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The Case of the Waterproof Motor Control Center

I was minding my own business, sitting in my office one morning when an engineer from the Paper Mill called. "What are you guys doing over there?" was his opener. "My lights just dimmed and it got very quiet over here."

"I haven't been listening to the radio, but I thought everything was OK over here.", I replied. "I'll go up to the control room and get back to you".

By the time I got to the Utilities control room, lots of people had called asking what had happened. We looked over the board, checked the DCS, but didn't find anything out of the ordinary. The problem is on their end, we concluded.

Soon, we had an answer. One of the Maintenance Managers in the Paper Mill called back and told us they had water in an MCC. In preparation for an important tour the operators were cleaning up the operating floor and walls. An operator had used a high pressure wash up hose on the wall and the door to an Electric Room!! Enough water entered the electric room to travel over curbing, down the cable tray, and dripped into a 4160 V Switchgear compartment on the ground floor!! Since this was an indoor unit, and not weatherproof, the water had caused it to flashover.

At this point I figured they could probably use a little help, so I headed over to the paper mill. When I got there I found some frantic radio traffic asking to shut off one of the distribution feeders. I interjected, and explained that they were asking for an obsolete number, and told them the number they really wanted. A crowd had gathered, trying to isolate the substation. Since its not a procedure the paper mill personnel usually do, they needed some guidance. Since they did not yet know the extent of the damage (or how much water might still be present in the gear) they decided to isolate power upstream at the distribution feeder while isolating this particular transformer. We racked out the breakers, got the Kirk keys, and opened the line side switch. Once we had the line side disconnect open, tested, and grounded, we turned the distribution feeder back on.

With it safely isolated and locked out, we started assessing the damage. The good news is it was relatively minor (as flashovers go). The load side connection at the cable lugs had the most damage. The compartment had a lot of dirt, but the busbar connections were intact.

There was a crack in the line side bus insulation, so we knew we would have to take that apart to inspect it. In general, most of the damage had been confined to the cables themselves.

Meanwhile, management had decided to take a maintenance outage on the paper machine a day early. We would have to get this substation repaired as quickly as possible (naturally), but the other maintenance work might run as long as we would.

Luckily, with the damage we had, we could pull a little slack out of the cables and reterminate the lugs. While a crew started on that work, we also called in another crew to overhaul the breaker, test the CT response, and test and inspect the relays. In this instance, we decided to err on the side of caution and inspect everything. When this crew started on the breaker, they found more water on the vacuum bottles. When we found that, it made the additional inspection and repair worth every cent!

By this time, several hours later, we had all the repair work underway and progressing well. Since it was my anniversary, I called my wife and told her I could leave. The paper mill engineer agreed to take over the repairs. I took my wife out, but naturally couldn't get my mind off the excitement. Since she knows me very well, she was not the least bit surprised when I suggested I should stop in the mill and check on the job. She cheerfully sat in the car while I went back in.

When they took apart the bus bar, they found some evidence of tracking and places where the copper had vaporized.

While repairing the damage, we found some corona around the line side bus insulation and on the micarta that supports the bus as it transitions from one cubicle to the next.

Photo 4160A

Picture 4160A shows the bus going through the micarta. Picture 4160B shows the bus insulation after it was removed. Notice the tracking in each picture.

Photo 4160B

During our infrared inspection PM we need to be also looking for this type of corona. In this case it didn't cause the incident that we had, but it might have caused its own trouble at a later date.

The rest of the repair went fine. The cables were reterminated, the CT checked out fine, the busbar was repaired with a shrink wrap sleeve, the relays were OK, and the breakers were overhauled. After a thorough visual inspection (no tools left behind!) and meggar test, we were ready to re-energize.

To re-energize, we decided again to be extra cautious. We wanted to minimize any inrush, and additionally decided it would be much easier and safer to remove the grounds with the power off. By turning off the power, we could avoid "working on or near energized parts" and not have to go get the safety equipment, which was stored in Utilities. Since I had since returned home (my wife would only be cheerful in the car for so long), the paper mill engineer and I discussed this over the phone. When I called the control room to make sure they understood the procedure, I was disappointed to discover they had E/I working on the wrong controls. Even though the breaker was tagged as obsolete on the panel, and the proper breaker number was marked, people were trying to use the wrong one. The right number was even marked on the substation that was being repaired. Once that confusion was cleared up, we were able to shut off the power, remove the grounds, and re-energize the feeder.

The lights went on, and the paper machine was OK. There are a few important lessons here.
1) Water and indoor substations don't mix. We have since installed extra flashing on our electric room doors, but there is no substitute for inattention.

2) Tagging does not always work. The proper breaker number is clearly marked on both the substation and the benchboard, yet people still ask for the wrong number.

3) There is no substitute for experience. We had the proper test equipment, procedures, and safety equipment. We also had lots of help and attention. What we didn't have were people familar with the procedures, the equipment, or where the equipment was stored. Since this repair involved activities usually taken care of by other crews, the people first on the scene were not the most knowledgeable. It is important to have clear communication and chain of command for an emergency repair of this type.

PowerJock Consultants
Stephen Richens, PE
Steve@PowerJocks.com