A high voltage direct current (HVDC) system with conventional line commutated converter (LCC) and multiple paralleled modular multilevel converters (MMCs) in hybrid cascaded connection at the inverter side, known as a hybrid LCC/VSC cascaded HVDC (HC-HVDC) system, effectively integrates the advantages of LCC, as well as MMC technologies. However, due to the master-slave control mode adopted in the project of paralleled MMC stations, current imbalance issues would inevitably occur among paralleled MMCs in the case of power change, DC voltage drop and short-circuit fault conditions, which could result in overcurrent in MMCs or even slow down the fault recovery process. To realize current of MMC group distributed evenly, a power-based compensation control is proposed in this paper. First, imbalanced current between power-controlled and voltage-controlled MMCs is deduced, then the imbalanced current is transformed into active power, and finally, obtained active power is compensated to the active power outer loop control. By compensating the active power reference value, the proposed control approach can achieve current balancing of paralleled MMCs under different conditions. The HC-HVDC system, based on the Baihetan-Jiangsu project, which is currently under construction, is developed in PSCAD/EMTDC to validate effectiveness of the proposed control approach. Results show that, power-based compensation control can effectively balance currents of paralleled MMCs, accelerate system recovery process, and improve system dynamics.

To mitigate commutation failures (CFs) of a line-commutated converter based-high voltage direct current transmission system (LCC-HVDC), an evolved thyristor based full bridge sub-module (ET-FBSM) and a compound LCC (CLCC) topology are proposed with the capability of commutation voltage compensation and DC power consumption. Under AC fault conditions, additional commutation voltage for LCC can be compensated, and rise of DC current can be suppressed, by switching the control modes of ET-FBSM, to reduce risks of CFs and improve the dynamic characteristics of LCC-HVDC systems. In this paper, working modes of the ET-FBSM and a coordinated control strategy between ET-FBSM and converter valve are presented, and the parameter selection approach for the ET-FBSM is provided. Then, a CLCC-HVDC system with ET-FBSMs embedded in the inverter station is developed in PSCAD/EMTDC to verify effectiveness of the proposed topology. Simulation results show the proposed topology can significantly reduce risks of CFs and improve dynamic performances of the overall system.

Inaccuracy of modeling for line commutated converter based high voltage direct current (LCC-HVDC) systems will degrade the reliability of small-signal stability analysis results. Taking a practical project as the study system, this paper establishes a modified dynamic model by modifying the current switching function, firing angle calculation and DC voltage calculation. Then, the accuracy of the modified model is verified by a detailed electromagnetic transient (EMT) simulation. Finally, with the modified dynamic model, the impact of the control parameters and AC system strength on the small-signal stability of the overall system is investigated by adopting the eigen-analysis. The results clearly demonstrate that the modified model shows higher accuracy than the original model.