First unit of Subansiri project synced to national grid after two decades

Guwahati, Dec 4: After a 20-year-long journey, the Subansiri Lower Hydroelectric Power Project began generating power from Wednesday, and one unit was synchronised with the national power grid at 6.12 pm.

The 2,000 MW project, being implemented by NHPC, will have eight units of 250 MW each, and the second unit started generating yesterday. Three more units are likely to be commissioned this month, while the entire project is likely to be commissioned by March 2027.

The consultant of the project, AN Mohammad, told The Assam Tribune that it was a defining moment in India’s hydropower journey as soon as one unit of the Lower Subansiri project was synchronised with the National Grid.

Mohammad said that the commissioning process commenced with Unit 1 and Unit 2, which underwent mechanical runs on October 26 and  November 6, respectively. The synchronisation of the remaining three units – Unit 1, Unit 3, and Unit 4 – will proceed upon completion of comprehensive testing during wet commissioning that shall deliver an additional 1000 MW of output by December 2025.

The commercial operation date (COD) will be determined shortly. The final four units are scheduled for sequential connection during 2026–27, further enhancing energy supply and supporting national infrastructure.

Upon completion, the Subansiri Lower HE Project is expected to contribute 7.421 billion units of renewable energy annually, strengthening the country’s energy sector and leaping forward to fulfil India’s carbon neutrality target.   

Giving details of the project’s history, Mohammad said that the Subansiri Lower HEP, which was envisaged in January 2005 at Gerukamukh on the Arunachal Pradesh-Assam frontier, has been a saga of resilience.

Construction came to a halt for eight gruelling years between 2011 and 2019, stalled by Assam’s protests and court battles over fears of dam safety and downstream ecological fallout.

But with fortified mitigation plans in place, the project construction roared back to life in October 2019, transforming scepticism into steady progress.

To quell these valid concerns, the Government of India assembled a cadre of expert panels. The Technical Expert Committee (TEC), formed by the Planning Commission in July 2012, meticulously debunked the Assam Expert Group’s hydrological worries.

The Dam Design Review Panel (DDRP), in June 2013, prescribed key enhancements for unassailable safety: widening the dam from 171 meters to 271 meters, extending upstream cut-off walls, adding downstream barriers against seepage, intensive foundation grouting, and rock anchors with cable tendons to anchor the abutments.

The National Committee for Seismic Design Parameters (NCSDP) also scrutinised and approved the project’s seismic safeguards.

A landmark “Cumulative Impact and Carrying Capacity Study of the Subansiri Sub-Basin, Including Downstream Impacts,” led by the Central Water Commission (CWC) in December 2014, underscored the need for ecological harmony.

It mandated a steady 240 cubic meters per second environmental flow to nurture the river’s aquatic life, ensuring the Subansiri’s pulse beats strong for generations.

The Subansiri itself is no stranger to chaos, notorious for its 90-km rampage through Assam’s plains, where floods and erosion have reshaped lives for decades.

The devastating 1950 earthquake rerouted its fury, unleashing landslides that temporarily dammed the river in Arunachal Pradesh. The eventual breach unleashed biblical deluges downstream. Yet, the project’s 1,365 million cubic meter (MCM) reservoir emerges as a vigilant guardian, designed to blunt such extremes.

It permits unimpeded monsoon surges while guaranteeing minimum non-monsoon flows to sustain downstream ecosystems.

A 15-meter flood cushion atop the reservoir with 442 million cubic meters acts as a vital buffer, while comprehensive riverbank fortifications – spanning 60 km downstream – have boosted the channel’s capacity by 15–20 per cent.

Complementing these are downstream upliftment initiatives worth over Rs 470 crores, bolstering health, education, sustainable livelihoods, and infrastructure for affected communities.

Beyond protection, the project pulses with promise: churning out 7.421 billion units of pristine, renewable energy each year to propel India’s clean energy crusade.

As a national endeavour, power allocation adheres to central hydropower norms. Assam, beyond its standard 208 MW slice, secures a preferential 300 MW allotment plus 25 MW of free electricity. Arunachal Pradesh, the gracious host, reaps a 12 per cent free power as per the national hydropower policy.

The remaining power is allotted to other states, including the Northeast states of India.

The project cost increased from Rs 6,285 crores at the 2002 price level to approximately Rs 26,000 crores at the current price level, primarily due to the extended construction period, escalation charges, and interest during construction (IDC), Mohammad added.