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1.4 DISTRIBUTION OF ENERGY FOR LIGHTING
Edison planned his first installation in a densely populated area of lower Manhattan in New York City, and decided that an underground system of distribution would be necessary. This took the form of a network supplied by feeders radiating from a centrally located direct current (DC)-generating station to various feed points in the network. Pilot wires were taken back to the generating station from the feed points 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 later date, a battery was connected in parallel with the generator to guard against a station outage (Figures 1.1 and 1.2).
Gutta-percha, which had proved to be a satisfactory material for insulating the telegraph cables, was not suitable for the lighting feeders because of the softening of the material (a natural thermoplastic) at relatively high operating temperature. Experience with other types of insulation had not been sufficient to provide any degree of satisfaction with their use. The development of a cable sufficiently flexible to be drawn into ducts was accordingly considered a rather remote possibility. Therefore, Edison designed a rigid, buried system consisting of copper rods insulated with a wrapping of jute. Two or three insulated rods were drawn into iron pipes and a heavy bituminous compound was forced in and around them. They were then laid in 20-foot sections and joined together with specially designed tube joints fromwhich taps could be taken if desired. The Edison tube gave a remarkably satisfactory performance for this class of low voltage service.
The low voltage and heavy current characteristics of DC distribution were limited to the area capable of being supplied from one source if the regulation was to be kept within reasonable bounds. The high first cost and heavy losses made such systems uneconomical for general distribution. Accordingly, they were developed in limited areas of high-load density such as the business districts of large cities.
In the outlying districts, alternating current (AC) distribution was universally employed. This type of distribution was developed largely as a result of the work in 1882 of Lucien Gaulard and J. D. Gibbs, who designed a crude AC system using induction coils as transformers. The coils were first connected in series, but satisfactory performance could not be obtained. However, they were able to distribute electrical energy at a voltage considerably higher than that required for lighting and demonstrate the economics of the AC system. This system was introduced in the United States in 1885 by George Westinghouse, and served as the basis for the development of workable systems. An experimental installation went into service at Great Barrington, Massachusetts, early in 1886. The first large-scale commercial installation was built in Buffalo, New York, the same year.
The early installations operated at 1,000 volts. Overhead construction was considered essential for their satisfactory performance and almost universally employed. This was also true of the street-lighting feeders, that operated at about 2,000 volts. In Washington and Chicago, overhead wires were prohibited, so a number of underground lines were installed. Many different types of insulation and methods of installation were tried with little success. Experiments with underground conductors were also carried out in Philadelphia. The 1884 enactment of a law forcing the removal of all overhead wires from the streets of New York City mandated the development of a type of construction that could withstand such voltages. It was some time, however, before the overhead high-voltage wires disappeared. In 1888, the situation was summarized in a paper before the National Electric Light Association [1] as follows:
No arc wires had been placed underground in either New York or Brooklyn. The experience in Washington led to the statement that no insulation could be found that would operate two years at 2,000 volts. In Chicago, all installations failed with the exception of lead covered cables which appeared to be operating successfully. In Milwaukee, three different systems had been tried and abandoned. In Detroit, a cable had been installed in Dorsett conduit, but later abandoned. In many of the larger cities, low voltage cables were operating satisfactorily and in Pittsburgh, Denver and Springfield, Mass., some 1,000 volt circuits were in operation. (Underground Systems Reference Book 1931, 2).

1.4照明電能的傳輸

愛迪生在紐約曼哈頓人口密集地區(qū)開始了他的第一次電力照明計劃，他決定使用地下電力傳輸系統(tǒng)。這個電力網絡從發(fā)電站輸入直流電能到網絡中的各個節(jié)點。并通過引流線回歸到供電站，為整個系統(tǒng)的供給端提供一個相對的電壓。通過供電的通斷實現控制。后來，又在系統(tǒng)中并聯(lián)了一個電池，做為發(fā)電站斷電時的備用電源。

杜仲橡膠適用于電報線路，但也被證明不適用于照明線路。因為杜仲橡膠天然的熱塑性，在導體承受電流發(fā)熱時，會變得很軟。經試驗，其它的絕緣材料也不以有充分滿足要求。以導管的方式開發(fā)一個遠距離的靈活的傳送線路，也是不可能的。因此，愛迪生開發(fā)了一種剛性的，地下敷設的，以鋁桿為導體，黃麻纖維做為絕緣的電纜。一個鐵管中穿入2到3根絕緣線，并壓入大量的瀝青填充。每20英寸一段的電纜連接到一起，并用特殊設計的套管連接。如果需要，接頭上有可以打開的塞子。愛迪生的管狀電纜，在低壓傳輸方面，取得了極大的成功。

低壓和大電流的傳輸特性，限制了直流輸送的有效區(qū)域。過高的造價和損耗使用這種方式的傳輸沒有經濟性。因此，這種系統(tǒng)只能在大城市密集的商業(yè)區(qū)使用。

在其它區(qū)域，交流電被廣泛的應用。1882年盧西恩.加勒德和J.D.吉布斯利用感應線圈，設計了一個粗糙的變壓器。這個必明，極大的推動了電能傳輸的發(fā)展。這個線圈最初被串聯(lián)在線路中，但沒有取得良好的效果。但是它卻能夠提高電壓，在交流系統(tǒng)中，可以更加經濟的傳輸電能。在1885年喬治.威斯汀豪斯將這項技術引入美國，服務于政府的工作系統(tǒng)。1886年，在馬塞諸塞州的大巴靈頓，一個試驗線路開始運行。同年，在紐約的布法羅，第一個大規(guī)模的商來項目完成建設。

早期的線路運行在1000V的電壓。架空的線路被認為是一種安全可靠的結構，并被廣泛的應用。路燈照明供電能夠運行在2000V。在華盛頓和芝加哥架空電纜被禁用，所以敷設了大量的地下電纜。各種各樣的絕緣結構很少有成功的。地下導體的試驗在費城也同樣在進行著。在1884年，紐約頒布法律強制拆除架空線纜，因此開發(fā)一種能夠承受一定電壓的地下電纜勢在必行。在架空電纜消失之前，這項工作就已開始進行。在1888年，國家電燈協(xié)會以的報紙上就有如下描述:

在紐約和布魯克林，沒有地下布電系統(tǒng)。在華盛頓的試驗證明，沒有一種絕緣可以在地下穩(wěn)定運行兩年。在芝加哥除了鉛套電纜，其它的絕緣結構都是失敗的。在密爾活基，試驗了三種不同的絕緣結構，但最終都放棄了。在很多大一點的城市，低壓電纜可以穩(wěn)定的運行，一些1000V的電纜也能使用。（相關圖書1931.2《地下系統(tǒng)》