Mitigation of voltage sag using time-ratio-controlled autotransformer

This paper presents analysis of time-ratio-controlled autotransformer as a mitigating device for voltage sag disturbance. Control circuit based on load RMS and voltage sag sensing circuit based on peak detection are employed. Commonly dynamic voltage restorer and STATCOM are employed for mitigation of voltage sag. The proposed system has less number of switching devices in comparison to commonly used compensators. Simulation analysis performed in PSCAD/EMTDC for three-phase compensator and performance analysis of the system is presented for various percentages of sag. The THDs of the load voltage and current when the compensator is functioning during disturbance condition are within the limits. Also the compensator can maintain the load reactive power constant. Simulation analysis is performed for various sag conditions like constant voltage sag due to faults and variable voltage sag due to induction motor starting. Simulation results are presented to show the effectiveness of the mitigating device in compensating voltage sag.