Three-phase four-leg voltage source inverter with load impedance estimation and D-Σ digital control

This paper presents a division-summation (D-Σ) digital controlled three-phase four-leg voltage source inverter with load impedance estimation for uninterruptible power supply applications. The D-Σ digital control developed for current tracking can accommodate wide filter inductance variation, which has been successfully adopted in grid-tied applications. This paper proposes a load impedance estimation scheme to extend the D-Σ digital current tracking control to voltage tracking. Output load is estimated every switching cycle so as voltage tracking error can be converted to current tracking error and compensated with a D-Σ digital controller, achieving fast tracking response and low voltage distortion. In the design and implementation, the inverter filter inductances corresponding to various inductor currents were measured at system startup and stored in a single-chip microcontroller RX62T for scheduling loop gain every switching cycle. Experimental results measured from a 10 kVA inverter have verified the analysis and discussion.