Efficient approach to quantify commutation failure immunity levels in multi-infeed HVDC systems

Commutation failure immunity index (CFII) is a useful indicator to quantify the immunity level to commutation failures in line commutated converter-based multi-infeed HVDC systems. Instead of using simulation tools which inevitably lead to heavy workload and long computation time, the analytical expressions of 'local CFII' (LCFII) and 'concurrent CFII' (CCFII) are derived based on the minimum extinction angle criteria. Using these expressions, the impact of the system parameters on commutation failure immunity levels in the inverter side can be determined quickly and effectively. Moreover, the derived analytical expressions will make the parametric sensitivity analysis more comprehensive and complete compared to the traditional simulation-based methods. Furthermore, in order to give an insight into how inter-inverter interactions influence the concurrent commutation failure behaviours, an improved index 'weak coupling multi-infeed interaction factor' (WCMIIF) is proposed. The validity and accuracy of the proposed index and analytical expressions are verified by the simulated results based on a dual-infeed HVDC system in PSCAD/EMTDC, which indicate that LCFII has an approximately linear correlation with the local effective short-circuit ratio (ESCR) of the faulted ac system, and that with the WCMIIF unchanged, CCFII has an approximately linear correlation with the local ESCR and an inverse correlation with the multi-infeed interaction factor.