Digital control of pulse width modulated inverters for high performance uninterruptible power supplies.

摘要

This PhD research discusses digital control strategies for three-phase Pulse Width Modulated (PWM) voltage inverters used in Uninterruptible Power Supplies (UPS) for single unit and parallel unit systems. For the single inverter system, this research proposes a novel control strategy which utilizes the perfect Robust Servomechanism Problem (RSP) control theory to allow elimination of specified unwanted voltage harmonics from the output voltages under non-linear load conditions and to achieve fast recovery performance on load transient. This technique is combined with a discrete sliding mode current controller that provides fast current limiting capability needed for overload or short circuit conditions. For the parallel inverter system, a combination of two control methods is proposed: average power control method and droop control method. The average power method is used in order to overcome the sensitivity of load sharing to output voltage/current measurement errors and mismatch wiring impedances between units, while the droop method allows the control to still maintain proper load sharing in the event that inter-unit communication is lost. In addition to this, a harmonic droop scheme for sharing of harmonic content of the load currents is introduced based on the proposed single unit control. Control analysis, experimental and simulation studies, using two parallel three-phase PWM inverters, are presented to show the effectiveness of the proposed control strategies, both for single and parallel inverter systems.