Voltage source converter based high voltage direct current (VSC-HVDC) can participate in voltage regulation by flexible control to help maintain the voltage stability of the power grid. In order to quantitatively evaluate its influence on the voltage interaction between VSC-HVDC and line commutated converter based high voltage direct current (LCC-HVDC), this paper proposes a hybrid multi-infeed interaction factor (HMIIF) calculation method considering the voltage regulation control characteristics of VSC-HVDC. Firstly, for a hybrid multi-infeed high voltage direct current system, an additional equivalent operating admittance matrix is constructed to characterize HVDC equipment characteristics under small disturbance. Secondly, based on the characteristic curve between the reactive power and the voltage of a certain VSC-HVDC project, the additional equivalent operating admittance of VSC-HVDC is derived. The additional equivalent operating admittance matrix calculation method is proposed. Thirdly, the equivalent bus impedance matrix is obtained by modifying the alternating current (AC) system admittance matrix with the additional equivalent operating admittance matrix. On this basis, the HMIIF calculation method based on the equivalent bus impedance ratio is proposed. Finally, the effectiveness of the proposed method is verified in a hybrid dual-infeed high voltage direct current system constructed in Power Systems Computer Aided Design (PSCAD), and the influence of voltage regulation control on HMIIF is analyzed.

To enhance power flow regulation in scenarios involving large-scale renewable energy transmission via high-voltage direct current (HVDC) links and multi-infeed DC systems in load-center regions, this paper proposes a hybrid modular multilevel converter–capacitor-commutated line-commutated converter (MMC-CLCC) HVDC transmission system and its corresponding control strategy. First, the system topology is constructed, and a submodule configuration method for the MMC—combining full-bridge submodules (FBSMs) and half-bridge submodules (HBSMs)—is proposed to enable direct power flow reversal. Second, a hierarchical control strategy is introduced, including MMC voltage control, CLCC current control, and a coordination mechanism, along with the derivation of the hybrid system’s power flow reversal characteristics. Third, leveraging the CLCC’s fast current regulation and the MMC’s negative voltage control capability, a coordinated power flow reversal control strategy is developed. Finally, an 800 kV MMC-CLCC hybrid HVDC system is modeled in PSCAD/EMTDC to validate the power flow reversal performance under a high proportion of full-bridge submodule configuration. Results demonstrate that the proposed control strategy enables rapid (1-s transition) and smooth switching of bidirectional power flow without modifying the structure of primary equipment: the transient fluctuation of DC voltage from the rated value (UdcN) to the maximum reverse voltage (-kUdcN) is less than 5%; the DC current strictly follows the preset characteristic curve with a deviation of ≤3%; the active power reverses continuously, and the system maintains stable operation throughout the reversal process.

The controllable line-commutated converter (CLCC), which achieves forced commutation and effectively prevents commutation failures, has seen initial engineering applications in high-voltage direct current (HVDC) transmission systems. This paper analyzes fault characteristics and develops a valve control strategy for single-phase-to-ground faults on the valve side of the CLCC converter transformer. First, mathematical models of the DC-side circuit and its control system are established. Next, based on the fault location and changes in current paths during the fault, the transient characteristics of the valve current are analyzed, accounting for the effects of the control system. Furthermore, a relationship describing how faults affect the extrema of the valve current during commutation is derived. Finally, a method for optimizing the valve control strategy is proposed based on predicting the times at which the valve current reaches its extrema. This prediction is obtained using a current transfer time model derived from an equivalent circuit describing current transfer between the main and auxiliary branches of the CLCC. Simulation results show that the proposed method can effectively reduce the current stress imposed on the arrester by fault-induced abnormal commutation, improve the current waveform during commutation, and enhance the operational stability of the system during faults.