Analysis on the Dynamic Characteristics of ±1100 kV Ultra HVDC Integrated AC System

±1100kV Ultra HVDC has the advantages of large capacity and long distance power transmission. Nevertheless, it will bring greater impact on the AC/DC transmission system under uncertain disturbance. Based on 2020 planning power network in China, the model of AC/DC transmission system has been built in the electro-magnetic transient program, and the dynamic characteristic of ±1100 kV Ultra HVDC system when AC/DC fault happens and the HVDC modulation applied to improve the AC system operating performance have been investigated. The commutation failure is more likely to happen in ±1100 kV UHVDC system, and the reason has been theoretically analyzed. Moreover, the principle of HVDC control strategy on failure process and recovery procedure has been mastered. The control optimization measures have been put forward: 1) To optimize the logic of commutation failure prediction; 2) To optimize the RAML (the rectifier alpha min limiter) and VDCOL (voltage dependent current order limiter) functions in the rectifier; 3) To perfect the control of the extinction angle of inverter; 4) To modulate the reactive power in order to suppress AC voltage oscillations in the inverter. The results can provide technical reference for the ±1100kV HVDC transmission project.