High resistance grounded wye system questions.

[iwire]

Where are these systems used?

What is the purpose?

Is there anything I really should know if I come across one?

Thanks!

 

[GoldDigger]

They are used for much the same reason that ungrounded systems are used, namely so that a first fault to ground will not bring down all of the loads on the system until the fault is repaired. This can be very important for some industrial processes that will have equipment damage if the power goes off suddenly, or will lose valuable production time.
The HRG gives you a positive indication of a ground fault and also prevents the system conductors from floating up to twice the nominal voltage from ground in case of certain small arcing faults during normal operation.

 

[iwire]

Is the advantage over ungrounded delta the fact you get to have a wye system?

 

[iwire]

And thank you.

 

[GoldDigger]

Is the advantage over ungrounded delta the fact you get to have a wye system?

Pretty much. The NEC, for the most part, prefers that a grounded system have a neutral conductor being grounded.
I suppose you could instead do an HRG on one corner of a delta and still be code compliant, but I do not know offhand of anyone who has done that, since it requires higher voltage rated breakers and other components than the corresponding wye system.

And you are welcome, for what my advice is worth.

 

[winnie]

Is the advantage over ungrounded delta the fact you get to have a wye system?

Sort of.

With an HRG system you are not permitted to use the 'grounded' circuit conductor, so you can only feed 'delta' loads. You are limited to the same uses as an 'ungrounded' delta system. I believe that you need to use full rated breakers; 'slash' rated breakers are only acceptable in solidly grounded systems.

But you get the benefit of a good ground reference. Unless you have a low resistance fault, the line to ground voltages are held at the line-ground voltages that you would see in a normal wye system. In the event of an arcing ground fault you don't get the 'pumping' action that can happen with an ungrounded system.

-Jon

 

[iwire]

Sort of.

With an HRG system you are not permitted to use the 'grounded' circuit conductor, so you can only feed 'delta' loads.

OK, that blows my idea of the advantage.

But you get the benefit of a good ground reference. Unless you have a low resistance fault, the line to ground voltages are held at the line-ground voltages that you would see in a normal wye system. In the event of an arcing ground fault you don't get the 'pumping' action that can happen with an ungrounded system.

Thanks.

 

[big john]

I'd say 95% of the HRG systems I see are on generators. I believe the logic behind them is a belts-and-suspenders type of thinking where even though your protection should operate fast enough to save the genny, the cost of rewinding can be so severe that places want a physical check in place to limit fault current.

The rare cases where I run into HRG distribution it's just being used as a safer alternative to ungrounded.

 

[iwire]

I'd say 95% of the HRG systems I see are on generators. I believe the logic behind them is a belts-and-suspenders type of thinking where even though your protection should operate fast enough to save the genny, the cost of rewinding can be so severe that places want a physical check in place to limit fault current.

That seems to make sense and if you need a wye I imagine you can supply a delta - wye transformer and set that up with a direct bond.

 

[Tony S]

Are we talking LV or MV here? There is a very big difference and purpose for each.

 

[iwire]

Are we talking LV or MV here? There is a very big difference and purpose for each.

I am a low voltage guy. I max out at 600 volts, more commonly 480 volts.

 

[Tony S]

OK I?m dropping out of this. MV IT (impedance terra) is more my field.

LV IT would be a very special application in the UK other than for generating plant where for obvious reasons it is used.

In essence both situations boil down to fault limitation.

 

[Lost_RFTech]

Off the top of my head the two primary justifications for an HRG system are:
1.) Safety. Arc flash is not as big a concern when you have a phase to ground fault relative to a solidly grounded system.
2.) Orderly fault recovery. Systems are designed to alarm in the event of a ground fault but remain energized for a specified period, e.g. 24 hrs., before opening a protection device. This would allow orderly coordination of corrective action.

A significant thing to be aware of is that the grounding system design (or lack thereof) must be taken in to consideration when setting up devices such as VFD's.
Be aware that some systems such as the brand I-Gard provide trouble shooting tools to help pinpoint a fault.

There is much debate about the merits of an HRG system. As a guy who is around the field equipment I welcome the reduced arc flash hazard.

I work in a facility with a mix of transformer secondaries and grounding. Grounded delta (intentional), grounded delta (unintentional), solidly grounded wye, ungrounded delta, and a new HRG system. Systems and facilities as new as two years old, some that date back to the turn of the century, and I ain't talking about the most recent one. Think wooden beams and wooden pegs...

To date, we have had no problems with the HRG outside of some VFD config confusion, and a case where we ignored the ground fault alarm and suffered an unplanned uncoordinated shutdown 24 hrs. later. I am aware of at least one case where we had a workmanship problem that caused a ground fault at a conveyor motor where we were able to conduct an orderly shutdown and resolve the problem with minimal impact. I know there have been other similar cases but I have not been directly involved.

There is some interesting reading, both pro and con, on the internet. Search for the I-Gard system.