[已经解决 ] 1.4 DISTRIBUTION OF ENERGY FORLIGHTING
P: 2016-11-09 15:41:42
1.4 DISTRIBUTION OF ENERGY FORLIGHTING
Edison planned his firstinstallation in a densely populated area of lower Manhattan in New York City,and decided that an underground system of distribution would be necessary. Thistook the form of a network supplied by feeders radiating from a centrallylocated direct current (DC)-generating station to various feed points in thenetwork. Pilot wires were taken back to the generating station from the feedpoints to give the operator an indication of voltage conditions on the system.Regulation was controlled by cutting feeders in, or out, as needed. At a laterdate, a battery was connected in parallel with the generator to guard against astation outage (Figures 1.1 and 1.2).
Gutta-percha,which had proved to be a satisfactory material for insulating the telegraphcables, was not suitable for the lighting feeders because of the softening ofthe material (a natural thermoplastic) at relatively high operatingtemperature. Experience with other types of insulation had not been sufficientto provide any degree of satisfaction with their use. The development of acable sufficiently flexible to be drawn into ducts was accordingly considered arather remote possibility. Therefore, Edison designed a rigid, buried systemconsisting of copper rods insulated with a wrapping of jute. Two or threeinsulated rods were drawn into iron pipes and a heavy bituminous compoundwas forced in and around them. They were then laid in 20-footsections and joined together with specially designed tube joints from whichtaps could be taken if desired. The Edison tube gave a remarkably satisfactoryperformance for this class of low voltage service.
Thelow voltage and heavy current characteristics of DC distribution were limitedto the area capable of being supplied from one source if the regulation was tobe kept within reasonable bounds. The high first cost and heavy losses madesuch systems uneconomical for general distribution. Accordingly, they weredeveloped in limited areas of high-load density such as the business districtsof large cities.
Inthe outlying districts, alternating current (AC) distribution was universallyemployed. This type of distribution was developed largely as a result of thework in 1882 of Lucien Gaulard and J. D. Gibbs, who designed a crude AC systemusing induction coils as transformers. The coils were first connected inseries, but satisfactory performance could not be obtained. However, they wereable to distribute electrical energy at a voltage considerably higher than thatrequired for lighting and demonstrate the economics of the AC system. This systemwas introduced in the United States in 1885 by George Westinghouse, and servedas the basis for the development of workable systems. An experimentalinstallation went into service at Great Barrington, Massachusetts, early in1886. The first large-scale commercial installation was built in Buffalo, NewYork, the same year.
Theearly installations operated at 1,000 volts. Overhead construction wasconsidered essential for their satisfactory performance and almost universallyemployed. This was also true of the street-lighting feeders, that operated atabout 2,000 volts. In Washington and Chicago, overhead wires were prohibited,so a number of underground lines were installed. Many different types ofinsulation and methods of installation were tried with little success. Experimentswith underground conductors were also carried out in Philadelphia. The 1884enactment of a law forcing the removal of all overhead wires from the streetsof New York City mandated the development of a type of construction that couldwithstand such voltages. It was some time, however, before the overheadhigh-voltage wires disappeared. In 1888, the situation was summarized in apaper before the National Electric Light Association  as follows:
Noarc wires had been placed underground in either New York or Brooklyn. Theexperience in Washington led to the statement that no insulation could be foundthat would operate two years at 2,000 volts. In Chicago, all installationsfailed with the exception of lead covered cables which appeared to be operatingsuccessfully. In Milwaukee, three different systems had been tried andabandoned. In Detroit, a cable had been installed in Dorsett conduit, but laterabandoned. In many of the larger cities, low voltage cables were operatingsatisfactorily and in Pittsburgh, Denver and Springfield, Mass., some 1,000volt circuits were in operation. (Underground Systems Reference Book 1931, 2).
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