480 volt 3-phase

[Roy Matson]

QUOTE=brucechase;303834]It's not dangerous. I have seen and worked on many.

Well for one thing it's 480 to ground instead of 270. This must put more stress on the insulation of a motor designed for a balanced 480 V system.
I come from a region where we had 230/400 then jumped to 3.3 kV with very few transformers. It was a bit of a shock coming to NA where we have 120,208,240,480,575,600 and several more that escape me for the moment. I deal a lot with vendor packages, with all the Voltage choices it's very easy to end up with a motor that's not compatible with the supply.
What is the rational behind grounding one phase?
Regards
Roy

 

[Jim Dungar]

The correct term is a 480Y/277V , or simply wye, distribution system, not balanced 480V system. electrically a balanced system means all of the individual voltage measurements are the same.

The predominant conductor insulation systems, other than in flexible cords, in North America are rated 600V. For many decades the standard industrial system in Canada was 600V delta, only resultantly have they been installing 600Y/347V systems. The biggest problem is many electronic components are only rated for 300V to ground, which is why many VFD's cannot be used in Canada.

And yes, a phase to ground fault can blow equipment apart. That is why we have "fault current" ratings on equipment. Although faults to ground usually cause arcing and fires which actually may not cause protective devices to open.

If a fault to ground is not cleared, then current will flow on all grounded surfaces, and through any person in parallel with a surface. Remember current takes all paths based the inverse %diff of their resistances.

 

[DickDV]

brucechase, in a perfect world, you might be able to say that a floating delta or corner grounded power system is "safe". But, in the real world, it isn't.

First, as mentioned above, much of the newer equipment is designed for balanced-to-ground power. In my experience with AC and DC drives, this is most definitely the case.

Second, even tho the insulation is nominally 600V, under dirty, wet, or condensing atmosphere conditions, stressing insulation at 480V results in considerably more leakage current than stressing it at 277V. This leakage can and often is around disconnects, etc. I almost lost my life to such a situation 12 years ago where a corner grounded 480V system leaked around an open disconnect and I got rectified 480VAC (yes, that's 700VDC) arm-to-arm across my chest. An unforgetable experience, to say the least!

Third, a floating power grid holds considerable capacitive charge and, when the first ground occurs, the discharge current from this capacitance can result in fault currents far in excess of expected levels resulting in unexpected hazards and a lot of "collateral damage".

Hope that clarifies the hazard exposure a bit.

 

[DickDV]

As to why this bizarre system is being promoted, I can see two weak reasons, neither of them being justification for the additional hazards.

First, as mentioned above, the system will continue to operate through the first ground fault. This is perceived as being more reliable.

Second, in the case of the corner grounding, you eliminate the cost of one conductor to get a neutral. In effect, you have two hots and a neutral (the grounded third phase) and you can operate single phase 480V loads from the two hots to the "neutral" without the fourth wire.

The electric utilities generally transmit hi-line power using floating delta sources and for good reason. The hazard issue isn't there because only their own properly trained and equipped personnel are exposed to the system. And, the fact that a tree limb or similar ground can contact the system without releasing the fuses is a big plus. But, even there, the system capacitance can produce a large flash when these grounds touch the wires.

Bottom line: just say no to floating or corner grounded 480V systems. The hazards just are worth it.

 

[monkeyhead]

As to why this bizarre system is being promoted, I can see two weak reasons, neither of them being justification for the additional hazards.

Seems like a third reason that gets cited a lot in favor of delta is that if you lose a coil, you can re-configure and run an open-delta at 57% of the transformers original load.

 

[brucechase]

QUOTE=brucechase;303834]It's not dangerous. I have seen and worked on many.

Well for one thing it's 480 to ground instead of 270. This must put more stress on the insulation of a motor designed for a balanced 480 V system.
...Roy

No, there is no more stress on a motor that is on a 480 corner grounded system vs a Wye system. The insulation is rated for the phase to phase (and turn to turn) voltages. I do not see a higher failure rate of motors on a 480 corner grounded system.

brucechase, in a perfect world, you might be able to say that a floating delta or corner grounded power system is "safe". But, in the real world, it isn't.

First, as mentioned above, much of the newer equipment is designed for balanced-to-ground power. In my experience with AC and DC drives, this is most definitely the case.

Second, even tho the insulation is nominally 600V, under dirty, wet, or condensing atmosphere conditions, stressing insulation at 480V results in considerably more leakage current than stressing it at 277V. This leakage can and often is around disconnects, etc. I almost lost my life to such a situation 12 years ago where a corner grounded 480V system leaked around an open disconnect and I got rectified 480VAC (yes, that's 700VDC) arm-to-arm across my chest. An unforgetable experience, to say the least!

Third, a floating power grid holds considerable capacitive charge and, when the first ground occurs, the discharge current from this capacitance can result in fault currents far in excess of expected levels resulting in unexpected hazards and a lot of "collateral damage".

Hope that clarifies the hazard exposure a bit.

I agree if the equipment needs a balanced to ground system, then it should designed for that. Drives and similar equipment needs it and should have it. That can be accomplished with a transformer is necessary.

I don't think that there is "considerable" more stress on a system that is 480 Volts to ground VS 277 volts to ground. If a disconnect has that much problem, then it would have shocked someone at 277. And that could have (and will be) just as lethal. I've seen many people injured and (and one killed) on 277 lighting.

I personally don't like floating systems. I would like a reference to ground other than capacitive coupling.

Second, in the case of the corner grounding, you eliminate the cost of one conductor to get a neutral. In effect, you have two hots and a neutral (the grounded third phase) and you can operate single phase 480V loads from the two hots to the "neutral" without the fourth wire.

Single phase 480 can operate on 2 legs of a corner grounded system or a wye system. You don't need a neutral for either system, there is no difference.

The electric utilities generally transmit hi-line power using floating delta sources and for good reason. The hazard issue isn't there because only their own properly trained and equipped personnel are exposed to the system. And, the fact that a tree limb or similar ground can contact the system without releasing the fuses is a big plus. But, even there, the system capacitance can produce a large flash when these grounds touch the wires.

The point of this should be one thing only and that is SAFETY.

Everyone who works on any electrical system above 50 Volts need to be qualified. This doesn't only apply to utilities.

You should know what kind of system you have, you should be trained on that system, you should know the hazards involved with working on that system, you should know what personal protective equipment you need to work on that system (and wear it), and you should know how to make it safe. In other words (actually the words of OSHA) you need to be a qualified person before you even approach LIVE or EXPOSED voltages.

 

[DickDV]

You must live in a world much closer to perfection, brucechase, than I do.

I simply don't agree that insulation sees the same stress at 480V than at 277V. And especially, under wet or humid conditions, the leakage is often not a linear relationship to voltage, the higher voltages leaking much more than would be expected.

As to your comment about motor failures on corner grounded systems, I think it is clear that we were not talking about total insulation failures but rather, hazards to humans. It doesn't take a total insulation failure to kill people, even properly trained people.

On that training subject, you make a good point about the utility peoples' training. By extension then, if you are going to operate your facility with a floating or corner ground source, then special training would be required. In particular, anyone disconnecting a circuit from a floating or corner grounded system should be required to physically ground all disconnected conductors before working near them. That's what utility highline people are trained to do and maybe it should be SOP for electrical maintenance people too. (Did I mention something earlier about being in a near-perfect world?!!!)

Oh well, I don't want to argue about it. From where I stand, floating and corner grounded systems are significantly more hazardous than center grounded wye systems. Extra precautions are in order.

 

[brucechase]

You must live in a world much closer to perfection, brucechase, than I do.

You've said this twice now, but you are missing the point. If safety is not first and if you do not understand what you are working on, then you have no businees getting anywhere near it. It has absolutely NOTHING to do with a perfect world. Way too many people do not respect the fact that they are working on live systems and can get killed.

I simply don't agree that insulation sees the same stress at 480V than at 277V. And especially, under wet or humid conditions, the leakage is often not a linear relationship to voltage, the higher voltages leaking much more than would be expected.

At these voltages, you really aren't getting the ability to track down conductors. You don't see problems like that until well over 1000 volts. I would understand more if you said insulation was cracked and in poor condition, but just dirty and tracking down a conductor isn't what I would expect.

As to your comment about motor failures on corner grounded systems, I think it is clear that we were not talking about total insulation failures but rather, hazards to humans. It doesn't take a total insulation failure to kill people, even properly trained people.

No you are wrong. I was talking about motor failures from Roy Matson. I even quoted Roy Matson and my response was to his statement.

On that training subject, you make a good point about the utility peoples' training. By extension then, if you are going to operate your facility with a floating or corner ground source, then special training would be required. In particular, anyone disconnecting a circuit from a floating or corner grounded system should be required to physically ground all disconnected conductors before working near them. That's what utility highline people are trained to do and maybe it should be SOP for electrical maintenance people too. (Did I mention something earlier about being in a near-perfect world?!!!)

Yep. Every person working on an electrical system above 50 Volts has to be trained. It doesn't matter if it is a grounded delta, grounded wye, floating or whatever. It is required by OSHA and the company is responsible if someone gets hurt.

Nowhere did I say that disconnecting a circuit on a grounded delta or a floating system would require the same precautions as working on systems normally energized above 1000 volts. That is where you MUST ground the conductors that you are working on. I'm not sure how you are attributing one to the other.

Oh well, I don't want to argue about it. From where I stand, floating and corner grounded systems are significantly more hazardous than center grounded wye systems. Extra precautions are in order.

Don't want to argue either, but I think you are taking my statements out of context. Extra precautions are in order working around any electrical system that is not locked out. It is not a perfect world in which people should be trained on the system, the hazards associated with that system, and the protection needed to save a life. To just assume something about an electrical system is what will get people killed.

 

[BobB]

Just finished a generator power station in Canberra (Ozz) for NASA (radio telescope - deep space tracking station). The new power station generates at 480V 60Hz. It is linked to an old power station that generates at 2400V 60Hz. There is also a rotary converter that converts the HV down to 2400V 60Hz. The 2 generator power stations are linked via transformers.

The new power station has Woodward EGCP3 digital generator controllers, the old power station has Woodward analogue generator controllers. There is a Woodward LSIM module between the 2 stations to allow load sharing between the digital and analogue controllers. This is only for active load.

We looked at cross currant compensation between all the generators for reactive load sharing but it was far too complex and messy. We opted to run the digital controllers at a fixed reactive load percentage.

Another complication is that they run the whole lot up against the mains (co-generation) and can sit the generators against the mains for days/weeks aty a time. They also usually run on generator when space shuttles/satellites etc are being launched as it is more reliable than the mains.

Further complication is that NASA run these stations all over the world with the star point of the generators tied together but NOT GROUNDED! Apparently they run a high resitance, high voltage earthing scheme and grounding the generator star points for a neutral interferes with the earthing scheme they use.

The net result is that there are transformers everywhere because there is no neutral. I am not sure if the HV/LV distribution transformers have the LV side star point grounded and as a neutral or not.

Pretty strange design - not my cup of tea.

It turned out to be a pain in the tail job but we did not expect it to be easy. It is all working now.

I had not seen this type of system before. I have seen neutral earth switches, earthing transformers etc etc.

They do not have a higher than normal incidence of winding, or any other, failures than our standard MEN system in Australia does. I have not investigated whether there is a higher chance of human damage or not - will have to when I have time - probably tear 2020 (if I live that long).