July 2001

A Smarter Power Grid

Continued from page 2

By Peter Fairley

Power Chips

Electronic control over the grid has been a long time coming. The core problem has always been to switch high levels of electricity-and to do it fast enough. The potential solution dates back to the late 1950s, when General Electric pioneered the thyristor, a cousin of the transistor.

Like transistors, thyristors turn the flow of electrons through an integrated circuit on or off. Thyristors are more efficient for handling big power loads because, unlike transistors, once turned on they stay on-allowing energy to flow with little resistance. But the solid-state "latch" that's the key to this ability also renders the thyristor thousands of times slower to switch than the transistor, limiting its ability to regulate the high-speed action of the electric grid. That began to change in the 1980s with the arrival of a speedy hybrid device called the gate turn-off thyristor, which employs its own dedicated circuit of transistors to electronically open and close the thyristor's latch. These and other advances have enabled power electronics to be used increasingly in consumer applications such as smoothing out the flow of juice from small-scale power generators (" Power to the People ," TR May 2001 ).

Now, thanks to the Electric Power Research Institute, a utility-funded R&D consortium in Palo Alto, CA, and demonstrations with utilities and large engineering firms such as Siemens, ABB and Mitsubishi Electric, power electronics is finally ready for heavy lifting in the transmission grid. The first payoff of these systems will be to make the grid less vulnerable to voltage sags and surges, as well as to noise in the power signal.

While the electric grid was originally interconnected to increase reliability and reduce cost, that's turned out to be a mixed blessing. Interconnection means the most expensive generators can be kept off if others-even several hundred or several thousand kilometers away-can fill the need more cost-effectively. The bad news is that the grid can also transmit disturbances, making the whole system harder to control. Fluctuations can work their way around the grid like the wave among fans at a football stadium. And just as the wave works better at a crowded arena, an electric disturbance becomes more pronounced at higher power levels and with increased power transfers. "Unfortunately, by the nature of the physics involved, the higher the power flows the more dynamically unstable you become," says Karl Stahlkopf, vice president of power delivery at the Electric Power Research Institute.

Because high-power transmission is so unstable, operators must often limit a line's load to as little as 60 percent of its ultimate thermal capacity (the point at which the wire overheats, sags into trees or onto the ground, and shorts out). Power electronics is beginning to reclaim this lost capacity using programmable processors that can patch over a surge or sag within a small fraction of a second. That's a big advance over conventional grid controls, which can be as slow as manually adjusting a transformer or as unsophisticated as automatic breakers that sense a disturbance and "trip" a transmission cable off line, sending a tsunami of power surging through neighboring circuits.

The first of these power processors were installed in 1995 by the Electric Power Research Institute, Siemens and the Tennessee Valley Authority in northeast Tennessee. By smoothing out the flow, electronics eliminated the need for a new power line, saving at least $14 million. It was a modest but encouraging first step, and Siemens and ABB have recently sold three more commercial power processors to Texas utility Central and South West. Meanwhile, Mitsubishi Electric recently installed one in Vermont and is engineering the first of four units destined for San Diego Gas and Electric. By making it safe to draw hundreds of extra megawatts from distant sources over existing transmission lines, San Diego's devices will help overcome the shortage of local power generation that has left power companies throughout California vulnerable to outages and price swings.