Direct Current (DC)

The flow of electric charge in only one direction

Alternating Current (AC)

The movement of electric change that periodically reverses direction

Direct current (DC) is the preferred technology for moving large amounts of power across long distances. DC results in overall higher efficiency and reliability than an equivalently-sized alternating current (AC) system moving the same amount of power.

Historically, the transfer of electricity between regions has been over high voltage alternating current (AC) transmission lines (typically along overhead wires and pylons), which means that both the voltage and the current on these lines move in a wave-like pattern along the lines and are continually changing direction. In Europe, this change in direction occurs 50 times per second (defined as 50 hertz [Hz]). The electric power transmitted over AC transmission lines is exactly the same as the power we use every day from AC outlets, but at a much higher voltage.

Unlike an AC transmission line, the voltage and current on a direct current (DC) transmission line are not time varying, meaning they do not change direction as energy is transmitted. DC electricity is the constant, zero-frequency movement of electrons from an area of negative (-) charge to an area of positive (+) charge.

The first commercial electric power system built by Thomas Edison in the late nineteenth century carried DC electricity, but given some early advantages, AC power eventually became the primary power system worldwide. Some of these advantages are no longer applicable (e.g., technology has advanced to allow efficient conversion from AC to DC), and DC transmission is the preferred solution for moving large amounts of power over long distances with considerable social and commercial advantages.

HVDC delivers power from one grid system to the other by converting power to DC, using power electronic switches called thyristors. The resulting DC power is then is transmitted over hundreds of miles along a pair of buried cables before being converted back to AC.

After the DC power is converted back to AC it is transformed to the common voltage of the grid to which it is being connected.

A major advantage of DC power lines is their efficiency – less energy is lost as it is transmitted and there is no need for reactive compensation along the line. Because DC (Direct Current) flows steadily through the wires without changing direction many times each second and through the entire conductor rather than at the surface, DC (Direct Current) transmission lines typically lose less power than AC transmission lines.

DC converter station looks like a warehouse building which occupies relatively little space. It is almost entirely noiseless and is completely pollution free.

The Advantage of DC

  • More efficient: Over long distances, DC transmission can move more power with less electrical losses than an equivalent AC transmission line.
  • Lower Cost: Higher efficiency means a lower transmission cost.
  • Improved Reliability: HVDC transmission can enhance system stability, allow the operator complete control over power flow, and facilitate the integration of wind from different resource areas.
  • Smaller Footprint: DC transmission lines require narrower right-of-way footprints, using less land, than equivalent AC lines.

 

DC is the large scale power transmission technology of the future

Fact Sheet

High Voltage Direct Current electricity – technical information

Courtesy of National Grid UK