The sun is producing more electricity. For India’s power system, that is good news. But, for a coal-fired power plant, it can mean a very different operating day.
During strong solar hours, the plant may be asked to reduce generation. As the sun sets and demand rises, it has to increase generation again. If this happens frequently, the plant’s equipment is exposed to repeated changes in temperature, pressure and load.
This has sharpened the debate over flexible operation of thermal plants.
A Central Electricity Authority (CEA) Committee examining the wear and tear, O&M, and plant-life implications of flexible and two-shift operation offers a nuanced assessment. For the units examined, it did not find significant damage attributable to operation at 55% Minimum Technical Load (MTL) with conservative ramping. At the same time, it does not establish that sustained 40% operation or repeated cycling across the wider fleet will have no long-term consequences.
The more important question may therefore be how thermal plants are made to flex, rather than simply how low they can operate.
Why 40% and 55% MTL?
MTL is the lowest level at which a thermal unit is expected to operate while meeting specified technical and reliability requirements.
CEA’s 2023 framework requires coal-based thermal units to operate at 55% MTL, while providing for measures to progressively achieve 40%. The lower target is linked to the changing generation profile as renewable capacity grows: thermal plants need to back down during strong solar hours and ramp up as solar generation falls in the evening.
The scale of this movement is already substantial. Grid-India told the Committee that evening net-demand ramping is around 60 GW, with thermal generation contributing about 50 GW and hydro about 10 GW. All-India demand ramping is generally 250–300 MW per minute and has reached 500 MW per minute on some days.
Is minimum load really the main concern?
NTPC has raised concerns about boiler tube leakages, thermal fatigue, and equipment stress from flexible operation, particularly repeated ramping. It has argued that some consequences of repeated cycling may only become visible over several years.
The Committee’s analysis presents a more complicated picture. Equipment deterioration can have several causes, including age and operating history, grid-driven cycling, start-ups and shutdowns, human and procedural errors, and combustion management.
CEA and Intertek examined operating data from NTPC’s Farakka Unit-2 and Jhajjar Unit-3. At Farakka, a 39-year-old unit, outages over five years had multiple causes, while its ramp rates were generally conservative. The Committee found no significant damage that could be attributed directly to operation at 55% MTL with conservative ramping.
NTPC’s concern is essentially about cumulative effects that may emerge over several years, while the Committee’s analysis found that the available data did not establish a direct causal relationship between flexible operation and the failures examined.
The report’s more significant observation is that frequent, high-magnitude ramping can cause severe wear and tear, particularly when a unit is subjected to another ramp before it has stabilised.
This shifts the debate from simply 40% versus 55% to questions of frequency, ramp magnitude, stabilisation time, and which units are repeatedly called upon to provide flexibility.
What does this mean for 40% MTL?
India has already demonstrated 40% operation in selected units through flexibilisation studies and tests. The more difficult question is what sustained, repeated operation at that level means for equipment life, reliability and O&M across a diverse fleet.
The Committee supports moving towards 40% through appropriate retrofits, control modifications, operational measures and training, while recommending unit-specific studies and testing.
It also calls for more operating data from units experiencing frequent ramping, followed by analysis of equipment degradation and maintenance trends.
This is important because thermal units differ significantly in age, design, operating history, and maintenance conditions. A single fleet-wide assumption may therefore be inadequate.
What about older plants?
The report proposes two-shift operation as a possible option for some older thermal units: shutting down during strong solar hours and restarting for the evening peak.
It identifies units preferably of 210 MW or less and more than 35 years old as potential candidates for feasibility studies and pilots, estimating around 34 GW of potential two-shift capacity.
But the report does not treat this as a ready solution. Technical constraints, life consumption, retrofit requirements, reliability and tariff implications need to be tested.
NTPC has raised concerns that the consequences of repeated cycling on ageing units may take years to become visible. The lack of suitable pilot units has also limited empirical validation.
New plants should be designed for a flexible grid
The implications extend to future capacity. BHEL, Intertek and Grid-India have supported standard specifications and tender requirements for flexible operation at 40% MTL for new and incoming coal-based units.
If thermal plants are going to operate alongside increasing renewable generation, flexibility needs to be considered at the design and procurement stage, rather than added later through retrofits.
Flexibility has a cost
Repeated backing down, ramping and cycling can increase O&M, auxiliary consumption, heat-rate degradation, oil consumption and equipment life consumption.
The Committee recommends expanding the existing compensation mechanism for operation at 55% MTL to include incremental O&M costs of up to 5%, along with the existing components relating to auxiliary consumption, station heat rate and oil consumption degradation. It also recommends better coordination among system operators and adherence to required MTL schedules during solar hours.
This raises a wider regulatory question: if flexibility is being required to support the power system’s renewable integration, should its incremental cost be explicitly recognised as the cost of providing a system service?
The answer will not be thermal alone
The Committee recommends greater use of hydro, gas, pumped storage and BESS for finer balancing, alongside better forecasting, controls, monitoring and dispatch.
The emerging system will therefore need to match different flexibility requirements with the resources best suited to provide them.
For thermal generation, that means understanding the capability and limitations of individual units and avoiding unnecessary or repeated cycling of the same plants.
So, does flexible operation damage thermal plants?
The CEA report does not settle the debate, but it does narrow it.
It finds no significant damage directly attributable to 55% MTL operation with conservative ramping in the Farakka unit examined.
At the same time, it identifies frequent, high-magnitude ramping, start-ups, shutdowns and operating practices as important factors in plant wear, while leaving the long-term effects of sustained 40% operation and two-shift cycling open for further evidence.
The next phase therefore needs more than a lower MTL target. It needs plant-level evidence, pilots, appropriate retrofits, better monitoring, disciplined dispatch, clear technical standards for new capacity and a compensation framework that recognises the cost of flexibility.
The real question for India’s thermal fleet is:
How much flexibility can each plant provide, how frequently, at what cost, and for how many more years?
As renewable generation grows, flexibility will increasingly have to be designed, measured, dispatched and paid for, rather than being treated simply as an obligation attached to thermal generation.
As India pushes thermal plants towards deeper flexibility, where should the line be drawn between system need and plant life?
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