By Sudarshan Varadhan and Rajendra Jadhav

CHENNAI/MUMBAI, April 4 (Reuters) - Prime Minister Narendra Modi's appeal to citizens to switch off lights across India on Sunday to mark the coronavirus fight is generating a lot more work and some tough challenges for India's power sector workers.

On Friday, Modi appealed to Indians to turn out their lights for nine minutes at 9 p.m. on Sunday to show solidarity amid the coronavirus lockdown with a show of lamps, candles and flashlights.

The appeal, however, set off alarm bells for those in charge of India's power grid, prompting a flurry of orders to officials manning generation plants and managing grid load, amid concerns that a surge in voltage due to a steep dip in demand could harm the grid and cause widespread outages.

India's Power System Operation Corp (POSOCO), which oversees the national power grid, ordered all senior officials to be present at generating stations, substations and load despatch centers across India between 6 p.m. and 10 p.m. on Sunday.

It advised engineers to start reducing output from baseload plants such as coal-fired power stations just ahead of 9 p.m. and ramp up generation from hydro and gas plants, typically used to address peak power demand, to manage the anticipated gyration in demand.

POSOCO expects consumption to dip by over 10% when Indians switch off lights, it said in an advisory sent to grid operators across the country, calling the expected reduction in load and rapid recovery, "unprecedented".

POSOCO's parent body, Power Grid Corp of India, asked regional electricity transmission centre employees to be on "high alert," as the lights out plan could "lead to outage of grid elements due to grid constraints".

India's Ministry of Power sought to allay concerns though, and said in a statement, "The Indian Electricity grid is robust and stable and adequate arrangements and protocols are in place to handle the variation in demand."

Modi has ordered India's 1.3 billion people indoors to avert a massive outbreak of coronavirus infections, but the world's biggest shutdown has left millions without jobs and forced migrant workers to flee home to their villages.

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India's power consumption has already plunged amid the lockdown to stem coronavirus cases - which rose to nearly 3,000 on Saturday. Some states are worried about the impact of the "lights off".

Nitin Raut, the power minister of Maharashtra, a western state which consumes the most electricity in India, appealed to citizens to light lamps and candles, while keeping lights on to ensure proper functioning of the grid.

"Already the electricity demand and supply equation has been stretched," said Raut, adding the possibility of grid failures "could not be ruled out." (Reporting by Sudarshan Varadhan in Chennai and Rajendra Jadhav in Mumbai; Editing by Euan Rocha and Ros Russell)