Backup considerations for line current differential protection

Line current differential (87L) protection relies on communications for the exchange of current values and, if applied over asymmetrical channels, on external time sources for current alignment. Proper engineering of 87L schemes calls for a backup strategy that considers the loss of communications and/or the loss of external time sources. This paper reviews various channel and time backup strategies for 87L protection schemes and considers utility practices and regulatory constraints related to line protection redundancy, forced line outages, preferred balance between protection dependability and security, ability to provide adequate protection coverage with distance or overcurrent elements, and availability of an independent pilot channel for directional comparison backup schemes.