Commutation failures (CF) are very frequent dynamic events in HVDC power systems. In this paper, a simplified thyristor lumped-charge model is used to analyze the change of the stored charges in the device during turn-off and the commutation process in an HVDC inverter is studied theoretically. The influences of external circuit parameters, as well as thyristor device parameters on CF, are examined. Based on the removal process of the stored charges in thyristor devices, the duration of the phases in turn-off process of the device is given and a novel criterion of commutation failure is proposed.
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