Control method for operating a synchronous machine

A controller for controlling a machine field excitation of a synchronous machine configured to supply power to a grid, the controller comprising:

• a control circuit;
• a control signal output configured to connect to a power converter of the synchronous machine;
• and
• a torque input configured to receive a torque signal indicating a torque of a synchronous generator of the synchronous machine,
• wherein the control circuit is configured to:
• determine a torque for the synchronous machine based on the received torque signal,
• calculate a derivative of said torque,
• determine whether the calculated torque derivative is within a predefined stable operation torque derivative range for the synchronous machine, wherein the stable operation torque derivative range is a range within which a stable operation of the synchronous machine occurs, and
• when the torque derivative is not within the predefined stable operation torque derivative range, output a control signal from the control signal output to modify the machine field excitation to bring the torque derivative within the predefined stable operation torque derivative range.