Forbes

Since 1994, Northern States Power Co. (NSP) of Minnesota, USA is operating a Static Var System (SVS) supplied by Hitachi Energy in its 500 kV power transmission network.

The SVS, located at Forbes, is part of NSPs Manitoba-Minnesota Transmission Upgrade Project, the purpose of which is to increase the power interchange capability between Winnipeg and the Twin Cities on existing transmission lines.

This solution for increased power transmission capability was chosen instead of building a new line as it was found superiour with respect to increased asset utilization as well as minimized environmental impact. The main purpose of the SVS is to improve the generation and transmission system‘s dynamic response to network disturbances. It also provides improvement during steady-state conditions by supplying adequate reactive power support. With the SVS in operation, the power transmission capability of the transmission system has been increased by some 200 MW.

Without the SVS, power transmission capacity of the NSP network would be severly limited, either due to excessive voltage fluctuations following certain fault situations in the underlying 345 kV system, or to severe overvoltages at loss of feeding power from HVDC lines coming from Manitoba.

The SVS has a dynamic range of 450 Mvar inductive to 1000 Mvar capacitive at 500 kV, making it one of the largest of its kind in the world. It consists of a Static Var Compensator (SVC) and two 500 kV, 300 Mvar Mechanically-switched Capacitor Banks (MSC). The large inductive capability of the SVC is required to control the overvoltage during loss of power from the incoming HVDC at the northern end of the 500 kV line.