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A submarine power cable is a major transmission cable for carrying electric power below the surface of the water.〔 (Underwater Cable an Alternative to Electrical Towers ), Matthew L. Wald, ''New York Times'', 2010-03-16, accessed 2010-03-18.〕 These are called "submarine" because they usually carry electric power beneath salt water (arms of the ocean, seas, straits, etc.) but it is also possible to use submarine power cables beneath fresh water (large lakes and rivers). Examples of the latter exist that connect the mainland with large islands in the St. Lawrence River. ==Design Technologies== Most power systems use alternating current (AC). This is due mostly to the ease with which AC voltages may be stepped up and down, by means of a transformer. When the voltage is stepped up, current through the line is reduced, and since resistive losses in the line are proportional to the square of the current, stepping up the voltage significantly reduces the resistive line losses. The lack of a similarly simple and efficient system to perform the same function for DC made DC systems impractical in the late 19th and early 20th centuries. (Available devices, such as the rotary converter, were less efficient and required considerably more maintenance.) As technology improved, it became practical to step DC voltages up or down, though even today the process is much more complex than for AC systems. A DC voltage converter often consists of an inverter - essentially a high-power oscillator - to convert the DC to AC, a transformer to do the actual voltage stepping, and then a rectifier and filter stage to convert the AC back to DC.〔"Introduction to Modern Power Electronics" By Andrzej M. Trzynadlowski〕 DC switch gear is larger and more expensive to produce, since arc suppression is more difficult. When a switch or fuse first opens, current will continue to flow in an arc across the contacts. Once the contacts get far enough apart, the arc will extinguish because the electric field strength (volts per meter) is insufficient to sustain it. In AC circuits, current drops to zero twice during each AC cycle, at which time the arc extinguishes. If the distance between the contacts is still relatively small, the voltage will re-initiate an arc. Since DC is constant and these zero-crossing events do not occur, a DC switch must be designed to interrupt the full rated voltage and current., leading to larger and more expensive switching equipment.〔"The electric power engineering handbook" By Leonard L. Grigsby〕 The voltage required to re-initiate an extinguished arc is much greater than the voltage required to sustain an arc. DC power transmission does have some advantages over AC power transmission. AC transmission lines need to be designed to handle the peak voltage of the AC sine wave. However, since AC is a sine wave, the effective power that can be transmitted through the line is related to the root mean squared (RMS) value of the voltage, which for a sine wave is only 0.7 times the peak value. This means that for the same size wire and same insulation on standoffs and other equipment, a DC line can carry 1.4 times as much power as an AC line.〔"Advances in high voltage engineering" By D. F. Warne, Institution of Electrical Engineers〕 AC power transmission also suffers from reactive losses, due to the natural capacitance and inductive properties of wire. DC transmission lines do not suffer reactive losses. The only losses in a DC transmission line are the resistive losses, which are present in AC lines as well. For an overall power transmission system, this means that for a given amount of power, AC requires more expensive wire, insulators, and towers but less expensive equipment like transformers and switch gear on either end of the line. For shorter distances, the cost of the equipment outweighs the savings in the cost of the transmission line. Over longer distances, the cost differential in the line starts to become more significant, which makes high-voltage direct current (HVDC) economically advantageous.〔"High voltage direct current transmission" By J. Arrillaga〕 For underwater transmission systems, the line losses due to capacitance are much greater, which makes HVDC economically advantageous at a much shorter distance than on land.〔"AC/DC: the savage tale of the first standards war" By Tom McNichol〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Submarine power cable」の詳細全文を読む スポンサード リンク
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