Willmar Municipal Utilities investigates cause of 2-hour power outage last month
WILLMAR -- Willmar Municipal Utilities is investigating the sequence of events and equipment failure at the south substation that caused a two-hour power outage May 18 in Willmar and parts of Kandiyohi County, according to Bruce Gomm, utility general manager.
Gomm discussed the outage Monday night with the City Council. The outage was the largest in city history since the utility began keeping detailed records in 1993.
The outage occurred while utility employees were performing a planned and budgeted equipment replacement and upgrade at the 40-plus-year-old substation, Gomm said. The substation feeds not only all of Willmar but a large portion of the county.
"A lot of power for the region goes through that substation,'' Gomm said. "It's a major event and we scrambled to get it back up as fast as we can. The cards were stacked against us.''
The work began at 5:30 a.m. when power from a 230,000-volt Great River Energy transmission line from Granite Falls was switched to a 115,000-volt line that connects to Paynesville. The power was switched to take the larger line out of service and allow the repair work to proceed.
Gomm said Great River Energy was in charge of the switching procedure and Willmar linemen were following GRE's detailed and specific switching procedure. Gomm said the line crew and power plant personnel completed the upgrades by about 5:30 p.m., or about 2½ hours ahead of schedule.
"It all went better than expected, actually,'' said Gomm. "We were ahead of schedule. We initially didn't expect to be complete until 8 p.m.''
He said the crew was switching the power back to the normal 230-KV line when during the switching procedure a relay caused the substation to "trip out.'' A relay monitoring the line status sensed an abnormal condition and set in motion a protection scheme to shut down the line segments. This caused a cascading effect, which shut down the substation.
"We were following their switching procedure and we're still investigating what sequence of events caused the trip that happened. I can't say much more than that, other than it's extremely important for us to understand why that happened,'' Gomm said.
The problem was compounded when a battery bank, which would have provided control power to restart the substation, failed within 60 seconds of the outage.
"Before we were in a position where we figured out exactly what had happened, and exactly what we needed to do to bring the station back, we were dealt a second blow where we didn't have control power anymore to perform the switching,'' said Gomm.
"It was a challenge to figure out on the fly. It took some investigating and some quick thinking on behalf of our line crew to hook up a portable generator they had been using earlier in the day to the battery charger and try to recharge the battery enough to get the switching going,'' he said.
At 7:36 p.m., power was restored to over 99 percent of Willmar customers. At 8 p.m., all service was restored.
Gomm said a temporary battery was in place within a few days and a permanent replacement was ordered while the battery failure is investigated.
After the outage, Gomm said staff reviewed the battery testing protocol. He said batteries at all substations are tested monthly. An investigation indicated the utility could have been performing a more thorough test, which Gomm said has since been implemented.
Gomm said employees did a very good job in responding to the outage. He said several office employees came in to take calls and inform major customers of power status.
The power plant was 75 percent ready to provide power to the town when the outage ended. Gomm said the plant would have been powering the town within that 2-hour period.
Afterward, Gomm said he was told people heard banging or explosions downtown. Gomm said the sound was coming from the power plant as boiler steam was released by pressure relief valves.
Gomm said Willmar will tie into a new substation that GRE will be constructing in 2012 in the Spicer area to provide a second transmission source.
Gomm said electricity is an integral part of life.
"Because it's so readily available and off so infrequently, a lot of people can take it for granted and forget how important it is to them,'' he said.