Transformer neutral

[GoldDigger]

i agree

When a transformer is used for step-up rather than step-down, there is sometimes a confusion as to whether the description is primary-secondary or HV-LV, which will put the Delta and Wye in opposite orders for the same transformer. For step down transformers the two orderings are identical.
I don't think the primary voltage was ever mentioned.

 

[pfalcon]

As a factory it could well be ungrounded 480 Delta. Or it could be the equivalent 480 Wye with a High Resistance ground. Both operate as though they're ungrounded 480 Delta. The High Resistance ground makes it easier to check for phase-ground faults.

Ungrounded Delta is used in many factories so that a single phase-ground fault doesn't bring the whole factory down. Common practice.

The system still requires a local ground for the machine tools. This is often provided by a buried system connected to the building steel and/or the 4th bar in the buss. Machine tools that require a neutral provide their own transformer with a Wye secondary with the neutral tied to the grounding electrode system.

The 277 measured to ground is an "artifact" of this technique. Magnetic feedback from the machine transformers and the building primary pull the building Delta back to "center". As Golddigger said, this is unstable. Though the effect is substantial, a local anomaly could pull the building voltage off center. This effect would only occur proximate to the anomaly.

 

[pfalcon]

... The secondary is 480/277 volts, 3-phase 4-wire. ...

In case my prior post was a bit confusing - the fourth wire would be attached to the building GES not the transformer secondary. It would be a wire run ''with'' the 3-wire but strictly speaking it's not really the 4th wire. It's the local ground for your machine tools.

 

[templdl]

Just curious. If the secondary is in fact a 480v delta then it would be safe to assume that it's 3ph/3w with no X0. If the primary is a eye then the the H0 is most likely grounded. As such could it be that the installer brought a EGC from the H0 to the secondary assuming that it would act as an X0 and EGC for the secondary and an the assuming that the 3ph3w now has an EGC?
That's the only way I can make any sense of this even that it will not work as the installer intended.
I'm trying to think outside the box a bit.

In case my prior post was a bit confusing - the fourth wire would be attached to the building GES not the transformer secondary. It would be a wire run ''with'' the 3-wire but strictly speaking it's not really the 4th wire. It's the local ground for your machine tools.

I'm backing out of this. We are now at a point where you have an ungrounded 3ph3w delta and now are expecting that that the "4th wire" when run with the other 3wires will actually provide a ground for the newly derived 3ph3w ungrounded delta system.

 

[ggunn]

I'm backing out of this. We are now at a point where you have an ungrounded 3ph3w delta and now are expecting that that the "4th wire" when run with the other 3wires will actually provide a ground for the newly derived 3ph3w ungrounded delta system.

Ground, yes, but the phase voltages relative to it are anybody's guess.

 

[GoldDigger]

Ground, yes, but the phase voltages relative to it are anybody's guess.

And the OP has confirmed that he has line-to-neutral loads connected to a non-existent neutral. (Effectively just to ground, since there is no neutral on the secondary side of the transformer. Those loads will collectively establish the phase-to-ground voltages.

The lighting is all 277 volt, for example.

The only possible source of 277 volts on the secondary side is line to nonexistent neutral = common point of all 277 volt loads = ground.

 

[kwired]

Ungrounded systems still have equipment grounding conductors and grounding electrode system, the intent is to have no potential between all non current carrying components, whether it be by inductance, capacitance, or a ground fault from a system conductor. Once a system conductor is faulted to this grounding system you now have an unintentional grounded system, a second fault on another phase will open overcurrent devices quickly.

 

[pfalcon]

I'm backing out of this. We are now at a point where you have an ungrounded 3ph3w delta and now are expecting that that the "4th wire" when run with the other 3wires will actually provide a ground for the newly derived 3ph3w ungrounded delta system.

No. Not what I said.

The OP needs to find out what industrial installation technique they used. Since the building primary is Delta the building must have a grounding electrode system present.

My guess from here is that they ran a four bar buss and therefore labeled panels as "4-wire", which they aren't. A four bar buss at such a site will be a 480 3-wire system with a GES tied to the fourth bar - not a neutral.

Alternately they ran a three bar buss. Then when they make a machine drop they bring a 4th wire from the GES tied building steel. In which case someone probably saw four wires in the drop and again presumed it must be "4-wire", which it's not.

A lot of machine tools use the ground for protection and leave the three-phase floating. They don't use a neutral in any fashion.

On the other hand, if you need the neutral such as for 277 lighting then you have to install a Delta-Wye transformer for the lighting. That allows you to create a neutral on the secondary side.

In an industrial installation someone not paying attention could wrongfully treat the GES like a neutral. There's typically enough magnetic linkage that the 3-wire system centers its voltage so it appears to be 277 to ground. It'll function most of the time since the linkage is typically very strong. It usually shows up as short bulb life due to the voltage instability.

Same with a machine tool. If someone presumed that it was 4-wire just because the drop has four wires coming down, then the machine components will have a shorter life span and frequent controller faults. Just because the magnetic linkage makes the 3-wire read 277 to ground doesn't make the GES a neutral. But if you measure thinking it's a 4-wire and see that reading you may fail to understand why it's down all the time.

I've been at my industrial site for 30 years. I know it's 3-wire with GES building steel. In all that time I've never seen or heard of anyone reading less than 250V or more than 284V to ground on a functioning buss.

 

[kwired]

If you have an ungrounded system you are supposed to have ground fault detection and indication. I can't say I have experience with installing or using such a system, but if someone were to attempt to connect 277 volt loads to this system wouldn't that cause the ground fault indication to show there is a fault?

 

[jim dungar]

If you have an ungrounded system you are supposed to have ground fault detection and indication. I can't say I have experience with installing or using such a system, but if someone were to attempt to connect 277 volt loads to this system wouldn't that cause the ground fault indication to show there is a fault?

The most simplest GF detector, delta power systems, is three light bulbs (loads) connected into a grounded-wye configuration.

Any balanced loads connected in a grounded-wye, would simply function as the GF does. Unbalanced loads could be viewed as faults, depending on other circuit parameters, such as size of load versus system leakage current.

Loads connected in an ungrounded-wye would experience L-N voltages that varied based on the load balance.

During an actual L-G fault, any loads connected in any wye would experience up to the full L-L voltage instead of the L-N it expects.

 

[kwired]

The most simplest GF detector, delta power systems, is three light bulbs (loads) connected into a grounded-wye configuration.

Any balanced loads connected in a grounded-wye, would simply function as the GF does. Unbalanced loads could be viewed as faults, depending on other circuit parameters, such as size of load versus system leakage current.

Loads connected in an ungrounded-wye would experience L-N voltages that varied based on the load balance.

During an actual L-G fault, any loads connected in any wye would experience up to the full L-L voltage instead of the L-N it expects.

So with the three light bulbs - they would operate @ 277 across each lamp. But in a ground fault situation, the voltage on the faulted phase becomes same potential as the neutral of the wye connected lamps. The lamp on the grounded phase would have no voltage across it and would not light, the other two lamps would have 480 volts across them and would burn brighter and also have a need for a 480 volt rating otherwise they likely wouldn't last for very long.

This neutral point is not a solid neutral in the supply windings, if one were to add unbalanced 277 volt load mistakenly then I would think you would have some change in the ground fault indication lights that may prompt someone to look into why there was a change. I do agree if you added balanced 277 load then nothing abnormal should appear in the fault indication.

 

[pfalcon]

The current standard is to use a High Resistance Wye. A high resistance is tied from the neutral to ground but not run with the 3-wire buss. This allows the three lights to work. The resistance pulls the three phases to 277 at the transformer. A phase-ground fault shifts the voltages. A light that was wired to the faulted phase goes out. Though they use monitored voltage and amperage readings that go to computer systems now rather than lights.

There's a way to do it on a Delta as well but I never got into that one.