1200 volts on a 600 volt system
1200 volts on a 600 volt system
(OP)
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??
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??






RE: 1200 volts on a 600 volt system
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
RE: 1200 volts on a 600 volt system
RE: 1200 volts on a 600 volt system
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.
RE: 1200 volts on a 600 volt system
Keith Cress
kcress - http://www.flaminsystems.com
RE: 1200 volts on a 600 volt system
Also, have you had a look at the voltage wave form? Is the voltage consistently 1200 or does it vary?
RE: 1200 volts on a 600 volt system
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.
RE: 1200 volts on a 600 volt system
What voltage-to-ground is measured on the other 2 phases?
RE: 1200 volts on a 600 volt system
RE: 1200 volts on a 600 volt system
RE: 1200 volts on a 600 volt system
Maybe their voltmeter is on the wrong scale.
RE: 1200 volts on a 600 volt system
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: ground for an open delta 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?
RE: 1200 volts on a 600 volt system
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
htt
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 1200 volts on a 600 volt system
RE: 1200 volts on a 600 volt system
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?
RE: 1200 volts on a 600 volt system
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
RE: 1200 volts on a 600 volt system
RE: 1200 volts on a 600 volt system
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 1200 volts on a 600 volt system
Keith Cress
kcress - http://www.flaminsystems.com
RE: 1200 volts on a 600 volt system
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
RE: 1200 volts on a 600 volt system
RE: 1200 volts on a 600 volt system
Yes!
But
Regardless of the measurement issue there is a serious situation that must be resolved.
I would concentrate on isolating the fault. We can speculate later as to the reason for the measurement issue.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 1200 volts on a 600 volt system
Amen to that! It's just amazing that ungrounded systems still persist.
RE: 1200 volts on a 600 volt system
Outdoor 3 phase 600 volt power cable running up a wooden pole.A vehicle had backed into this cable and had damaged it.
Aside:
Our own calibrated voltmeter readings to-day indicated approx. 600 volts to ground on 2 phases and 0 volts on the third phase.With the damaged feeder disconnected,the voltage dropped to approx.347 volts to ground on all 3 phases.
RE: 1200 volts on a 600 volt system
RE: 1200 volts on a 600 volt system
gb7, now would be a good time to add high-resistance grounding to the 600-volt system.
Typically, it sized at ±1 ampere per MVA of serving transformer, with machine-tool control-power transformers with primaries connected grounded-wye, and secondaries in broken delta, with a resistor. For 1 ampere of system charging current, the transformers would be ±0.75kVA, and the resistor would be rated ±44Ω at 1.1kW. This assembly could be connected to any point on the 600V system—even a small branch circuit; minimizing the need for a system outage.
Voltage (or absence of it) across the resistor would be an indirect measure of 600V-system insulation quality.
John P Nelson has an outstanding paper on the subject regarding its use in the petrochemical industry. See—www.neiengineering.com/pdfs/paper1JN.pdf
RE: 1200 volts on a 600 volt system
Bill
--------------------
"Why not the best?"
Jimmy Carter