Enabling wind turbines to operate at high wind speeds

Enabling wind turbines to operate at high wind speeds

Wind turbines are typically designed to shut down when wind speed reaches a level which could result in damage to the turbine. In high windspeed and gale prone areas this means a regular curtailment of production. New developments allow reduced operation under these conditions rather than complete curtailment, with associated advantages for network stability and energy production.

At very high wind speeds, typically above 25 km/h, most wind turbines cease power generation and shut down. The wind speed at which shut down occurs is called the cut-out speed, or sometimes the furling speed. Having a cut-out speed is a safety feature which protects the wind turbine from damage. Shut down may occur in one of several ways. In some machines an automatic brake is activated by a wind speed sensor. Some machines use furling to twist or “pitch” the blades to spill the wind. Still others use “spoilers”, drag flaps mounted on the blades or the hub which are automatically activated by high rotor revolutions, or mechanically activated by a spring loaded device which turns the machine sideways to the wind stream. Normal wind turbine operation usually resumes when the wind drops back to a safe level [1].

Shut down using any of the mechanisms can have negative effects including:

  • A sudden loss of generating power: If several windfarms, or a large number of turbines are affected, the sudden loss of output could result in network stability problems.
  • Loss of income from the wind farm: In high wind speed areas this can be significant.

To prevent frequent shutdowns and restarts, which contribute to fatigue loading of the turbine, hysteresis is often applied, so that the wind turbine starts up only when the average wind speed reaches a value lower than the shutdown wind speed, i.e. the turbine will only restart after the wind speed has fallen below a level less than the cutoff speed by several m/s. This is not a problem where wind speeds rarely exceed the cutoff speed, but can be problematic in areas of high wind speeds dominated by short gusts of wind above the cutoff speed and high turbulence [2]. The net result is a loss of production as the turbine switches in and out of operation. The effect of hysteresis on a typical power curve of a modern wind turbine is shown in Fig. 1.

Fig. 1: Effect of high wind speed shutdown hysteresis [1].

Fig. 1: Effect of high wind speed shutdown hysteresis [1].

The wind turbine will shut down when the average wind speed reaches a certain value denoted V4 in the figure. The typical shutdown wind speed is 25 m/s. When the average wind speed drops below the shutdown value to value V3, the wind turbine starts again. Energy production is lost in the transition between V4 and V3.

Operation above cut-out speed

Wind turbines are enabled to operate at speeds above the cut-out speed by derating. Derated operation is the ability of a wind turbine or an entire wind plant to operate below its maximum capacity during times of high wind speed. Derating uses a range of control methods, from pitch control of blades to generator torque control in order to operate a wind turbine at below its maximum capacity. Solutions to this problem allow the turbine to continue generating at wind speeds higher than the cut-off but at reduced levels. All approaches use a smooth ramp control of power with increasing wind speed. Two approaches are possible:

  • Ramp rate control: In this system the output is gradually reduced in accordance with a preset method as wind speed increases to zero output at wind speed with zero probability. This eliminates the hysteresis effect as operation continues backwards and forwards as wind speed increases and decreases. Ramp rate control is illustrated in Fig. 2.
  • Dynamic control: Instead of predefined wind-power ramp control a dynamic approach is used that relies on the wind turbine state estimation and worst case wind speed prediction. The algorithm assesses all wind speeds with defined characteristics and chooses the one that produces the maximal loads. Wind turbine power or rotor speed setpoint is then adjusted to ensure that even in such an extreme event design driving loads will remain in the predefined envelope [1].

Fig. 2 shows the difference between the two approaches.

Fig. 2: Comparison of soft cut-out strategies [2].

Fig. 2: Comparison of soft cut-out strategies [2].

Control mechanism details

Systems use blade pitch-control systems to achieve a smooth ramp-down of power when winds get to speeds of about 25 m/s and threaten to overload a turbine. By pitching the blades away from the wind, the turbines can keep operating at a lower power until the storm-force wind subsides.

The maximum wind speed is determined by the controls of the wind turbine. Above the rated speed the blade speed is controlled by varying the pitch. The maximum speed is the wind speed at which it no longer becomes possible to maintain the blade speed at maximum power. Control above the maximum wind speed is affected by varying the pitch to reduce the blade speed, i.e. operating in a non-optimal configuration which still generates some power, and gently ramping up and down until the point is reached where even this is no longer possible and turbine is stalled.

Advanced system controls use a combination of blade speed and torque control. This type of control is only applicable to double conversion systems where the output frequency is not dependant on rotational speed. Reduced output is achieved by slowing rotational speed, which reduces the output power of the alternator, allowing the blade speed to be controlled to a manageable value.

Fig. 3: High wind ride through controls [3].

Fig. 3: High wind ride through controls [3].

Examples

Siemens “high wind ride through” system

With high wind ride through, the wind turbine will gradually reduce power output instead of shutting down completely. This results in a more stable power output at high wind speeds. As a result, the operating range of the wind turbine at high wind speeds is extended, while remaining load neutral. This is achieved by intelligently pitching the blades out of the wind as soon as the rated power output is reached and by limiting rotational speed in proportion to the increase in wind speed and turbulence intensity. The gradual derating eliminates abrupt cutouts, which significantly improves grid stability. This is an advantage, especially for larger wind farms where commitment to a certain level of energy production is often required.

Enercon “storm control”

The Enercon system ramps down speed using pitch angle control to vary blade speed. The power curve diagram showing operation with storm control (Fig. 4) demonstrates clearly that the wind turbine does not shut down automatically when a certain wind speed is exceeded, but merely reduces power output by slowing down the turbine’s rotational speed.

Fig. 4: Enercon’s “storm control” [4].

Fig. 4: Enercon’s “storm control” [4].

This is achieved by slightly pitching the rotor blades out of the wind. Once the wind speed drops, the blades turn back into the wind and the turbine immediately resumes operation at full power. This prevents yield-reducing shutdown and start-up procedures.

Retrofitting

As the systems are primarily based on the control portion of the turbine, it would appear that the function could be retrofitted to existing wind turbines.

References

[1]    L Horvath: “The influence of high wind hysteresis effect on wind turbine power production at Bura-dominated site”, www.ewea.org/ewec2007/allfiles2/498_Ewec2007fullpaper.pdf
[2]    M Jelavic: “Wind turbine control beyond the cut-out wind speed”,https://bib.irb.hr/datoteka/619383.EWEA13_clanak.pdf
[3]    Siemens: “High wind ride through: providing more predictable output”,www.energy.siemens.com/br/pool/hq/power-generation/renewables/wind-power/Flyer-WindPower.pdf
[4]    Enercon: “Wind energy converters”, www.enercon.de/p/downloads/EN_Productoverview_0710.pdf

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