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Analysis of Commutation Failure in LCC-HVDC Systems During Unbalanced Conditions Using Sequence Components

This paper takes close research on the impact of unbalanced faults in the occurrence of Commutation Failure (CF) in LCC-HVDC inverters, by finding the probability of CF with the phase shift that takes place in case of unbalanced faults. The analyses of the unbalanced system are conducted by using sequence components, a relationship between the phase shift and unbalance ratio is implemented to describe the degree of unbalance in the system. The unbalance ratio is used to quantify the amount of the phase shift that will cause CF in the system, and the probability of CF with respect to unbalance ratio is obtained. Moreover, a mathematical model is implemented to calculate the critical amount of unbalance degree that will cause CF in the system, with different faults applied, the relationship between commutation failure and phase shift is obtained. The mathematical model is verified using PSCAD/EMTDC.