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Providing grid service -- Wind integration

As the size of wind turbines and wind farms grow, transmission operators want greater assurance that wind projects will not trip offline when voltage sags in the power system outside a wind farm. GE Wind Energy says the low-voltage ride through (LVRT) capability it is adding directly to its 1.5 MW turbines will take care of this problem, giving the turbines the ability to stay connected even during a voltage disturbance.

The new technology, says GE, will make the power generated by these turbines more competitive. It may also give them a leg up on competition in the US.

LVRT was added to the generators at the request of a customer concerned about the ability of wind turbines to stay connected when the grid needs power most. "This enables wind turbines to provide the same service to the grid as other energy generators do," says GE's Mary McCann. "This fits into the system and doesn't tax it in any way. Our goal is not just to supply wind turbines, but to provide overall service to the grid."

If voltage dips by more than 25% to 30%, most small induction generators like wind turbines will trip offline. GE says its product will keep its turbines online even if voltage drops up to 70% for 100 milliseconds. It declines to say whether it has applied for a patent for its LVRT.

While GE may have the only wind turbine that provides this capability, it is not the only solution to the voltage dip problem, says Charles Stankiewicz of American Superconductor. In 2002, his company introduced the voltage regulating D-VAR system and installed the device at two wind projects -- the 135 MW Foote Creek project in Wyoming (Windpower Monthly, June 2002) and the 2.6 MW Basin Electric Cooperative project at Minot, North Dakota (Windpower Monthly, February 2003). The projects use Mitsubishi and Nordex turbines, respectively.

D-VAR holds the voltage coming out of a wind farm stable within a limited band, even as the power output fluctuates. That helps to integrate the facility into the local transmission system, but it also works in the other direction by instantaneously boosting voltage at the distribution level and protecting the entire wind farm from tripping offline when grid voltage drops. The GE solution takes care of that internally, Stankiewicz says, while D-VAR takes care of it at the distribution feeder. However it is provided, the capability is important to both transmission operators and utilities, he says.

"If you're relying on 1 MW wind turbines and you have 50 or 60 of them, that becomes a big chunk of a utility's generation," Stankiewicz says. "If you can't count on this generation, you will need some back-up on an instantaneous basis, and that will cost the utility more."

Stankiewicz predicts that once transmission operators know of this capability, they will change their rules for wind turbine interconnection, calling for the lower voltage protection. "Transmission organisations like to have things gold plated and this will become a precedent," he says.

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