A voltage sag supporter utilizing a PWM-switched autotransformer

This paper describes a novel distribution-level voltage control scheme that can compensate voltage sag and swell conditions in three-phase power systems. Faults occurring in power distribution systems or facilities in plants generally cause the voltage sag or swell. Sensitivity to voltage sags and swells varies within different applications. For sensitive loads, even a voltage sag of short duration can cause serious problems. Normally, a voltage interruption triggers a protection device, which causes the shutdown of the entire load. In order to mitigate power interruptions, this paper proposes a scheme for voltage sag support based on a pulse width modulated (PWM) autotransformer. The proposed scheme is able to quickly recognize the voltage sag or swell condition, and it can correct the voltage by either boosting the input voltage during voltage sag events or reducing the voltage during swell events. The key advantage in this voltage boost topology is that it uses only one controlled switch per phase to boost the input voltage, thereby resulting in minimum cost per kVA. The paper presents the new voltage control topology, as well as the voltage detection method, the snubber design, and the commutation logic for the thyristor bypass switch. Experiments have been carried out to verify the validity of the proposed scheme.