If everything equipment makers said about technological breakthroughs were true, we’d all be living with The Jetsons.
But every once in a while a pitch grabs an editor’s attention. ABB’s claim that it had solved a “100-year-old electrical riddle” is a good example.
The Swiss company says it has developed the “world’s first” circuit breaker for high-voltage direct current and that its hybrid HVDC breaker can interrupt power flows “equivalent to the output of a large power station within 5 milliseconds,” about 60 to 80 times far faster than the blink of an eye.
Such a breakthrough would allow power to be moved longer distances more efficiently, facilitating the delivery of renewables from remote regions rich in wind and solar resources to distant load centers, e.g., cities, and it could greatly expand the uses of DC transmission lines.
ABB traces the riddle back to the “current wars” between Thomas Edison and George Westinghouse that date back to the dawn of the electric power industry. Edison championed DC power, claiming that AC power was too dangerous for consumer use, even staging electrocutions to make his point. But Westinghouse won, and today the world runs on AC power.
Since Edison, DC power has largely been relegated to niche applications, such as underwater connections, that are point-to-point with converters on either end.
But plans to use DC transmission lines are proliferating, driven in large part by the logistics of delivering renewable power. Improvements in breaker technology could facilitate even wider deployment.
ABB spokesman Antonio Ligi compared HVDC lines to a dedicated highway between two cities. But with ABB’s hybrid breaker, it is now possible to have exits on the highway, he said. So instead of stringing a DC line from, say, Montana to Minneapolis with one “on” and one “off” ramp, the line could be interrupted several times to allow the power to flow to load centers along the way.
“If ABB is ready, some of our projects could be ripe for the application” of the new breaker technology, Wayne Galli, vice president of transmission and technical services at Clean Line Energy Partners, said.
Clean Line is developing four DC transmission projects that are designed to deliver power from resource areas in the Midwest to load centers to the east or west.
With current technology it is not possible to isolate a fault in a DC line, as can be done with an AC line. A fault in a DC line would require the entire line to shut down. Putting a circuit breaker on a DC line would enable the fault to be isolated.
Galli said that could be very important for Clean Line’s projects that originate in regions with “weak grid interactions,” that is, regions where transmission lines are few and far between and required voltage support can be tenuous. “We are evaluating it,” Galli said, but “we still have to see if the economics work out.”
Industry experts say the breaker technology represents a breakthrough and that it appears to be viable, but noted that the costs could slow adoption.
“ABB has changed the game,” Norman MacLeod, technical director of HVDC at Parsons Brinckerhoff said. “DC can now do what AC guys have been doing for 120 years.”
ABB’s Ligi acknowledged that the hybrid HVDC breaker has not yet been brought to market. The next step is going to be deploying the technology in a pilot project. That is in discussions, Ligi said, but he declined to give details. Regarding cost, he said it is difficult “to put a number at this stage as pricing will depend on numerous factors.”
The other players in the field, Alstom and Siemens, are also working on DC breakers, but ABB is ahead of them, MacLeod said.
Alstom is working on a DC circuit breaker–until recently MacLeod worked at Alstom–and expects to finish testing in March 2013. Alstom confirmed it is working HVDC breaker for introduction in March 2013.
Siemens, he said, is tight lipped about its efforts. And, in fact, Siemens did not return requests for interviews.
But even after ABB’s DC circuit breaker hits the market, the cost could slow its deployment. “The hybrid breaker is the size of a tennis court and costs millions and millions of euros,” MacLeod said.
One of the attractions of DC transmission is its greater efficiency over long distances.
DC line losses are about half of those of AC lines, depending on voltage and distance. The cross-over point is about 800 kilometers (about 500 miles), the distance beyond which the economics tilt toward DC, MacLeod said.
In addition to lower line losses, DC lines do not require the equipment that AC lines do to maintain voltage. For instance, among other things, reactive power, which can be a costly and contentious issue for AC lines, is not an issue with DC lines.
DC power lines also require a smaller footprint than do AC lines, which require three lines for each of its three phases. DC power has only one polarity, noted Kiran Kumarswami, a manager with consulting firm ICF International. So a single DC cable could replace three AC cables required to deliver the same power, he said.
It is DC power’s single polarity, in fact, that presents the technical challenge of creating a DC circuit breaker. AC power cycles, that is, it travels in three waves that are schematically depicted as sine curves. An AC circuit can be broken when the power passes through the zero point of the curve. DC power does not cycle, so there is no zero point.
“For DC power you have to do it the hard way; make it go to zero with power electronics so you can open a mechanical switch to isolate the circuit, but you still have to deal with the energy so the third part is a surge arrestor to absorb the energy,” MacLeod explained. That is why it is called a “hybrid” breaker; there are three different parts. “It’s a complicated beast,” MacLeod said.
So far, ABB has tested its hybrid breaker, but it has yet to be proven in a commercial application. To be tested, the circuit breaker would have to be in place 24/7, and “it could be waiting years to do its job which just takes 5 milliseconds,” MacLeod said.
As noted, cost could be an impediment to deployment until greater adoption drives down costs. But as MacLeod said, the hybrid breaker would make it possible to put Norwegian hydropower, English wind power and Spanish solar power into a DC grid and move it all around the Europe.
Going back to the highway and exit ramp analogy, it would be possible to build a DC line to move wind power from Wyoming to load in Las Vegas with an exit in Salt Lake City. But the project would be expensive, requiring eight hybrid circuit breakers.
Not that such a grid would be electrical nirvana. “Once you start mixing these switches into the network, how do you coordinate them?” Marija Ilic, professor of electrical and computer engineering At Carnegie Mellon University, asked. “Breakers react to flow and don’t care about coordinating the wider area,” she said.
Time will tell if and when the greater efficiency of HVDC will warrant the investment in hybrid breakers to make a DC line that rivals, or even outperforms, an AC line. But for now, ABB seems to be the first out of the gate in reaching that goal.