HVDC additional transient control is an essential means to elevate the transient stability of AC/DC interconnected systems and the inevitable requirement of intelligent transmission. Two-stage transient additional control and a smooth switching strategy are proposed. By establishing an extended state observer for unbalanced power in the system disturbance, the real-time and accurate estimation of unstable power is realized, and an adaptive emergency power support controller is constructed. The mathematical model of the system is identified by using Total Least Squares-Estimation on Signal Parameters via Rotational Technique (TLS-ESPRIT). Based on the singular value reduction theory, the HVDC additional Active Disturbance Rejection Damping Controller (ADRDC) that can inhibit the low-frequency oscillation of the system is designed. Finally, two switching strategies are designed based on the change in the state of the network fault switch and the change in the power angle oscillation curve. The three-in-feed HVDC system is built in PSCAD, and the simulation analysis is carried out. The simulation results confirm the validity of the proposed method.

To improve the fault ride-through capability of the VSC-HVDC system and ensure the safe operation of the equipment, it is necessary to design the temporary blocking scheme of the converter valve of VSC-HVDC. Taking the Chongqing Hubei Back to Back VSC-HVDC Project as the research object, this paper first analyzes the fault types of the VSC-HVDC system, obtains the fault types of the AC fault ride-through; and then, according to the requirements of system fault ride-through, three-valve controlled temporary blocking schemes are designed: global blocking, phase splitting blocking and bridge arm separated blocking. Finally, based on the RTDS real-time simulation platform, the actual valve control and control protection systems are connected. Comparative simulation tests are carried out to verify the three temporary blocking schemes. The simulation results show that the bridge arm separated blocking can ensure the fault-passing ability of the converter valve and has good electrical characteristics. In practical projects, the bridge arm separated blocking scheme is recommended for valve-controlled bridge arm overcurrent protection.

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