A method to damp oscillations in a synchronous alternating current grid, the method comprising: receiving current from the synchronous AC grid through a phase of a n-phase supply line; and characterised by supplying the current received from the synchronous AC grid to a phase of a synchronous motor; detecting a sub-harmonic oscillation in the current received from the synchronous AC grid, the sub-harmonic oscillation being an oscillation having a frequency lower than the fundamental electrical grid frequency of the AC grid; and damping the sub-harmonic oscillation by: shunting a portion of the current away from the phase of the synchronous motor during at least a portion of an upper-half of the sub-harmonic oscillation, and/or supplying compensation current from a partial power converter to the phase of the synchronous motor during at least a portion of a lower-half of the sub-harmonic oscillation, wherein said partial power converter has a power rating less than a power rating of the synchronous motor; wherein: a current vector (304,306) is a transformation into two dimensions of measurements of currents on a plurality of phases of the n-phase supply line, a rotor electrical angle of the motor is a transformation of voltage measurements at the phases of the n-phase supply line into the two dimensions, the rotor electrical angle is an indication of an angular position of a rotor of the synchronous motor; time 1 is a time at which an angle between the current vector and the rotor electrical angle is smallest; time 2 is a time at which the angle between the current vector and the rotor electrical angle is largest; the upper-half of the sub-harmonic oscillation is a time period during which a magnitude of the current vector exceeds a magnitude of the current vector at time 1 and time 2; and the lower-half of the sub-harmonic oscillation is a time period during which the magnitude of the current vector is less than the magnitude of the current vector at time 1 and time 2. receiving current from the synchronous AC grid through a phase of a n-phase supply line; and characterised by supplying the current received from the synchronous AC grid to a phase of a synchronous motor; detecting a sub-harmonic oscillation in the current received from the synchronous AC grid, the sub-harmonic oscillation being an oscillation having a frequency lower than the fundamental electrical grid frequency of the AC grid; and damping the sub-harmonic oscillation by: shunting a portion of the current away from the phase of the synchronous motor during at least a portion of an upper-half of the sub-harmonic oscillation, and/or supplying compensation current from a partial power converter to the phase of the synchronous motor during at least a portion of a lower-half of the sub-harmonic oscillation, wherein said partial power converter has a power rating less than a power rating of the synchronous motor; wherein: a current vector (304,306) is a transformation into two dimensions of measurements of currents on a plurality of phases of the n-phase supply line, a rotor electrical angle of the motor is a transformation of voltage measurements at the phases of the n-phase supply line into the two dimensions, the rotor electrical angle is an indication of an angular position of a rotor of the synchronous motor; time 1 is a time at which an angle between the current vector and the rotor electrical angle is smallest; time 2 is a time at which the angle between the current vector and the rotor electrical angle is largest; the upper-half of the sub-harmonic oscillation is a time period during which a magnitude of the current vector exceeds a magnitude of the current vector at time 1 and time 2; and the lower-half of the sub-harmonic oscillation is a time period during which the magnitude of the current vector is less than the magnitude of the current vector at time 1 and time 2. shunting a portion of the current away from the phase of the synchronous motor during at least a portion of an upper-half of the sub-harmonic oscillation, and/or supplying compensation current from a partial power converter to the phase of the synchronous motor during at least a portion of a lower-half of the sub-harmonic oscillation, wherein said partial power converter has a power rating less than a power rating of the synchronous motor; a current vector (304,306) is a transformation into two dimensions of measurements of currents on a plurality of phases of the n-phase supply line, a rotor electrical angle of the motor is a transformation of voltage measurements at the phases of the n-phase supply line into the two dimensions, the rotor electrical angle is an indication of an angular position of a rotor of the synchronous motor; time 1 is a time at which an angle between the current vector and the rotor electrical angle is smallest; time 2 is a time at which the angle between the current vector and the rotor electrical angle is largest; the upper-half of the sub-harmonic oscillation is a time period during which a magnitude of the current vector exceeds a magnitude of the current vector at time 1 and time 2; and the lower-half of the sub-harmonic oscillation is a time period during which the magnitude of the current vector is less than the magnitude of the current vector at time 1 and time 2.