A variable speed generator-motor system comprising a power converter connected to a DC power supply , which comprises a positive pole(P)and a negative pole (N), and a three phase synchronous machine having damper windings and connected to an AC side via three terminals (R,S,T), wherein the three terminals (R,S,T) are connected to neutral points (Ux,Vx,Wx) of three sets of three-terminal reactors and are configured to be connected to a three-phase transformer , first terminals (Up,Vp,Wp) of the three-terminal reactors are connected to machine side terminals (a) of three two-terminal arms on the positive pole side of a modular multilevel converter, MMC, that are formed by serially connected k two-terminal unit converters that are configured to output an arbitrary voltage via an energy storage element having voltage source characteristic, where k is a natural number equal to one or larger, and positive pole side terminals (b) of the arms are star-connected to the positive pole (P) of the DC power supply , second terminals (Um,Vm,Wm) of the three-terminal reactors are connected to machine side terminals (b) of three two-terminal arms on the negative pole side of the modular multilevel converter, MMC, that are formed by serially connected unit converters , and negative pole side terminals (a) of the arms are star-connected to the negative pole (N) of the DC power supply , the variable speed generator-motor system further includes: a first controller configured to adjust a frequency and an amplitude of current supplied from the power converter to the synchronous machine to a fixed value; a second controller configured to synchronize a frequency of current supplied from the power converter to the synchronous machine with a rotational speed of the synchronous machine and to adjust an amplitude of the current to a value proportional to the frequency; the variable speed generator-motor system being configured to use either the first controller or the second controller ; and a switch configured to short-circuit a field winding of the synchronous machine with a resistor when the first controller is used, and to connect the field winding of the synchronous machine to an excitor when the second controller is used, and the variable speed generator-motor system is configured to use the first controller when the synchronous machine is started from rest, and to switch control to be performed by the second controller once the frequency reaches a set value, and wherein the variable speed generator-motor system is further configured to temporarily stop the MMC power converter when the frequency reaches the set value, in order to allow the connection of the exciter through the switch . a first controller configured to adjust a frequency and an amplitude of current supplied from the power converter to the synchronous machine to a fixed value; a second controller configured to synchronize a frequency of current supplied from the power converter to the synchronous machine with a rotational speed of the synchronous machine and to adjust an amplitude of the current to a value proportional to the frequency; the variable speed generator-motor system being configured to use either the first controller or the second controller ; and a switch configured to short-circuit a field winding of the synchronous machine with a resistor when the first controller is used, and to connect the field winding of the synchronous machine to an excitor when the second controller is used, and the variable speed generator-motor system is configured to use the first controller when the synchronous machine is started from rest, and to switch control to be performed by the second controller once the frequency reaches a set value, and wherein the variable speed generator-motor system is further configured to temporarily stop the MMC power converter when the frequency reaches the set value, in order to allow the connection of the exciter through the switch . <EOS>