R.K. Choudhary, Former Member, Bihar Electricity Regulatory Commission
From 1980 to 2010, India’s power generation was predominantly based on fossil fuel-based power plants (such as coal, gas and oil), hydroelectric plants and nuclear stations. The primary focus during this period was ensuring reliable power supply to the grid, with secondary emphasis on improving plant efficiency.
Until 1980, many thermal power plants operated at unit sizes of 210 MW or less, based on subcritical steam parameters of 150 ata/530°C/530°C. The efficiency of these units was generally below 30 per cent. In 1984, Tata Power Company Limited commissioned the country’s first 500 MW unit at Trombay in Mumbai, still subcritical but operating at higher steam parameters of 170 ata/537°C/537°C.
Around the same time, concerns about greenhouse gas (GHG) emissions began gaining global traction. The 1997 Kyoto Protocol marked a significant international effort to reduce CO2 and other GHG emissions contributing to global warming.
Up until the early 2000s, electricity tariffs in India were determined based on plant load factor. Generators were incentivised to maximise generation, even when grid frequency was on the higher side (sometimes exceeding 52 Hz, particularly in the eastern region), often by biasing turbine governors. To address this, the Central Electricity Authority, after stakeholder consultations, introduced the Free Governor Mode of Operation to stabilise grid frequency within the range of 49-51.5 Hz. Simultaneously, in 2005, the Central Electricity Regulatory Commission implemented the availability-based tariff mechanism. Together, these reforms brought the frequency closer to the optimal 50 Hz ± 0.5.
As environmental concerns escalated, particularly with regard to emissions from coal-fired plants, the Ministry of Environment, Forest and Climate Change began pushing for cleaner technologies. The twin goals were:
Enhancing plant efficiency to reduce generation costs, and Reducing CO2 emissions to combat global warming.
This led to a push for supercritical technology, which operates at higher steam pressures and temperatures. The first supercritical units in India – 800 MW at CGPL Mundra (a Tata Power subsidiary) and 660 MW at NTPC Sipat – were commissioned in 2012. Supercritical units operate at pressures above 225 kg per cm² (ata). Compared to subcritical units, their higher steam parameters (for instance, 280 ata/600°C/600°C) enable thermal efficiencies of around 42 per cent, up from 37 per cent, resulting in lower coal consumption and CO2 emissions per unit of electricity generated.
Parallel to this, several hydroelectric plants were commissioned and served both as baseload and peaking sources, depending on system requirements. These developments collectively helped stabilise the Indian grid, maintaining frequency within permissible limits.
The rate of change of frequency (df/dt), or grid inertia, remained sufficient during this era due to the prevalence of high-inertia rotating machines like steam turbines. Such machines, when tripped, can take up to 10 minutes to coast down from 3,000 revolutions per minute (rpm) to low turning speeds (around 200 rpm), helping maintain frequency stability during disturbances.
High renewable energy penetration presents new challenges. Renewable energy sources, being inverter-based, contribute little to grid inertia, unlike conventional rotating machinery. This raises concerns, particularly in light of recent grid events in Spain and Portugal, which may be partially attributable to low-inertia systems.
While battery energy storage systems (BESSs), pumped storage projects (PSPs) and other ancillary services are critical for managing variability and ensuring grid stability during renewable energy generation peaks or intermittency, a balanced energy mix is essential.
Research is ongoing in advanced ultra-supercritical technologies operating at even higher steam parameters (300 ata/720°C/720°C), along with carbon capture and storage solutions. These hold promise for the future, with commercial viability within reach.
Hence, a diversified and balanced energy mix is vital – combining renewable energy with environmentally friendly conventional sources – to ensure a robust and stable power system. Although renewable energy appears cheaper (with solar/wind generation at around Rs 2.5 per unit), the cost advantage diminishes once the expense of necessary ancillary systems (BESS, PSPs, etc.) for grid stability is factored in. This holistic approach also promotes optimal utilisation of India’s diverse natural resources such as solar, wind, coal, hydro and nuclear. We must adopt a comprehensive strategy to achieve this balanced and resilient energy mix by 2047.