Why a one-way current can travel farther
Electricity moving through a wire loses some energy as heat. The amount depends strongly on current: doubling the current produces about four times the resistive heating. Power lines therefore carry electricity at very high voltage, which lets them deliver substantial power with less current—and less heating.
High-voltage direct current (HVDC) adds another advantage on certain routes. Direct current flows steadily in one direction; alternating current (AC) repeatedly reverses direction. AC systems can carry reactive power, energy that oscillates between the grid and electrical equipment rather than being delivered to customers. That extra current still contributes to heating. HVDC avoids this particular burden, so it can be an efficient choice for moving large amounts of power over long distances.
The tradeoff happens at the ends
Most power grids use AC, so an HVDC link needs converter stations at both ends. These stations change AC into direct current for transmission, then change it back for the receiving grid. The equipment is costly and consumes some energy, meaning HVDC is not automatically the best choice for every line. Its lower transmission losses have to make up for the expense and losses of conversion.
The balance is especially favorable for very long routes and submarine cables. In long AC cables, electrical effects cause current to flow even when that current is not delivering useful power, limiting how far the cable can run efficiently. Direct-current cables avoid that specific limitation. The result is a practical way to connect distant power systems or carry electricity beneath the sea—at the cost of specialized conversion equipment at each end.
