Clock synchronization for line differential protection

The present invention is concerned with time synchronization between two geographically separated stationary clocks 1, 2, in particular clocks 1, 2 at two ends of a power transmission line. In particular the method deals with synchronizing a first and a second clock 1, 2 located respectively at a first and a second end of an AC power line wherein the two clocks 1, 2 are initially synchronized to within a period of a line frequency of the power line. The method is comprised of producing a first representation 3 such as a waveform of an oscillating power line quantity q such as the AC current by measuring or recording the power line quantity at the first end of the power line i.e. recording a sufficient number of samples q(t1) of the oscillating quantity q within a oscillation period, and time-stamping the first representation 3 by the first clock 1. The method is further comprised of producing a second representation 4 of the same oscillating power line quantity q by measuring the power line quantity at the second end of the power line, and time-stamping the second representation 4 by the second clock 2. A further step comprises comparing i.e., aligning, using the respective time-stamps, the first and the second representation 3, 4 to determine a clock offset 19 between the first and the second clock 1, 2, and adjusting based on the comparison one or both of the first and second clock 1, 2 or at least the first clock 1 to reduce the determined clock offset 19.