Better Load Response

Flexibilising TPPs for stable grid operations

The power sector scenario in India is changing significantly in line with the country’s commitment to explore low-carbon pathways for energy generation and emission reduction. As per its Intended Nationally Determined Contribution (INDC), India has committed to install 40 per cent of its total installed power generation capacity based on non-fossil fuel resources by 2030 with international support on technology transfer and financing. This includes the government’s ambitious target of installing 175 GW of renewable capacity by 2022. India has also committed to reducing its emission intensity per unit of GDP by 33-35 per cent from the 2005 levels by 2030.

As a result of these commitments, the share of renewable energy in the installed capacity is expected to increase to 33 per cent by 2022 vis-à-vis 19 per cent at present. The share of coal-based power is projected to decline to 48 per cent from 58 per cent during the same period. That said, coal will continue to remain the mainstay of India’s energy sector owing to its relative abundance and the limited availability of alternative fuels such as gas, and energy storage solutions. Coal-based generation will continue to be the major source of reliable power for the safe and stable operation of the grid till proper storage solutions are developed and commercialised. However, the flexible operation of coal-based power plants will be critical for their sustenance in the changing power scenario and will require a realignment of strategies by utilities.

Business case for flexibilisation

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. With increasing renewable generation, coal-based power plants may not be operated continuously to provide baseload power. Thus, power plant cycling will be required. Cycling refers to the operation of power generating units at varying load levels (demand), including on/off and low load variations, in response to changes in system load requirements. Every time a power plant is turned off and on, the boiler, steam lines, the turbine and auxiliary components undergo immense thermal and pressure stress, running the risk of serious damage. To avoid the critical risks of process safety, increased costs, higher probability of equipment failure and reduction in unit life associated with cycling, it is essential to undertake effective management of thermal power generation. This builds a business case for flexibilisation as power plant cycling costs will impact the generator’s profit margins.

Preparation for flexibilisation

As per Power System Operation Corporation Limited’s (POSOCO) 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 automated generation control (AGC), run at the minimum generation level using gas as fuel, and operate at sliding pressure.

NTPC has taken pioneering steps in ensuring flexible operations at some of its thermal power plants (TPPs). In September 2017, the utility installed a condensate throttling-based primary frequency control solution at its Dadri TPP (Stage II, Unit 6). Condensate throttling is used for immediate generation of additional power to ensure frequency control. Further, AGC was operationalised on a pilot basis at Dadri Stage II, in January 2018, while another AGC pilot is under way at the Simhadri TPP. Besides, NTPC is undertaking flexibilisation studies for several other stations.

To flexibilise its TPPs, NTPC is carrying out test runs at different units to determine the existing flexibilisation capability and potential limitations. Each power plant start-up is monitored at the control centre by a team of experts and feedback for improvements is provided to the station after full and partial load operations are analysed, and static and dynamic stability of components is determined. Besides, all new units being set up by NTPC have increased flexibilisation capability, with advanced alloys, thinner materials capable of faster temperature rise, and lower stresses.

Further, two units of NTPC and one unit each of the Damodar Valley Corporation, Gujarat State Electricity Corporation Limited, Andhra Pradesh Power Generation Corporation Limited and Maharashtra State Power Generation Corporation Limited are being considered for 55 per cent minimum load operation in line with the Central Electricity Regulatory Commission’s guidelines.

Leveraging digitalisation

The generators also need to leverage digitalisation for supporting flexibility in plant operations. A fleet-wide flexibilisation of power plants will require the digitalisation of the overall commercial operations and maintenance strategy. Digitalisation will be essential for bringing down the levellised system cost of flexible power. Some of the key digitalisation applications are process automation/ boiler auto-tune; online predictive tools for predicting failures, providing maintenance advisory, profiling critical parameters to avoid tube leakages; advanced monitoring for combustion stability; lifetime monitoring and control; lifetime assessment; and strategic maintenance. Other digital applications for power plants include online coal analyser, fleet monitoring, predictive tools for forecasting cycling costs to enable a low-cost fleet strategy, and digital tools for additional safety and training resources.

Barriers to flexibilisation

There are various barriers to flexibilisation such as lack of a robust regulatory framework, geographic concentration of renewables, limited participation of state utilities in flexibilising TPP operations, and coal quality issues. On the regulatory front, market readiness to support flexible operation is limited as most of the capacity is tied up under long-term power purchase agreements, which offer little scope for flexibility in the operation of coal-based power plants. Besides, the state utilities are reluctant to reduce their minimum load levels. There is a need to provide regulatory incentives to ensure that a larger fleet of plants take up flexibilisation on priority.

In addition, introducing flexibility in power plant operations requires a paradigm shift in maintenance practices as varying load cycles put stress on the power plant machinery and thus TPP owners need to adopt an operations-oriented maintenance approach. The existing fleet of TPPs was designed for baseload operations, taking into account the energy deficits of 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. The retrofits for minimum load operation would require an investment of Rs 100-150 million per unit. The Ministry of Power and power utilities are considering obtaining funding for the same from the Power System Development Fund or the Coal Cess Fund.

Further, there are concerns regarding the operation of TPPs equipped with flue gas desulphurisation (FGD) systems at reduced loads. It is uncertain if the FGD equipment will be able to perform efficiently when the unit is running at low loads.

The way forward

Steps are being taken to introduce flexibility in the power system to deal with the growing share of renewables. As per POSOCO, pilot projects for AGC are under way at four locations in the western, southern, eastern and north-eastern regions. Further, frequency bias mode and tie-line bias frequency mode operations are being studied besides including additional units under AGC. After successful testing, this technology is planned to be implemented across India in around 100 power generation plants. A total of around 65,000 MW is envisaged to be brought under AGC. A full-scale AGC project, planned by 2022, will enable efficiency and grid security in the Indian power system, making it ready to handle the targeted 175 GW of renewable energy capacity.

However, policy and regulatory measures need to be taken to further encourage flexibilisation initiatives. The policy should be aimed at minimising the levellised cost of electricity rather than just focusing on renewable energy tariffs and this requires redesigning the electricity market. There is also a need to promote and prioritise non-variable renewable energy sources such as hydro, biomass, geothermal, solar integrated with storage, and hybrid power plants. Further, a centralised renewable energy forecasting mechanism should be integrated with system operations and advanced decision-making systems need to be implemented to enable system operators to respond faster to changing grid conditions. Lastly, energy storage solutions need to be introduced to store electricity from variable renewable sources when supply exceeds demand in order to increase system security and flexibility. n

With inputs from a presentation by A.K. Sinha, Additional General Manager, NTPC, at a recent Power Line conference

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