Improvement in input current waveform of high efficiency AC-AC converter for online UPS

This paper proposes a switching method and a LC design method in order to suppress the distortion of the input current which occurs in a high efficiency AC-AC converter. The proposed AC-AC converter which is used in online-type Uninterruptible Power Supplies (UPS) is constructed by a T-type three-level rectifier and a T-type three-level inverter. The switching loss of the proposed AC-AC converter is drastically reduced because the switching frequency is reduced to six times of grid frequency. In order to suppress the distortion of the input current, elimination methods of the output surge voltage and the discontinuity of the input current are proposed. For decreasing the surge voltage, a current commutation strategy which uses an overlapped switching in order to maintain the current path is applied. Besides, in order to eliminate the discontinuous current on the input side, LC components in the circuit are designed in the condition that the input phase angle is maintained above 30 deg. As a result, the output surge voltage is suppressed to almost zero and the total harmonic distortion (under 40th harmonics of the fundamental frequency) of input current is reduced by 51%.