Wind turbines can be classified into 3 groups as “small scale”, “medium scale”
and “large scale” in terms of their power output capacity. Wind turbines with power ratings lower than 100 kW are called as small scale where the turbines with power ratings between 100 and 700 kW are called as medium scale.The large scale wind turbines have the power output capacity of greater than 700 kW.
Figure 4.4 Operating regions of a typical wind turbine
The maximum power which can be produced by a wind turbine is the rated power of it, and the wind speed at which the turbine reaches rated power output is called as the rated wind speed. Above this, there is a maximum wind speed, called as cut-out wind speed, at which the turbine is designed to shut down in order to save mechanical parts of the wind turbine from harmful effects of high wind speed. The lowest wind speed at which a wind turbine will operate is known as the cut-in wind speed. At or above the rated wind speed, the power output remains constant whatever the wind speed (below the cut-out wind speed), but below the rated wind speed the output power varies with the wind speed. (Boyle, 1996, pp.268-269)
Table 4.1 Descriptions of Operational Regions for a Typical Wind Turbine Operating
Region
Operational Description:
Power Output vs. Wind Speed
Wind Speed Range
Region - I - Wind speeds too low to produce usable electric power.
0 to cut- in wind speed;
0 to 4 m/s.
Region - II - Production of electric power increasing with wind speed.
Cut- in to rated wind speed;
4 to 13 m/s.
Region - III -
Production of electric power at constant, rated power level. Wind turbine blades purposely made less efficient as wind speed increases.
Rated wind speed to cut-out wind speed;
13 m/s to 25 m/s.
Region - IV -
No electric power output. Winds too energetic to justify added strength and cost for the small number of hours per year beyond cut-out wind speed.
Cut-out wind speed to survival wind speed; 25 m/s to rated survival wind speed.
As the blades of the wind turbine rotate through circular path, they sweep through a disc- like area which is referred to as the swept area. This value can be normally calculated by area formula for circles;
r2
A =π⋅ (4.1)
where r is the rotor radius.
Figure 4.5 Rotor diameter vs. power output
The power that a wind turbine can extract from the wind at a given wind speed is directly proportional to its rotor’s swept area. It is extremely important that the maximum swept area is presented to the wind and this is achieved by making sure that the rotor’s axis is aligned with the direction from which the wind is blowing. As the wind does not always blow from the same direction, a mechanism of some kind is needed to realign the rotor axis in response to changes in wind direction. This aligning or slewing action, about a vertical axis that passes through the center of the tower, is known as yawing.
A wind turbine blade has a distinctive curved cross-sectional shape, which is rounded at one end and sharp at the other. The shape of the blade’s cross-section is the key how modern wind turbines extract energy from the wind. This special profile is known as an aerofoil section and is already familiar as the cross-sectional shape of aeroplane wings.
Figure 4.6 Swept area by rotor blades
Due to the aerodynamic forces on rotor blades, a wind turbine converts the kinetic energy of wind flow into rotational mechanical energy. These driving aerodynamic forces are generated along the rotor blades, which need specially shaped profiles that are very similar to those, used for wings or aeroplanes. With increasing airflow speed, the aerodynamic lift forces grow with the second power and the extracted energy of the turbine with the third power of the wind speed, a situation which needs a very effective, fast acting power control of the rotor to avoid mechanical and electrical overloading in the wind turbine’s energy transmission system.
Modern wind turbines use two different aerodynamic control principles to limit the power extraction to the nominal power of the generator. The most passive one is the so-called stall control, the active one pitch control. Stall control is a traditional way and has restrictions. Pitch control is more flexible and has opportunities to influence the operation of the wind turbine. (German Wind Energy Institute, DEWI, 1998, p.44)
4.3.1. PITCH CONTROL
Pitch control is an active control system, which normally needs an input signal from the generator power. Always when the generator’s rated power is exceeded due to increasing wind speeds, the rotor blades will be turned along their longitudinal axis (pitch axis), or in other words, change their pitch angle to reduce the angle of attack of incoming air flow. Under all wind conditions, the flow around the profiles of the rotor blade is well attached to the surface, thus producing aerodynamic lift under very small drag forces. Therefore, turbine blades reach the optimum pitch angle, at which it will produce the maximum power at that wind speed.
Pitch controlled turbines are more sophisticated than fixed pitch stall controlled turbines, because they need a pitch changing system. (German Wind Energy Institute, DEWI, 1998, p.45)
The advantages of the pitch controlled wind turbines are;
• Allow for active power control under all wind conditions, also at partial power.
• Straight power cur ve at high wind speeds.
• They reach rated power even under low air density conditions (high site elevations, high temperatures).
• Higher energy production under the same conditions (no efficiency reducing stall adaptation of the blade).
• Simple start-up of the rotor by simple pitch change.
• No need of strong brakes for emergency rotor stops.
• Decreasing rotor blade loads with increasing wind above rated power.
• Feathering position of rotor blades for low loads at extreme winds.
• Lower rotor blade masses lead to lo wer turbine masses.
Figure 4.7 Pitch Control
4.3.2. STALL CONTROL
Stall control is a passive control system, which reacts on the wind speed. The rotor blades are fixed in their pitch angle, and cannot be turned along their longitudinal axis. Their pitch angle is chosen in a way that for winds higher than rated wind speed the flow around the rotor blade profile separates from the blade surface (stall). This reduces the driving lift forces and increases the drag. Lower lift and higher rotational drag act against a further increase of rotor power. (German Wind Energy Institute, DEWI, 1998, p.44)
The advantages of stall controlled wind turbines are;
• No pitch control system.
• Simple rotor hub structure.
• Less maintenance due to fewer moving machinery parts.
• High reliability of power control.
Figure 4.8 Stall Control
In last years, a mixture of pitch and stall control is appeared, the so-called active stall. In that case the rotor blade pitch is turned in direction towards stall and not towards feathering position (lower lift) as it is done in normal pitch systems.
The advantages of this system are;
• Very small pitch angle changes necessary.
• Power control under partial power conditions (low winds) is possible.
• Feathering position of rotor blades for low loads at extreme winds.
The main issues in deciding between pitch and stall control are listed in Table 4.2.
Table 4.2 Pitch vs. Stall Issues
Issues Pitch Stall
Energy Capture Better in principle Compromised power curve Control With
Fixed Speed
Difficult in high wind speeds Generally satisfactory, although design uncertain Control With
Variable Speed
Better power quality, lower drive train loads
than any stall option
Requires proving
Safety Complete rotor protection Needs auxiliary systems for over-speed protection Cost More cost in rotor systems Less cost in rotor, but more
in braking system
Large wind turbines almost exclusively use pitch or stall control. In a few instances, yawing out of wind is used as a back up safety procedure or as contributory to control.
Recently, some manufacturers have used stall in conjunction with variable speed operation. The one configuration that has now been unanimously rejected is fixed speed pitch control. This combination produced very large transients in the power
output when controlling power. This rejection is, however, rather interesting since it was, in the early days, a popular choice.
Figure 4.9 Stall & Pitch controlled power schemes
As shown in Figure 4.9, pitch controlled power scheme results almost zero oscillations. Beside, stall control scheme shows some unwanted fluctuations causing power losses.