MCC Got toasted

[Electrogrunt]

At the facility I work at, we just had one of the RVAT starters catch on fire. We are still not sure what drove this, but what happened is that as the transformer was cooking, it spread an acidic residue throughout the MCC, impregnating the insulating busbar supports with a semi conducting material. After my co worker and I found that the busbars had about 1/2 MOhm between them, we decided to tear the whole thing apart, and clean every thing. This involved about 18 hours of work until we got the resistance into the 50 Mohm range. Still not great, but we will change out these supports during our next shutdown. This residue also affected the plc rack, so that none of the I/O cards worked, and the outputs were sending signals to start, with no command. Our area manger tried valiantly, although vainly to resurrect these cards by pouring alcohol on them, and heating them over portable lights. We suggested that they wait until the programmers could be contacted in the morning, and it was not until then that a resolution was found; replace all of the I/O cards, as well as the main processor card. Has anyone seen something like this happen, and were you able to determine what caused it? Also, as this happened in the evening, it was the assistant plant Manager who opened the main disconnect, a gutsy move that probably prevented a more serious outcome.

 

[iceworm]

At the facility I work at, we just had one of the RVAT starters catch on fire. We are still not sure what drove this, but what happened is that as the transformer was cooking, it spread an acidic residue throughout the MCC, impregnating the insulating busbar supports with a semi conducting material. ...

I can't help a lot. Couple of questions
Control transformer in an MCC bucket that died?
If so, did it have primary and secondary fusing?
If yes, were the fuses correctly sized? Probably unable to check - BBR

ice

 

[heavyaslead]

Harmonics

Harmonics

Harmonics comes to mind...

Need more detail to offer a better opinion.

 

[GoldDigger]

I can't help a lot. Couple of questions
Control transformer in an MCC bucket that died?
If so, did it have primary and secondary fusing?
If yes, were the fuses correctly sized? Probably unable to check - BBR

ice

RVAT = Reduced Voltage AutoTransformer.
The autotransformer carries the applied line voltage at all times and provide a buck winding/tap to provide a lower voltage to the motor during starting, then changes taps with a vacuum contactor once the motor is at a high enough speed.
It is probably the autotransformer, mounted remotely from the starter assembly, which cooked. It could be pretty large if feeding a large motor.
Might be oil filled. (PCBs anyone?) The vapor which left a residue was either from the decomposition of the oil or from the vaporization of the wire insulation. Or some combination.

PS: If the deposits came from an organic substance which was oily in nature, I can see that alcohol alone would not have been a good enough solvent. Not to mention that the stray resistances on the board during the time before the shutdown could easily have permanently damaged the components.

 

[zbang]

This residue also affected the plc rack, so that none of the I/O cards worked, and the outputs were sending signals to start, with no command. [...] and it was not until then that a resolution was found; replace all of the I/O cards, as well as the main processor card. .

I'd be replacing the whole thing; you don't know what gunk got into/onto the backplane or other components. The added cost should be well worth the peace of mind.

 

[Electrogrunt]

Thanks for the responses. We still don't know what happened to the starter that caused it to burn. The resistance between phases has climbed, which is good, but other than getting the MCC back on line, not much thought was given to why this happened. Ironically, the same electrical manager who tried using alcohol to fix the I/O cards, also rushed the whole process. As a result, another problem developed due to a loose connection, which was glowing red hot when we found it. Working with the lead electrician, we took our time and retoqued every connection we could reach. I think this MCC is cursed, as soon as it was built, its 500 HP 1336 plus 2 had components fail. These were fixed, and it worked for another 5 years. It finally failed, and was replaced with an AB 755. This drive also failed within 3 weeks, and was replaced with a new unit. So far it has held up, but I am waiting for the next disaster in this MCC.

 

[tes]

Thanks for the responses. We still don't know what happened to the starter that caused it to burn. The resistance between phases has climbed, which is good, but other than getting the MCC back on line, not much thought was given to why this happened.

Hard to be positive without being at the site, but it sounds like the system stayed in start and did not transition into run. This would have caused the auto-transformer to overheat, which would then begin to release that wonderful smoke and stink of burning electrical components. From your description of the condition of the MCC, and the tracking problem on the insulators, I would say that this is what happened. You should look into the starter timing controls, and replace the TR1 relay. Youi might also consider installing a thermostat device on the auto-transformer which would shut the system down if it hangs in start again and begins to overheat. Most systems will transition into run in 3 to 5 seconds.