July 31: The biggest power outage in history struck 600 million people in India on Tuesday as three grids collapsed one after the other around 1pm.
Bengal was among the half of India that was affected by the blackout after noon. Rarely have so many people separated by so many miles been bound together by the same trauma because of the failure of something that has been taken for granted.
The Telegraph tries to answer your questions.
What is a power grid?
It is an interconnected network of transmission lines and sub-stations hooked on to generating stations on the one hand and load centres (or distribution companies) on the other. The generating stations supply electricity to the grid through the transmission lines. The load centres then draw the power from the lines and send it to your homes and other end-consumers.
The network is called a grid because it resembles a framework of criss-crossed lines.
A delicate balance between generation and load must be maintained at all times to prevent a failure.
How many grids does India have?
Five. Northern, eastern, northeastern, southern and western. The state-owned Power Grid Corporation, which operates more than 95,000 circuit kilometre transmission lines, runs the grids. Of these, the northern, eastern and the northeastern grids collapsed on Tuesday.
When does a grid collapse?
A grid collapses if there is excess withdrawal of power by member states or oversupply by the generating unit.
Who drew more on Tuesday?
Not yet clear. The suspicion is on northern states like Uttar Pradesh, Rajasthan, Haryana and Punjab. But all these states have denied doing so. They are being watched closely because farmers there need power for irrigation as the monsoon has been weak. Hydel power generation has also come down because of the poor rains, which has made these states more dependent on thermal power.
Are all grids inter-connected?
Yes, except the southern grid, which bailed the southern states on Tuesday. There are, however, plans to connect the southern grid. The inter-connection helps supply of electricity from a power-surplus to a deficit region.
Is Bengal also to blame?
Not really. Bengal, a power surplus state, was actually helping out northern India. Distress power from Bhutan was also flowing to the north through the eastern grid. But some state or states in the north are suspected to have drawn more than their share, which triggered the chain collapse.
Why didn’t the eastern grid collapse on Monday but did on Tuesday?
On Monday, the eastern grid was pumping power to the north. The suspicion is that when the northern grid collapsed again on Tuesday, there may have been “overdrawing” by the eastern and northeastern grids.
Why do you keep saying ‘suspected’? You are not sure?
No conclusive proof has been made public yet. “Data up to a milli-second has to be studied to know why the grid collapsed and if it was because of excess withdrawal of power and, if so, by which state,” an official in Delhi said on Tuesday evening.
What is overdrawing?
Taking more electricity from the grid than declared to the grid operator the day before in a detailed report on projected use for every 15 minutes of the day. Mostly because of the late arrival of the monsoon, the load on the northern grid increases during the paddy season in Uttar Pradesh, Haryana and Punjab and they sometimes overdraw to meet their agricultural needs.
Who sets the drawing limit?
The Central Electricity Regulatory Commission sets the drawing limit for each state, which is based on the agreement reached by them with the generating firm .
If someone overdraws, why are the grids collapsing?
This is a bit complex. Please read with patience. “The grid remains in steady state as long as there is an overall load-generation balance,” said Anil Kulkarni, a power systems dynamics specialist at the Indian Institute of Technology, Bombay. “When a zone of the grid overdraws power, at times, certain parts of the network can become insecure.”
If an additional disturbance or a disruption is superposed on a transmission line located in an insecure zone, the resulting tripped line can have a cascading effect, Kulkarni said. Typically, a region of the grid may experience one or two disturbances each day.
But the loss of a line can add load to other sections of the network which may also trip. Such trips may isolate a region of the grid, creating an island-grid, which is using up more power than it is generating. In such situations, the turbines spinning in the power generating stations tend to slow down, pulling down the frequency.
Most steam turbines are designed to operate at frequencies between 52 Hz and 47 Hz. As turbines slow down, the frequency dips. When the frequency drops below 47, the turbines just shut down.
Isn’t there any isolation system to prevent the spread of the collapse?
Yes. The architecture is so created that tripping does not shut down the entire system. It seems the systems meant to prevent such collapse did not act on time on Monday and Tuesday
Then how was the western grid spared?
The grid must have taken rapid action. “There are standard emergency control measures that engineers have designed to prevent a precipitous fall in the frequency — like under-frequency load shedding,” Kulkarni said. But if a grid failure is imminent, action might need to be taken within seconds.
How long does it take to restore power after a grid failure? Why does it take so long?
The restoration takes between eight and 12 hours, usually. The longest time, six to eight hours, is taken to restart generation in the thermal power generating units. During such a collapse, the generating stations are disconnected from the grid and shut down. The units need several hours to be up and running again. Also, there is the danger of another grid failure because of system overload if the entire power generation is routed to the grid at one go. The generating units are restarted in phases over several hours.
In case of a hydro power station, restarting does not take much time.
Whose responsibility is it to run the grids without hitches?
Under the leadership and monitoring of the Power Grid Corporation and the National Load Dispatch Centre, the responsibility is shared by numerous generation, transmission and distribution utilities.
If states overdraw, can’t they be penalised?
There is a financial penalty against overdrawal. In India, the grid frequency-linked penal measure is called the Unscheduled Interchange or the UI rate.
In its amendment to the rules pertaining to UI, the Central Electricity Regulatory Commission has proposed to slap a charge at the rate of Rs 1.65 per unit drawn at a grid frequency not below 50 Hz. The penal rate is Rs 4.50 per unit if the overdrawal happens at a grid frequency not below 49.80 Hz. The penalty is Rs 9 per unit if the overdrawal happens at a grid frequency below 49.7 Hz.
However, for some high power-deficit states, the overdrawal even with the penalty is more economic than buying power from the market. According to sources in the state-run power sector, the northern states are frequent violators of the grid frequency norms, especially Uttar Pradesh, Haryana, Punjab and Jammu and Kashmir.
The UI penalty has failed to deter these states, which overdraw and then default on the UI payment with the grid operator.
So, is the UI system useless?
No. Rather, the mismatch between the voracious appetite for energy and the inability to stomach measures unpalatable in the short term but rewarding in the long term has blunted the effectiveness of the penalty system. Remember, the last big blackout in India was in 2002. The introduction of financial penalty was the main reason why overdrawals had come down in between. Besides, improvements in the load despatch centres and the integration of the grids helped prevent frequent collapses of the grid.
Can the grids collapse again?
Yes. What happened on Monday in the northern grid and on Tuesday in the three grids were accidents that can take place again. But with better checks and balances by the grid operator against overdrawal and stern monitoring by the load dispatch centres in close co-ordination with the Union power ministry, there could be a better chance of preventing a recurrence in the near future.