480V, Corner-grounded Delta 3

[jmbelectrical]

I am involved in a project in which an existing building is being stripped and renovated in its entirety. Its incoming utility service is 480V, corner-grounded delta.

As the project is presently designed, we are stepping this down to 208/120Y and serving all of the building's loads via the 208/120V distribution system. Aside from serving the step-down transformer, the 480V side is mostly going unused. The size of the step-down transformer is increasingly becoming an issue, as the existing electrical room can only accommodate so much equipment. I am considering shifting some loads (A new elevator and some new HVAC equipment, in particular) to the 480V side of the distribution system, but I have some concerns.

I know that a typical three-phase load with a rated voltage doesn't "care" whether a voltage is line-to-line or line-to-neutral, so long as the voltage's value is correct. But what about equipment involving the use of VFDs, soft starters, and solid-state electronics? Is it safe to serve such equipment from a 480V, corner-grounded delta system? Or does a possibility exist that some component is going to be expecting a reduced line-to-neutral voltage, as would be the case in a Y-connected system?

Are there any other potential issues that I may not be considering?

 

[davidbeach]

If the step down transformer has a delta winding on the 480V side (all most certain to be true), and the wye-point of the 208V side is solidly grounded then nothing on that side will know about how the 480V system is grounded. But don't connect any VFD or other such equipment to that 480V system.

I'd see if a new service from the utility was possible, either a 480V wye or a 208V wye system. They might be more willing to help than might be expected, particularly if that's a very rare connection on their system.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[cranky108]

Not knowing what type of building this is or what it is being made into, giving advice is difficult.
I do know there are some single phase lighting systems that can work with 480V, but depending on the building use this might be limited to parking lot lighting.

It is also possible that for a large building, to have more that one electrical room, in which case the 480 might be what you want.

 

[jmbelectrical]

The building is part residential, part commercial (think of a dormitory building or a hotel). It's also approximately 10,000 square feet, so it's fairly small (to me, anyways). Unfortunately, we are limited to reusing an existing electrical room. No opportunities exist for the expansion or addition of electrical rooms.

I'm not entirely sure what compelled the original engineer to specify the use of a 480V, corner-grounded delta utility service when the property was designed and constructed 30 or 40 years ago. Given its size and its use, I would have likely specified a 208/120Y service.

 

[cranky108]

Just saying, I have seen 480 all over a mile oil field, and 208/120, over a huge factory.

Better to right size the service.

 

[TugboatEng]

Davidbeach, I'm curious, can you elaborate on why the 480v side can't be used if the low voltage side is grounded?

 

[waross]

I know that a typical three-phase load with a rated voltage doesn't "care" whether a voltage is line-to-line or line-to-neutral,

?????
Do you mean line to neutral voltage?
Corner grounded vs four wire delta vs symmetrical wye point grounding?
Some equipment checks the health of the system by monitoring the line to neutral voltage.
Such equipment will not operate properly on other than a symmetrical wye point ground.

Davidbeach, I'm curious, can you elaborate on why the 480v side can't be used if the low voltage side is grounded?

Where did David say that?
David did warn against connecting VFDs to a corner grounded 480 Volt system.

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

 

[TugboatEng]

The second sentence started with "but" so I thought it was tied to the sentence before.

May I ask what the problem running VFD's on corner grounded delta is? I have some delta systems in my fleet but they're all ungrounded. I'm just trying to learn new things here.

 

[davidbeach]

Most VFDs are expecting a grounded-wye source and respond badly to an ungrounded source during a ground fault. For those devices the corner grounded system looks like a permanent ground fault on an ungrounded system. Some VFDs can be provisioned to run on an ungrounded (and therefore a corner grounded) system, but they must be specifically procured for that use. I'm sure the shipboard equipment suppliers understand that.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[TugboatEng]

Our vessels with VFD's run 480 wye ungrounded and no neutral (3 wires). There is still functional ground fault detection within the VFD. Would wye vs delta make a difference in this case?

 

[davidbeach]

No. Commodity VFDs require a balance in the phase-ground voltages, even if there is no neutral connection. That's what you get when you order a VFD without clearly indicating the intended use. There are, as I understand, some that can be field modified to allow operation on ungrounded systems. Others are designed from the ground up to operate on a delta system without concern for where ground is relative to any phase; they cost more. But if you know you need them you can get them, but most commercial electricians wouldn't know to get one suitable for that corner grounded system.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[itsmoked]

Frequently VFDs have input filters that reference to center neutrals. Showing up with corner grounding will toast those filters so you have to remove a special screw that disconnects the filter. The other issue is with center grounding all the VFD's bits and pieces are expecting 277V to earth. With corner grounding you're forcing some of the bits and pieces to 480 to earth.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TugboatEng]

Sorry to keep dragging this on but it's just something I've never heard of and now I'm interested. I can understand the troubles of corner grounded and the higher than expected line-earth voltages. Our system is wye but the neutral is not connected in the generator. When I say ungrounded, the system is still bonded. A meter from line to ground does show 277v but I believe that is only because the system has ground fault indicating lights which are powered by 3 transformers in a wye configuration with the center grounded. We are running Allen Bradley Powerflex 700S and 753 models. I know they require no modification to function correctly.

 

[jraef]

VFDs and modern DC drives "care" about delta power systems, most UPS will care (assuming a 480V UPS), servo amps will care, all based on what Keith mentioned. They are designed for "world markets" and the internal components are designed expecting no more than 277V to ground. Even if you use versions that allow you to remove the ground reference for the protective devices and filters, you are also removing the protection and filtering that those devices provided, so you are risking the long term viability of them and in some cases, voiding their UL listing, which may have insurance consequences.

Soft Starters or any other kind of motor starters will not care, most lighting systems will not because they are typically single phase anyway. But what you DO have to pay attention to is ratings of circuit breakers and other devices. Many IEC designed devices that say they are rated for 480V are really what we call "slash rated", meaning they are rated for use on 480/277V system, but they CANNOT be used on 480V delta systems. So basically even without electronics involved, using 480V delta system means you must be extra vigilant on what goes into your facility. Recently, all circuit breaker mfrs had to redesign their products to meet some new regulations on component recyclability, so because only North America uses Delta power distribution, they are all designed for Wye system, relegating NA users to be availability of older legacy products which, for now are still available, but because the volumes are dropping, they will become more and more expensive as they are phased out.

In a nutshell the days of 480V delta systems being viable for a modern facility are numbered if not already over. In your case you may want to look closer at replacing that entire setup with a single 208Y120V system that has enough capacity.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[TugboatEng]

I just had a partial epiphany. On an ungrounded y system when there is a ground fault it becomes like corner grounded Delta as there will be a line to ground voltage of 480 volts.

 

[itsmoked]

That's true Tug. Oh and the same problems come up with center grounded delta.

Hadn't thought of that availability aspect Jeff. Thanks for pointing that out.

I'm doing a lot of work at a place that has center grounded 480V. I was actually able to use Slash rated breakers.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

You do know that those 277/480 Volt rated breakers will be seeing 416/480 Volts on one phase?

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

 

[itsmoked]

Why?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The voltages to ground on a 277/480 Volt wye system are 277 Volts.
The voltages to ground on a 240/480 Volt four wire delta are, 120V, 120V, and 416V.
Does that make a difference to the slash rating/approval?


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

 

[FacEngrPE]

I had this same discussion with my lead EE a few weeks ago.

The correct slash rated breaker for this connection is 240/208, which is not an available combination. A 480/208 slash rating would be valid, but a 240/120 would not be (except for possibly single phase panels connected only to the bottom leg).