Abstract
This thesis presents an SPWM-based division-summation (D-Σ) digital control for a three-phase parallel-connection uninterruptible power supply (UPS) system. Compared to analog control, it is easier to implement digital control to achieve system functions, which can minimize circuit complexity and reduce the volume of hardware circuits at the same time. A controller with the D-Σ digital control is designed to cancel the variation effects of dc-bus voltage, switching period and inductance. This study will derive D-Σ digital control laws first and express them in general forms for readily software programming. In the circuit implementation, the inductances corresponding to various inductor currents were measured at the start-up and stored in the controller for scheduling loop gain cycle by cycle. This research adopts D-Σ digital control avoiding conventional abc to dq frame transformation so that the calculation process can be significantly simplified. In the meantime, the variation of dc-bus voltage and inductance are taken into account in the control-law derivation, which allows wide inductance variation and reduces the core size accordingly. From the perspective of current sharing, the paralleled inverters control the variation of inductor currents based on voltage and current feedback signals directly and the inverters operating in parallel share them equally. Thus, there is no need to have an external controller to achieve output voltage regulation and equal current distribution. Finally, the paralleled system consisting of three three-phase inverters has been simulated and implemented to verify the feasibility of the proposed control scheme.