Delta-Delta vs Wye-Delta Step Up

[MD88]

Hello, I have a situation where we have a 4.16kV/480V delta/wye w/ 5A NGR, and we want to step it up to 25kV.

If we had the money and time, I would order a 480/25kV delta/wye (or a 4.16/25kV delta/wye actually)… but all we can find is 480/25kV delta-delta and wye/delta (ie: a step down transformer run in reverse).

On the 25kV delta side we’d put in a zig zag and NGR, but I was hoping to get some feedback on the primary 480V side (wye vs ungrounded wye vs delta).

I’ve read countless threads so far but I can’t seem to figure out which is best.

Thanks

 

[waross]

Avoid a grounded-wye/delta arrangement.
A floating-wye/delta may suffer switching transients, but the issues with the grounded-wye/delta are much worse.
I would go with the delta/delta.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[MD88]

@waross right I did include the grounded wye primary option by mistake, definitely do not want that. If all switching is 3 pole, is there a real risk of switching transients on the wye delta option? For what it’s worth, the distance between the existing step down and this step up will be max 50ft. Would an RC snubber be a good idea to include?

 

[wcaseyharman]

I’m guessing the delta-delta would be better, unless you want to add another ground source on your 480V system ( possibly screwing up your ground coordination and increasing the available ground fault current.) If you float the neutral it probably won’t matter much either way.

 

[waross]

My bad experiences were all with grounded-wye/delta.
I understand that with three pole switching, each phase has a different point on wave.
If one phase takes maximum inrush current, it may drive the voltage on the other phases high.
I may be wrong.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[cranky108]

FYI, the higher voltage side is always called the Primary side. You referenced the 480v primary, which should not be the primary.

 

[wcaseyharman]

Not to distract the topic, but I believe in the US the primary winding is the winding connected to the normal source of power. Most transformers are stepping down power to lower voltages so the high voltage winding is always the primary, the major exception are generator step up transformers, where the primary is defined as the low side winding. It isn’t totally an academic nitpicking exercise either, the standards have different requirements as far as voltage capability between the primary and secondary which affect how you specify and operate the transformer, at least for generating stations.

 

[waross]

We really have to agree to disagree.
Some segments of the electrical field call the high voltage side the primary.
Some segments of the electrical field call the input side the primary.
No-one is going to change.
Thank you for pointing out the ambiguity, cranky.
There may be less possibility for confusion if we forego the use of the term "Primary" in regards to transformers and use instead the terms "Input Side" or "High Voltage Side".
I was taught in the mid 50s that the input side was the primary.
It is only in the last 10 or so years that I have heard the high voltage side defined as the primary.
Avoid confusion.
Avoid the term "Primary".

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[xnuke]

Another vote for delta on the LV side, as it sounds like you have a HRG system at 480 V. You don't want to put in a grounded wye on that side of the transformer with a HRG in the system.

I also recommend avoiding both the terms "primary" and "input side" as I've been working a lot with transformers connecting low-voltage battery energy storage systems to medium-voltage distribution systems. With bidirectional power flow, which side is the "primary" or "input?" I've been sticking to "LV side" and "HV side."

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

With bidirectional power flow, which side is the "primary" or "input?" I've been sticking to "LV side" and "HV side."

That works for me!

--------------------
Ohm's law
Not just a good idea;
It's the LAW!