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Space vector pulsewidth modulation (SVPWM) based three-phase voltage source inverters provide a widely used interface between electric grids and wind turbine systems. PI controllers, predictive algorithms and real-time sampling techniques are often used to overcome the shortcomings of SVPWM. These techniques depend highly on accurate measurements of inverter voltages and currents, thus making sensors the key elements in the control process. Among these sensors, the dc link voltage (Vdc) sensor is critical: if it sends out a signal with a significant error, the output current will be distorted. The Authors have developed a combination of PI and predictive methods, using them simultaneously to control a three-phase grid-connected inverter. Under this new control scheme, the PI controller is given a new task of monitoring and controlling Vdc. As a result, the output current of the inverter is of high quality, and more importantly, Vdc can be double checked for its correctness of measurements. When the Vdc sensor fails or its signals are corrupted, the Vdc PI controller will become a Vdc controller, adding an extra protective function for the reliable operation of wind turbine inverters.
Date of Publication: May 2008