intermittent arcing faults on ungrounded systems - 480vac
intermittent arcing faults on ungrounded systems - 480vac
(OP)
We've heard some interesting things on thie forum about intermittent arcing faults on ungrounded systems.
Does this apply to 480vac systems or only to higher voltages? My memory was that a part of the phenomenon had to do with establishing and extinguishing an arc as the voltage to ground (accross capacitance to ground) oscillated in one of the phases. But also it seems that this type of arcing might not be possible in 480 volt systems due to very samll propensity for arcing.
Also, are there any features incorporated into the design of modern ungrounded systems that alleviate this problem? The reason I'm asking is... my facility has ungrounded 480-volt system for vital loads... but we have not experienced these problems over the course of 12 years since facility waw built.
Does this apply to 480vac systems or only to higher voltages? My memory was that a part of the phenomenon had to do with establishing and extinguishing an arc as the voltage to ground (accross capacitance to ground) oscillated in one of the phases. But also it seems that this type of arcing might not be possible in 480 volt systems due to very samll propensity for arcing.
Also, are there any features incorporated into the design of modern ungrounded systems that alleviate this problem? The reason I'm asking is... my facility has ungrounded 480-volt system for vital loads... but we have not experienced these problems over the course of 12 years since facility waw built.






RE: intermittent arcing faults on ungrounded systems - 480vac
The upper limit I'm familiar with is 4160V, but a couple of decades ago the IAS Transactions carried a paper about formal [and successful, in the authors’ view] experiments on high-resistance grounding done with 15kV-class distribution. Based on the cited ANSI reference below, it is a valid application for generator/GSU-transformer combinations, and they go up to what, ~23kV?
The purpose of the resistor is to offset ‘charging current’ from the three phase-to-ground capacitors that the system insulation naturally forms in cables and windings. It can be easily measured in an operational system.
A formal description states, “the power dissipated in the resistor is equal to, or greater than, the zero-sequence reactive voltampere loss in the zero-sequence capacitance of the [motor] windings, [cables and busses] and the windings of the transformer.” (ANSI C37.101 §5.1)
For the sake of other readers, if one fault develops, then you’ve got to watch out for is a second fault on another phase. At this point, overcurrent devices start operating, often in pairs. If allowed to progress to this degree, the whole concept is now useless. You need to have fixed monitoring equipment for this application, and respond to it in a timely manner, to be effective in giving high-level service continuity. Suffice it to say, the more stuff connected to a single bus, the more difficult it is to trace a fault. In plants with MV distribution, it’s normal to have multiple ~750-1500kVA secondary unit substations scattered about to serve localized LV loads. This inherently limits circuit lengths and numbers of motors for each unit sub.
Recent coverage of this issue is—High-Resistance Grounding of Low-Voltage Systems: A Standard for the Petroleum and Chemical Industry, IEEE Transactions on Industry Applications, Vol. 35, No. 4, July/August 1999.
RE: intermittent arcing faults on ungrounded systems - 480vac
Here is an interesting discussion of ungrounded system with ground fault from IEEE 62.92.3-1993.
"The voltage and currents between phase conductors, ground, and neutral are affected by the equivalent capacitance of the circuit and equipment windings to ground". With no intentional conductive path to ground, this capacitance establishes a return path for ground fault current as follows: from ground, through the capacitances of the unfaulted phases, to system neutral, and out the faulted phase to the fault. If fault resistance is low, the predominant impedance is capacitive; current zero occurs at the fault at voltage crest. It becomes possible for high voltage to reionize the arc path and forthe arc to restrike. Such an intermittent fault may be established with the arc restriking every half cycle, equivalent to switching a capacitor every half cycle. A cuumulative builduip may the occur if the recovery rate of insulation strength increases after each extinction (a situation not likely to occur in open air arc, but prevalent within confined spaces within multiconductor cables, raceways and machine windings). High transient peak voltages will occur and will be limited either by the insulation recovery rate and strength at the fault, or by the system insulation strenth."
#1 - can anyone explain that in English.
#2 - They talk about a neutral... is the same thing possible in our delta system?
RE: intermittent arcing faults on ungrounded systems - 480vac
#1 - can anyone explain that in English. I think that’s essentially being described at this reference: About two-thirds of the way through Thread238-6870 I posted an incident that was published in Beeman’s 1955 Industrial Power Systems Handbook that discussed a sustained 1200-volt measurement, effectively pushing the system ‘neutral’ point way outside the 480V phasor triangle. I’m guessing that’s what eats insulation for breakfast. With resonant series-LC networks and motor/transformer iron, it’s probably a real nasty harmonic-filled waveform. Seriously, in a staged ground fault at 480V [Don’t try this at home, kids.] the arc noise is fairly quiet, but it sounds very unusual.
#2 - They talk about a neutral... is the same thing possible in our delta system?
Various IEEE papers and standards talk about using a separate, dedicated zigzag autotransformer or wye-delta bank to establish a neutral for resistance grounding. The relative kVA rating of the bank can be quite small, because you only need to handle charging current; typically less than 5 amps for a low-voltage system.
RE: intermittent arcing faults on ungrounded systems - 480vac
RE: intermittent arcing faults on ungrounded systems - 480vac
I wonder what other factors make some ungrounded systems susceptible to this phenomenon and some apparently not? Perhaps it has to do with the capacitance etc of the system? Or maybe a very specific type of intermittent arcing fault is required to initiate the event.. which only occurs in certain types of equipment?
RE: intermittent arcing faults on ungrounded systems - 480vac
If you have an ungrounded system, and are not checking for grounds, and have not had a problem, I would say you are just lucky. What is your rate of unexplained motor failures?
RE: intermittent arcing faults on ungrounded systems - 480vac
From an insulation-damage/transient-overvoltage standpoint, it was pure, unadulterated Beeman. [They'd done it that way since the 40s, and they were not about to change.] You wouldn't believe the manhours spent pulling motors, baking them overnight to get megger readings >1M and reinstalling. I got good with varnished-cambric, rubber and vinyl tape, though.
It got comical at times. Of course, sub-50-IQ hose jockeys almost always kept everything wet, and no one seemed to know about NEMA 4/4X enclosures.
[ref Chapter 6, DONALD BEEMAN Editor, Industrial Power Systems Handbook, McGRAW-HILL BOOK COMPANY, 1955]
RE: intermittent arcing faults on ungrounded systems - 480vac
RE: intermittent arcing faults on ungrounded systems - 480vac
Thanks!
electricpete
RE: intermittent arcing faults on ungrounded systems - 480vac
1. There some good posting covering this and related topics in this Forum
2. To </images/new.gif> electricpete (Electrical) Jul 18, 2001 marked by ///\\\.
We've heard some interesting things on thie forum about intermittent arcing faults on ungrounded systems.
///Probably overvoltages caused by arcing, damaging electrical equipment and causing EMI emissions.\\\
Does this apply to 480vac systems or only to higher voltages?
///The phenomena caused by ungrounded systems, frequently used in more remote past, are common to all voltage levels, in general.\\\
My memory was that a part of the phenomenon had to do with establishing and extinguishing an arc as the voltage to ground (across capacitance to ground) oscillated in one of the phases. But also it seems that this type of arcing might not be possible in 480 volt systems due to very samll propensity for arcing.
///It may be realizable, but it is difficult since one has difficulties to discern small arcing from various noises.\\\ Also, are there any features incorporated into the design of modern ungrounded systems that alleviate this problem?
///There is some effort developed to use the digital signal processing to detect arcing and trip the protective devices.\\\
The reason I'm asking is... my facility has ungrounded 480-volt system for vital loads... but we have not experienced these problems over the course of 12 years since facility waw built.
///The problems still may came; especially, when the insulation becomes to deteriorate, and current creepages become higher. Then, you may contemplate the high resistance grounding or whatever may be next better and feasible.\\\
RE: intermittent arcing faults on ungrounded systems - 480vac
I have seen high-resistance grounded systems used up to 15 kV, 400 A (although this is arguably a low-resistance grounded system). The rule of thumb of 1 A/MVA of transformer capacity arose because the capacitance charging current of cables and equipment in systems is usually within this range, and the neutral grounding resistor must be sized greater than the charging current. I have also seen this rule of thumb modified to read 0.5 A/1000 kVA for low-voltage systems and 1 A/1000 kVA for medium-voltage systems.
However, this is only part of the story. An adequate tripping ratio is required to ensure operation of the ground-fault relay on arcing faults and on impedance-limited or part-winding faults. I have seen figures of anywhere from 5 to 10 times the charging current recommended to allow an adequate tripping ratio. The minimum I would use is a 5 A resistor on an LV system.
A high-resistance grounded system can easily be added to any ungrounded system by either grounding an accessible neutral point, or the neutral point of a zigzag transformer connected to a system without accessible neutral, through the resistor.
If you use two resistors and a timed contactor to short one resistor out, you can "pulse" the ground fault current and use a clamp-on ammeter to trace cables to find the fault location. This can save the expense of purchasing separate ground-fault relays for each feeder, and works well if your electrical code allows you to run your system without tripping. In Canada, we can alarm on ground fault only on systems 5 kV and less, and 5 A and less.
You can monitor ground fault current at the neutral only, or at feeders by using a core-balance (zero-sequence) CT. If your system is such that you have to trip on fault, selective coordination is easy to do, using time-delay only ie. the pickup current on all relays can be identical, just progressively increase the delay on upstream relays.
There is a Canadian company, Federal Pioneer (now a branch of Groupe Schneider/Schneider Electric), that makes a relay system that will monitor several feeders. Upon occurrence of the first ground fault, the system alarms only. Upon occurrence of a second ground fault, which of course is now a phase-to-ground-to-phase fault with no resistor limiting of the fault, it will trip the ground fault on the feeder with the lowest-priority instantaneously. This relay probably works with ungrounded systems as well, and could help limit damage when the ground fault escalates (as it inevitably will, given enough time, due to the overvoltage stress on unfaulted equipment).
RE: intermittent arcing faults on ungrounded systems - 480vac
RE: intermittent arcing faults on ungrounded systems - 480vac
RE: intermittent arcing faults on ungrounded systems - 480vac
We do have ground detection in the form of 3 lights which have unequal intensity when a ground appears. I'm not that familiar with the history of how many grounds we have found and cleared (I can't recall of any), but I have only heard of one motor failure. This is among probably 100 480 volt motors which operate periodically for short periods of time over a period of 10 years.
I believe this is standard design for safety buses in nuclear plants. At the time of design this was felt to be more reliable in terms of ensuring that motors would be available when needed. We also have features like motors that alarm (vs trip) on overload, but that's another story.
I am still curious how it is that our design seems to work so well in spite of all the horror stories from other ungrounded systems.
RE: intermittent arcing faults on ungrounded systems - 480vac
RE: intermittent arcing faults on ungrounded systems - 480vac
RE: intermittent arcing faults on ungrounded systems - 480vac
RE: intermittent arcing faults on ungrounded systems - 480vac
....." unless you have a serious problem with insulation degredation due to damage, age and heat related failure, etc."
True, On a 480 volt system, you should not have arcing, until you start having insulation problems. But where you do have an insulation problem, you can have an arcing contidion, when it is not a solid ground.
In an older plant you do have a problem with insulation degredation due to damage, age and heat related failure, etc. We have had to replace 480 volt cable runs, due to ground faults. The insulation had deteriorated, due to age, and moisture, and steam leaks (heat)in the area over a year ago. Most grounds are found in either a motor or in the pot head of a motor. We may average two or three grounds a month. Thus it is important to detect the grounds and clear them ASAP.
RE: intermittent arcing faults on ungrounded systems - 480vac
An important corollary to LV hi-res grounding is the single-pole interrupting rating of overcurrent devices, covered in: George D. Gregory, Single-Pole Short-Circuit Interruption of Molded-Case Circuit Breakers, IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 35, NO. 6, NOVEMBER/DECEMBER 1999
RE: intermittent arcing faults on ungrounded systems - 480vac
Well worth reading. Again it is a reference to having a voltage anomalie when a ground occurs on such a system.
http://www.southwire.com/tech/pubs/pcu/600vtoolkit.html
RE: intermittent arcing faults on ungrounded systems - 480vac
Lots of good info. Many many horror stories about 480vac ungrounded systems gone berzerk. Meanwhile, here we stand with probabaly 1,000 or more motors on 480vac ungrounded systems and I've never seen anything remotely resembling this. And by the way, it's a pretty standard design among nuke plants and I have heard anything from the other plants.
So now the question: why would some plants be susceptible and some not?
I can only come up with a limited number of ideas:
1 - you guys are lying. Probably not... there are too many of you saying it.
2 - All of those stories involved motors much inferior to ours... maybe 1950's motors... maybe open drip proof motors out-doors?
3 - Maybe there is a difference in the systems. We have discussed that capacitance can affect the behavior of resistance grounded systems... maybe it plays a role in ungrounded systems as well? Our plants have no large capacitances: transformers within a few hundred feet of motors => relatively short cables, no power factor correction caps, no surge caps at this voltage level.
Can anyone familiar with the problems occuring on ungrounded systems tell me if those systems had any of these factors?
RE: intermittent arcing faults on ungrounded systems - 480vac
RE: intermittent arcing faults on ungrounded systems - 480vac
What's your maintenance response on the first (not second) experience ground fault on your ungrounded system?
1. Correct the ground fault that day.
2. Correct the ground fault during the next scheduled shutdown.
3. Correct the ground fault after the next unscheduled ground fault.
4. Um, how do we know if we have a ground fault?
The closer you are to #1, the less often you'll experience problems with overvoltages, etc.
Also, you nuke plants usually build things to be rather bulletproof compared to some other installations (with good reason), which would make it seem natural that you'd experience fewer insulation failures. That said, it surprises me that you would not employ more resistance/reactance grounding methods to provide for better ground fault relaying.
Just curious, besides lights, what other measures do you typically provide on your ungrounded systems -- thumpers, remote alarming, anything else?
RE: intermittent arcing faults on ungrounded systems - 480vac
it's when we lost the ground reference that problems really showed up...
am I off base here?
RE: intermittent arcing faults on ungrounded systems - 480vac
RE: intermittent arcing faults on ungrounded systems - 480vac
Does anyone know the new address of that southwire article mentioned by jburns. I was able to get into www.mysouthwire.com By registering you get access to many docs, but I didn't see that one.
RE: intermittent arcing faults on ungrounded systems - 480vac
RE: intermittent arcing faults on ungrounded systems - 480vac
electricpete — Perhaps significant headroom can be gained with a warm, dry, indoor environment {including periodic exorcisms.}
Spent time in a wet-process plant with fourteen 750-1500kVA secondary unit substations—all but one with 480V ungrounded {wye!!} secondaries. The classic account in the 1955 Industrial Power Systems Handbook by Donald Beeman is very real. Old habits [since World War II as far as anyone could tell] die hard. 6L6.net//localuser/busbar/dat/beemaIPSH6z.doc
RE: intermittent arcing faults on ungrounded systems - 480vac
IEEE Std 141-1993 IEEE Recommended Practice for Electric Power Distribution for Industrial Plants, (Red Book),
Section 7.2.1 Ungrounded Systems describes the current industry standard views on ungrounded systems with References to:
IEEE Std 142-1991 (Green Book)
GET-3548 available on http://www.ge.com
RE: intermittent arcing faults on ungrounded systems - 480vac
Pablo raises a good question of the effect of lights. In our case lights are driven of secondary of grounded wye primary pt's. This certainly might have some effect. Do other people that have seen the problem have similar setup? Pablo - why would we ever lose this ground reference? (unless there were overvoltages blowing pt fuses to begin with... which seems to contradict the assumption that the pt's help prevent the overvoltage).
Yes, we clear our grounds usually at most within a day or so. Do the plants that have the problems ignore their ground indications?
Based on busbar's previous comments I got a copy of Beeman. There are at least 3 scenario's mentioned. The first is associated with arcing. Does everyone here think that arcing does not apply to 480vac systems?
jb - I have reviewed the color books and they clearly express a strong opinion against the reliability of ungrounded systems. Can you by any chance provide the direct link to the ge document... I cannot find it.
This is an issue which will remain of keen interest to me. We have the systems, they operate well, and I am 100% sure we have no intention of adding a ground to these existing systems. But it remains a concern/question.
Back to the issue of pt's. Is it possible that ferroresonance is involved in some of these cases? (perhaps the initial ground provides the mild overvoltage which pushes the pt into it's ferroresonant range)
I notice we don't have any ferroresonance suppression resistors in our 480vac ungrounded systems but we have them in our 25kv isophase bus which at times is ungrounded. Any thoughts on why that is?
RE: intermittent arcing faults on ungrounded systems - 480vac
Although it has nothing to do with ferroresonance, sometimes 480V PTs connected ø-g are improperly specified with a nominal 277V-primary rating instead of 480V. When a ground fault occurs, two of three lights get REALLY bright for a little while, with 480V on the primary of a 277V PT until the fault is cleared.
(Note thread inception of 18 Jul 2001)
RE: intermittent arcing faults on ungrounded systems - 480vac
btw - I think that is very relevant to the issue of ferroresonance, which depends on saturation characteristics.
RE: intermittent arcing faults on ungrounded systems - 480vac
I am now comissioning a plant in Germany and the low voltage system in use is the new European 690V voltage.
The system here is defined as "IT system" meaning that the neutral is not grounded.
The system has the required Insulation monitors that will alarm on the occurence of first ground fault.
The plant has now been in service for six months and we are experiencing a high number of Variable Speed Drives failures which are in my opinion caused by the so called "arcing ground faults".
I have not much experience with these systems as they are not nowadays so much in use in Canada where I come from.
Condulting the Merlin Gerin Technical Publications I have noticed that French codes require the use of surge suppressors (with optional resistor) between the transformer neutral and earth, has anybody tried this on their systems? Did anybody experience problems with VFD's on such IT systems?
Regards,
Denis R. Boudriau
RE: intermittent arcing faults on ungrounded systems - 480vac
Denis — Older [and current] material describes the near-absolute need for a neutral-to-ground continuous-rated resistance whose value is somewhat below zero-sequence capacitive reactance of the associated low-voltage system.
A decent {online! free!!} paper on the high-resistance grounding is www.neiengineering.com/papers/paper1JN.pdf, although written in ANSI parlance, it briefly discusses application with drives and power-system high-frequency characteristics. Simple tuning of grounding resistor with L-C components is discussed.
To me, high-resistance-grounded LV systems serving power semiconductors need 'kid-glove' treatment, especially with rapid effort [via human intervention] at first-fault clearing.
There are also related Schneider Electric “Cahier Techniques” [173(+177)+178+204] pdfs www.schneider-electric.com/cahier_technique
RE: intermittent arcing faults on ungrounded systems - 480vac
Thanks for the interesting paper proposed, I did not have the time to read it yet as I have to go and work on the VFD problem today. As for the Cahiers Techniques I already consulted them and they are quite interesting and practical.
From what I could see from Merlin Gerin "Cahiers Techniques" the French specifications require the surge suppressors and and optionnal resistor of around 1k ohms therefore too high to be below the zero-sequence capacitance reactance of the system. It must probably still have some "damping" effect on arcing faults overvoltages.
This LV system design with ungrounded system was actually the result of the customer's request, not our design.
We normally use the high resistance scheme with LV systems, normally the Federal Pioneer DSP MKII or equivalent. The customer apparently has another plant in Germany in which ungrounded LV system are used but they are 380V not 690V, I think that makes a big difference. (690V is a relatively new voltage in Europe)
In any case I have yet to see if IEC and VDE (German)specs require anything similar as the French specs.
Happy New Year to all!
Denis