Abstract :
Very large systems of a few years ago can now hardly be classed as of medium size. A 1,000-light generator, that is, of approximately 100 h.p. capacity, was a monster machine only a few years ago. Now we are designing 20,000 h.p. units. The size of generators gradually grew larger and larger, but as the cities took up this new power the growth was so rapid that many systems were obliged to tie in with others, making the regulation of the voltage on the lights very poor. Then the multiple voltage and the three-wire systems came into use. The three-wire system aided wonderfully in giving better regulation, but it was due to the modern alternating current systems that electric lighting became so successful. This was not without a bitter fight, however, between the direct and alternating current advocates, which lasted for many years. The alternating-current systems had a sad history, due largely to the fact that there were then no alternating-current motors, and therefore practically no load on the system during the day. This was costly. The frequencies used at that time were too high, being 125 and 133 cycles per second. It was realized, however, that there were vast possibilities in the alternating-current system. At this period, the rotary converter, which changes direct into alternating current, or vice versa, made its appearance. A combination of the alternating and direct-current systems was first tried out on a large scale by the New York Edison Company, and then by the Chicago system with a double current generator. The direct current was used near the station and distributed from three-wire mains, while the outlying districts were supplied with 3-phase, 25-cycle current at 6,600 and 13,200 volts. This alternating current was transformed at the generators to 6,600 or 13,200 volts and then re-transformed at suburbs to low voltages, and connected to rotary converters, giving three-wire direct current as near the station. As these stations developed in size the q- estion naturally arose as to the limiting size of an economical station. Calculations were made showing that this limit was from 10,000 to 20,000 kw. These calculations were, however, based on one character of load. If a station has a very varied load, such as light, power, and railway, there is practically no limit to the economical size which can be built. Mr. Insull, of Chicago particularly brought out this argument and was willing to try it in the Fisk Street Station in Chicago, one of the largest in America. This station now has connected up 180,000 kw., equal to almost 250,000 h.p. On such large systems, problems arise which are very difficult of solution and which were not even heard of in the smaller stations where the voltages were low, capacities limited, and the net-work and bus bars were of comparatively high resistance. There are high-frequency disturbances on the lines all the time, and enormous capacities behind any break-down. In fact, with a modern turbine station not equipped with special reactance coils for limiting the current, if a short circuit should occur at the bus bars, and the normal rating of the station were 250,000 h.p., there might be concentrated for a moment at the point of short circuit over 12,000,000 h.p. Apparatus such as switches, circuit breakers, etc., can hardly be made to take care of such enormous powers and it has been found necessary to install reactance coils in each generator circuit and also in the bus bars between groups of generators. These reactances, in order not to saturate, or “lay down”, on such enormous currents are constructed with non-magnetic cores. They are also subject to enormous mechanical strains, up to hundreds of tons, when the short circuit current is thrown upon them, so that their design is very difficult. A comparison of these disturbances to waves of water facilitates an understanding of this subject by the non-technical man. With a small body of water, it takes a hurricane to ra
