The total renewable energy capacity in India has crossed 100 GW and accounts for about 24 per cent of the overall installed capacity. The share of renewables in the total power generation mix is low at present but is set to increase exponentially in the coming years. This is expected to pose challenges for thermal power plants (TPPs). There are issues of variability and uncertainty in power generation from renewable energy sources. While solar is still predictable during the day and for most of the year, wind is intermittent as it varies with daily and seasonal weather patterns.
NTPC Limited has developed a numerical weather prediction-based forecasting software in collaboration with the Indian Space Research Organisation for day-ahead forecasting of solar power generation with a certain accuracy. In order to deal with the challenges of variability, uncertainty and geographical limitation pertaining to renewables, the existing TPP fleet needs to be flexible in ramping up and down in line with system requirements.
Many pilot studies and field tests have been conducted at various NTPC power stations in a controlled environment by experts, with the objective of studies quantifying the impact and identifying the constraints in machines as far as flexible operations are concerned. These studies have been centered on three attributes of flexibility – minimum technical load, ramp rate and start-up time.
Currently, only NTPC and a few state utility stations are functioning up to the mandated 55 per cent technical minimum. If the minimum operation of state-controlled TPPs is also reduced, there would be much greater flexibility in the Indian grid. The provision of 55 per cent technical minimum load of thermal units needs to be adopted by all state electricity regulatory commissions under Section 86 of the Electricity Act, 2003. Also, there is a need for an incentive mechanism for investing in upgrading the systems to make them more flexible. The plants were initially designed as base-load units and their upgradation is required to achieve flexibilisation.
With respect to the ramp rate, it has been mandated that if the stations do not achieve a 1 per cent ramp rate, they will lose 0.25 per cent of their equity. This has compelled the stations to comply with the mandate, but no TPP has tried to achieve more. An incentive mechanism for achieving higher ramp rates can address this. In the studies, each and every start-up has been monitored to reduce the start-up time.
The technical issues observed in the pilot studies are excessive fluctuations in steam temperatures, high drum level swings during ramping, flame disturbance during ramping and at minimum load, occasional furnace pressurisation, stalling of primary air fans at low loads and low boiler flue gas exit temperature, leading to acid corrosion. Following these studies, NTPC has changed its operations and maintenance (O&M) practices and has invested in the competence building of engineers.
Challenges due to flexibilisation
The impacts of variable power on the system range from difficulty in load frequency control, the requirement of an enhanced transmission network and its underutilisation to an increase in the requirement of ancillary services. These factors result in increased system operation cost and transmission cost. Meanwhile, generators are impacted as there is a lower plant load factor (PLF) due to ducking of load curve, high ramping requirement, two shifts and cycling of plants, increased forced outages, higher O&M cost, lifetime reduction of equipment, poor heat rate and high auxiliary power. The cycling of TPPs to cater to variability has a huge cost involved due to increased wear and tear, increased thermal stresses, and reduced operating efficiency. The life of the equipment decreases with the increasing number of start-ups and stops. The primary flexibility challenges to TPPs are combustion and flame stability, increased boiler tube failures, ash deposits at low loads, tuning to process control loops, variations in coal quality, and thermal fatigue of failure of thick-walled components.
There are three main categories of TPPs under NTPC – pit-head plants, middle energy charge rate (ECR) plants and high ECR plants. Pit-head plants have a low ECR and are always scheduled at 100 per cent. So, they do not face the problem of variability and flexibilisation. A security-constrained economic despatch was implemented on these plants for a couple of years, and all of them had a PLF of 100 per cent. High ECR plants have coal transportation costs that are greater than or equal to the coal cost and run at a technical minimum. Even these do not face the issue of variability.
However, in middle ECR plants, the minimum load keeps fluctuating throughout the day. Flexible operations in TPPs lead to a much higher rate of deterioration of plant components. This has been observed in increased failure rates and more frequent replacement of components (especially the thick ones such as boiler drums and steam headers). These components have been designed for specific fatigue life in terms of the total start-ups and stops during the life cycle. Even with the change of plant operation from baseload to load cycling, these machines continue to run, but the components depreciate at a faster rate. Thus, there is an increased tendency of failures. There are also operational problems such as ash deposition in the gas path due to reduced flue gas velocity in part loads, issues with flame stability due to deterioration in combustion and many other issues related to combustion at lower loads.
Cost of flexing
The Central Electricity Authority (CEA) had published a report in 2019, based on the studies carried out at NTPC stations, to determine the cost of flexibilisation in terms of capex and opex. According to the two studies carried out by Siemens for NTPC Dadri and by GE for Talcher Kaniha, the capex would be Rs 200 million for Dadri and Rs 500 million for Talcher Kaniha. The opex per kWh would be 26 paise for 200 MW units and 34 paise for 500 MW units. In both studies, the plants were to function at 40 per cent minimum technical load.
Further studies are being carried out at the Dadri and Simhadri TPPs. The opex is high because of the degradation of heat rate, higher O&M cost due to depreciation of components and start-up oil consumption. Losses are expected to be more with a higher load and will rise considerably with frequent start-ups.
The flexibilisation of TPPs involves several challenges such as a reduction in plant efficiency, increased risk of failures, and power system tripping due to frequent start-ups and shutdowns. On the whole, it increases the cost of operations. However, monitoring, analytics and automation (such as developing an asset health index, predictive analytics practices, and implementation of customised dashboards) can address these challenges. Smaller-sized TPPs could be used for higher flexibilisation since flexible operation costs decrease with unit size. The opex can be based on benchmarked costs to compensate for the increased costs and the capex can be reimbursed on actual basis after examination. There is a need for regulatory interventions, such as a revised tariff structure and a grid code, for the supply of flexible power generation.
Based on a presentation by Achal Kumar Arora, General Manager, Operation Services SIIS, NTPC, at a Power Line conference