Design of a State-Feedback Controller for Series Voltage-Sag Compensators

Voltage sags have become a major power quality issue encountered by industries in recent years. The voltage-sag compensator, based on a transformer-coupled series-connected voltage-source inverter, is among the most cost-effective solutions to protect sensitive loads. Many voltage-sag compensators adopt open-loop control strategies to increase the response speed for the sag compensation. However, the L-C filter at the inverter output could introduce oscillations in transient, and the load-voltage distortion could appear due to nonlinear loads. This paper presents a full-state-variable feedback control scheme to accomplish fast ride-through compensation, active damping of the output filter, and improved disturbance rejection capability to maintain the waveform quality of load voltages. The feedback and feedforward controllers of the proposed scheme is implemented in the synchronous reference frame, and all the cross-coupling terms are identified. Detailed explanations are presented, and the effectiveness of the proposed scheme is verified by laboratory test results.