Mitigation of unbalanced voltage dips using Static Series Compensator

The Static Series Compensator (SSC) is suited to protect sensitive loads against voltage dips. Because most of the power system faults are single- or double-phase, the control algorithms of the SSC should be adapted for unbalanced dips. This paper proposes two control strategies to improve the dynamic performance of the SSC. The first strategy uses a fast technique for separating positive and negative sequence components of the supply voltage, which are then controlled separately. Thus, two controllers are implemented for the two sequences, each based on vector control. The second strategy is based on using only a positive sequence controller and increasing the switching frequency. Consequently, the negative sequence due to the unbalanced dip is transformed into variations in the positive sequence. As the switching frequency increases, the ability of the controller to follow those variations improves. The validity of the proposed strategies is demonstrated through PSCAD/EMTDC simulation, when the grid is subjected to unbalanced three-phase voltage dips.