The vast majority of overhead power lines, including high-tension transmission and local distribution wires, carry Alternating Current (AC). AC forms the backbone of the global power grid, moving power from centralized generating stations to homes and businesses. While AC is the standard, the modern electrical grid also incorporates High-Voltage Direct Current (HVDC) systems for specialized transmission needs. This hybrid network capitalizes on the strengths of both current types for reliable, large-scale power delivery, especially over long distances or underwater.
Understanding Alternating and Direct Current
The difference between Alternating Current (AC) and Direct Current (DC) is the direction in which the electric charge flows. Direct Current, found in batteries and solar panels, flows steadily in a single direction, meaning the voltage remains constant over time.
Alternating Current, by contrast, periodically reverses its direction of flow. The current moves back and forth, like an oscillating wave, with the voltage constantly changing from positive to negative and back again. In the United States, this reversal happens 120 times every second, resulting in a frequency of 60 Hertz. This oscillating nature gives AC its unique properties for power transmission.
Why Alternating Current Dominates the Power Grid
The reason AC was chosen as the standard for the power grid lies in the simple device known as the transformer. A transformer relies on a changing magnetic field to operate; because AC naturally reverses direction, it creates the necessary alternating magnetic flux to induce a voltage in a secondary coil. This allows AC voltage to be easily manipulated.
Power loss during transmission is proportional to the square of the current, so current must be reduced dramatically to minimize loss over long distances. Transformers enable the voltage to be “stepped up” to hundreds of thousands of volts at the power plant, simultaneously lowering the current for efficient travel. Once the electricity reaches a city or neighborhood, transformers efficiently “step down” the voltage to safe, usable levels. Direct Current cannot use a simple transformer to adjust its voltage because it lacks the necessary oscillating property. This ease of voltage regulation made AC the practical choice for building a large, interconnected electrical system.
Where High-Voltage Direct Current is Used
Despite AC’s advantages, High-Voltage Direct Current (HVDC) is employed where its benefits outweigh the complex conversion equipment required. HVDC systems are more efficient than AC for transmitting power over very long distances, typically beyond 600 to 800 kilometers, due to lower power losses. This efficiency stems from the absence of reactive power losses and the lack of a “skin effect,” which plagues AC transmission by forcing current toward the outer surface of a conductor.
HVDC is also the preferred technology for power transmission through undersea or underground cables. In these confined environments, AC cables suffer from high capacitive losses, where the cable acts like a capacitor, draining power over distance. Because DC current does not have this effect, HVDC cables can transmit power much farther underwater, often becoming more cost-effective than AC beyond 50 kilometers.
A specialized use for HVDC is connecting two power grids that are not synchronized, such as systems operating at different frequencies (like 50 Hz and 60 Hz). The HVDC conversion stations convert the AC power to DC for transmission, isolating the two systems before converting the DC back to AC at the receiving end. This allows for stable power sharing between otherwise incompatible networks.