NOVELTY - The system (3) has a Rogowski sensor (19) adapted to be installed at a power supply conductor (9) of a rotor (11) of a rotating machine (13). The sensor measures a signal of a rotor current at a variable frequency, where the rotor current controls a rotational speed of the rotating machine. A processing unit measures the signal over a wide frequency band of few tens of hertz to multiple tens of hertz, preferably from one-tenth hertz to sixty hertz. A memory unit saves calibration parameters. A Rogowski coil defines coil turns of the power supply conductor.    USE - Measuring system for use in a control device (claimed) of a protection system for measuring a current of a rotor of a rotating machine i.e. driving generator, which is used in an energy source e.g. tidal wind mill and hydroelectric station, in an electricity network.    ADVANTAGE - The Rogowski sensor is adapted to be installed at the power supply conductor of the rotor of the rotating machine, and measures the signal of the rotor current at the variable frequency, where the rotor current controls the rotational speed of the rotating machine, thus enabling reliable measurement of the current of the rotor, and providing optimal protection of the rotating machine. The system avoids disturbing of the sensor by eddy currents and allows fast change of the currents. The system avoids the need for an iron core to saturate, and allows linear measurement of the current signal even when the system is subjected to large currents including disturbances caused by high powers pulses. The memory unit saves the calibration parameters, thus avoiding the need for calibration for installation of the sensor or for maintenance actions, and hence adapting the sensor to any types of installations.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a control device.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of a protection system of a rotating machine.    Measuring system (3)    Power supply conductor (9)    Rotor (11)    Rotating machine (13)    Rogowski sensor (19)