4 or 5 legged core. If not the smaller transformers must include a tertiary winding. It often adds about 25% to the cost.
Here's what I understand about how they would be applied in applications other than utilities. Y-Ys are often used because of zero phase shift in UPS applications where there can't be any phase shift when paralleling loads.
Often times it is misunderstood that a Y-Y in required when the supply is a 480Y/277 and you want to step down to 208Y/120 where a conventional 480v delta-208Y/120 should be used.
Another common way to get a zero phase shift is with a delta primary and a wye zig-zag secondary.