In the circumstance of electronic transformers used in a digitized substation， line differential protection faces many new problems， mainly those about the data sharing for the protection units， the synchronization of data sampling between two line differential protection units in different substations， the mutual operation of protection units with other units in the process layer. A solution for clock signal synchronization based on the Ping-Pang principle is proposed with an analysis made of a solution for the performance and mutual operation based on the synchronous clock differential protection.