A new method of channel monitoring for fiber optic line differential protection

This paper puts forward a new method of channel monitoring for the optic fiber longitudinal differential protection. It involves following approaches: the differential protections at two ends of line communicate according to designed message frame format; calculate and store the communication channel delay of each message frame; estimate the stability of channel delay according to the criterion inequality, conclude the communication channel unstable if the channel delay meets the criterion inequality and closedown the protection. The method avoids the risk during routing switching by monitoring the channel delay jump and considers maturely ensuring the differential protection´s reliability during communication channel routing switching, therefore eliminates the maloperation accident of protections. The criterion focuses on the channel delay jump but not the channel delay itself and has adaptation toward channels with diverse delays. The application results verify that the proposed method has high sensitivity and reliability.