An Electrical Grid Is Pressed to Its Limit
An Electrical Grid Is Pressed to Its Limit
By MATTHEW L. WALD Published: July 31, 2012
WASHINGTON — The Indian electrical grid, said Arshad Mansoor, the senior vice president for research and development at the Electric Power Research Institute in Palo Alto, Calif., is like "a whole bunch of rubber bands." Cutting some, he said, might make no difference, but cutting another one could make the web fall apart.
"You don't know which one is stretched, and their system is really stretched," said Dr. Mansoor, whose undergraduate degree is from a university in Bangladesh and whose master's and Ph.D. are from the University of Texas, Austin. The American grid has more resilience, he said, because it has excess capacity that can be turned to very quickly if a single generator or power line fails, but India is woefully short of capacity.
The area blacked out in India is effectively a single, interconnected grid, which means that power is transmitted through it almost instantaneously, as are imbalances, explained Raj Rao, an American electric company executive who is a frequent visitor to India and is familiar with the system there.
Mr. Rao, who is the president and chief executive of the Indiana Municipal Power Agency, based in Carmel, Ind., agreed that India is chronically short of generating capacity, a problem exacerbated by a shortage of coal. But shortages of generating capacity do not produce widespread power failures; power system operators manage those with rotating blackouts.
The United States had an extensive blackout in the summer of 2003 that stretched from Detroit to New York City. Mr. Rao and others cautioned that a definitive engineering analysis of the Indian blackout would take months, as it did after the American blackout in 2003.
But, he said, the most likely mechanism was a botched attempt to black out a small area temporarily. For example, he said, if the generating capacity was 120 megawatts and the available capacity was 100, that would require unplugging 20 megawatts of load.
"My hunch is, somebody fell asleep and they did not cut off the 20 megawatts," he said. "And that's where you run into trouble."
When demand on the generators runs higher than they can satisfy, they automatically disconnect themselves to prevent mechanical damage, and as each one drops off, it makes a cascade more certain, he said. The first generators would cut out in a fraction of a second, he said.
Mr. Mansoor noted that parts of the Indian grid were quite modern, including a class of automatic devices called relays that will shut down parts of the system if they sense an imbalance. These relays monitor the alternating current system (which in India is 50 cycles, compared with 60 cycles in North America).
If the frequency slows down because demand exceeds supply, relays will shut down power lines or transformers, to protect the system, he said, but if they cut off too much load, frequency will bounce back at too high a level, leading to a disturbance that propagates through the system.
Gopinath S Bangalore
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