A method of operating a power distribution system comprising: a main ac busbar; an emergency ac busbar; a hybrid drive system comprising: an electrical machine and a prime mover, the rotor of the electrical machine and the driving end of the prime mover being mechanically coupled to a load, e.g., by means of a mechanical linkage such as a gearbox; a first active rectifier/inverter having: ac input terminals electrically connected to the main ac busbar, and dc output terminals; a second active rectifier/inverter having: dc input terminals electrically connected to the dc output of the first active rectifier/inverter by a dc link, and ac output terminals electrically connected to the electrical machine; wherein, in the event of an electric power blackout situation, the method comprises the step of supplying power from the emergency ac busbar to the dc link to increase the dc link voltage; and is characterised by the steps of: increasing the dc link voltage to a partial dc link voltage that is less than the rated dc link voltage; operating the second active rectifier/inverter as an inverter to supply the partial dc link voltage to the electrical machine to increase the magnetic flux of the electrical machine to a partial magnetic flux that is less than the rated magnetic flux; supplying current from the electrical machine to the dc link through the second active rectifier/inverter to increase the dc link voltage to the rated dc link voltage; and increasing the magnetic flux of the electrical machine to the rated magnetic flux. a main ac busbar; an emergency ac busbar; a hybrid drive system comprising: an electrical machine and a prime mover, the rotor of the electrical machine and the driving end of the prime mover being mechanically coupled to a load, e.g., by means of a mechanical linkage such as a gearbox; a first active rectifier/inverter having: ac input terminals electrically connected to the main ac busbar, and dc output terminals; ac input terminals electrically connected to the main ac busbar, and dc output terminals; a second active rectifier/inverter having: dc input terminals electrically connected to the dc output of the first active rectifier/inverter by a dc link, and ac output terminals electrically connected to the electrical machine; wherein, in the event of an electric power blackout situation, the method comprises the step of supplying power from the emergency ac busbar to the dc link to increase the dc link voltage; and is characterised by the steps of: increasing the dc link voltage to a partial dc link voltage that is less than the rated dc link voltage; operating the second active rectifier/inverter as an inverter to supply the partial dc link voltage to the electrical machine to increase the magnetic flux of the electrical machine to a partial magnetic flux that is less than the rated magnetic flux; supplying current from the electrical machine to the dc link through the second active rectifier/inverter to increase the dc link voltage to the rated dc link voltage; and increasing the magnetic flux of the electrical machine to the rated magnetic flux. dc input terminals electrically connected to the dc output of the first active rectifier/inverter by a dc link, and ac output terminals electrically connected to the electrical machine; wherein, in the event of an electric power blackout situation, the method comprises the step of supplying power from the emergency ac busbar to the dc link to increase the dc link voltage; and is characterised by the steps of: increasing the dc link voltage to a partial dc link voltage that is less than the rated dc link voltage; operating the second active rectifier/inverter as an inverter to supply the partial dc link voltage to the electrical machine to increase the magnetic flux of the electrical machine to a partial magnetic flux that is less than the rated magnetic flux; supplying current from the electrical machine to the dc link through the second active rectifier/inverter to increase the dc link voltage to the rated dc link voltage; and increasing the magnetic flux of the electrical machine to the rated magnetic flux. increasing the dc link voltage to a partial dc link voltage that is less than the rated dc link voltage; operating the second active rectifier/inverter as an inverter to supply the partial dc link voltage to the electrical machine to increase the magnetic flux of the electrical machine to a partial magnetic flux that is less than the rated magnetic flux; supplying current from the electrical machine to the dc link through the second active rectifier/inverter to increase the dc link voltage to the rated dc link voltage; and increasing the magnetic flux of the electrical machine to the rated magnetic flux.