Intermittency and the HVDC Supergrid

by Howard Silverman

Smart grid or supergrid? Although the terms are sometimes used interchangeably, distinguishing them can be useful.

A smart grid uses digital communications networks to manage and monitor advanced electrical functions: integrating large quantities of distributed generation, shifting and scheduling demand loads, and dynamically pricing electrical delivery. 

A supergrid uses high-voltage, direct current (HVDC) transmission lines to send electricity over long distances.

Certainly, the technologies are complementary. Both would be incorporated in large-scale concepts like a SuperSmartGrid across Europe, North Africa and West Asia, or a Unified Smart Grid across the U.S. or North America.

Still, they assume different strategies for dealing with intermittency: the periodic availability of renewable energy resources like wind and solar.

A smart grid would use supply and demand management and pricing to smooth the intermittent availability of renewables. A supergrid would rely on a long-distance footprint, covering a portfolio of sources and availabilities.

A supergrid could also be built in stages, link-by-link transmitting from where energy is generated to where its needed. Dozens of HVDC projects have been developed around the world. Last December, 10 European countries signed an agreement on a North Sea supergrid to take advantage of offshore wind. Offshore proposals along the U.S. East Coast are also under discussion ("Offshore Wind Power Line Wins Backing," NYT).

One can't help but notice the discrepancy in media coverage between the smart and super grids. Plenty of attention to smart grids, which are in initial phases of development. Little attention to supergrids, segments of which have been around for decades.

Here are some of the best sources on supergrids:

• A Grid Integration of Renewables Workshop, hosted by the TomKat Center for Sustainable Energy at Stanford (video and presentations), January 2011.

• A Scientific American article by Matthew Wald, "How to Build the Supergrid," November 2010.

• A Financial Times-published interview with supergrid proponent Gregor Czisch, June 2010.

• A New Scientist article by David Strahan, published on his blog as "Green Grid," March 2009.

• And a critique by John Thackara, "Renewable Energy: Salvation or Snake Oil?," February 2011.

And some key points:

Strahan: "[A] major advantage of HVDC is that it can operate over much greater distances underground and under water than AC. That’s because AC produces powerful alternating electric fields that cause large additional energy losses if the line is buried or submerged, whereas for DC this ‘capacitance’ effect is practically negligible."

Wald (citing one of the article's sources): "If the (HVDC) line is operating at 345 kilovolts (kV) – the level used along many backbone wires today – 19.8 MW of power is lost. At 765 kV, the highest voltage operated in the U.S. (though not widely used), only 3.45 MW is lost – about one sixth of the losses at 345 kV. For a 1,100-kV line, the loss is a mere 1.91 MW."

Wald: "Direct current is desirable for point-to-point transmission, with no stops in between. It is already used between hydroelectric dams in northern Quebec and New England and between dams on Oregon's Columbia River and southern California. In these cases, direct current was selected because it is efficient and is controllable. Alternating current follows the path of least resistance, buzzing along random wires like water on a mountaintop trickling down various streams to a pool at the base. A direct-current line is like a pipe from top to bottom, with a pump that can be adjusted in real time."

Wald: "Despite the advantages, direct-current lines are worthwhile only if installed over long distances. That is because special converter stations are needed at each end to change alternating current to direct current and back again. The conversion requires massive electronics that eat up around 1 percent of the electricity at each end. According to Andrew Phillips, director of transmission at the Electric Power Research Institute, with declining costs, the break-even point has moved down to about 300 to 350 miles, compared with 500 miles 15 years ago. … The complication is that tapping into a line at a midpoint, the equivalent of a highway interchange, is extremely expensive."

Strahan: "One disadvantage of HVDC lines is the need for converter stations where they connect to an AC grid. These are big and expensive: for a 3000MW line the converter stations at either end would cover 9 football pitches and cost around $200 million each. But once the link is longer than about 600km, the extra cost is increasingly outweighed by the energy savings, making it economic to transport power over vast distances that with AC would be expensive and technically difficult."

Czisch, at the grid integration workshop (from presentation notes): "There is no technical or economic problem which must prevent us from changing our electricity supply to one that is based 100% on renewable energies."

John White, at the grid integration workshop, on the difficulty of developing collaboration among regional utilities: "Transmission is like sovereignty. Transmission in how they protect themselves from each other."

Nick Jenkins, at the grid integration workshop, on multi-terminal HVDC in the North Sea: "You have major issues associated with faults – when faults occur – on the DC system, because you have no DC circuit breakers. ... You cannot buy an HVDC circuit breaker at this capacity with a five-year warranty, which is what you need."

Thackera: "These supergrids look neat and hygienic ... but their implementation begs a host of questions. Among these is the process by which decisions are being made. The Energy Report states blandly that 'the grid will allow power to be transferred from one part of the continent to another' – but allowed by whom, and in whose interests?"

See also: "Mark Jacobson and Mark Delucchi: Wind, Water and Solar"