All digital controlled three-phase UPS without inverter-transformer

A fully digital-controlled three-phase UPS (uninterruptible power system) is developed which does not have an inverter-transformer. To remove the transformer, a different type of main circuit from the conventional UPS is adopted. One phase of the input AC is regarded as the common voltage of the main circuit. A DC voltage stabilizer solves problems caused when this transformerless circuit is applied to the UPS. A digital controller with two RISC (Reduced Instruction Set Computer) processing microcomputers to operate the circuit is also proposed. Experiments show the THD (total harmonic distortion) of the input current is less than 5%, and that of the output voltage is less than 3.6% under nonlinear load.