Ultra-high voltage AC (UHVAC) transmission is playing an increasingly greater role as transmission systems expand in the some countries in the eastern hemisphere, but its prospects for becoming part of the North American grid appear less certain.
Defined as voltages at 800-kV and above, UHVAC made its first inroads in China in 2009, when that country’s power system energized a 1,000-kV transmission line. Recently, the Power Grid Corporation of India (PGCI) announced it has “successfully tested” a 1,200-kV transmission circuit that will connect to 1,200-kV class transformers and circuit breakers that were delivered in late 2011.
Those operating voltages are almost double the highest voltages in North America, where 765-kV is the highest voltage currently in use.
“The need for UHVAC in the U.S. has not been as great as the need in those other countries because of the rate of [their] growth,” Jeff Fleeman, American Electric Power’s (AEP; NYSE:AEP) director of advanced transmission studies and technologies told TransmissionHub on April 9.
“From 1969 thru 1983, we ran tests on a small scale version [of UHV] because we anticipated we would expand to ultra-high voltage,” he said, “but that never came to pass largely because the growth of electric demand in the U.S. declined from 7% to 8% percent a year to under 3% a year.”
That meant electricity demand, which had been doubling approximately every decade, would only double about every 25 years, Fleeman said.
“Because of that, there wasn’t as much need to get ahead of that growth with an even higher overlay voltage, so we were able to meet our growth in demand with the technology we had in hand,” Fleeman said. “So even though we were capable of doing something more, we couldn’t really justify the need for UHV.”
In addition, distance from source to sink is a major factor in China, according to Fleeman. “They have massive sources of hydro power in the western part of the country, and in the northwest, they have massive sources of coal, but all the load is in the east. So they have to get from the source to the sink across 1,000 kilometers,” or 621 miles, he said.
While those distances are similar to those of the transmission build-out needed to bring wind energy to market in the U.S., Fleeman said the only certainty is that additional lines will be needed.
“With the RPS standards that are coming in, people are going to need to get access, and some areas don’t have as much renewable [energy] available to them except from remote sources, so transmission is the only practical way to get [that energy],” he said. However, he doesn’t think the need for more lines will necessarily mean higher-voltage lines.
Not that the industry didn’t study increasingly higher transmission voltages.
“There was a natural progression of voltages in the U.S. as we grew,” Fleeman said. “There was a tendency to overlay a voltage that was two- to three times as high as the previous one. One of the ways we got to 765-kV is that was about 2-1/2 times 345-kV.”
One barrier to UHVAC development in the U.S., is that system operators would have to incorporate higher voltages into the lower voltages of the existing system while China and India are working with more of a “blank slate,” according to Fleeman.
“In China, they don’t need to repeat [our] history. If UHV is available as a technology, they are free to use that rather than the progression we went through,” he said.
While higher voltages bring advantages, including a four-fold increase in the power-carrying capacity for every doubling of the voltage, they also have drawbacks.
“You’ve got a lot of eggs in one basket,” Fleeman said. A single 1,200-kV line can carry 6,000 to 8,000 MWs, he said, “[S]o if you lose that line – and it’s a very long line so the probability of an outage on that line is not small – it’s a real challenge to your grid. That’s surmountable, but it’s something you have to design for.”
Therefore, Fleeman said, a single UHV line carrying that much power can’t be built without a significant amount of disruption; a network – a grid – with multiple paths for power flow must be built.
Public acceptance an issue
Fleeman also predicted it would be difficult to site UHVAC lines in the U.S.
“It’s challenging enough to site a new 765-kV line [and] it would be even more challenging to site an even higher-voltage line,” he said.
While AEP’s transmission fact sheet says a single-circuit 765-kV tower over flat terrain averages about 150 feet tall, the 1,200-kV test tower being used by PGCI is 126 meters, or 413 feet, tall. “A lot of folks don’t like seeing that on their horizon,” Fleeman said.
Fleeman said using existing voltages, up to and including 765-kV, may be a better strategy than trying to increase the voltage to the next level because there isn’t as much demand for us to go that high.
But with the rapid changes in the electric industry, he said, “Never say ‘never’.”
Based upon the industry’s history, the next step Fleeman would expect would be for areas of our country that have 500-kV to advance to 1,000-kV or 1,100-kV because that would represent an approximate doubling of the voltage.
“For us, at 765-kV, we’d probably want to double that. If UHV were to happen, it’s more likely to happen as an overlay to 500-kV.”