Submarine Power Cable - Design Technologies

Design Technologies

Most power systems use alternating-current. This is due mostly to the simplicity of the AC transformer, which allows AC voltages to be easily stepped up and down. 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 simple DC transformer made DC systems impractical in the late 19th and early 20th centuries. As technology improved, DC transformers became possible, though even today they are much more complex than AC transformers. A DC transformer often consists of an oscillator or inverter to convert the DC to AC, an AC transformer to do the actual voltage stepping, and then a rectifier and filter stage to convert the AC back to DC.

DC switch gear is also more expensive to produce, since arc suppression is more difficult. When a high voltage AC line is switched off, the voltage will arc across the switch contacts. Once the contacts get far enough apart, the arc will naturally extinguish itself since the voltage drops to zero twice during the AC sine wave cycle. Since DC is constant and doesn't cycle to zero, a DC switch will draw a much longer arc, and suppressing this arc requires more expensive switching equipment.

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.

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.

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.