The influx of renewables poses a threat to grid stability and security owing to the intermittent nature of renewable power generation. In order to ensure grid stability, the balance between load and generation has to be maintained continuously. For this, conventional thermal power plants (TPPs) have to be flexible enough to ramp up and down quickly, and provide frequency control to balance the power grid. However, this could have a negative impact on plant operations as TPPs have certain technical and commercial limitations.
Need for flexibilisation
The rapidly changing energy mix is a key driver for the flexibilisation of TPPs. The share of renewables in the total energy generation has increased significantly over the past decade, from 0.4 per cent in 2006-07 to 10 per cent in 2017-18. It is expected to further increase to 16 per cent by 2022 given the government’s ambitious renewable energy capacity addition targets. Variability in renewable generation patterns and the associated uncertainty affects grid operations. The problem is aggravated due to the varying renewable resource potential across states. A fluctuating input creates uneven load on the grid as it seldom matches the demand, thus creating a demand-supply imbalance.
Another driver for flexibilisation is slow demand growth. While thermal power capacity has grown at a compound annual growth rate (CAGR) of over 11 per cent in the past five years, the demand has grown only at 4 per cent. As a result, the plant load factor (PLF) has declined considerably from about 70 per cent in 2012-13 to 59 per cent in 2017-18, and is set to decline even further.
Another factor impacting PLFs is the increase in the “peak-to-off-peak” ratio, which has resulted in lower despatch and greater instances of reserve shutdowns. This has also led to the shortage of ramping resources.
Role of coal-based TPPs in flexibilisation
A coal-based power plant’s flexibility can be determined by its technical minimum limit, ramp-up and ramp-down rates, and the number of startups and shutdowns. TPPs are required to ramp up and down generation to cope with the sudden variations in renewable energy output or any grid-related contingency. The spinning reserves also need to respond with a sufficient ramp rate during the automatic generation control (AGC) process to match with the control signals.
As per Power System Operation Corporation Limited’s report, “Flexibility Requirement in the Indian Power System (2016)”, some of the key features of a flexible coal-based power plant are its capability to start up and shut down up to two times daily, maintain AGC, run at the minimum generation level using gas as fuel, and operate at sliding pressure.
The new fleet of power plants based on supercritical technology is more flexible than the old plants. Further, operators are reluctant to ensure flexible operations owing to the degradation of the heat rate that occurs when the TPP is running below the technical minimum. In this context, the Central Electricity Regulatory Commission has issued amendments to the Indian Electricity Grid Code. As per these, the technical minimum scheduled for the operation of an interstate generating station unit is fixed at 55 per cent of the unit’s installed capacity. The regulations also specify that generators operating below 55 per cent of the installed capacity will be compensated in terms of the heat rate.
Also, the Central Electricity Authority’s Technical Standards for Construction of Electrical Plants stipulate a ramp rate of 3 per cent for TPPs but the actual ramp rates declared by the plants are much lower. This would necessitate retrofitting of key technology and equipment in these plants.
Some of the physical modifications required to achieve flexibilisation are reduction in the mill size; increase in the number of mills; installation of advanced burners and indirect firing systems in boilers; use of special alloys for improved material strength; installation of external steam preheating systems to reduce the start-up time; and elimination of thick-walled components such as drums. Operational modifications include laying down procedures for boiler ramp rates, controlling the temperature movements during plant start-up and shutdown, and undertaking rigorous inspection of plant components.
Gas-based power plants are extremely flexible in terms of adjusting the power generation output and can be ramped up and down at a fast rate. These power plants can be started up in less than an hour and can, therefore, be deployed as non-spinning reserves in case of contingencies. However, given the shortage of domestic natural gas, most of the gas-based power plants in the country are stranded operating at a sub-par PLF of 23 per cent, as of February 2018.
Introducing flexibility in power plant operations requires a paradigm shift in maintenance practices. The existing fleet of TPPs was designed for baseload operations, taking into account the energy deficits in the past. It is difficult to deal with the demand for frequent start-ups and shutdowns, and operate below the designed level without adequate retrofits in mechanical, process engineering and control systems.
In addition, varying load cycles put stress on the power plant machinery and thus TPP owners need to adopt an operation-oriented maintenance approach. Under baseload operation, the condition-based maintenance approach of TPPs works well as there is enough time between the first signs of damage and the action taken to repair the affected component. However, in the case of partial-load operation, the power plant equipment experiences greater stress/fatigue, which can cause irreversible damage to weaker components.
Therefore, a regular assessment of power plant condition should be carried out by third-party agencies to detect existing weaknesses and analyse the residual life of the plant. Continuous online monitoring of the key equipment must also be undertaken as part of the maintenance strategy.
However, retrofits and operational modifications in power plants to introduce flexibilisation require a significant upfront investment.
The way forward
Taking cognisance of the need for flexibility in TPP operations, developers are taking steps in the right direction. For instance, in September 2017, NTPC Limited installed a condensate throttling-based primary frequency control solution at its Dadri’s TPP (Stage II, Unit 6). Condensate throttling is used for immediate generation of additional power for frequency control. A task force under the Indo-German Energy Forum is also studying flexibilsation options for NTPC’s Simhadri power plant.
In order to promote flexibilisation, the regulators need to devise an appropriate framework that incentivises flexible operation of power plants. In this regard, the NITI Aayog’s draft National Energy Policy points out that policy enablement will play a key role in the development of flexible generation sources. It adds that mechanisms for fair price discovery and compensation to flexible resource providers will be established going forward. The flexible operation of TPPs will help create synergies between conventional and renewable energy sources, and even the stranded gas-based capacity may find a market.