The Vacuum-Insulated, Varying-Capacitance Machine

The prospects for a varying-capacitance, vacuum-insulated, rotating machine have been examined analytically for the generation of HVDC power. Such a machine offers two potential advantages over conventional rotating machinery: 1. It is capable of very high efficiency (losses in the machine itself - excluding diodes or thyristors which might be necessary in some applications - would be less than 0.1%). 2. It is inherently suitable to high voltage operation. It appears that these machines, utilizing the best present-day techniques, are not an advantageous alternative to conventional magnetic synchronous machines for power frequency applications. For the generation of HVDC power, however, the situation is quite different: the vacuum-insulated, varying- capacitance machine would be comparable in size to the conventional magnetic machine and would possess the important advantages of producing HVDC power directly, and with higher efficiency. Assuming that 10 kV/mm could be insulated across the rotor-to-stator gap, such an HVDC machine would be 30-50% larger than a conventional synchronous alternator, but would, in effect, replace not only the alternator, but the power transformer and rectifying equipment. Losses in the vacuum-insulated HVDC machine would be 1/3 the losses of the conventional alternator-transformer -rectifier combination. If gradients of 25 kV/mm could be supported in the vacuum-insulated machine, it would be 20-30% smaller than the conventional alternative of a combination of magnetic synchronous machine, high voltage transformer, and rectifier. The design of a 7,000 kW,200 kV, dc generator, based on peak working gradients of 25 kV/mm, is discussed as an example.