The protective function, used in the inverter, mitigates commutation failures due to AC voltage dips. After a fault detection (three-phase or one-phase) an angle value (AMIN_CFPREV) is sent to the converter control in order to be deducted from the maximum delay angle limit (alpha_max). Practically, the commutation margin is consequently enlarged reducing therefore the risk of commutation failures.
The CLARK transformation is used for three-phase fault detection and the zero-sequence voltage evaluation. The detection is based on instantaneous values, which ensure fast reaction of the control system when AC faults occurs.
Ref. "A Novel Method to Mitigate Commutation Failures in HVDC Systems,"
Lidong Zhang, Lars Dofnas ABB Utilities Power System Technology,
Proceedings PowerCon 2002, Volume: 1 , 13-17 Oct. 2002, Page(s): 51-56 vol.1