A fault detection system for line-phases (101, 102, 103, 301, 302, 303, 501, A, B, C) of an electric power transmission system, comprising: a plurality of line current monitors (311, 312, 313), each line current monitor configured to be coupled to one of a plurality of line-phases (101, 102, 103, 301, 302, 303, 501, A, B, C) of an electric power transmission system, wherein the power transmission system preferably comprises three line-phases (101, 102, 103, 301, 302, 303, 501, A, B, C); each line current monitor (311, 312, 313) comprising: a phase detector configured to be coupled to a line-phase (101, 102, 103, 301, 302, 303, 501, A, B, C), and wherein the phase detector is further configured to perform a rotating frame transformation to provide in-phase (I d) and quadrature (I q) signals in a rotating frame based on in-phase (I α) and quadrature (I β) signals proportional to an instantaneous current in the line-phase (101, 102, 103, 301, 302, 303, 501, A, B, C); a loop filter (414, 514) coupled to the phase detector; and a controlled oscillator (415, 515) coupled to the loop filter (414, 514) and coupled to the phase detector, the controlled oscillator being preferably configured to provide a reference signal to the phase detector; characterized in that the loop filter is coupled to the phase detector and the controller oscillator forming a phase locked loop; one or more inter-phase communication modules (333, 1021, 1031, 1022, 1032, 1023, 1033) coupled to the plurality of line current monitors, preferably with electric isolation; and one or more fault detection modules (321, 322, 323), wherein each fault detection module is coupled to at least one of the line current monitors; and is configured to detect an error or a fault in the power transmission system; and wherein the one or more inter-phase communication modules (333, 1021, 1031, 1022, 1032, 1023, 1033) are configured to communicate the in-phase (I d) and quadrature (I q) signals of the plurality of line current monitors provided by the respective phase detectors to the one or more fault detection modules (321, 322, 323). a plurality of line current monitors (311, 312, 313), each line current monitor configured to be coupled to one of a plurality of line-phases (101, 102, 103, 301, 302, 303, 501, A, B, C) of an electric power transmission system, wherein the power transmission system preferably comprises three line-phases (101, 102, 103, 301, 302, 303, 501, A, B, C); each line current monitor (311, 312, 313) comprising: a phase detector configured to be coupled to a line-phase (101, 102, 103, 301, 302, 303, 501, A, B, C), and wherein the phase detector is further configured to perform a rotating frame transformation to provide in-phase (I d) and quadrature (I q) signals in a rotating frame based on in-phase (I α) and quadrature (I β) signals proportional to an instantaneous current in the line-phase (101, 102, 103, 301, 302, 303, 501, A, B, C); a loop filter (414, 514) coupled to the phase detector; and a controlled oscillator (415, 515) coupled to the loop filter (414, 514) and coupled to the phase detector, the controlled oscillator being preferably configured to provide a reference signal to the phase detector; characterized in that the loop filter is coupled to the phase detector and the controller oscillator forming a phase locked loop; a phase detector configured to be coupled to a line-phase (101, 102, 103, 301, 302, 303, 501, A, B, C), and wherein the phase detector is further configured to perform a rotating frame transformation to provide in-phase (I d) and quadrature (I q) signals in a rotating frame based on in-phase (I α) and quadrature (I β) signals proportional to an instantaneous current in the line-phase (101, 102, 103, 301, 302, 303, 501, A, B, C); a loop filter (414, 514) coupled to the phase detector; and a controlled oscillator (415, 515) coupled to the loop filter (414, 514) and coupled to the phase detector, the controlled oscillator being preferably configured to provide a reference signal to the phase detector; characterized in that the loop filter is coupled to the phase detector and the controller oscillator forming a phase locked loop; one or more inter-phase communication modules (333, 1021, 1031, 1022, 1032, 1023, 1033) coupled to the plurality of line current monitors, preferably with electric isolation; and one or more fault detection modules (321, 322, 323), wherein each fault detection module is coupled to at least one of the line current monitors; and is configured to detect an error or a fault in the power transmission system; and wherein the one or more inter-phase communication modules (333, 1021, 1031, 1022, 1032, 1023, 1033) are configured to communicate the in-phase (I d) and quadrature (I q) signals of the plurality of line current monitors provided by the respective phase detectors to the one or more fault detection modules (321, 322, 323).