History of windmill and wind turbine control

Wind turbine control has a long history which was probably initiated by the regulation of the rotational speed of the Persian windmills in the tenth century A.D using a series of shutters. Another early example of a windmill regulation device is the mill-hopper which was used to regulate the flow of grain in a mill depending on the speed of rotation of the millstone by about 1588.

The variability of the wind in both speed and direction was addressed and patented by the British blacksmith E. Lee in 1745 [154]. The drawing annexed by Lee's patent is depicted in Fig. 7.1. To compensate for wind speed variations, he invented a mechanism that pitched the blades as the wind speed increased and vice versa. The general principle of working of this device was that the force of the wind pitched the blades when the force magnitude exceeded that of the counterweight.

Figure 7.1: Drawing annexed by Lee's patent titled "Self-regulating Wind Machine" with "A, the case of the Machine, B, the Sails, C, the Regulating Barr passing thro' the center of the originall axis, D, the Chains from the Barr to the Sails, E, the Back Sails which keep the machine constantly in the wind, F, the weight which regulates the Sails according to the winds force, G, the Traveling wheel which moves on planks round the machine, H, the Regulator to which the weight is Fixed."

In addition, for keeping windmills pointed into the wind he developed the fantail: an auxiliary set of blades located behind and oriented perpendicular to the rotor. This yaw mechanism turned the cap of a tower mill automatically into the wind, thereby eliminating the need for manual changes in the windmill's orientation. The fantail thus had the ability to follow the changes in wind direction and is one of the earliest applications of feedback control.

Proportional feedback in the form of a centrifugal or fly-ball governor was used to regulate the speed of grain grinding windmills by controlling the force between the millstones around 1750 [5]. It must be noted that, in 1788, James Watt used a similar system for speed control of steam engines. The fly-ball governor and linkage kept the millstones apart, allowing the rotor to run unloaded up to a specified rotational speed. At this speed, the fly-balls rose up and allowed the millstones to move together and absorb torque. As the wind ramped up, the miller increased the flow rate of grain to absorb more torque. If the wind increased even more, it was necessary to turn the rotor out of the wind and stop it. Next, the sheets of canvas that were stretched over the wooden framework of the sails were reefed in order to reduce the swept area, and the milling process could be restarted. The spring sail, invented by the Scottish millwright A. Meikle in 1772, replaced the sheets of canvas with a series of wooden shutters. The openings of the shutters could be adapted manually by pulling on a chain or rope attached to a system of levers to "spill" the wind. One difficulty with both methods was that the windmill had to be stopped in order to adjust the settings in case the strength of the wind altered. This problem was resolved in 1807 by W. Cubit who introduced the patent sail. In this design all the shutters of all the sails were controlled automatically by a counterweight suspended outside the windmill.

In the windmill era there was, in principle, thus no need to closely control the rotational speed. In fact, allowing the windmills to operate at variable speed was highly advantageous as it increased the total energy extracted from the wind. The same holds true for the turbines that harnessed the wind for battery charging at the start of the electric era. The majority of these turbines were equipped with a DC generator, had blades with a fixed pitch angle and were operated at variable speed. In the early to mid-1970's, the need for supplying AC power to the grid changed the demands on wind turbine control because of the the fixed relationship between the speed of the AC generators and the fixed utility grid frequency. Consequently, the majority of the machines were operated at constant speed and had pitchable blades to level off excess power.

The control of the constant speed of commercial turbines has been done predominantly using PI-controllers with additional lead-lag and notch filters, while in simulation studies more advanced control strategies were proposed and explored (including optimal control [93, 179, 205] and gain-scheduled controllers [158]). The majority of the simulation studies followed a format of applying (advanced) control system design to not yet validated and often over-simplified turbine models, followed by simulation based performance comparisons. These performance comparisons are almost invariably based on measures of pitch angle, shaft speed and power variations. Fair comparisons between the industry standard control system and more advanced controllers, however, are scarce. Knudsen et al. [138] compared a robust controller with the existing Pi-type controller of a 400 kW commercial wind turbine. The robust controller, designed on the basis of an identified turbine model, achieved a reduction of pitch activity and some potential for fatigue load reduction compared to the existing controller. The comparison confirmed that improvement in performance can be achieved by proper control. The interested reader is referred to La Salle et al. [245] for a more thorough review of constant speed pitch controlled wind turbines as late as the 1990's.

The majority of the wind turbines now in operation are three-bladed, stall regulated constant speed turbines 1[172]. It must be stressed that in this case the blade pitch angle is fixed and, as a consequence, no control possibilities exist at all. Recall that stall utilizes the inherent aerodynamic properties of a rotor blade to limit the aerodynamic power. In the sequel, however, we will focus our attention on the closed-loop control of grid-connected, variable speed pitch-regulated turbines since this configuration has the highest potential to reach cost-effectiveness in the (near) future.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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