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Wind turbine technology is discussed in detail in Part 1. Turbine technology is also covered briefly in this section, to provide some background to the information that follows.
The traditional Danish stall-regulated wind turbine concept uses an induction generator. Its rotational speed is fixed by the frequency of the electricity network to which it is connected. The blades are fixed - inother words they do not pitch - so the output power and structural loads in high winds are limited by passive stall regulation. Unfortunately this concept, though cheap, simple and reliable, has several negative effects on the electricity network:
- Lack of power control, meaning that system frequency cannot be controlled, which is achieved relatively simply by conventional power plants;
- Limited control of reactive power, making it more difficult to control network voltages.
- During network disturbances (such as a sudden fault on the network), a wind turbine is likely to aggravate the situation.
The fixed-speed, pitch-regulated concept (in other words the possibility to control active power output, by pitching the blades) resolves the first of these issues and the limitations of the stall-regulated wind turbine concepts can be mitigated with the addition of terminal equipment in the substation.
The development of variable-speed wind turbines, using power-electronic converters, was undertaken largely to reduce mechanical loads. This introduces additional control of reactive power as a by-product, and in the majority of cases also reduces the wind turbine’s effect on the network during a sudden fault.
The larger the power-electronic converter (relative to the size of the wind turbine), the greater the control over reactive power. So variable-speed, pitch-regulated wind turbines, based on the full-converter principle now allow the desired control of wind turbines within the required limits.
The currently available wind turbines do not make full use of this capability, and also grid codes do not yet take advantage of the full capabilities. As wind penetration increases, and network operators gain experience with the behaviour of their systems, grid codes will possibly become more demanding. New technical requirements should be based on:
- A detailed assessment of requirements;
- The technical potential of all the power plant’s technology; and
- The optimal way in which to meet these demands, both technically and economically.
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