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United States

Creating grid stability with new control technology

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Advances in wind technology are gradually opening up development in areas where weak electricity distribution systems have prevented new projects. Modern wind plant, through real time voltage control, can actually add stability to a weak local grid by smoothing voltage spikes, rather than compounding an existing problem.

Wind turbines equipped with volt-amps reactive (VAR) control technology provide dynamic power factor control -- the turbines individually supply reactive power to the grid as and when central voltage monitoring and control equipment tells them it is required in order to iron out a spike in the system.

"This technology provides further justification for utility investment in wind power, particularly in rural areas with weak grids," says electrical engineer Tom Wind of Iowa-based Wind Utility Consulting. "These turbines do more than generate power -- they provide utility managers with a tool to support their transmission and distribution systems."

Without such power electronics, the operator of a weak grid is forced to import reactive power whenever it is needed, among other things to prevent voltage spikes tripping the wind turbines.

But too much reactive power consumption impacts a grid's voltage stability margin, risking a power failure. Furthermore, voltage management using traditional capacitors only provides voltage adjustment about every five seconds -- too slow to cope with sudden wind power variations. The new technology enables four adjustments per second, allowing increased control. The first large scale wind plant to use power electronics was the Lake Benton II Wind Facility in Minnesota, US, using Enron Wind 750 kW turbines, reports Windpower Monthly's sister publication, WindStats Newsletter. More recently Enron's power electronics are being tested with the1.5 MW turbines in the 30 MW Montfort Wind Farm, Minnesota.

"VAR control technology allows utilities to have better voltage consistency," says Enron Wind's Bob Gates. "The wind system supports the local grid so that it isn't necessary for the utility to supply VARs."

Indeed, it was excess voltage spikes occurring on the ageing rural grid of Minnesota utility Xcel Energy that led to the utility's strict condition that the power must be stabilised before it would agree a purchase contract for the output from Lake Benton II. Xcel (formerly Northern States Power) reported interconnection problems following the addition to its system of the Enron equipped 107 MW Lake Benton I plant in 1998, saying the voltage from this and earlier turbines strung along Buffalo Ridge varied significantly. As a result, wind generation sometimes causes flicker and dips. To eliminate the usage of reactive power from Xcel Energy, the Lake Benton II turbines were designed to produce reactive power. "Without VAR control technology turbines, we'd experience a lot more trips, and that means lost revenue and lost energy," says Xcel's Charles Grunewald.

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