Abstract
This thsis presents a division-summation (D-Σ) digital control based three-phase four-wire inverter with capacitor-current compensation, and fast outer voltage loops. The inverter is adopted to uninterruptible power supply applications and can supply unbalanced, linear and rectified loads. With the proposed capacitor-current compensation scheme, the D-Σ digital control which was originally developed for current tracking can be adopted to generate sinusoidal voltage output. The adopted D-Σ digital control can accommodate wide filter inductance variation and achieve fast tracking response when including the fast outer voltage loops. In the thsis, the control laws for the inverter are derived in detail first. In the design and implementation, the inverter inductances corresponding to various inductor currents are stored in a single-chip microcontroller for scheduling loop gain cycle by cycle, but the capacitor currents are estimated online to determine control laws (duty ratio) which are accompanied with fast outer voltage loops to improve voltage tracking accuracy. The major contributions of this research can be summarized as follows. One is deriving the control law based on D-Σ digital control. It can take care of inductance variations, so that the inverter allows a wide inductance value change, effectively reduce the size and losses of the inductor core. The other one is estimating load current and using the fast outer voltage loop to achieve fast tracking action to load changes. Simulated and experimental results measured from a 10 kW inverter have verified the analysis and discussion.