Description of the Wind Turbine component in Schematic Editor.
The Wind Turbine component is a Schematic Editor component which models a small wind turbine with a single-phase grid-tied average model inverter. The model represents the aerodynamic and mechanical behavior of an actual wind turbine in a simplified way. You can choose between pre-parametrized turbine models of 5.2 kW and 8.9 kW nominal power, or enter custom wind turbine parameters by choosing the "Custom" option in the Wind Turbine combobox. The 5.2 kW and 8.9 kW models are based on NREL models available on GitHub. These two models use internal Cp(β, λ) tables and have fixed blade pitch angles. The 5.2 kW and the 8.9 kW models have an internal wind speed limit at 16 m/s and 20.5 m/s, respectively as their Cp(β, λ) tables are based on wind speed with these limitations.
Selecting the "Custom" option will allow you to insert custom wind turbine data. There are two aerodynamic models available for Cp(β, λ) - Heier and Slootweg. You can use an externally defined model by selecting the "Extern" option and connecting your model to the "Cp(β, λ)" input of the Wind Turbine component.
In Table 1 the schematic symbol and properties window of the Wind Turbine are shown.
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Wind Turbine internal structure
The wind turbine model is built with three main parts, the aerodynamic and mechanical models, the wind turbine Control System, and the grid-tied inverter. The aerodynamic and mechanical models process the wind time series input, accounting for single-mass rotor inertia, and provide the rotor speed, counterbalanced by the torque value provided at the "Gen Torque" input. In other words, the rotor speed will be increased by the energy captured from the wind, but the Gen Torque will resist this change, as expected in an actual wind turbine.
The structure of the Wind Turbine aerodynamic and mechanical model is shown in Figure 1.
In the region of operation where wind speeds result in lower than nominal power, the Control system implements a Maximum Power Point Tracking (MPPT) algorithm. The provided Gen Torque is according to the following Control law:
where
In the case of modern high power wind turbines, when the rotor reaches the nominal speed and the wind speed is above the rated, a blade pitch controller is a common choice to limit the wind energy harnessed. However, small wind turbines do not use blade pitch control, and the energy capture is limited by blade design. In this operating region, the Gen Torque is kept constant at its nominal value. The wind turbine Gen Torque is converted to an equivalent electric current by considering the DC bus parameters. The DC bus nominal voltage depends on the selected grid voltage for the inverter. For a 120 V grid, the DC bus nominal value is set to 400 V, whilst for 240 V grids it is set to 800 V.
The grid-tied inverter is fed by a DC bus, the voltage of which is provided by a current source that is dynamically set by the wind turbine rotor torque. If the wind turbine available torque increases, the DC bus voltage will tend to increase because the current source will provide a higher current. The same idea applies if the wind speed decreases and the wind turbine provides less energy to the DC bus. The grid-tied inverter’s active Power reference is taken from the DC bus voltage. If voltage increases above nominal, a PI controller increases the active Power reference for the inverter, supplying more power to the grid. Conversely, if the DC bus voltage decreases, the PI controller will decrease the active Power reference and the Inverter will supply less power to the grid. In this way, the DC bus voltage is regulated around its nominal value and all energy produced by the wind turbine is fed to the grid. The grid-tie inverter uses the \(sin(\omega t)\) and \(cos(\omega t)\) signals provided by the Power Meter component as grid voltage signals on the controller. The control system algorithm works in \(\alpha - \beta\) stationary frame.
The internal structure of the Wind Turbine is shown in Figure 2.
References
[1] J.G. Slootweg, S. W. H. de Haan, H. Polinder and W. L. Kling, "General model for representing variable speed wind turbines in power system dynamics simulations," in IEEE Transactions on Power Systems, vol. 18, no. 1, pp. 144-151, Feb. 2003, doi: 10.1109/TPWRS.2002.807113.
[2] HEIER, S. Grid Integration of Wind Energy onshore and offshore conversion systems. 3. ed. [S.l.]: John Wiley and Sons Ltd, 2014.
[3] Rinker, Jennifer and Dykes, Katherine. 2018. WindPACT Reference Wind Turbines. Golden, CO: National Renewable Energy Laboratory. NREL/TP-5000-67667. https://www.nrel.gov/docs/fy18osti/67667.pdf.