Power Distribution Transformer
Power Distribution Transformer
(OP)
Gentlemen: Please forward any ideas.
13.8KV/480,3PH/3W,1000 KVA, Delta-Y, Resistance Grounded serving a Motor Control Center. No Neutral Conductor.
Measured Secondary Voltage Phase to Phase 480V, all Phases.
Measure Phase Currents 80 A, 80 A, 85 A.
Measure Secondary Voltage Phase to Ground at the MCC:
P1 - 420 V
P2 - 380 V
P3 - 77 V
Any ideas about whats going on with this transformer and distribution system ?
13.8KV/480,3PH/3W,1000 KVA, Delta-Y, Resistance Grounded serving a Motor Control Center. No Neutral Conductor.
Measured Secondary Voltage Phase to Phase 480V, all Phases.
Measure Phase Currents 80 A, 80 A, 85 A.
Measure Secondary Voltage Phase to Ground at the MCC:
P1 - 420 V
P2 - 380 V
P3 - 77 V
Any ideas about whats going on with this transformer and distribution system ?






RE: Power Distribution Transformer
I'd suggest you take it out of service and look for the ground.
dpc
RE: Power Distribution Transformer
The grounding resistor is most likely connected from the 1000kVA-transformer XO bushing to system ground bus. It’s probably running fairly hot (and must be rated for continuous duty). The idea is that the 480V system will continue to operate with a single phase-to-ground fault, UNTIL another fault develops on another phase. If the fault not located, the second ground fault them you may have two overcurrent devices operating simultaneously. Current during a solid fault should only measure a few amps maximum on a 1MVA 480V system.
Do the voltage readings to ground change periodically? If they move back to balanced for a time then the fault may be downstream of a magnetic starter that cycles through its operating process. The fault can be found by a methodic process of elimination, isolating sections of the 480V system.
The resistor is crucial for damping transient overvoltages caused by capacitive phase-to-ground current that exists naturally—and will limit their damage to insulation. Instead of constantly swapping meter leads for phase-to-ground readings, you can just monitor voltage across the resistor. With normally good insulation, voltage across the resistor should be close to zero. (Conversely, you would expect about 277V with a solid ground fault.) This is good justification for having consistent and well-identified phases throughout the system.
Check Thread238-3370 Thread238-6870 Thread237-7403 Thread238-6870 Thread238-9005 Thread238-10998
RE: Power Distribution Transformer
RE: Power Distribution Transformer
We are a little embarrased by this incident as we know what the described measurements indicate. We just could not find the groundfault.
However, after further investigation we found that the grounding resistors and associated controls mounted in the Electrical Equipment Room were mislabeled. Essentially, we were checking out the wrong MCC because of this labeling problem.
Happily, with the newly discovered knowlege we were able to quickly located a groundfault on the P3-Leg of the correct MCC in an analyzer shelter HVAC load.
Elaboration on the lessons learned from this incident are unnecessary. I am sure you guys know there are many.
Regards
Lighting Bolt
RE: Power Distribution Transformer
You can make your own pulsing circuit w/500V/10A contactor + small timer control circuit to pulse the contactor on & off to switch a test resistor in parallel with the existing grounding resistor. W/O knowing the existing grounding resistor value it's hard to say what value of test resistor to use. Measure the current & voltage across the grounding resistor and calculate new value for the parallel test resistor.
f. ex if grounding resistor is 27.7 Ohm (10A @277V) a 100 Ohm 500W test resister would increase the fault current by ~3A.
The goal is to increase the fault current at the fault by lowering the grounding impedance enough to get a measurable current change that can be measured w/ amp probe. Usually 3 to 5 A is enough. Every time contactor closes fault current will increase. This can de detected on the faulted phase and traced downstream to the fault source w/o shutting down entire system.
RE: Power Distribution Transformer
As far as pulsing systems for high-resistance grounding fault location are concerned, I've found them to be a colossal waste of time. The low-range big-jaw ammeters look good in the literature, but in my experience, the fault/leakage current divides and goes a hundred different directions. For me, much more effective is a widely visible set of “ground lights” that be seen while observing processes and equipment cycling. But then a 2500kVA unit sub feeding 135 motors can require some tedious searching, and make timely fault isolation difficult and sometimes ignored.
Supporting manufacturers’ information at:
http://www.schneider-electric.ca/www/en/products/ground/F0980AB9901EPR1.pdf
http://www.geindustrial.com/products/manuals/GEI-72116.pdf
RE: Power Distribution Transformer