Generating fixed voltage and frequency from a generator driven with a variable speed, optimizes the power-extraction

To maximize the power extraction from a source, the load-impedance must match the internal impedance of the source. For wind and hydroelectric turbines the efficiency is maximized, when the mechanical impedance of the turbine matches the converting-impedance between the flow and turbine. As the efficiency of turbines is dependent on RPM, controlling the RPM operating point, e.g. by placing two three-phase inverters back to back between the generator and the grid, can increase the efficiency of the system. By flow-fluctuations, the maximum efficiency for turbine systems with large inertias, the mechanical impedance of the turbine requires the operating point to deviate from its steady state value. Some of the associated control issues are discussed. The author presents a concept to maximize the power extraction from a flow, using a standard induction motor. The concept can be used for wind, hydraulic and ship turbine generator systems