What We're Thinking

WHY HVDC?

Posted on 01/23/2012 by Dr. Wayne Galli,P.E.

In our last blog, Michael Skelly, talked about the importance of conveyance infrastructure as a secret sauce for economic development.  Just this morning I heard a story on the radio about the famous engineer Octave Chanute.  While Chanute is best known for rendering help to the Wright brothers in their quest for heavier-than-air flight, Chanute was primarily one of the most brilliant railway engineers of his time.  Chanute designed many firsts in railway bridges; especially the notable Kinzua Bridge built in 1882 which was hailed as the “eighth wonder of the world” as it held the record for the tallest railway bridge for two years subsequent.  What makes this bridge unique in the world of infrastructure is that in 1881, demand for coal in Buffalo, New York reached about three million tons per year.  Chanute saw that a bridge spanning the Kinzua Valley would be a much more cost effective option to move coal from the coal fields of Pennsylvania to Buffalo versus building around the valley.  At this point in history, it was this bridge that would continue to fuel the booming economy of the northeast United States.

Today, we are faced with a similar challenge.  Clean energy is essential to fuel our economy and keep it moving forward.  The lowest cost alternative is land-based wind from the Plains States of the U.S.  The most effective means of moving large amounts of electric energy over long distances is through direct current (DC) technology.  Of particular note is the amount of HVDC currently being installed in China.  Similar to the U.S., China’s best resources for energy lay far inland and population centers are near the coast.  In order to develop these resources to fuel the great demand for energy, China has already installed 11 projects with 35,000 megawatts of HVDC transmission capacity and has plans to install an additional 33 projects with 217,000 MW of HVDC transmission capacity over the next two decades.

So, you may ask, “Why then is the overwhelming majority of the bulk electric grid alternating current (AC)?”  Well, that’s relatively simple to answer.  Around the same time as Chanute was building the Kinzua Bridge, Thomas Edison commissioned the first central power plant in the United States – the Pearl Street Station in Manhattan, New York.  In the early years of the electric industry, Edison had done extensive work on developing direct current (DC) based electricity distribution systems and no practical AC motors existed.  However, George Westinghouse began investing in AC technology and eventually hired Nikola Tesla to promote AC systems.  It became clear that as the demand for electricity grew, the concept of higher voltages for transmission and distribution made sense for efficiency reasons.  However, at this point in history, it was difficult and almost impossible to change voltage levels in a DC distribution system.  With AC, a transformer (using the principle of induction) is used to readily change voltages and hence lower current and reduce losses associated with transmission; at the point of delivery, a second transformer is used to easily step voltage down to a level that is safe and practical for use by a consumer.  So, as the industry grew, and large central generating stations became the norm, AC emerged as the winner in the “war of the currents.”  However, just as technology evolved in the late 1800’s, technology has again evolved to the point where there are many efficiencies associated with HVDC that can now be leveraged.

In modern times, the ability to easily convert from AC to DC (a process called rectification) and from DC back to AC (a process called inversion) has been achieved through the use of high-power solid-state devices.  Not only are the conversion processes well understood at large power levels, they are also very efficient.  Existing large scale converter stations have losses of less than 0.7%.  In addition to efficient conversion processes, there are other efficiencies and benefits to be gained from utilizing HVDC in the appropriate applications. 

One of the main efficiencies is that when you utilize DC, you actually lose less energy in the overall transmission process (line losses).  Energy lost during the transmission of electricity is in the form of heat.  Namely, the wires (conductors) and components in contact with the conductors get hot due to the flow of current.  That heat is generated by resistance in the conductor – resistance is analogous to friction.  The heat generated is proportional to the square of the current flowing in the conductor.  With AC, the current tends to move away from the center of the conductor in a process known as the “skin effect.”  This phenomenon results in the conductor having a higher resistance to alternating current than it does to direct current, thus creating higher line losses for a given conductor when used in AC versus DC.  In projects like ours at Clean Line, the efficiencies achieved by utilizing DC are on the order of two to three times better than if we were to try to build an equivalent transfer capability with an AC system.

There are other efficiencies that are brought to light when DC lines are used for long distance transmission of power.  The discussions of these are relegated to future posts for more thorough explanation; however I will highlight a few of them below in the context of our projects:

  • Long AC lines become limited by a physical characteristic called the Surge Impedance Loading (SIL) that prevents them from being efficiently loaded to their full capability without additional equipment. 
  • HVDC lines are completely controllable.
  • HVDC lines use about one-third the right-of-way requirements that an AC system uses to move an equivalent amount of power.
  • HVDC has the ability to provide stabilizing power to the grid during disturbances.

HVDC is the right solution for the challenge at hand.  Trying to move the best renewable power in the U.S. to market in any other way would be shortsighted.  Finally, I would encourage you to check out the websites of ABB, Alstom, and Siemens to see what is going on around the world in HVDC.