Mechanism of DC capacitor voltage control of 6-pulse VSI-based SSSC

For a three-phase power system of inductive lines with SSSC (Static Synchronous Series Compensator), the AC and DC currents in one period, as well as the DC capacitor voltage, of the SSSC based on six-pulse VSI (Voltage Sourced Inverter) are calculated for both inductive and capacitive compensation modes. Combined with PSCAD/EMTDC electromagnetic transient simulation, the impact of currents on the charging and discharging of DC capacitor and the conductive period of its switching devices are analyzed. The calculative, simulative and analytic results show that, when the controllable serial voltage injected by SSSC is perpendicular to line current, the capacitor current has no effect on the charging and discharging of capacitor, and there is only reactive power exchange between SSSC and system; when the controllable serial voltage injected by SSSC is incompletely perpendicular to line current, the capacitor current has effect on the charging and discharging of capacitor, and there are both reactive and active power exchanges between SSSC and system.