Supply of single-phase loads from central stations

Abstract

American central stations, contrary to European practise, have extensively adopted single-phase distribution from polyphase stations, balancing the loads among the different phases by grouping of the single-phase feeders or the distributing substations. Voltage regulation for lighting circuits has been supplemented by individual regulators. Generators with good single-phase characteristics were sometimes employed. In other cases generators were installed of larger kilovolt-ampere rating than the kilowatt capacity of the steam unit. Large customers, using electric welding machines, electric furnaces, etc., assist the balancing by dividing their load between the two or three phases. In cases of railroad companies' generating stations, the single-phase power is furnished from three-phase generators of special design. In cases of purchased power from central stations, different methods of supply may be available. In connection with the supply of service to the Western portion of the New Haven Railroad electrification by the New York lighting companies, four methods were considered contemplating the supply of power — (1) directly from the 25-cycle system busbars; (2) from a separate section of the 25-cycle system busbars; (3) from a 60-cycle station with frequency changers at the delivery point; (4) from special 25-cycle generators installed in the latter station. On account of the requirement of parallel operation with the railroad power plant, the first method would not give the necessary load control. The second method was dismissed on account of the requirements of balancers and also longer transmission lines than the other methods. The third plan necessitated a large investment in special apparatus and gave poor efficiency of conversion. The fourth method was adopted as it gave complete control of the load and voltage, and maximum efficiency of transmission. The larger original investment in new generator capacity was partly compensated for by its value to the companies as a standard equipment for supplying their future demands and other three-phase existing loads in the immediate districts. The equipment is described.