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The Physics of Elecrtrical Failure: Ungrounded Systems
3

The Physics of Elecrtrical Failure: Ungrounded Systems

The Physics of Elecrtrical Failure: Ungrounded Systems

(OP)
I am admitted technocrat... one, who upon learning that 4,378,573 people were married last year, asks the question "why is it not an even number!"  In other words, I'm a detail nut... requiring factual explanation of electrical failures.

Now I would like to respond to the 3 M's... misconception, misunderstanding, and malarkey... existing in past threads covering "electrical faults!"  Initially I would like to address: a) ungrounded systems; b) arcing ground-faults; c) unwarranted fuse blowing; and any others that posters agree will fit the thread.  This post will start with the dreaded "ungrounded systems.  Others will follow:

a) Ungrounded Systems.
First of all it's a misnomer.  While it is true that such systems are not deliberately connected to a grounded reference point, their phases are still capacitively connected to "ground."  All electrical components have ground capacitance... motors, heaters, power-factor correction capacitors, surge arrestors, bus duct, transformers, and the most significant, cables.  And, as long as capacitance is equal on all three phases, then the system ground becomes the electrical-neutral-point.  As a result, all three phase-to-ground voltages are equal, i.e., Vpp/sqrt3.

A little history.  I cut my engineering teeth in one of those industries that employed an ungrounded 480V system.  Justification was based on the "false belief" (quotes are mine) that "an inadvertent ground-fault won't cause an undsirable outage!"  See NEC for exact verbage.  It was anr old plant, 50+ years.  Yes, all substations were equipped with ground-detectors in the form of wye-connected 480-V bulbs connected between phase and ground!  One day, I noticed all bulbs were out in one substation, but only two fuses blown.  While the ph-to-ph voltages were the same, the grounded phase, say 'A' showed 0-V, but the "sound" phases 'B' and 'C' showed 800-V, and 1,200-V, respectively.  Furthermore, I learned that everytime this happened, 3, 4, sometimes more, motors would fail within 1 to 3 weeks.

Now about the intermittent arcing phenomena... While I agree that intermittent phase-to-ground exacerbates the problem, I assure you, arcing is not involved, at least not by my definition.  The root-cause is transient-overvoltage, not current.  In short, the 'sound' phases are exposed to extreme overvoltages, resulting in insulation breakdown.  As a consequence, a phase-to-phase fault occurs (now current is involved), blowing fuses or tripping breakers.

Several observations were made in the thread regarding ungrounded systems that didn't exhibit such failure.  It has nothing to do with luck, nor good-maintenance.  The parameters that lead to this phenomena are very large cable systems, hence large ground-capacitance and old systems having degraded system insulation.  One mention was made about a Navy system being OK, but I have two observations by Navy personnel with just the opposite observation.  Also, I would venture an educated guess that such a fault receives very prompt response, compared to land-based systems.

Regarding the plant I was in.  A change was made to a grounded system and motor failures disappeared.

BTW, this phenomena also explains most of the mysterious MOV failures on VFD systems.
 

RE: The Physics of Elecrtrical Failure: Ungrounded Systems


My personal best for this scenario was a steady-state reading of >1800V on a 1000V multimeter applied to a “480V” system.  Lucky for me the meter didn’t detonate.  

Wet-process industries seem to be most intolerant of ungrounded LV systems, and it’s almost humorous to find plantfuls of 480Y unit substations with NO connections whatsoever to their XO bushings.  Surge protection alone seems a poor solution for this type of transient [or sustained] ground overvoltage.  Unless they are of epoxy-potted machine-tool varieties, small control-power transformers seem notorious for failure exposed to the ungrounded-system mode.

A classic account of perils is in Beeman’s 1955 text, excerpted at  67.115.161.42/dat/beemaIPSH6z.doc

John P Nelson has a recent paper that also briefly mentions harmonics on ungrounded systems at neiengineering.com/papers/paper1JN.pdf  [A searchable version is available from IEEE.]  
  
Resistors connected to smaller grounded-wye/broken-delta transformer banks [id est, PT sets] are good for ground detection/relaying over a wide range of voltage classes.  For reliable annunciation, the “59G” type of relay needs to be able to pickup at <0.1 p-u, but continuously withstand at least 1.73 p-u voltage—third-harmonic restrained.
  

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

Polygamy.

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

Stevenal:  thanks for restoring my hope for mankind.  The first thing that occurred to me was marriage outside the species.

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

(OP)
stevenal, and peebee,

Huh?

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

We were responding to the first question in your post.

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

(OP)
Sorry, but you took me by suprise!

In the short time I've been a member, I haven't seen much humor associated with this forum.  Perhaps, an engineer thingy?

So keep'm coming!

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

Humor still catches me off guard too.  

Satire, cynicism, and irony abound, as is evident by inspection from more of my posts than I care to admit.  But you're right on that true humor in engineering and on this site is rare.

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

Suggestion: The ungrounded system on ships has relatively low voltage, e.g. 450V, or perhaps 11kV on aircraft carriers. The power distribution systems on high voltage side do experience flash overs and the transients would be very detrimental to the grid stability. Therefore, the grounded systems are more suitable for the power distribution systems. Also, the ships do not have that many loads, e.g. motors, as the industrial objects may have. Also, the cost of production was also involved in the private enterprise system. The NAVY is the government system where some overvoltages and damages due to ungrounded systems have been going unnoticed in the huge DOD budgets.

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

How many times did Jennifer Lopez marry last year?  If she was married twice, and each of her husbands married once, then that would be three persons married during the year (an odd number).

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

Do you have any experience with High Res. Grounding  and morts that may fail from insulation over-voltages?

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

I meant motors!

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

(OP)
Edjh2,

I have not seen any, at least in my years involved with Exxon HPI plants.

On the other hand, I've seen numerous failures related to arcing ground-faults, in solidly-grounded LV systems!

RE: The Physics of Elecrtrical Failure: Ungrounded Systems


edjh2 — Please search the electrical forums {fora?} of this site.  It has been extensively discussed here.  
  

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

(OP)
etal,

Now the following might not meet the criteria of all on this forum, but some might find it "interesting!"

Of course Jbartos is right-on regarding the military's reluctance to accept civilian practice.  Know why?

And now the rest of the story!  I remember (that memory thing again with significant events) one ship in particular having a rather unigue power distribution system.  Four large generators (ea, 1,000 kw) and four propulsion drives (ea, 1,000 kW)were connnected in series by a single 1,000Vdc, 1,000-Amp, 2-wire loop.  Machines were switched in and out depending on propulsion requirements.  I'ts no longer afloat, but its name was transferred to the Coast Guard rescue cutter in the movie "The Perfect Storm!"  Guess its name?

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

Shortstub - Not being very familiar with your setup and this scenario, I am trying to work through understand your initially described scenario.

The fuses which blew I assume are the primary fuses protecting your pt's?

They blew I assume because of high current due to high voltages to ground on non-faulted phases?

The voltae measurements were taken on line side of the blow fuses.. connected to the energized 480v transformer winding?

The measurements show normal phase-to-phase but phase to ground are 0, 800, 1200?

One thing that comes to mind is I cannot even draw a phasor diagram which satisfies this.  If Vag=0 and Vcg=1200, then I expect Vac=Vag-Vac=1200.

What is the explanation for that? Harmonics interfere with the measurement?  Erratic time varying measurements? Or am I missing something else?

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

Typo alert - Should have been: "I expect Vac=Vag-Vcg=1200"

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

(OP)
Electricpete,
Good pickup!  You are right in questioning my observation.

The event I mentioned took place about 48 years ago.  What I should have said, was, although the voltage measurements between "ground" and the "sound" phases showed very high magnitudes (between 800-1,200 Volts, and probably not coincident) the phase-to-phase measurements were balanced @ about 480!   It was the phase-to-ground overvoltages that led to subsequent insulation failure, on a number of motors.

Another "mea culpa", the 480 "bulbs" I mentioned were actually 2x240 Volts.

I tried to emphasize that the phenomena is not necessarily dangerous in all ungrounded systems... but dependent on system size (ground-capacitance magnitude) and insulation age (deterioration).  And more importantly, not restricted to 480 and greater voltages.

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

Heres what I know:
1.  "Ground" is like a reference point.  
2.  Although I have read about it and heard about it, I still have no idea what "ungrounded" means.

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

Oranjeep:

Grounded = "Connected to earth or to some conducting body that serves in place of the earth" per NEC Article 100.

Ungrounded can mean theres no intentional OR unintentional connection to ground (or neither) depending on the context.

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

My underatanding has always been that none of this matters provided the system is installed and operated to minimize (peak phase-to-ground voltage over time causing insulation breakdown and future failures x production time lost to the immediate effects of a single fault).  If in a particular situation that is accomplished by a high resistance ground indicator with a good tracking policy, or a solid ground with a fast diagnostic/repair crew really makes no difference.  The economics should dictate, so the question becomes "what are the economics"?

Pechez les vaches.

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

I mostly agree with you, lengould:  the right questions to ask are "what are the functional needs/desires/economics".

But keep in mind there's code requirements for solid grounding on some systems too. . . .

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

Suggestion: Reference:
1. Donald Beeman "Industrial Power Systems Handbook," First Edition, McGraw-Hill Book Company, Inc., New York, 1955.
Chapter 6 by L. J. Carpenter, and L. G. Levoy, Jr.
"System Grounding"
It reads smoothly as a bestseller.

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

There is a solution to the transient over voltage spikes and phase voltage instability in an ungrounded power system. Phaseback was designed for just such a problem. We have them in our facility with all 35 of our 2000 kva power transformers protected and haven't had the instability generally associated with ungrounded delta power systems. The link is:

http://www.applied-energy.us

I hope this helps
Bill

RE: The Physics of Elecrtrical Failure: Ungrounded Systems

Some electrical utilities have measured parasitic AC voltage as high as 2,000 volts on an ungrounded 480 volts system. The usual culprit for overhead services was capacitance coupling from higher voltage lines. There are some other culprits. What makes the parasitic voltage go away is to connect three 100,000 ohm or 1 megohm resistors from each phase to ground. These resistors have to have a hefty power rating to drain off the parasitic voltage without overheating. 1 megohm resistors are a rather old trick. !00,000 ohm resistors would need a 40 watt or higher rating to drain off parasitic voltages without overheating.

I have seen a ground monitor that used say 500 va or 1 kva control transformers and 130 volt 200 watt light bulbs. Ground detection light bulbs in this size range will absorb all parasitic overvoltages.

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