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Square Butte

ABB upgraded the old analog control system, installed in the mid 1970's, with the modular-based MACH control system.

Minnkota Power Cooperative's and Minnesota Power's Square Butte 500 MW ± 250 kV DC HVDC transmission system was originally built by General Electric and commissioned in 1977.

The bulk power transmission link stretches between Center, North Dakota and Duluth, Minnesota, a distance of about 750 km, and was built to reinforce the AC power system in North Dakota.

In 2002, ABB received an order to upgrade the old analog control system installed in the mid 1970's with a modular-based MACH control system. The upgrade was coordinated with a total shutdown of the generator unit feeding the HVDC link, so the installation and testing could be performed during a bipolar shutdown.

During the outage, the HV thyristor valves were disconnected from the old system and reconnected to new computerized equipment. The new system minimizes the number of control components, eliminating thousands of aging individual parts that had the potential to fail.

The generator outage started on September 4, 2004 and after an exceptionally smooth installation and testing period, all testing was completed and HVDC bipole back in full operation on October 18, 2004.

In addition to improving the systems overall reliability, the control upgrade also improved transmission line monitoring capabilities.

MACH is a control and protection system designed specifically for HVDC and FACTS (flexible AC transmission systems). MACH2 systems are also designed to provide unequalled calculation capacity in order to fine-tune performance of the converter firing control system during system disturbances.

Main data

Commissioning year: 1977 (by General Electric); upgrade 2004
Power rating: 500 MW
No. of poles: 2
AV voltage: 230 kV (both stations)
DC voltage: ±250 kV
Length of overhead DC line: 749 km
Main reason for choosing HVDC: Long distance, network stability
Application: DC links in AC grids, upgrade

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