Abstract: A gas turbine engine driven auxiliary electric power unit which includes a turbine (9), an air compressor (7) for supplying compressed air to the turbine and driven by the turbine, fuel supplying means (2) for supplying gaseous fuel to the turbine, a generator (10) driven by the turbine for generating electrical output power which is fed to a main electric user supply (14), first detecting means (15) for monitoring turbine shaft speed, second detecting means (19) for monitoring the output power of the generator, first modulating means (16,18,13) for modulating the turbine shaft speed to cause it to coincide with a predetermined shaft speed (17) by means of controlling current flow from the generator to the main electric user supply, and second modulating means (20,22,23) for modulating the flow of gaseous fuel which is supplied from the fuel supplying means to the turbine so as to cause the monitored output power to coincide with a predetermined output power (21).