Analytical modelling of point-to-point CCC-HVDC revisited

Weak AC grids are incapable of providing the reactive power needed by the line-commutated converter (LCC) converters to maintain an acceptable system voltage, which leads to several operational challenges. Although voltage source converter-high-voltage DC (HVDC) can solve these issues, its power rating has not matched that of the LCC technology as yet. The available alternative is the capacitor-commutated converter (CCC) technology, which is currently in operation using back-to-back configuration. In order to explore the capability of this technology for long-distance transmission, this study proposes a comprehensive non-linear state-space averaged phasor model of CCC-HVDC connected to weak grids on both sides of HVDC. To that end, the analytical model of CCC is revisited and the need of including the capacitor dynamics and the contribution of unbalanced capacitor voltages to get a closer match with the detailed switched model is investigated. The proposed model is used to present the root-cause analysis of the dynamic performance and quantify the interaction of various states in the system with different modes, which is not possible through a detailed switched model.