In cases where HVDC is selected on technical considerations, it may be the only practical option, as in the case of an asynchronous interconnection. However, for long-distance power transmission, where both ac and HVDC are practical, the final decision is dependent on the total costs of each alternative. The total cost of a transmission system includes the line costs (conductors, insulators, and towers) plus the right-of-way (R-O-W) costs. A dc line with two conductors can carry almost the same amount of power as a three-phase ac line with the same size of line conductors. However, dc towers with only two conductors are simpler and cheaper than three-phase ac towers. Hence, the per-mile costs of line and R-o-W will be lower for a dc line. Power losses in the dc line are also lower than for ac for the same power transmitted.
However, the HVDC system requires converters at each end of the line; hence, the terminal costs for dc are higher than for ac. The variation of total costs for ac and dc as a function of line length is shown in Figure. As illustrated, there is a break-even distance beyond which the total costs of the dc option will be lower than the ac transmission option. This is in the range of 500 to 800 km for overhead lines, but much shorter for cables. The break-even point is between 20 and 50 km for submarine cables and twice as far for underground cables.
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| Figure Transmission system cost as a function of line length for ac and dc systems. |