【正文】 流輸電比交流輸電在跨接方面具有很大的競爭優(yōu)勢。原動機將原始能量（化石燃料，核能，液壓）轉化為機械能，同步發(fā)電機再將機械能轉化為電能。 。 電力產生于發(fā)電廠（ GS），通過使用一系列獨特的電網(wǎng)設備包括輸電線路，變壓器，開關設備構成的復雜網(wǎng)絡傳輸給用戶。在 1954年，第一個現(xiàn)代的商業(yè)用高壓直流輸電是通過 96 公里海底電纜在瑞典大陸和 Gotland 島之間實現(xiàn)互聯(lián)。 第一個三相 12 公里 2300V輸電線路在于 1893 年在北美 California 南部投運。到 1886 年，直流系統(tǒng)的局限性日益突現(xiàn)，只能在很短的距離內從發(fā)電機向外送電。負荷是包括白熾燈在內的 110V供電的電纜系統(tǒng)。 在 19 世紀 90 年代 ,關于電力工業(yè)應采用直流還是交流作為標準的相當大的爭論。這些高壓類（ HV）的標準電壓等級是： 11 13 161 和 230 kV，特高壓（ EHV）有 34 500 和 765kV幾個電壓等級 。 主要包括三相交流系統(tǒng)的穩(wěn)定工作電壓，發(fā)、輸電設施使用三相設備。初次分配的電壓一般在 千伏和 千伏。發(fā)電器電壓通常是在 11 到 35 kV范圍內。隨著轉化設備的成本降低、體積變小和可靠性的提高，高壓直流輸電的使用實現(xiàn)了穩(wěn)定增長。 在早期的交流輸電中頻率不統(tǒng)一。 由于 和法國巴黎的 開發(fā)了變壓器和交流輸電技術，由此產生了交流電力系統(tǒng)。 ? Voltage levels can be easily transformed in ac systems, thus providing the flexibility for use of different voltages for generation, transmission, and consumption. ? AC generators are much simpler than dc generators. ? AC motors are much simpler and cheaper than dc motors. The first threephase line in North America went into operation in 1893a 2,300 V, 12 km line in southern California. Around this time, ac was chosen at Niagara Falls because dc was not practical for transmitting power to Buffalo, about 30 km away. This decision ended the ac versus dc controversy and established victory for the ac system. In the early period of ac power transmission, frequency was not standardized. Many different frequencies were in use: 25, 50, 60, 125, and 133 Hz. This posed a problem for interconnection. Eventually 60 Hz was adopted as standard 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 p