Model Development of a Wind Turbine System with a Continuously Variable Transmission for Design of Region 2 Speed Control

Variable speed operation enables wind turbine systems to increase their aerodynamic efficiency and reduce fatigue loads. An alternative to the current electrically-based variable speed technologies is the continuously variable transmission (CVT). A CVT is a transmission whose gear ratio can be adjusted to take on an infinite number of settings within the range between its upper and lower limits. CVT research in wind turbine applications predicts an improvement in output power and torque loads compared to fixed-speed machines. Also, a reduction in the harmonic content of the currents is anticipated by eliminating the power electronics. This paper develops a model that combines CVT and induction generator dynamics with the FAST wind turbine simulator to create a high fidelity CVT wind turbine model for simulating the system’s performance in MATLAB/Simulink. A proportional-integral (PI) controller provides the CVT with an input for the rate-of-change of the speed ratio, and available outputs include the FAST-measured torque loads, electric power, and line currents created by the transient induction generator model. Variable-speed wind turbines can capture more power than fixed-speed turbines operating in the same wind conditions. Variable-speed turbines can also reduce the mechanical stress on some wind turbine components compared to fixed-speed turbines. Variable-speed operation is typically accomplished using power electronics in one of two configurations. In the rectifier/inverter approach, the power electronics rectify the variable-speed (variable-frequency) alternating current produced by the generator, and then invert it to match the grid frequency. The second approach uses power electronics (PE) in the feedback loop of a wound rotor induction generator (DFIG) in which the slip is controlled to permit the generator’s rotor to spin at a variable rate. However, cost and inverter reliability concerns for small- and medium-scale turbines create an opportunity for an alternative to the power electronic technologies, and the continuously variable transmission (CVT) is presented as an attractive option. A CVT is a gear box whose gear ratio is dynamic; it can be controlled to take any value within its upper and lower limits. CVTs can be found in automobiles, farm equipment, snowmobiles, and motor scooters. The variable gear ratio allows a machine to operate at its ideal rotor speed for optimum performance while driving a variable-speed load or being driven by a variable-speed source. The CVT, DFIG, and rectifier/inverter technologies are each used to connect a variable speed wind turbine to a fixed frequency electric grid. The difference between them is the point within the system where this conversion takes place. A wound-rotor induction generator, unlike most generators, can convert incoming variable-speed rotational energy into constant frequency alternating current (AC) via slip control. The rectifier/inverter strategy rectifies the variable-frequency AC output of the generator to produce variable-speed electric current, and then inverts it to grid frequency AC. Unlike these approaches, a CVT performs its frequency conversion on the mechanical side of the generator. These differences are shown in Figure 1, where the red ellipses indicate the point at which constant frequency operation begins. Several studies predict that a CVT wind turbine would improve energy capture