Performance Boundaries and Related Improvement Method of Sub-Oscillation- Suppression Capabilities in Continuously Adjustable Phase-Shifting Transformer

The increasing penetration of power electronic devices has elevated the possibility of sub-synchronous oscillation (SSO) in the power grid. At the same time, the continuously adjustable phase-shifting transformer (CAPST) exhibits a certain capability for SSO suppression while regulating power flow. To clarify the SSO suppression mechanism and enhance its suppression capability, this study first analyzes the oscillation suppression mechanism of the CAPST, quantifies the boundaries of its oscillation suppression performance, and reveals inherent limitations of CAPST, including inadequate SSO suppression capability and the inability to simultaneously achieve effective power flow regulation and SSO suppression. To further extend the SSO suppression performance boundary, a capacitive series-compensated phase-shifting transformer (CSCPST) is introduced, accompanied by a design method for the series compensation capacitor. Additionally, to address the associated issue of power-flow reversal, a hybrid open-closed-loop power flow regulation strategy is proposed. Finally, simulation studies are conducted on both the CAPST and the CSCPST. The results verified the limitations of CAPST in SSO suppression capability and confirmed the superior performance of the CSCPST in enhancing SSO suppression and simultaneously achieving effective power flow regulation and SSO suppression.