As northeastern North America finally recovers from the world鈥檚 worst ever electricity blackout, investigators are starting to narrow down its cause. Under suspicion are faulty computer warning systems, poor maintenance of power lines and human error.
Most of the 60 million people affected by last Thursday鈥檚 massive outage now have power restored. Public transport systems have resumed normal service, but big businesses, including car manufacturers General Motors and Ford, have cut their usage to help the system recover. A significant number of power stations are still closed down.
The task of determining what happened will be far from easy. The blackout involved dozens of companies and thousands of systems. Millions of pieces of data need to be trawled through.
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Attention has initially focussed on a failure of power lines near Cleveland, Ohio. The lines are owned by the FirstEnergy Corporation, which reported 鈥渦nusual electric conditions and disturbances鈥� up to four hours before the blackout.
FirstEnergy has admitted that a warning system was not working, though monitoring equipment was. However, on Monday the company denied their system was to blame for the widespread outage. 鈥淲hat happened on Thursday afternoon is a very complex situation, far broader than the power line outages we experienced on our system,鈥� a company statement said.
Investigators are keen to learn whether the company used the telephone hotlines to alert neighbouring regions to this threat, or took steps to mitigate the problem. 鈥淭here are no automatic systems to handle major disturbances,鈥� comments Daniel Kirschen, an expert in power systems security at the University of Manchester Institute of Science and Technology in the UK. 鈥淚t is done manually by human operators, so the question is did they try to take the necessary action to avoid the outage.鈥�
Laws of physics
Although the event that triggered the cascade of failures is still unknown, Bikash Pal, a power engineering expert at Imperial College London, has used data on the fluxes in the power grid in New York to built up a picture of a likely sequence of events.
The problems began, he says, when the high voltage power lines running through Ohio tripped for some unknown reason. The power network was heavily loaded across the region that day because of high demand, and so there was no room to divert the power supply safely elsewhere. At some point before 1611 EDT on Thursday, 鈥渢he voltage output for customers in New York dropped to 80 per cent of the normal value,鈥� Pal told 快猫短视频.
But the laws of physics state that if the power demand remains constant and the voltage drops, then the current must increase. In turn, because the resistance of the power lines is constant, the increase in current causes the voltage to drop, completing a feedback loop.
Pal says the voltage continued to fall until it hit 50 per cent of the normal value, at which point the cut-off systems that protect local power networks were tripped. This isolated the New York network from the regional grid, and caused New York鈥檚 21,000 megawatts of electricity to surge back into the grid.
This led to a domino effect all the way back to the power stations producing the electricity. The returning energy caused the turbines driving the electricity generators to spin faster, up from the normal 60Hz to 65Hz. 鈥淭his 鈥榝lywheel effect鈥� is very dangerous,鈥� says Pal, and so safety circuits shut down the power stations.
鈥淯ltimately, this is all the fault of a privatised electricity power network,鈥� says Pal. 鈥淲hen there is no commercial interest to look after the power network, then it gets neglected.鈥�
But Kirschen disagrees. 鈥淓nhancing the electricity transmission network will help the economics, but not necessarily the security of the network. The question is whether the software that is supposed to warn operators did indeed warn them of the imminent disaster.鈥�