High Voltage: AC/DC no Longer at War

alternating-current-direct-currentThomas Edison may have actually been way ahead of his time when he advocated direct current (DC) transmission as the optimal electricity delivery method. While alternating current (AC), pioneered by Nikola Tesla and George Westinghouse, would win the War of the Currents in the 1880’s, DC has potential to revolutionize the modern-day power grid. The hybrid HVDC breaker, recently developed by technology company ABB, could provide the stability needed for high-voltage direct current (HVDC) power to expand into a “supergrid” of reliable, renewable energy.

As its name implies, DC is the flow of electric charge in one direction, from generation to endpoint. DC has the advantage of being reliable, but the primary problem that has always plagued widespread transmission of DC is loss of power along conducting cables. In Edison’s time, there was no cost or resource efficient way to transmit DC power farther than a mile away from a generating station. While increasing voltage in DC lines would have allowed for more efficient long distance transmission, there was no safe way to decrease direct current voltage to a safe level once the energy reached its destination. In contrast, AC was able to distribute power far away from generating stations at higher voltages. AC voltage was then reduced via transformers before it entered houses or buildings for use. This is the same way power grids currently operate in most parts of the world. Surprisingly, the core technology has progressed little since Tesla’s time.

High-voltage DC (HVDC) is an updated version of Edison’s DC; it safely transmits high voltage power with little resistance along power lines. HVDC has been in use for years all over the world, but only to transmit bulk power over very long distances. Converters that turn DC into AC power for use in current electric grids already link these bulk transmission lines to population centers in several parts of the world where energy needs to be transported far from the generation source. The new HVDC breaker, however, introduces the possibility for far more widespread use of reliable and efficient HVDC lines.

The HVDC Breaker, if properly implemented, will make it possible to interrupt power to HVDC transmission lines in milliseconds, avoiding the widespread power outages that have made DC transmission impractical in the past. A prime reason DC “supergrids” are not common is the risk of widespread blackouts should any failure occur in the network.  The HVDC breaker can identify and quickly isolate any damage to transmission lines without power interruption to the rest of the grid. The reduced risk of power outages offered by this technology  could mean the renewable energy industry will see pervasive installation of these transmission lines and breakers. HVDC power lines have the potential to link renewable energy sources to the grid and transmit them from the generation source to population centers located far away. DC can also balance the natural instability of renewable energy and deliver it evenly, reducing the need for fossil fuel generated power plants. HVDC cables are also far better suited for running underground and under water than AC cables, which increases the potential for viability of offshore wind farms.  With a breaker control to ensure grid integrity, HVDC can become a feasible power delivery solution for all types of renewable electricity generators.

Thus far, the HVDC breaker has only been tested in a laboratory setting, so the reality of a stronger AC/DC supergrid is still far off. Even so, there is no doubt that electric grid infrastructure will benefit from an HVDC network. Imagine an electric grid where it is possible to control electricity flow, even during times of extreme stress or damage to the grid. DC has the capability to pull excess power from connected AC grids and re-direct it to where it is needed, thus improving grid reliability. Until we see the results of widespread HVDC use, more conventional means of grid stabilization, such as demand response will remain essential. The blending of DC and AC electricity, however, could prove that neither Edison, nor Tesla was wrong in their bet for the best electric current out there. The War of the Currents, may actually be ending in a draw.

Jessica Kennedy
Energy Curtailment Specialists, Inc.

Jessica can be found on Twitter and Google+.

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