I have come across a transformers with one phase grounded....I know this might sound funny but since there is no difference in potential in that particular phase it will read 0 volts when I check it with my meter from that phase to ground..My question is why is it gounded like that and please explain in detail....??..I remember the instructor at school telling us about transformers like this but my memory has gotten the best of me....
Transformers with one phase grounded
- Thread starter msilva
- Start date
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- Location
- Massachusetts
Re: Transformers with one phase grounded
In general one terminal of most every transformer is required to be bonded to the grounding system.
If you did not bond the transformer to ground circuit breakers and fuses would not operate when one of the transformers conductors makes contact with grounded objects.
Some industrial operations run ungrounded systems. To do this they must install ground fault monitors.
In general one terminal of most every transformer is required to be bonded to the grounding system.
If you did not bond the transformer to ground circuit breakers and fuses would not operate when one of the transformers conductors makes contact with grounded objects.
Some industrial operations run ungrounded systems. To do this they must install ground fault monitors.
gregoryfast
Member
- Location
- Alaska
Re: Transformers with one phase grounded
Without knowing the voltages you found, my guess is that you found a 3 wire grounded delta system. This system is used to prevent rotating phase to ground faults which can result in very high voltages.
Without knowing the voltages you found, my guess is that you found a 3 wire grounded delta system. This system is used to prevent rotating phase to ground faults which can result in very high voltages.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: Transformers with one phase grounded
Not so fast Gregory
Not so fast Gregory
Not true. A grounded delta is just what Bob said it provides a low impedance path to open the OCPD devices. It won't prevent a fault quite the contrary it will allow a fault. In a ungrounded delta system the fault would only happen when the second grounding of a ungrounded circuit conductor would occur. This is done to prevent a unnecessary shutdown of equipment that could cause more harm that the fault would. And as Bob said with a ungrounded system it would be required to install a ground fault monitor to alert someone to have it repaired at the first point the system can be safely shut down.
buzzlectric
Member
- Location
- Iowa
Re: Transformers with one phase grounded
we have a couple of these , we call them "corner grounded delta " and we are still trying to figure out how 3 phase equipment runs on it.
we have a couple of these , we call them "corner grounded delta " and we are still trying to figure out how 3 phase equipment runs on it.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: Transformers with one phase grounded
Buzz
Having one phase grounded does not change the way the 3 phase works. keep in mind that transformers are isolating so after every one there would be no reference to earth or any other grounding but by bonding one of the phase conductors. All three phase conductor still are 120? out of phase with each other and the rotational effect is still there. but because of the grounded phase you just have two polls on a breaker instead of three. (you don't need to protect a grounded phase)
Buzz
Having one phase grounded does not change the way the 3 phase works. keep in mind that transformers are isolating so after every one there would be no reference to earth or any other grounding but by bonding one of the phase conductors. All three phase conductor still are 120? out of phase with each other and the rotational effect is still there. but because of the grounded phase you just have two polls on a breaker instead of three. (you don't need to protect a grounded phase)
gregoryfast
Member
- Location
- Alaska
Re: Transformers with one phase grounded
Maybe I didn't explain myself very well. The grounded corner delta is a way to have a 3 wire system and trip the OCPD rather than energize building parts. This is how it prevents the capacitive buildup of high voltages when a part of an armiture goes to ground on an electric motor. As the armiture spins around it makes contact with the metal frame. This causes high voltages to devleop.
Here is a ~50-year-old account of the perils on LV ungrounded systems?as opposed to high-resistance grounded systems. This all-time classic description is detailed in a chapter on system grounding in Beeman?s "Industrial Power Systems Handbook". [
"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
Manager, Industrial Power Engineering / Industrial Engineering Section,
General Electric
McGRAW-HILL BOOK COMPANY, INC. 1955
Maybe I didn't explain myself very well. The grounded corner delta is a way to have a 3 wire system and trip the OCPD rather than energize building parts. This is how it prevents the capacitive buildup of high voltages when a part of an armiture goes to ground on an electric motor. As the armiture spins around it makes contact with the metal frame. This causes high voltages to devleop.
Here is a ~50-year-old account of the perils on LV ungrounded systems?as opposed to high-resistance grounded systems. This all-time classic description is detailed in a chapter on system grounding in Beeman?s "Industrial Power Systems Handbook". [
"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
Manager, Industrial Power Engineering / Industrial Engineering Section,
General Electric
McGRAW-HILL BOOK COMPANY, INC. 1955
paul
Senior Member
- Location
- Snohomish, WA
Re: Transformers with one phase grounded
The sawmills around here are corner grounded delta's. That way if there is a ground fault, the head saws don't shut off while trying to ram-rod a log through, which is a recipe for disaster. Instead the light goes off, they do a controlled shutdown and search for the ground fault to repair it.
I friend of mine was on one such crews searching for the ground-fault at a local mill that took four guys 8 hours just to find it. It was at the weather-head on the service conductors.
The sawmills around here are corner grounded delta's. That way if there is a ground fault, the head saws don't shut off while trying to ram-rod a log through, which is a recipe for disaster. Instead the light goes off, they do a controlled shutdown and search for the ground fault to repair it.
I friend of mine was on one such crews searching for the ground-fault at a local mill that took four guys 8 hours just to find it. It was at the weather-head on the service conductors.
- Location
- Illinois
- Occupation
- retired electrician
Re: Transformers with one phase grounded
Paul,
That sounds like an ungrounded system. A ground fault on a corner grounded system is a line to line fault and should take out the OCPD.
Don
Paul,
That sounds like an ungrounded system. A ground fault on a corner grounded system is a line to line fault and should take out the OCPD.
Don
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