Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 100

An important feature of this K_P –1/λ curve is that the power, initially, falls off once stall has occurred and then gradually increases with wind speed. This feature provides an element of passive power output regulation, ensuring that the generator is not overloaded as the wind speed increases. Ideally, the power should rise with wind speed to the maximum value and then remain constant regardless of the increase in wind speed: this is called perfect stall regulation. However, stall‐regulated turbines do not exhibit the ideal, passive stall behaviour.

Stall regulation provides the simplest means of controlling the maximum power generated by a turbine to suit the sizes of the installed generator and gearbox. The principal advantage of stall control is simplicity, but there are significant disadvantages. The power vs wind speed curve is fixed by the aerodynamic characteristics of the blades, in particular the stalling behaviour. The post‐stall power output of a turbine varies very unsteadily and in a manner that, so far, defies prediction (see Figure 3.62, for example). The stalled blade also exhibits low vibration damping because the flow about the blade is unattached to the low‐pressure surface, and blade vibration velocity has little effect on the aerodynamic forces. The low damping can give rise to large vibration displacement amplitudes, which will inevitably be accompanied by large bending moments and stresses, causing fatigue damage. When parked in high, turbulent winds, the fixed‐pitch, stationary blade may well be subject to large aerodynamic loads that cannot be alleviated by adjusting (feathering) the blade pitch angle. Consequently, the blades of a fixed‐pitch, stall‐regulated turbine must be very strong, involving an appropriate cost penalty.