Stall control consigned to history books

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As with all the grid codes, specifications for wind power in Denmark require the use of turbines which can control output on demand, ruling out the use of stall controlled machines in favour of those which operate with pitch control

The ubiquitous stall controlled wind turbine -- thousands of which have been installed worldwide over the past 25 years -- is all but banned in Denmark by rules for grid connection of wind plant implemented last year by the country's two transmission system operators (TSOs), Eltra and Elkraft System. The rules forbid the connection of wind turbines to the grid network if their power output cannot be regulated on demand. Only pitch regulated wind turbines, or turbines with some form of blade pitching, can meet the requirement.

For Denmark's two remaining wind turbine manufacturers, Vestas and Siemens Power Generation, the rules put an almost total stop to sales of their smaller stall-regulated models, both at home and on export markets with similar grid codes. Also larger NEG Micon turbines, which Vestas has retained on its product program after the merger of the two companies, the NM 900 kW, NM 1000 kW and NM 1500 kW models are being forced out of most of the world's main wind power markets. In Denmark, the only market left for stall regulated machines is if a site can be found where the combined wind capacity at the point of connection is less than 1.5 MW. Here the requirement for power control is waived.

Denmark's TSOs have produced two specific grid codes applying to wind power, one for connecting plant to transmission lines with voltages of 110 kV and above and one for connections to the wires at 100 kV and less. The codes make similar demands, but in practice it is the code for connecting into the lower voltage system that applies. It sharpens the technical requirements with regard to reactive power, regulation of power output, reaction of wind turbines to network faults, voltage quality, external control of wind turbines, and data transmission to and from an external control room. The purpose is to "maintain the technical quality and balance within the integrated electricity supply network."

Denmark's wind lobby regards the rules as stringent, but necessary. "It's correct to describe the demands are tough, but that's a reflection of the growing proportion of electricity supply coming from wind plant," says Strange Skriver, technical consultant to the Danish association of wind turbine owners. "It is obvious that with 20% of electricity supply coming from wind power any risk, for example, of all the wind turbines tripping is unacceptable. The larger proportion of wind on the system, the tougher the demands. And even though the tightening of the rules looks restrictive, I believe we can live with them. Nearly all turbines -- except stall regulated -- can meet the demands today, aside from a few small things that can be adjusted."

The most revolutionary part of the grid code demands that the power output of wind turbines be fully controllable in a range of 20% to 100% of nominal rated capacity. Only turbines with blades that can be feathered to the wind (pitched) can comply with the demand. In addition, such power control must be able to be carried out by an external instance, such as a TSO. In principle the rule, which apply to turbines installed after the grid code came into force, gives electricity companies the right to reduce and increase the output from wind turbines at their discretion.

Abuse unlikely

Although the wind turbine owners association objected to this requirement, the likelihood of TSOs abusing that authority is minimal, assures the association's Asbjørn Bjerre. "In Denmark we have a law which gives wind power priority access to the network. That law must be adhered to, whether or not some energy companies in specific situations have other business interests," he says. Bjerre adds that the opposition to wind power demonstrated by the utility majors in Germany is no longer seen in Denmark. The entire Danish energy sector has a collective aim to secure 50% of Denmark's electricity from wind power by 2025 -- and TSO Elkraft has declared that the goal is not only technically feasible, but makes macro-economic sense for the country.

The Danish grid rules also require that wind turbines tolerate a certain level of disturbance on the network without tripping off it. Short circuits lasting milliseconds have previously often caused wind turbines to stop operation. The new rules require that future wind turbines remain online even if they register one or several short circuits of 100 milliseconds.

The rules further demand: that wind turbine owners reach agreement with the TSO on balancing power requirements before grid connection; that it is the turbine owner's responsibility to secure fault-free operation, also if the turbine is subject to "unusual operating conditions" on the network; that a wind turbine automatically disconnect if voltage or frequency lie outside the network's operational settings; and that a wind turbine is dimensioned to withstand winds of a least 25 metres a second, and that it must not be programmed to stop at less than that wind speed.

The new grid rules have yet to be tested in practice. Almost no new wind turbines have been installed in Denmark over the past year, following a major change to the market framework which drastically reduced payment for wind power. A four-year program of financial incentives to stimulate replacement of old turbines with state-of-the-art technology has not got into its stride.

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