Voltage control of grid-connected induction motors using floating capacitor H-bridge convertors

A system is described where a floating H-bridge converter is located in each phase between the utility grid and an induction machine. These converters are used to adjust the phase and magnitude of the motor voltage relative to the utility grid by injecting a series voltage in each phase. A control strategy is described where the phase and magnitude of the injected voltage is controlled, and the motor phase voltage is estimated without direct measurement. The resulting voltage injection control can achieve a range of objectives: grid VAR control (leading, unity and lagging), control of the floating H-bridge DC voltages, and motor voltage control. The latter feature can operate the machine under its rated conditions during grid voltage sags and swells. The proposed method is implemented in hardware and experimental results are presented.