I. Introduction
As one of the most widely used flexible ac transmission systems (FACTS) devices, the thyristor-controlled series capacitor (TCSC) can regulate the impedance smoothly in a wide range by adjusting the firing angles of the thyristor. TCSC also can be used to regulate its impedance between capacitive state and inductive state, which is important to stability control of power system [1]–[12]. Since TCSC mode-switching is a switch between two operating mode and it's usually activated during a large disturbance in power system, it may interact with system supplies and the transient process of mode switching may have negative impact on system stability. Thus, it is necessary to develop a mode switching control that is not only fast and precise but also a moderate and dynamic process.