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Both the local and concurrent commutation failures (LCF and CCF) are concerned threats to the secure operation of multi-infeed LCC-HVDC (MIDC) systems, which will be largely impacted by the voltage stability. However, this impact has not been clearly revealed yet by earlier works due to the lack of the related analytical evaluation method. Thus in this paper, the analytical evaluation method related to the LCF and CCF with the voltage stability in MIDC systems is firstly proposed considering the complicated inter-inverter interactions. This is realized by deriving the analytical expressions of the LCF and CCF immunity indices (LCFII and CCFII) as the functions of the voltage stability factor (VSF) based on the system quasi-steady-state model. Moreover, the proposed method is used to clearly evaluate the impact of the voltage stability on the LCF and CCF by comprehensively conducting the parametric dependence analysis of the LCFII, CCFII, and VSF versus various system parameters and operation variables. The analysis results indicate that in some certain situations, increasing the voltage stability of MIDC systems however imposes an unexpected negative impact on the LCF and CCF. Finally, the simulation results of the PSCAD/EMTDC program based on a tri-infeed LCC-HVDC system validate the theoretical developments. [ABSTRACT FROM AUTHOR]

The adverse impacts of commutation failure (CF) of a line-commutated converter (LCC)-based high-voltage direct current (HVdc) system on the connected ac system are becoming more serious for high-power ratings, for example, the development of ultra-HVdc systems. Aiming to solve the problem of CF particularly for higher power/current LCC HVdc systems, this paper proposes a new method, which utilizes a thyristor-based controllable capacitor (TBCC), to eliminate CFs. The topology of the proposed TBCC LCC HVdc and its operating principles are presented. Then, mathematical analysis is carried out for the selection of component parameters. To validate the performance of the proposed method, modified LCC-HVdc and capacitor-commutated converter (CCC)-based HVdc systems based on the modified CIGRE HVdc system are modeled in a real-time digital simulator. Simulation studies for zero impedance single-phase and three-phase faults are carried out, and comparisons are made with both LCC-HVdc and CCC-HVdc systems. Furthermore, voltage and current stress of the TBCC are investigated and power-loss calculations are presented. The results show that the proposed method is able to achieve CF elimination under the most serious faults while the increase of power losses due to the TBCC is small. [ABSTRACT FROM AUTHOR]