A power flow control system for a power transmission line comprising: an impedance injection module coupleable to a phase line of the power transmission line or a plurality of impedance injection modules arranged in series on the phase line of the power transmission line and/or in parallel on a plurality of parallel legs (21a, 21b) of the phase line of the power transmission line, each impedance injection module including a current transformer for sensing the current I L in the phase line of the power transmission line or in its respective leg of the plurality of parallel legs (21a, 21b) of the phase line of the power transmission line, a Rogowski coil for sensing the rate of change dI L /dt of the current in the phase line of the power transmission line or in its respective leg of the plurality of parallel legs (21a, 21b) of the phase line of the power transmission line, a microprocessor and a memory; a bypass switch coupled around the impedance injection module or the plurality of impedance injection modules, respectively; wherein the memory of each impedance injection module contains instructions executable by the processor to combine outputs from the current transformer and from the Rogowski coil in a predetermined manner; and to compare the combination with a pre-determined threshold value to characterize a fault that may have occurred, and to close the bypass switch if the comparison is indicative of a fault. an impedance injection module coupleable to a phase line of the power transmission line or a plurality of impedance injection modules arranged in series on the phase line of the power transmission line and/or in parallel on a plurality of parallel legs (21a, 21b) of the phase line of the power transmission line, each impedance injection module including a current transformer for sensing the current I L in the phase line of the power transmission line or in its respective leg of the plurality of parallel legs (21a, 21b) of the phase line of the power transmission line, a Rogowski coil for sensing the rate of change dI L /dt of the current in the phase line of the power transmission line or in its respective leg of the plurality of parallel legs (21a, 21b) of the phase line of the power transmission line, a microprocessor and a memory; a current transformer for sensing the current I L in the phase line of the power transmission line or in its respective leg of the plurality of parallel legs (21a, 21b) of the phase line of the power transmission line, a Rogowski coil for sensing the rate of change dI L /dt of the current in the phase line of the power transmission line or in its respective leg of the plurality of parallel legs (21a, 21b) of the phase line of the power transmission line, a microprocessor and a memory; a bypass switch coupled around the impedance injection module or the plurality of impedance injection modules, respectively; wherein the memory of each impedance injection module contains instructions executable by the processor to combine outputs from the current transformer and from the Rogowski coil in a predetermined manner; and to compare the combination with a pre-determined threshold value to characterize a fault that may have occurred, and to close the bypass switch if the comparison is indicative of a fault. to combine outputs from the current transformer and from the Rogowski coil in a predetermined manner; and to compare the combination with a pre-determined threshold value to characterize a fault that may have occurred, and to close the bypass switch if the comparison is indicative of a fault.