Electrical Systems for Rapid Transit Railroads

Technological advances have made possible new and improved electrical systems for rapid transit railroads. Two new circuit elements contribute to this. One of these, the thyristor, or controlled silicon rectifier, is a lineal descendant of the advances in solid-state physics that led to the transistor 21 years ago. Tjhvrstors are still in limited use because of cost, and the costs are gh because of limited use. The same situation existed with respect to the transistor in the earlier years of its availability. As in the case of the transistor, it may be expected that further advances in performance and reductions in cost will occur in the future. The second circuit element is a more recent development. This is the parametric transformer which utilizes variation of inductance instead of flux for energy transfer. The inherent properties of the parametric transformer are of substantial advantage in electric traction since they provide overload protection, filter action, voltage regulation, and a 90-deg phase shift. This last property permits single-phase-polyphase energy conversion in either direction and maintains balance on the polyphase system. This latter property is also advantageous for main-line railroad electrifications. The application of these circuit elements permits single-phase power distribution which in turn can permit use of regenerative braking without causing a problem in reverse energy flow that would add a major item to cost of a dc substation. Control is combined with on-board energy conversion.