Arcing in MCC

[gordonl]

I've recently had an MCC tub arcing at the bolts which fasten the tub to the frame. We found that the tub grounding system was quite poor so we added grounding jumpers to each tub. We couldn't find anywhere on the tub, front or back, where there had been a phase to ground fault, and the upstream ground fault protection never tripped.

My question is: Could induced voltage in the tub have been enough to cause the arcing. The arcing was quite severe at one of the two bolts, causing welding to the panel and black burn marks, along with black burn marks at approximately 10 other bolts between sections of the tub, and the tub and the frame and rails. The tub contains a 600A switch and fuses which feed the MCC. The max load on the MCC is two 200HP motors @ 600V, simultaneous start. (Simultaneous start is now prevented) The motors are on hydraulic pumps, and the starts are relatively tame.

Thank You,
Gord

 

[Shortstub]

Gordonl,
Where is the neutral CT installed?

1) Is it between the transformer phase winding "wye" point and the neutral groundED conductor of a 4-wire system?
2) Is it between the neutral groundEd connection in 1) and the groundING conductor to the ground-plane or ground bus-plane?
3) What is xfmr size?

 

[peebee]

You should realize that your busduct enclosure is the only ground conductor required by code, and you should focus your ground checks on the continuity of that enclosure to ground. That said, where does the other end of your #3 ground terminate at (hopefully, not a ground rod driven next to the MCC)?

Here's some suggestions:

1. Check resistance of MCC to ground via the busduct. Don't rely on visual inspections. Anything over, say, 5 or 10 ohms should be improved.

2. Check ground-neutral bond at the source panel or transformer. Resistance/voltage should be very low.

3. Check for ground currents at the MCC with a gaussmeter.

 

[gordonl]

Neutral CT in located in the neutral bushing, so between wye point and ground connection. (3 wire system)

 

[GusD]

Hi Gordonl
You said that bonding should be OK now.The burnt bolts and consequent damage has been repaired.
I don't know how long ago you corrected this problem,
perhaps it is too soon to claim victory, if there are no further signs of burnt bolts.
As to your question "can induction" cause this problem? For the people that have experienced first hand the effects of induction problems with electrical
equipment,this problem is not that rare.
Electrical Systems are protected for most of the other possible faults,there are no protections for induction caused problems.

GusD

 

[Shortstub]

gordonl,
Now we're getting somewhere. How is the #3 (did you mean #3/0 AWG) grounding cable routed to the CT? And, the transformer size? Your data indicates about 4 to 500 kVA.

peebee,
DC resistance won't do the job. The ground-return path is an AC impedance.

An easy way to measure impedance is to supply the transformer primary with a low voltage source, say 600V, and run phase-to-phase and phase-to-ground current tests at the MCC. The tests will yield the positive and zero-sequence impedances, Z1 and Zo, respectively. Then the ratio of zero-sequence to positive sequence will indicate if the arc-fault phenomenon is still present.

 

[jbartos]

Suggestion: The return ground path should be adequate for the projected current that is supposed to safely carry. This means that the proper installation of return ground path connections, especially through the paint and good two locknuts where applicable, should be implemented. The ground conductor should improve the ground path quality noticeably.

 

[peebee]

Good catch, shortstub. I tend to use "resistance" and "impedance" somewhat interchangeably, more often than I should. I would argue that if the resistance of his ground path is OK, then the total impedance is probably OK, too. But you're right that measuring the total impedance would be more accurate.

Either way, measuring resistance or impedance, would be better than an eyeball inspection.

 

[gordonl]

Transformer is 2.5MVA. Wye point ground is about 70' long running to sub ground, as well as connected to ground grid. (Not run in tray with power cable) The transformer neutral CT is in the bushing of the transformer.

The run to the MCC is two #3awg run in teck cable. The ZSCT on the MCC feeder has no grounds running through it.

 

[Shortstub]

The addition of the internal #3AWG wire proved the inadequacy of the Bus-Duct enclosure as the "ground-return" path.

Now there is another inadequacy. Based on the size of your transformer, I believe the #3AWG is not large enough to accomodate the potential ground-fault current! The cable will certainly fuse if you are relying on a 51 relay for protection!

 

[peebee]

Shortstub, re: "The addition of the internal #3AWG wire proved the inadequacy of the Bus-Duct enclosure as the "ground-return" path." -- I don't follow you there, please explain.

The #3 does sound undersized, though.

Gordonl: Where is the other end of the #3 bonded to? To a switchboard ground bus?

 

[Shortstub]

Peebee,
Gordonl's Apr 21 post mentioned a earlier severe arcing fault in the bus-duct of this sub. This is indicative of an ineffective ground-return path. Ground-current tends to flow in a path in close-proximity to the phase conductor. Tarc-fault (presumably degree of damage) of the enclosursub.

Mathematically the Zo to Z1 ratio is very large, resulting in too small an equivalent rms current to be "seen" by the 51relay. He said that the fault had to become ph-ph before the upstream (presumably the hi-voltage side of the transformer) responded.

Gordonl,
How is the #3 conductor routed to the transformer's neutral bushing? Via magnetic or non-magnetic conduit?

 

[jbartos]

Suggestion: The grounding conductors tend to be somewhat on the higher side or exaggerated, e.g. 4/0, 500MCM, and 1000MCM in substations.

 

[gordonl]

Bit of a misunderstanding, the transformer neutral connection is 500MCM connected to the ground grid in the transformer station, with a riser upto the substation ground bus. The riser is run seperate from the new power cables which run up to the bus in tray.

The #3awg ground is used from the sub to the MCC, in the teck cable. To runs in parallel, thus two #3awg in parallel to MCC in question.

 

[Shortstub]

Gordonl,

If the #3AWG wire is the groundING-conductor associated with the 3-wire bus-duct supplying the MCC, you still have a problem! Is the teck cable containing the #3AWG conductor, routed so that it is in close proximity to the bus-duct?

The 500MCM groundING-conductor to the main sub grid, is ineffective in this situation.

 

[gordonl]

Let me start from scratch with the current system description. A 2500kVA transformer feeds a sub approx 70' using power cable in tray. (Used to be bus duct.) Grounding from the transformer to the sub is 500MCM run seperate from the power tray.

From the sub to the MCC there is 2-3C-500MCM Teck with #3awg bonding cable.

All of the grounding and bonding conductors are connected to ground busses, columns, and grids.

So total ground return path is:
a)two #3awg ground conductors from MCC to 600V sub for about 250'
b)1 500MCM from sub ground to transformer neutral for about 70'
c)as well as parallel paths through columns, building ground loop, and earth.

 

[peebee]

So there's no busduct anywhere anymore?

 

[gordonl]

no

 

[Shortstub]

Gordonl,
You probably measured ground resistance, and are confident there is no problem. Measure phase-to-ground impedance using an AC current. I believe you will discover a much higher AC impedance!

But, nuff said from me!

 

[GusD]

Hello Gordonl

One could speculate forever as to the causes of your problem.History tell us, that similar problems are normally corrected by adequate bonding.If the problem persists after you have taken this first step,then you
may try the many suggested cures.
I would also make sure that my ground relays are properlly calibrated.

Good luck

GusD

 

[jbartos]

Suggestion: There may have also been electromagnetic induction, harmonics, unbalance load, etc. or a combination of those. The monitoring of the ground current in the ground conductor could help to find the root cause.