Control algorithms for a unified power quality conditioner based on three-level converters

This work is focused on improvements on the control algorithms of a unified power quality conditioner (UPQC) that employs two three-level neutral-point-clamped converters. The applied control strategy is based on instantaneous power definitions (active and non-active currents), together with a robust synchronizing circuit Phase-Locked-Loop (PLL), to detect the phase angle of the fundamental positive-sequence component of the system voltage. With this control strategy, the shunt conditioner is able to compensate unbalances and harmonics in the load currents, and to correct the power factor, whereas the series conditioner is able to compensate problems related to the system voltages, like sags, swells, unbalances, and distortions. In other words, the UPQC, with the proposed control strategy, consists in a universal solution capable to mitigate most power quality problems in an electrical installation. The back-to-back connection of three-level converters in the UPQC requires a special control algorithm to ensure adequate dc-link voltage regulation and capacitor-voltage equalization. A trustworthy model of the UPQC employing three-level neutral-point-clamped converters, connected in a three-phase three-wire system, was investigated in order to verify the performance of the UPQC with the improved controller.