In researching some information on ungrounded systems and intermittent overvoltages in reading thru. IEEE-242 [Buff Book] 8.2.5 talks about the potential for overvoltages to ground that can be as high as 6-8X phase voltage. Since these systems are “ungrounded” is the 6-8X based off the elevated voltage to ground of the unfaulted phases? For example, on a 480V system the unfaulted phases would elevate to 480V is the 6-8X based on this voltage level?
Ungrounded Overvoltages
- Thread starter Mike01
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I expect that they are talking about what happens on a re-striking arcing ground fault on an ungrounded system. This does show up in the line to line voltage between the line with the fault and the other two lines. It can damage equipment.
There is some technical information about this available here
There is some technical information about this available here
Thanks Don good info....so in a 480V ungrounded that means at 6x it could evelate above 2500V....wow can really do some dammage...
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the IEEE green book also speaks to ungrounded systems, says that motors are most often damaged as the motor windings can't take to over voltage
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ajcobb.cobb
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- Kentucky
My experience with ungrounded 480 systems has taught me that because it is ungrounded doesn't mean it won't clear fault but in over voltage from dc arcing or induced voltage from very close high voltage transmission lines the only result has been higher chance to damge equipment. Portable welder and similar equipment are unaffected due to auto transformation inside equipment but motors are seriously effected. Like one temp powered in to a large a.c. unit which cost thirty thousand dollars in damage after said in some that was damage by overvoltage. It was attached to a ofc 287 feet in the air where steel had already been erected making it hard to get crane to. So if your equipment uses harmanics, has a motor not surge protected or with phase monitoring control system which will open circuit in over voltage due to abnormal sine waves, or generally is not a welder then bond it properly and ground nuetral to xo at source. The reason to do this also is to balance out loads which is the neutral job whether being connect to equipment or not. Also to remove over voltage due to induction of nearby transmission line it must be connect to a very low impendence point. It may still induce small voltoge regardless if grounded at xo and bonded to ground rod because the induce votage sources common neutral may be on another phase some wave. Not allowing electrons to return to source. In situation to remove high dc pulse a ground rod at equipment or other bonding means at equipment to allow this removal of electricity but the equipment can not be bonded to clear fault and must be labeled as warning ungrounded metallic parts and a radius established based on nfpa70e. I know this is in depth and may or may not of helped. I only tell ya these situations from personal experience and how and the only way to resolve them. Thanks
winnie
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- Springfield, MA, USA
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- Electric motor research
My understanding is that the line to line voltages remain relatively constant, but that the system to ground voltage is what gets elevated to 6-8x the l-l voltage.
Remember that the l-l voltage is 'forced' to a specific value by the supply transformer, but since the system is 'ungrounded' the over-all system to ground voltage is undefined and can be raised by fairly low power source.
In a re-striking ground fault, you have the capacitance of the entire system periodically discharging through the inductance of the wiring up to the fault. You have an accidentally created 'boost' switching power supply which charges up the _faulted_ leg to high voltage relative to ground. The rest of the system is in fixed voltage relation to the faulted leg, so the entire system ends up at high voltage relative to ground.
The high voltage to ground then damages insulation by punching through to ground, opening the door to further l-g and l-l faults.
-Jon
Remember that the l-l voltage is 'forced' to a specific value by the supply transformer, but since the system is 'ungrounded' the over-all system to ground voltage is undefined and can be raised by fairly low power source.
In a re-striking ground fault, you have the capacitance of the entire system periodically discharging through the inductance of the wiring up to the fault. You have an accidentally created 'boost' switching power supply which charges up the _faulted_ leg to high voltage relative to ground. The rest of the system is in fixed voltage relation to the faulted leg, so the entire system ends up at high voltage relative to ground.
The high voltage to ground then damages insulation by punching through to ground, opening the door to further l-g and l-l faults.
-Jon
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