| A variable speed generation (VSG) system converts energy from a variable resource such as wind or water flow into variable rotational mechanical energy of a turbine or a similar device that converts translational kinetic energy into rotational mechanical energy. The mechanical energy is then converted into electrical energy by an electrical generator. Presently available and proposed generators include systems based mainly on dc machines, synchronous and induction machine technology as well as reluctance machines. While extracting more energy from the resource, most proposed VSG systems suffer a cost disadvantage due to the required rating of the power electronic interface. This cost penalty may eventually render the additional energy capture meaningless. Thus, reducing the cost of the power electronic hardware is essential for VSG systems to achieve viable and competitive {dollar}/kWh ratios when compared to fossil fuel-based generating systems.; A variable speed constant frequency (VSCF) system and controller are proposed that utilize a doubly-fed machine (DFM) as the energy conversion device. The system includes a power converter that provides the current excitation for the control winding of the DFM. Both the magnitude and frequency of the excitation is determined by an adaptive model-based controller which maximizes the power flow from the mechanical turbine to the electrical grid and reduces the generator losses by maintaining the maximum efficiency point throughout the mechanical input power range.; The proposed strategy has been experimentally verified in controlled laboratory conditions for a proof-of-concept brushless doubly-fed machine (BDFM) system of 1500 Watts power rating. Issues relating to power converter development and its incorporation in the system have been investigated. The controller and circuit design of a four quadrant, AC/AC power converter is presented and a novel sensorless current controller for the active rectifier stage is presented in detail.; While presented for a wind turbine application, the philosophy of the control algorithms are equally applicable for any variable speed application (motoring, generating, with and without gearbox, etc.) Also, the control can be augmented to not only maximize power and efficiency, but also provide for harmonic and reactive power compensation via the energy conversion system. |
