The outage was caused by a porcelain insulator that failed on an oil circuit breaker (OCB) in the substation just north of the power plant, east of Highway 4.
Last Wednesday, right at noon, a power outage interrupted many peoples’ lunch, including the Sleepy Eye line crew. The outage was caused by a porcelain insulator that failed on an oil circuit breaker (OCB) in the substation just north of the power plant, east of Highway 4.
The OCB is a device that protects the large substation transformer from damage should a short occur in the underground wiring between the plant and the substation. It appears that the porcelain insulator—on top of the OCB where the wire attaches—cracked, which allowed the current to come in contact with the metal can that is the body of the OCB, shorting it out and causing a substantial fireball for a brief moment. This, in turn, took out the three fuses on the high voltage side of the transformer, leaving approximately half the town without power.
The city electric crew hurried back to the plant, having just left for lunch, and started generators to get the lights back on. Approximately a half hour later, a relay on one of the generators tripped, causing all generation to go down. By then it was apparent that the electric load was low enough for the second substation transformer to carry the load of the whole town. The crew switched all the load to the #2 transformer for the rest of Wednesday and most of Thursday when repairs made to the #1 transformer were sufficient to move load back to it.
These types of failures are never timely, but on a normal hot August day, the #2 transformer would never carry the load of the whole town on its own. Cool temperatures, gray skies and intermittent rain held the demand for electricity down on Wednesday and Thursday, allowing the repairs to be made without running extra generation for those two days.