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 the transformer-coupled series-connected voltage source inverter, is among the most cost-effective solution to protect sensitive loads. Many voltage sag compensators adopt open-loop control strategies to increase the response speed for the sag compensation. But the L-C filter installed at the output of the voltage source inverter could introduce oscillations in transient, and the voltage disturbance could appear due to varied load conditions. In addition, the coupling components could appear due to coordinate transform for simplified operation process. These are unclear how errors can affect the performance of the voltage sag compensation. A closed-loop control scheme is proposed to accomplish active damping without using any damping resistors, and raise the resonant frequency of the system to provide effectively disturbance rejected ability. The entire controller is implemented in the synchronous reference frame, and all the cross-coupling terms under the synchronous reference frame are identified and de-coupled in the controller. Detailed explanations are presented, and the effectiveness of the proposed scheme is verified by laboratory test results.