Originally constructed in 1978, Great River Energy’s high-voltage direct-current system is one of a few of its kind in the world and one of the electric generation and transmission cooperative’s most valuable assets. The co-op’s high-voltage direct-current (HVDC) system consists of a 436-mile (702-km) HVDC transmission line and two converter stations, one at each end of the line, that convert electricity from alternating-current (AC) power to direct-current (DC) power and vice versa. Nearly all the power is delivered from Coal Creek Station — the co-op’s largest power plant — in Underwood, North Dakota, U.S., to Minnesota, where the utility’s 28 member co-ops are located.
After eight years of planning and preparation, Great River Energy recently upgraded the unique HVDC system, successfully completing one of the most significant transmission projects in its history. The utility and ABB — the manufacturer of its HVDC system — completed the upgrade from March 7, 2019, to May 17, 2019, during a power plant outage that was planned for regular maintenance. As part of the upgrade, all the equipment in the converter stations was replaced with today’s technology. This included replacing the conversion equipment (valves), controls, cooling towers and smoothing reactors as well as updating the DC yards. All the work either took place inside or directly supporting the converter stations, not on the transmission line itself.