A hangar at a major airport suffers damage and loss of power due to an earthquake. Investigation of the 15kV switchgear shows it is twisted. A review of the distribution system reveals serious weaknesses.
California is earthquake country. A big one occurred, causing devastating results within a 5-mile radius of the epicenter. The center is within 6 miles of our regional airport, where more serious damage occurred. Two of the north side buildings were without power, and a smell of burning insulation and ozone lingered on the site. Reviewing the condition of the airport's high-voltage distribution system, the maintenance electrician reported a lot of arcing near one of the 15kV switchgear sections. Not qualified to work on the 15kV system, his crew managed the 600V lines.
Two 3750kVA, 69kV/13.8kV transformers, one on the south and one on the north ends of the facility, supplied power to the airport. Feeders ran to 13.8/4. 16kV substations throughout the complex. The switchgear feeding the north-side buildings sparked our interest. Each had substation and circuit breaker sections serving various loads throughout the building.
The designer housed this switchgear inside a separate building. The earthquake created cracks in the floor: one running under the gear at an angle of 45. This crack continued outside and ran along a manhole. The manhole contained cables entering and leaving the building. With one tripped breaker, it was obvious the circuit to the buildings was out of service. The electrician attempted to reclose the breaker, but nothing happened. All this, with the still lingering smell of burning insulation and ozone, initiated a call to us.
Looking down the face of the twisted switchgear section, we noticed the bottom of it was about 4 in. out of line from the top. We immediately enlisted a high-voltage contractor to assist us with the examination. He checked the tripping operation of the suspect breaker; Yes, it was tripped. He then opened the section door, attempting to hand-crank the breaker down on its rails. It would not budge.
All the other breakers were closed and in operation, so why could we still smell ozone? The earthquake had caused a twist to the entire switchgear section! Amazingly, all but one breaker had continued to function after the earthquake twisted and turned about the gears.
Our concern was now for the entire airport site. How near was the system to total collapse? We proceeded to inspect the airport's entire electrical system for visible damage. The equipment throughout the site survived all the shaking with no significant damage. Only a couple meters had broken glass covers. It seemed the switchgear hut, which we initially inspected, had received the most damage.
The contractor then de-energized the section, and attempted to remove the breakers from their sections. The stabs were, of course, spring-loaded, which permitted them to retain contact with the bus. The earthquake displaced the buswork by 1 in. and chipped some of the insulators. Because the equipment was at a 5KV level, it was probably more tolerant of movement than higher-voltage equipment would be.
The next step was to force the other breakers to release, pulling them out to inspect for damage. After much tugging, pulling, and bending, the high-voltage contractor pulled off the rear panels. He then disassembled the parts for inspection.
Obtaining replacement parts for the damaged components took about two weeks. Meanwhile, we had to find capacity from other locations to continue serving the loads. While shutting down nonessential loads, we checked the underground cables in several manholes, all of which smelled of ozone when we removed the covers.
This lead to a major discovery. The underground cables were in very bad condition. While doing a high-voltage insulation check at the cables near the first substation, we found the test started to break down the insulation at 1 and a half times rating. They were supposed to read 3 times the rating. We found there had been no maintenance checks on these cables for 15 years! Repairs were made only when there was a failure. With no systematic review of the condition of the cables, they just carried the current over the years; no one thought anything about it.
Although the disaster caused other damage to the surrounding buildings, overall the airport had come through with minimal damage. The Airport Commission faced major expenses for expansion; with no budget for the repairs. Fortunately, the region was classified as a disaster area, making Federal and State funds available.
The questions moved from "what happened?" to "how do we fix it?" Assisting in implementing emergency repairs, we also located the electrical system's weaknesses due to the lack of effective maintenance procedures. Using our report on the damages and weaknesses to the electrical system, the Airport Commission took appropriate corrective measures. This situation resulted in initiating a needed major upgrade program to this busy airport's electrical system.