Neutral Resistive Grounding

[allscott]

BUMP;

I decided to resurrect this thread because I think it's important and I had to deal with this again today. The posts made by Lancie, DickDV, Jiri, and Leadfoot are all worth reading again for anyone who has to deal with drives and power systems.

I got called first thing this morning to a drive that had an overvoltage fault that would not clear. Checked phase - phase and all three are OK. Start checking phase to ground and the voltages are all over the place.

Sure enough we had a phase go to ground at the PDC and the ground fault annunciator was blinking as bright as it could in a room nobody ever enters. Spent most of the day tracking down and fixing the ground fault. I know where I work these annunciators are checked only randomly when someone happens to be in the room. How long it was there I don't know.

I spent the latter part of my day trying to explain this to some of the other plant management (some technical some not) my recommendation of switching to a hard grounded system. I remembered this thread and printed it out for them, hopefully they will read it tonight.

Any additional insight from those in the know would be appreciated.

EDIT:
The tx I was working on was a wye secondary 3MVA high resistive ground.

 

[DickDV]

There is middle ground between a hard ground and a floating or high resistance network----a low resistance grounded wye system.

The center of the wye secondary is grounded through a resistor that is of a low enough resistance to release the fuses or breakers but high enough to limit ground fault current and the resulting collateral damage and personnel hazards.

Of course, production has to stop until the ground is located and repaired but locating it is far easier due to the location of the released fuse.

Anyone who has had to locate a grounded phase somewhere in a large facility with a floating network knows what a headache that is!

Since the time of these above posts, a new problem specific to the reliability of AC drives has surfaced. On very small drives with three phase inputs, the fusing is typically 10amp or even smaller. If, in a floating network, there is more than potentially 10amps of phase-to-ground leakage plantwide, that leakage current will find a return path thru the internals of the AC drive back to the power network blowing the drive supply fuses.

I had a situation like this where I had four of these small 460V drives all fused at 10amps per phase. As soon as these drives were powered up, at least one and sometimes two input fuses released. The only way I could get the fuses to hold was to remove all grounding from the drive and dismount it from the baseplate. That way the fuses held but, with an ammeter between ground and the drive frame, the amps exceeded 10amps. Since it was outside my authority and ability to find the leakages in this large plant, we installed a delta-wye isolation transformer to feed the four little drives. End of fuse problem.

The longer I work out there, the more I dislike and even fear floating networks.

 

[allscott]

At least these drives were just blowing fuses!. I worked on a sytem about a year ago, PLFEX 70's, where the fuses didn't blow but the DC bus capacitors did. $$$$$$

 

[Alan Case]

What an interesting read. I can't believe that you can have an ungrounded system or a high resistance grounded system. All our reticualtion systems 415/240V has to be MEN. ie Multiple Earthed Neutral.
Regards Alan Case

 

[BobB]

Hi Alan

Did a generator job at Sydney Airport before the Olympics. 4.2 megs of generators at 11 kV - only one generator had the star point tied down and that was through a neutral resistor. I do not understand why but that is the way the HV engineer designed it.

Also had one at Herston (Brisbane) Hospital some years ago that was set up the same way.

 

[Lancie1]

What an interesting read. I can't believe that you can have an ungrounded system or a high resistance grounded system. All our reticualtion systems 415/240V has to be MEN. ie Multiple Earthed Neutral.
Regards Alan Case

Alan, I find it hard to believe also! I really find it hard to believe that there are engineers and plant managers that are planning to install MORE of these dangerous ungrounded or resistance-grounded systems. Organizations like the xxxx (deleted for liability reasons) are going to be responsible for many electricains and maintenance techs being killed or maimed for life (due to promoting grounding systems that are not well-maintained or monitored inside industrial plants).

The thing about ungrounded systems is that the original intention may be to monitor the ground-fault detectors and correct the phase-to-ground fault immediately, but over time this intention gets pushed to the side by more important things, like keeping the plant runnning. That always happens. I have seen many Delta-Delta plants, and never one that had an adequate ground-fault monitoring program. If a plant had a procedure where the ground-fault detectors are monitored daily, and a log maintained and signed off by a responsible party, and when a ground-fault is detected, a team is immediately assigned to find and fix it, then MAYBE..only maybe, that would be a safe system. How do you make such a procedure PERMANENT, so that it will be there through many changes of management?

Still, it will never be as safe for people as a hard-grounded delta-wye system. It comes down to People Safety versus Equipment Preservation, and keeping the plant running. Which is more important in the long run?

 

[mgvol]

This is an excellent reference for drives insatllation considerations using different power dist./grounding schemes:

http://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf

Also, when installing an AC drive: READ THE INSTALLATION MANUAL!
You don't want to know what your drive will look like if you have an ungrounded system and you don't remove the MOV's/Common mode capacitors.

 

[allscott]

This is an excellent reference for drives insatllation considerations using different power dist./grounding schemes:

http://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf

Also, when installing an AC drive: READ THE INSTALLATION MANUAL!
You don't want to know what your drive will look like if you have an ungrounded system and you don't remove the MOV's/Common mode capacitors.

Unfortunatley I know what they look like. Smoke, capacitor juice, etc....

After installing about 50 powerflex drives I thought I knew what I was doing without the manual. Number 51 was on a resistive ground transformer.

 

[Alan Case]

Hi Bob. If you have aread back through the previous posts you can see the logic behind why one leg only was tied to earth through a resistor. ie if you get phase to earth short the system will still run without blowing fuses. You should have an indication that his has happenned so it can be repaired before a second phase shorts to earth. When this happens there would be a big bang.
When I said our reticulation is MEN I meant the Australian 415/240 system HAS to have the neutral grounded at every installation.
Regards Alan Case

 

[Andybr]

Hi Bob. If you have aread back through the previous posts you can see the logic behind why one leg only was tied to earth through a resistor. ie if you get phase to earth short the system will still run without blowing fuses. You should have an indication that his has happenned so it can be repaired before a second phase shorts to earth. When this happens there would be a big bang.
When I said our reticulation is MEN I meant the Australian 415/240 system HAS to have the neutral grounded at every installation.
Regards Alan Case

Alan
While your logic would be correct for LV systems this is probably not the case for Bob's HV setup. On HV systems the resistor is normally present purely to limit fault current. There would almost certainly be an earth fault relay in the system which would open the breaker(s) in the event of a fault. When I say probably I should add that I am just hedging my bets. I have never seen a system which did not trip on an earth fault but I would not swear that such a system could not be used.
Andybr

 

[BobB]

Yep Andy, GE-Multilin SR489 generator protection relays. The engineer did the cals.

 

[stephen83]

What is the best to detect if you have a proper grounded supply system with out having to in to these HV transformer rooms... They make me nervous.. Lol...

 

[iant]

Before you go anywhere near HV do the required course
IT IS YOUR LIFE.
I am an 'A' Grade Electrician and Have a HV Ticket - it is site related.

Grounding and apparatus in HV substations are required to be checked regualarily.
This is done with the correct equipment.
Access or opperation of this equipment is for Qualified Electrical Operators (HV ticket).
NOT ELECTRICIANS OR ENGINEERS
If you are an electrician than stick LV until you are trained.
You do not need to be an electrician to have a HV ticket.

 

[stephen83]

I forgot to mention that i do hold a HV ticket and a "A" electrician.... i did my HV couurse about two years ago but never had a need to use it at my plant. And yes i hate those HV room.. it makes me a little nervous.... so back to my question... what is the best method to ensure that i have a proper grouded supply?

 

[iant]

Sorry for sounding negative stephen,
But as you have done the course you understand my concerns-
I was placed in an awkward position 25 years ago - but stuck to my guns at the time
and refused to operate a 22kv cfs
I have been trained and had several refreshers and redone one since.

So If i appear to be telling you how to suck eggs Please understand I would do this to anyone and expect the same in return - SAFETY IS # 1


If you are concerned with the gruonding of your HV TS the only quick way is a HANDS OFF visual inspection. Check what you can see.
If you need to go further then you need to get Probably ODG or similar to test for you.
You of course need to supply them an Access permit.
The Companies I have worked for, Including myself, Have Insisted on Two Operators being present during all tasks except for actual switching operations (Safety)

However is it the LV you are worried about

I might ask if it would have been better to Post a new thread instead of revitalising a 4 year old one also