Tuesday, September 06, 2011
Getting a Grip on the Grid
By MATTHEW L. WALDNY TImes
One of the crucial factors that limits the capacity of the electricity grid is the temperature of the lines; as their metal is heated by the current flowing through them, the lines get longer, and they sag, and can contact trees or other objects.
That development helped set off the great Northeast blackout of August 2003.
But the utility companies have never had a precise idea of what the temperatures are, or where their lines are in relation to obstructions below. They use a complicated formula for calculating the temperature that involves the angle of the sun, the wind speed and direction in relation to the lines, and the air temperature.
Now, though, utilities are testing a new technology that could reduce the guesswork. The company marketing the technology, Utility Risk Management Corporation of Stowe, Vt., says it could increase the useful capacity of some lines by 15 percent, without any of the cost or regulatory difficulty of building new transmission.
A prominent grid expert involved in testing the technology agreed, but said that in some cases it could lead to reducing the capacity, if better data showed that the temperature had been underestimated and the maximum safe loading was lower than what is assumed today.
With or without new technology, the companies that own transmission lines have been ordered by the North American Electric Reliability Corporation, an industry group that has governmental powers, to survey all of their lines in the next three years and get a better idea of the clearances between the bare wire of the transmission lines and the natural and man-made objects on which they might droop.
The standard technique is to equip a helicopter with Lidar , a remote sensing technology that is like radar but uses light instead of radio waves.
The Vermont company, known by its initials, U.R.M.C., has married the Lidar gun to a thermal sensor, so a helicopter can get the temperature as well as the position. Once those are known, a layer of approximation is eliminated.
“Our industry has been artificially constraining our transmission lines because of a formula the industry has been using since the 1940s,” said Adam Rousselle, the chief executive of the company. And demands on the power grid are growing, Mr. Rousselle said, because of restructuring that has let independent companies add generation stations around the country, and because of state requirements for renewable energy. Wind and solar “farms” tend to be located in out-of-the-way areas.
By starting with a hard number instead of the one derived by formula, an engineer can calculate how much the cable would sag with varying amounts of current running through it.
The Lidar system can pinpoint birds and mice, and has no trouble finding power lines, he said.
In independent testing conducted in June by the Electric Power Research Institute at a high-voltage laboratory in Lenox, Mass., that has previously been used to test robot power line inspectors, the heat measurement system generally correlated very well with temperature measurement devices installed on test lines. And both were better than the approximation arrived at by the formula.
A transmission expert, Dale A. Douglass of Power Delivery Consultants, said it would be a big benefit to get more work out of existing lines, because building new ones was so difficult. “Utilizing what we have is a big deal,” he said.
Whenever new transmission capacity appears, said Dr. Douglass, whether because of new construction or because of refinement of existing calculations, consumers may benefit because the system can make better use of low-cost electricity supplies shipped over longer distances.
And some generators will benefit, because new markets will open to them. But some will lose, he said, because distant generators will be able to compete with them.
Dr. Douglass, who has worked as a consultant for the Electric Power Research Institute, and for U.R.M.C. and various other companies, said that using the old formula required knowing the wind speed and direction over the course of a transmission line that could stretch for miles, and sometimes utilities did not go to the expense of getting weather measurements close to the line.
“They’d use airport data,” he said. “But you never see power lines at airports.”
Some utilities could discover, with more precise measurements, that their power lines cannot carry quite as much as they are now rated for, he said. But whichever way the rating was adjusted after getting more accurate measurements, he said, the system should be less prone to line failure and blackout.