電力系統(tǒng)的演化外文翻譯-其他專業(yè)-文庫吧資料
2025-01-27 03:23本頁面
【正文】 d in North America, although many other countries use 50 Hz. The increasing need for transmitting larger amounts of power over longer distances created an incentive to use progressively higher voltage levels. The early ac systems used 12,44, and 60 kV(RMS linetoline).This rose to 165 kV in 1922,220 kV in 1923,287 kV in 1935,330 kV in 1953,and 765 kV was introduced in the United States in 1969. To avoid the proliferation of an unlimited number of voltages, the industry has standardized voltage levels. The standards are 115, 138, 161, and 230 kV for the high voltage (HV) class, and 345, 500 and 765 kV for the extrahigh voltage (EHV) class. With the development of mercury arc valves in the early 1950s, high voltage dc (HVDC) transmission systems became economical in special situations. The HVDC transmission is attractive for transmission of large blocks of power over long distances. The crossover point beyond which dc transmission may bee a petitive to ac transmission is around 500 kV for around 500 km for overhead lines and 50 km for underground or submarine cables. HDVC transmission also provides an asynchronous link between systems where ac interconnection would be impractical because of system stability considerations or because nominal frequencies of the systems are different. The first modern mercial application of HVDC transmission occurred in 1954 when the Swedish mainland and the island of Gotland were interconnected by a 96 km submarine cable. With the advent of thyristor valve converters, HVDC transmission became even more attractive. The first application of an HVDC system using thyristor values was at Eel River in 1972a backtoback scheme providing an asynchronous tie between the power systems of Quebec and New Brunswick. With the cost and size of conversion equipment decreasing and its reliability increasing, there has been a steady increase in the use of HVDC transmission. Interconnection of neighboring utilities usually leads to improved security results from the mutual emergency assistance that the utilities can provide. Improved economy results from the need for less generating reserve capacity in each system. In addition, the interconnection permits the utilities to make economy transfers and thus take advantage of the most economical sources of power. These benefits have been recogn
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