Subsynchronous oscillations study of non-parallel operating AC/DC system with series capacitor

The first non-parallel operating AC/DC system with series capacitor being constructed in China is hybrid of the Hulunbeier-Liaoning ±500 kV HVDC lines and Yimin-Fengtun 500 kV AC transmission system. Its subsynchronous oscillation problem become complex because of the mutual effect of series capacitor compensation and HVDC. In order to analyze the SSO characteristic of this new grid structure, six cases derived from original system have been adopted: the islanded operating HVDC (Hulunbeier-Liaoning) system, the AC (Yimin-Fengtun) system without series capacitor and HVDC lines, the AC system with FSC and HVDC lines, the AC system with FSC+TCSC(two control strategy) and HVDC lines. Through the simulation of shaft torque and rotational speed of studied generators with multimass in different faults situations, the results show that the non-parallel operating AC/DC system can mitigate SSO caused by HVDC or FSC. The system characteristic is in association with the system structure, compensation type and control strategy of TCSC..