Comparative Characteristics of Autonomous Voltage Inverters for Uninterruptible Power Supplies

The article shows the relevance of developing and studying three-level autonomous voltage inverters (AVIs) for three-phase industrial uninterruptible power supplies (UPSs). It has been established that three-level AVIs with a clamping diodes and T-type AVIs are most promising for use in UPSs. The structures of one phase of an AVI with clamping diodes and a T-type AVI are presented, and a mathematical description of the switching functions of semiconductor switches of these inverters is given, including those ensuring a decrease in switching losses in dynamic modes for T-type AVI semiconductors. Power losses in semiconductor switches and the harmonic-distortion coefficient of the output current are selected as criteria for comparing the efficiency of AVI operation. A mathematical model for calculating power losses in steady-state and dynamic operating modes and the harmonic-distortion coefficient of the output current is presented. Dependences of the selected indicators of the AVI performance on the switching frequency of semiconductor switches and the modulation coefficient of the output voltage for different operating modes and load powers are given. It was found that the T-type AVI provides more efficient conversion of electric power in the entire range of switching frequency and modulation coefficient, while, with an increase in switching frequency, the difference in power losses between the T-type AVI and the AVI with clamping diodes increases, and the change in the difference in power losses on the modulation coefficient is weakly expressed. When using sinusoidal pulse-width modulation of voltage, both types of AVIs under consideration showed the same efficiency according to the harmonic-distortion coefficient criterion of the output current, while their advantage over the two-level AVI increases with a decrease in load and a decrease in switching frequency.