Delta Systems

[LincHawk]

Has anyone been having request for installing Delta/Wye transformers for new equipment installations? What I mean by that is that the current service supplying the building is 480v Delta from the power company and for the new equipment they want us to put a separate transformer in line with the equipment even though it’s still 480v but has the grounded conductor. Only we’re not using the grounded conductor. So I’m going 480v Delta to 480v Wye but am not using the grounded conductor. They’re claiming that this is causing issues with some of the equipment by not using this transformer if we just came directly off the 480v Delta service.


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[ptonsparky]

VFDs in particular do not like deltas. Read the fine manuals. You may not be using the grounded conductor as a normal current carrying conductor, 277 volt loads, but you do need the EG that should have been created at the secondary, or soon after.

 

[Besoeker3]

VFDs in particular do not like deltas.

Never been a problem for me. We have put in a lot of 12-pulse systems over decades. The usual configuration is Ddyn11. Both the star (Y) and delta sides worked just fine.

 

[Jraef]

Anything with 3 phase power conversion, such as VFDs , servo drivers and larger UPS systems, will not want to be connected to delta systems (or to resistance grounded wye systems for that matter). They are all designed to be connected to systems having a solidly grounded Wye so that the reference voltage to ground is always lower than the line-to-line voltage and that it remains consistent.

One reason is because they will have surge protectors (MOVs) on the line side of the rectifier that themselves are connected in wye and referenced to ground, and often Commin Mode Noise filter capacitors on the DC bus that are also referenced to ground. So if the voltage reference to ground is floating or higher than the components are designed for, as it is an a delta system, those components can fail catastrophically and cause collateral damage to the whole piece of equipment. Some mfrs provide you with a way to remove the ground reference connection for those devices, but that then also removes a good portion of their functionality, leaving the equipment vulnerable to other power related issues. So the “best practice” is always to install a delta-Wye isolation transformer to feed them, with the Wye point solidly grounded. It doesn’t matter that you don’t use the Neutral connection for anything and you only bring 3 wires to the feed, what matters is the consistent reference voltage to ground.

 

[Besoeker3]

Anything with 3 phase power conversion, such as VFDs , servo drivers and larger UPS systems, will not want to be connected to delta systems

That runs contrary to my experience.
We have put a lot of systems in where harmonic distortion limit compliance was a constraint.
OK. One drive fed from star, another from delta and that cancels out 6n+/-1.

But what do I know..........

 

[synchro]

Here's an article with additional commentary that backs up what Jraef stated in his post:

Why Use an Isolation Transformer? – Voltage Disturbance

voltage-disturbance.com

 

[Besoeker3]

Here's an article with additional commentary that backs up what Jraef stated in his post:

Why Use an Isolation Transformer? – Voltage Disturbance

voltage-disturbance.com

Actually, it doesn't.

 

[synchro]

Actually, it doesn't.

When I posted I hadn't seen your second post in this thread, and I certainly was not trying to contradict what you said in any way. What I wanted to point out (and which this article covered in more detail) is that a delta-wye transformer can be used to block the conduction of common-mode noise and interference. This seems like a very plausible reason for the OP's case where a dedicated delta-wye is being provided for new equipment installations.
It could also be that VFDs themselves were not having operational problems, but that controls or other portions of the equipment were having problems with common mode noise voltage on the main delta bus. Also, the OP didn't explicity mention that VFDs were being used, although they are so common now that there's a good chance they are present.

 

[Besoeker3]

When I posted I hadn't seen your second post in this thread, and I certainly was not trying to contradict what you said in any way. What I wanted to point out (and which this article covered in more detail) is that a delta-wye transformer can be used to block the conduction of common-mode noise and interference. This seems like a very plausible reason for the OP's case where a dedicated delta-wye is being provided for new equipment installations.
It could also be that VFDs themselves were not having operational problems, but that controls or other portions of the equipment were having problems with common mode noise voltage on the main delta bus. Also, the OP didn't explicity mention that VFDs were being used, although they are so common now that there's a good chance they are present.

Fair enough.
My background is industrial power electronics and specifically variable speed drives.

We didn't, as a rule, use transformers for the more mundane applications.
But we also did static kramers up to about 10MW, typically for compressors and fans. Oil refineries, cement works........

These controlled wound rotor motors and sent recovered power back to the supply, usually at 11kV