A differential protection system has a differential protection device at each end of a line section of an electrical power transmission line, such that it can selectively identify and safely disconnect even high-impedance faults on the monitored line section. The sensitivity of the system is improved by: detection of current vector measured values which indicate the current flowing in the line section by way of the differential protection devices at the ends of the line section during a learning phase; determination of a correction function from the current vector measured values, the correction function indicating a correction factor which is dependent on the amplitude of the current vector measured values of a selected differential protection device, the correction factor compensating for any amplitude difference and any phase difference between the current vector measured values of the differential protection devices; and adjustment of the differential protection system such that the current vector measured values detected after the learning phase by at least one of the differential protection devices are corrected using the correction function.