Excitation of large turbogenerators

The classic arrangement of a direct-driven d.c. exciter has proved, in general, a reliable, economic and satisfactory source of excitation supply for turbogenerators of moderate output. With increasing rating of individual units, the excitation power required exceeded that practicable for a direct-coupled d.c. machine and gearing was introduced to permit operation at a lower speed. These two factors, higher output and lower speed, inevitably resulted in greater physical size, with problems of access for maintenance purposes. Economics dictate that larger base-load units should operate for the maximum period between planned shutdowns and provide a stimulus to develop alternative sources of excitation power. The rotating-field a.c. exciter with static rectifier has been extensively adopted and forms the major feature of the paper; other schemes in use only on smaller units or on special or prototype machines are also discussed.