Abstract: A backup power supply system supplies power from a first ac power source in normal condition. Once the first ac power source is abnormal or fails, the system automatically turns off the first ac power source and turns on a second ac power source with null transfer time. When the first ac power source is normal, a dc/ac power inverter is controlled to supply an approximately null current such that a mechanical switch is allowed to be closed and no circulating current is generated between the first ac power source and the dc/ac power inverter. Since the mechanical switch is continuously closed, and requires no switching operation, the backup power supply system can supply the backup power with a null transfer time.

Abstract: A control method for the parallel-connected power converters includes a control circuit comprising a voltage feedback circuit and a current feed-forward circuit. The voltage feedback circuit, consisting of a voltage regulated control circuit and a waveform control circuit, is used to control the power converter's output voltage with constant amplitude and low total harmonic distortion. By adjusting the gain of waveform control circuit, it can regulate the current distribution among the parallel-connected power converters. The current feed-forward circuit separates the power converter's output current into the fundamental component and the harmonic component, and amplifies them by different gain. The amplified fundamental component acts as a fundamental resistor to suppress the circulating current among the parallel-connected power converters, and the amplified harmonic component acts as a harmonic damper for preventing from high frequency oscillation generated by the L-C filter.

Abstract: A backup power supply system supplies power from a first ac power source in normal condition. Once the first ac power source is abnormal or failed, the system automatically turns off the first ac power source and turns on a second ac power source with null transfer time. When the first ac power source is normal, a dc/ac power inverter is controlled to supply an approximately null current that a mechanical switch is allowed to being closed and no circulating current is generated between the first ac power source and the dc/ac power inverter. Since the mechanical switch is successively closed, and it requires no switching operation, the backup power supply system can supply the backup power with a null transfer time.

Abstract: A control method for the parallel-connected power converters includes a control circuit comprising a voltage feedback circuit and a current feed-forward circuit. The voltage feedback circuit, consisting of a voltage regulated control circuit and a waveform control circuit, is used to control the power converter's output voltage with constant amplitude and low total harmonic distortion. By adjusting the gain of waveform control circuit, it can regulate the current distribution among the parallel-connected power converters. The current feed-forward circuit separates the power converter's output current into the fundamental component and the harmonic component, and amplifies them by different gain. The amplified fundamental component acts as a fundamental resistor to suppress the circulating current among the parallel-connected power converters, and the amplified harmonic component acts as a harmonic damper for preventing from high frequency oscillation generated by the L-C filter.