Introduction general wind turbine model

A horizontal-axis wind turbine basically consists of five physical components, viz. rotor, transmission, generator, tower (including foundation) and control system. The rotor converts wind power into mechanical power, which is represented by the product of torque and angular velocity of the rotor shaft. This velocity is increased by the transmission in order to come to an angular velocity well-suited for the generator. The generator in its turn converts the mechanical power into electrical power. The transmission as well as the generator are housed in the nacelle. The tower plus foundation are needed to support the nacelle and besides that, they place the rotor into more windswept layers of air. Finally, the main goal of the control system is to enhance the closed-loop performance.

Judged from the point of view of control design, the most obvious shortcoming in most of the design codes listed in the previous chapter, is the lack of possibilities for integration of the design of a new wind turbine and the design of its control system. In order to enable this integrated design, we have developed a novel design tool called DAWIDUM. DAWIDUM is equipped with a Graphical User Interface (GUI) in order to simplify the operations involved with creating accurate dynamic models of wind turbines as well as facilitating controller design. DAWIDUM has been developed in the MATLAB®/SlMULINK® environment. For a detailed survey of the features of DAWIDUM the reader is referred to the User's Guide [187]. Implementation issues are also discussed in this guide.

Within DAWIDUM any wind turbine system is modeled as a set of bilaterally coupled modules as illustrated in Fig. 3.1. Bilateral couplings imply that the transfer of power is the result of mutual interaction instead of being imposed upon the system or upon the surroundings. To have one system impose power transfer to another system regardless of the state of the receiving system is physically not conceivable, and hence bilateral couplings should therefore be preferred. The general wind turbine model consists of the following five modules:

• Aerodynamic

• Mechanical

• Electrical

• Controller

The aerodynamic module converts the 3-D stochastic wind field generated in the wind module into aerodynamic forces. These forces are the input to the mechanical module which, in turn, converts these to velocities. The mechanical part of a generator is modeled in this module. The electrical module describes the electromagnetic part of a generator. Here mechanical power is converted into electrical power using torque set-points calculated by the controller.

Figure 3.1: Wind turbine modeled as a set of four interacting modules (i.e. aerodynamic, mechanical, electrical, and controller) and one input module (i.e. wind).

Notice that if the electromechanical torque equals the aerodynamic torque, an equilibrium is achieved, and as a result the wind turbine's rotational speed will be constant. Acceleration and deceleration to new speed set-points will be achieved by decreasing or increasing the electromechanical torque, respectively. Furthermore, if there are also mechanical means of controlling the aerodynamic torque, e.g. pitch control (indicated by the dashed arrow in Fig. 3.1), greater operational flexibility is achieved [106]. This can be easily seen by recognizing that pitch control can now be used to follow minute-to-minute fluctuations in aerodynamic power, while the (almost instantaneous) torque control can focus on fatigue load reduction.

It should also be mentioned that the presented model structure can be easily extended to being able to handle offshore wind turbines as well. The resulting model structure is shown in Fig. 3.2. The hydrodynamic module converts the wave field generated in the wave module into hydrodynamic forces. It is assumed that the structural dynamics are not influencing the wave field.

Figure 3.2: Wind turbine model structure of Fig. 3.1 extended with a wave and hydrodynamic module required for offshore applications.

The division of the complete wind turbine model into modules is based on the assumption that the modules are interacting via specified interaction variables. This creates a modular structure in which the user can easily exchange modules. Linked together correctly the modules will describe the complete wind turbine behavior. Hence, the user can compose a specific wind turbine configuration by choosing for each type of module (electrical, mechanical et cetera) the appropriate one out of the available DAWIDUM library. Moreover, any module can be modified or can be written totally new by the user as long as it is compatible with the presented model structure.

In the next sections we will discuss the main properties of the aforementioned modules as well as the interaction variables. We start with the wind module.

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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