Power Transformers Blog – Expert Safety and Usage Info

This MIDWEST blog is about a low voltage, high current transformer used in a hardening process. The transformer was physically located deep in the process equipment. It was very difficult for the owner to access the transformer for maintenance. They just could not maintain it during normal production schedules. The process operators were having more and more trouble controlling the output voltage and current. It got to the point they just could no longer regulated the output current as necessary to perform the metal hardening process. MIDWEST was contracted to remove and replace the transformer or repair it. Whatever could be done. When the transformer was being removed, the problem was found immediately.  There were massive copper bus bars on the secondary side of the transformer to carry the high currents. One of the output bus bar bolted connections had failed. It was held by only four bolts and the bolted connection had become loose. The copper at the connection was discolored and distorted from overheating. The connection contact surfaces were all destroyed by being overheated.  The damaged copper and damaged connection contact surfaces for this damaged transformer could not be repaired. The copper was damaged all the way into the winding. All the other connections were intact. The repair bill was over $50,000.00. This four bolt connection cost over $12,000.00 per bolt to repair.  The transformer replacement was not caused by a transformer failure, but by a simple bolted connection failure. MIDWEST frequently finds dry type transformer failures or replacement transformer projects are caused by the most probable failure mode because the needed preventive maintenance was not performed, because it would have been relatively expensive compared to routine, specified, maintenance. Everyone understands the value of the maintenance dollar. But it does not make sense to continually spend hard to get maintenance dollars on preventing very low probability failure modes and totally ignore the most probably failure mode all together, when the most probable mode would be catastrophic and the low probability failure modes have little consequence.  This is a nightmare for maintenance supervisors. It can be tough to sell critical maintenance procedures that are not found in standard maintenance specifications.

MIDWEST received an emergency call from a customer who had lost a 30 Mva 138 Kv to 13.8 Kv oil filled power transformer.  There was no storm or other unusual occurrence and they said it failed “last night.”  We are skeptical of transformers that appear to fail in the middle of the night when the sky is clear and nothing else unusual happened. We prepared to get a replacement reconditioned 30 Mva oil filled power transformer, and to arrange the resources to replace the transformer if necessary. Meanwhile MIDWEST Field Services went to check out the transformer. Differential relay protection took the transformer off line.  But we found the problem immediately in the form of a toasted raccoon and slightly blasted secondary bushing. We cleaned the secondary bushing such that it could be energized, tested the transformer, and figured someone at the Utility would want to use the fried raccoon as a wall ornament. But there were no takers. The transformer was okay. There was what we considered an unusual discussion as to why the raccoon was on the transformer. The consensus of management was that it did not make sense. Something more dramatic had to be in play, because one lousy raccoon could not be the cause of so much trouble and expense. The inquisition focused on the need for a good reason a raccoon would climb on top of the transformer. We explained that this was not an unusual occurrence. It was rather common. We explained that raccoons like the warm transformers and tend to climb over everything near their food supply. There were woods next to the substation. We recommended they put screen mesh along the bottom of the substation fencing and at the gate, to keep raccoons out.  And we suggested they move the park bench, used by employees as an outdoor lunch table, to an area away from the substation.