As candles are ready to be lit at 9 pm, power utilities asking people to switch off only lights

BENGALURU: The Power System Operation Corporation (Posoco) which operates the national load dispatch centre expects the load to fall by 12,879 MW across the five regions during the nine-minute light switch-off event this evening, according to its advisory to state load dispatch centres. The Posoco has advised load managers to go for a graded reduction in generation to prepare the grid for the nine-minute event which will begin at 9 pm.

Indian grid system is designed to operate at a frequency of 50 Hz (or cycles per second). A sharp drop below this level or a sharp rise above this level will hamper the grid stability. If the fall or rise is too sharp, it can set off a series of tripping of transmission lines resulting in outage of generating stations. Sharp fall in frequency used to be a general occurrence in the summer months in the 1990s with one state or the other within a region overdrawing from the grid causing the frequency to fall to precarious levels. It is another matter that such rogue states had to pay a fine. The trend, however, became a thing of the past after the power sector moved to merit-order dispatch or availability based tariff. In other words, tariffs payable by utilities is linked to the frequency.

As the load curve for March last week shows, India’s load hovers between 112551 MW and 101207 MW between 6 pm and 9 pm. The lighting load starts picking up after 6 pm. The Posoco estimates that, even if lighting load reverts to 6 pm levels with mass switch-offs of lights in response to Prime Minster’s call, the grid will see a drop of 11,344 MW. The difference, however, is unlike normal operation the drop of load of about 12-13 GW (1GW=1000 MW will happen in 2-4 minutes and recover in 2-4 minutes, nine minutes later. This is unprecedented, Posoco note says.

As part of their preparation for the grand event, all generating stations have synchronized their clocks, and after 6 pm, hydel output will be reduced. Hydel stations are the most flexible unlike thermal when it comes to raising or reducing the generation output. Thermal stations will meet the base load. After 9.9 pm, hydel generating stations will roar back to full life, and thermal scaling up.

“Lighting load makes up about 10-12% of overall load. In Karnataka’s context, the load is already down by about 3000 MW due to the lockdown. Of the 8500 MW- 9000 MW, if about 800 MW load suddenly drops, our engineers can manage it with adequate precaution,” says S Sumanth, former Director (transmission) at KPTCL and former MD of Mangalore Electricity Supply Company.

Distribution utilities are advising power consumers to only switch off lights during the candle-lighting event, and not other installations such as TV, AC and Fans. “As much as 60% of the load, in fact, comes from irrigation pump sets which most farmers will be using to water their fields,” Sumanth said, added: “Even if one in four do not participate in this event it may not add or subtract to this load change. However this exercise on the whole is not going to make any adverse impact on the grid system.”