Capacitor-current compensation with fast voltage loop for D-Σ digital control based three-phase four-wire inverter

This paper presents a division-summation (D-Σ) digital control based three-phase four-wire inverter with capacitor-current compensation. 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. In the paper, 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. Experimental results measured from a 10 kVA inverter have verified the analysis and discussion.