High Resistance Grounding Benefits?

[X49]

I have been asked to evaluate the possibility of using high resistance grounding (HRG) for a new facility. The primary reason is to reduce the possibilities of downtime and equipment damage.

The electrical service is 600V, 800A, supplied by a 500-750kVA utility transformer. The main loads are two 250hp VFD's. One is required to run at all times (though downtime will not be extremely expensive).

Will HRG help reduce potential damage to the VFD's or motors during a ground fault event? The VFD will be protected by fuses and the motor will be protected by the VFD's ground fault protection.

Another advantage to HRG is that a ground fault on the main bus will not cause the main breaker to trip, but is this even a likely occurence? Ground faults downstream should only trip downstream breakers/fuses if everything is coordinated properly. The main breaker is not required to have ground fault protection.

Any opinions would be welcome.

 

[lume7006]

Hello,

I think the usage of HRG will depend on several factors, for example:

1) If your production process needs to be continuos, without any interruption that may affect it.
2) If the whole system design has considered the overvoltages caused by the single phase to grund faults that will not be quickly cleared.
3) In come facilities like petrochemical and/or mining, where arcs maybe too dangerous and then the magnitude of the current is reduced to a very low value.


Typically, at the voltage level you are using th HRG, the single phase to ground current magnitude will be very small, then, none protection device (fuses or MCCB) will "see" that fault and that may become a problem.

I recommend you to perform a detailed analysis about pros and cons of HRG for your facilities and this has to include your protection schemes, equipment insulation levels and special requirement of the equipments to be fed, before doing something.

 

[dpc]

Low voltage HRG systems are safer than solidly-grounded systems. 70% to 80% of all faults start as ground faults. In a solidly-grounded system, these ground faults often escalate to three-phase faults. With an HRG system, this cannot happen, because there is insufficient energy in the ground fault. This reduces the risk of arc-flash incidents occurring (although the level of hazard is not reduced).

Equipment damage is greatly reduced especially for motor stator faults.

I have heard reports that some VFDs do not like the impedance to ground presented by HRG system - you should discuss with the VFD supplier.



David Castor

http://www.cvoes.com

 

[busbar]

You may want to review the paper at

http://www.neiengineering.com/pdfs/paper1JN.pdf.

It also has been extensively discussed in this forum.

 

[Live2learn]

Just I thought when I layout HRG low voltage systems I recommend (sometimes)the use of 2kV conductor to deal with the overvoltages. Anybody else do this?

 

[dpc]

No. Unless you're also buying 2kV insulation on your motor windings, I'm not sure you have help yourself much. The motor windings will generally fail on overvoltage before anything else.

David Castor

http://www.cvoes.com

 

[electricpete]

We have ungrounded 460 volt electrical systems at our plant. A consultant pointed to some IEEE document (I’m forgetting the exact reference) which said we should specify that our motors be designed for operation on an ungrounded system. I asked a motor rewind shop and a motor OEM what they would do different if this type specification were invoked. There response was “absolutely nothing”.

By the way the reliability of our 1000+ motors on this type of system has been good. Whatever is the mysterious transient intermittent arcing fault / overvoltage phenomenon that is associated with ungrounded system has not visited us. I’m not saying it doesn’t exist somewhere in some other facilities, but certainly not for us.


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[X49]

Thanks for the suggestions. I found that paper to be very useful.

It seems that the primary reliability benefit of HRG is that on solidly grounded systems with ground fault protection on the main, ground faults downstream from feeder breakers can trip the main GF protection unless the feeder breakers also have properly coordinated GF protection. On an HRG system, the facility remains in operation during this type of fault. Since my main breaker is less than 1000A, ground fault protection on the main breaker is not required, and therefore this is not a significant benefit. (HRG would only prevent a system shutdown in the case of a ground fault on the main bus (unlikely). Faults downstream will only trip downstream breakers, which is tolerable.)

The advantages of HRG with respect to equipment and personnel protection are harder to guage. My fault levels will be less than 15kA. Arc flash hazard is relatively low at Category 1 (<4 cal/cm^2, NFPA 70E). I would assume that on a solidly grounded system, the VFD's ground fault protection will adequately protect the motors during a GF event, and that the fuses will adequately protect the VFD's. Is this a safe assumption?

Since shutdowns are not terribly expensive in this application, and we are running a duty-standby configuration, I don't think HRG is justified.

 

[waross]

Hi Pete;
When I was taking my original training, years ago, couple of my instructors had worked in large industrial plants when ungrounded delta systems were the norm. Each plant had experienced a "discontinuous ground fault" or arcing ground fault. In each instance a large number of motors failed.
As I understand it the worst case is when a winding in a motor or transformer fails in such a way that the resulting arc to ground results in both a spark gap transmitter generating high frequency noise and the winding acting as an auto transformer and multiplying the high frequency voltage several times. This HF is superimposed on all energized power lines on the same transformer and the motors typically fail in the first few turns of the windings.
This was probably one of the reasons that grounded wye systems became common. (Another reason may have been the issues that arose with wye/delta connections.)
You may remember the old adage an try to share it very diplomatically with your superiors;
"Those who do not learn from history are doomed to repeat it!"
This is not a criticism, Pete, just some friendly information that may help to avoid a future disaster.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[thisisnotatest]

dpc,

I mentioned the 2kV insulation in regards to

http://www.eng-tips.com/viewthread.cfm?qid=232606&page=3

So in that discussion nothing ever mentioned the motor windinds? Therefore my conclusion would be that either specifiy 2kV conductors with 2kV motor windings or neither?

 

[electricpete]

Bill – Thanks for your concern, but there is nothing new to me in your comments. There are a variety of aspects highlighted in industry articles and guides including some which can be interpreted to support your view. Naturally code requirements trump everything else and I don’t think ungrounded systems are even an option under NEC…. but they certainly are allowed under NESC. Presumably part of the reason for that difference is related to the difference in level of monitoring and response that is provided at a power station (responding to ground alarms). There is a historical perception (right or wrong) that properly-designed, maintained and monitored ungrounded systems can be a more reliable alternative for continuity of power. Our experience with 1000+ 460vac motors at this facility for 20+ years (I’ve been the motor guy here for 10 of those years) suggests that perception is correct, but I am not telling anyone else what to do.

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[Skogsgurra]

HRG or insulated Neutral systems (aka IT grids) are - or rather were - popular in steel, pulp and paper industry in Scandinavia for decades. The primary reson was that you could have an earth-fault without stopping the process.

Insulation monitors, mainly Bender, were used to detect insulation faults and the thinking was that insulation faults should be rectified when signalled by the monitor. In practice, that seldom happened and when drives with measuring circuits that were referenced to ground were introduced, the isolation monitors became more or less useless.

There were also issues with the filters used with VFDs and the HRG systems caused more problems than they were supposed to solve. It is now a trend to ground the neutral in many steel works and pulp and paper mills. And there are very few - if any - problems with that.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

Correction

for continuity of power

should've been

for continuity of operation

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[electricpete]

I wanted to back up to the point of my post 13 Dec 10 11:48. It was not to discuss merits/demerits of ungrounded system. It was to point out thtat it is meaningless to tell a motor supplier/rewinder that a LV motor will be used on an ungrounded (or HRG) system or "specify" such duty. Because there is nothing in NEMA MG-1 or other standards that tells the motor manufacturer what he would do with such information. The NEMA MG-1 requirement is ac hi-pot test at a voltage of 2*VLL+1 (applied line to ground).

If you are buying a 460vac motor that's what it is. A 460vac motor with insulation rated 2kv is meaningless (undefined) as far as I know.

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