1200 volts on a 600 volt system 2

[ghostbuster7]

At approx. 4 pm everyday the phase to ground voltage jumps up from 347 to 1200 volts on 1 phase of a 600 volt delta system in a plant.

15-20 hours later the voltage drops back to 347 volts phase to ground.

Several large motors and circuit boards have alrerady been damaged.

Will be at site this week to investigate further.Could this be an arcing ground??

 

[waross]

It could be anything but it has to be fixed. Start by checking switching events at 4: PM and 15-20 hours later. Don't discount devices being switched of.
If this was 480 Volts I would suggest checking any devices with 240/480 Volt ratings to see if a series connection between windings was arcing to ground.
I would use a process of elimination to find the fault. Once found, it may be an arcing ground, but knowing that may not be much help finding the trouble.
Hint, you may get lucky carrying a small radio with you. Arcing grounds usually entail a lot of random RF radiation. If you find that an AM radio leads you right to the trouble and makes you look like a hero, you owe me a coffee and a doughnut at Timmy's. If you try the radio, do it quietly, it may not be of any use.

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

 

[dpmac]

When you say "everyday" does that mean this has been going on for days, weeks or months? Does it include weekends? When are the shift changes? Does the plant run 24/7?

 

[ghostbuster7]

Yes-everyday,7 days a week for approx.the last 8 weeks that it has been monitored.

Yes plant runs non-stop with various staggered shifts and staggered work production.Approx.700 employees doing various types of repair on heavy equipment(welding,frame bending etc.
Other buildings provide the production support services etc.

 

[itsmoked]

Got a bank of power factor correction caps lurking somewhere?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[alehman]

Excellent question.

Also, have you had a look at the voltage wave form? Is the voltage consistently 1200 or does it vary?

 

[ghostbuster7]

As usual, this place does not have a single line available.Will have to mentally put a single line together during the site visit walk around.At present ,I have no idea what components are connected inside this plant.(this is the traditional "black box" operation)

Waveform data by site personnel has not been recorded.The site monitor recorded consistent 1200 volt levels for these 15-20 hours each day.

Thanks for all your suggestions,will keep you posted.

 

[busbar]

What voltage-to-ground is measured on the other 2 phases?

 

[ghostbuster7]

Voltage to ground on the other 2 phases, according to site personnel was 340-350 volts

 

[davidbeach]

Nah, probably not that. The 1200V to ground was possible though highly improbable. 1200V, 350V, 350V to ground is not at all possible unless you also have considerably more than 600V phase to phase on at least two phase pairs.

 

[dpc]

I'd want to investigate how they are taking these measurements. These voltage readings are not making a lot of sense.

Maybe their voltmeter is on the wrong scale.

 

[rbulsara]

I would think any continous arcing fault for 15-20 hrs would have shown up by now in form of plume of smoke or flames. Or would have caused a permanent damage and wont keep coming up and going with such regularity.

It is probably related to switching off of some equipment which also is reconfiguring the grounding pt (even on the primary side)??

Many fact need to be gathered.

What do the phase to phase voltage read when one L-G is reading 1200V? Do they all remain 600V or different?

How is the delta system derived? What is the primary side of the delta secondary? Three or two transformers?

See somewhat related thread thread238-237266 and magoo2's attachment therein and see if any one of the setup resembles your system.

How come a delta system show balanced 347V L-G, perhaps a grounding transformer or a balance load such as wye connected caps?

 

[waross]

The voltage to ground on a large ungrounded 600V system can be expected to be 347 volts so the 350 volts looks good. I suspect that you have a piece of equipment failing with a high impedance arc to ground. The arc combined with the system capacitance is acting as a spark gap transmitter and superimposing high frequency RF on one phase. The high impedance of motor and transformer windings to RF is isolating this RF to one phase. It is probably something small as the magnitude of the RF energy does not seem to be enough to make much change to the voltage to ground onthe other phases.
I suspect a wye connected winding in a motor or transformer is arcing to ground near the internal wye connection and the winding is then acting as an auto transformer and boosting the RF to 1200 v.
If you have discharge lighting connected line to neutral that is a prime suspect. Either something turns on at 4 PM or you get a voltage spike at 4 PM that starts the arc.
I'll bet a box of Timbits to a Double Double that that is the problem. (Timbits, Double Double, Canadian vocabulary, not a big bet.)
Google.ca

http://www.timhortons.com/ca/en/menu/snacks.html

http://www.cbc.ca/arts/story/2004/06/30/doubledouble040630.html

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

 

[rbulsara]

It appears Bill may have seen this before. You better find the culprit soon..or he will end up getting the whole Dunkin Donut shop (or its Canadian equivalent!)

 

[ghostbuster7]

Answer to Questions:

Phase to Phase voltage is approx. 600 volts on all 3 phases during this event.

GE documented a case several years ago of measuring over 30,000 volts phase to ground during an arcing ground fault on a 480 volt delta system(20-30 motors blew during that timeframe).I believe this could be a similar type of occurence at a lower magnitude-the mechanism could be as described by Waross(Bill)
Does Timmy's sell gift cards?

 

[ghostbuster7]

Back from site,
1 phase is now solidly grounded 0 volts phase to ground
other 2 phases are 340-350 volts phase to ground(still 600 volts between the phases)

Now begins the hunt to find the grounded phase-can't turn any power off ,it will seriously impact the production

 

[davidbeach]

Draw a triangle with three sides each 600V long. Now pick one of the corners as being 0V to a fourth point labeled ground. How can you have 340-350V from the other two corners to this point labeled ground? There is a measuring problem, could be that the measuring device influences the measurements; you need to find a way to measure all 6 voltages (3*L-G, 3*L-L) simultaneously, preferably with phase angles and magnitudes.

 

[waross]

Hi David, is it possible to have 350 Volts, 60Hz to ground on two phases and a mixture of broadband high voltage RF on the third phase that the algorithm in the test instrument interprets as 1200 VAC? The energy would be low in relation to the normal leakage to ground on the other two phases. I'm thinking that a ballast may be developing an arcing fault in the winding and then acting as a auto transformer to boost the voltage up. Because of the high impedance of the ballast winding at high frequencies there may be little energy in the high voltage.

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

 

[itsmoked]

I wonder if everyday at the sacred time a VFD doesn't start up close by the measurement location..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[busbar]

Here is a ~50-year-old account of one of the perils in LV ungrounded systems.

Chapter 6 — System Grounding

About midafternoon one day in a West Coast manufacturing plant, normal operations became suddenly disrupted. The first evidence of trouble came in the form of a motor failure on the 480-volt system, then another, and still another in close succession. An inspection of switchboard voltmeters (measuring line-to-line volts) and ammeters indicated no unusual conditions. System equipment continued to fail. A test voltmeter was rigged up having a full-scale calibration of 1200 volts. Upon connecting it phase-to-ground, the pointer went off scale. A phase-to-ground potential on a 480-volt system of more than 1200 volts existed!

At once the incoming service transformers were suspected of internal breakdown between high- and low-voltage windings. As the last of these transformers was isolated and individually tested, it became evident that they were not at fault. System equipment continued to fail, and the situation was desperate.

A frantic group went into a huddle and decided that the only way out was to trip the main incoming service breaker, which would deenergize the entire system. At this point one of the workmen noticed a small wisp of smoke coming from a motor-starting autotransformer and, upon approaching, could hear a buzzing noise inside. This circuit was switched clear of the system, and the overvoltages disappeared. During the two-hour period that this arcing fault existed, between 40 and 50 motor windings had failed.

Finally it was found that the autotransformer enclosing case had been hashed in and was practically in contact with the coil. The spot where arcing had taken place was evident although not badly burned. An attempt was made to show the plant engineer what had been the trouble. A solid connection was made between the frame and the burned spot on the coil. Much to the bewilderment of the operating men and according to the expectations of the plant engineer, no more than the 73 per cent increase in the voltage to ground on the other two phases occurred. The main ingredient of the overvoltage (discontinuous conduction) had been omitted.

This is an actual case of severe prolonged experience of overvoltage of repetitive restrike origin on a 480-volt ungrounded system. This story is spectacular because of the magnitude of the disturbance and consequential damage. Similar occurrences of lesser extent are not uncommon, however, and there is evidence that they are more frequent than realized. It is a characteristic of ungrounded systems that they are subject to relatively severe transient overvoltages. This trouble can be avoided by proper grounding of the system, and other important benefits are also obtained.

Pp. 337-338 L. J. Carpenter and L G. Levoy, Jr.
Industrial Power Systems Handbook, DONALD BEEMAN, Editor
McGRAW-HILL BOOK COMPANY, INC. 1955