High Resistance Grounding Benefits?
High Resistance Grounding Benefits?
(OP)
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.
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.






RE: High Resistance Grounding Benefits?
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.
RE: High Resistance Grounding Benefits?
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
www.cvoes.com
RE: High Resistance Grounding Benefits?
You may want to review the paper at www.neiengineering.com/pdfs/paper1JN.pdf. It also has been extensively discussed in this forum.
RE: High Resistance Grounding Benefits?
RE: High Resistance Grounding Benefits?
David Castor
www.cvoes.com
RE: High Resistance Grounding Benefits?
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.
=====================================
(2B)+(2B)' ?
RE: High Resistance Grounding Benefits?
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.
RE: High Resistance Grounding Benefits?
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
RE: High Resistance Grounding Benefits?
I mentioned the 2kV insulation in regards to http://www
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?
RE: High Resistance Grounding Benefits?
=====================================
(2B)+(2B)' ?
RE: High Resistance Grounding Benefits?
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
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: High Resistance Grounding Benefits?
should've been
=====================================
(2B)+(2B)' ?
RE: High Resistance Grounding Benefits?
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.
=====================================
(2B)+(2B)' ?