Addressing Grid Variability: Ramp Up and Ramp Down Rates for Solar and Wind Power in India
India is rapidly increasing its renewable energy capacity, particularly solar and wind power, to meet its growing energy demands and reduce greenhouse gas emissions. However, the integration of intermittent renewable energy sources into the grid presents challenges in terms of grid variability. Grid variability refers to the fluctuations in electricity generation from renewable sources due to changes in weather conditions, such as cloud cover or wind speed. To address these challenges, ramp up and ramp down rates are crucial for ensuring grid stability and reliability.
Currently, India relies heavily on thermal generators, primarily coal-fired power plants, to meet load following needs and provide flexibility to balance supply and demand in the grid. However, these thermal generators are ramp-limited resources and cannot respond quickly to control signals to change their output. The Central Electricity Authority (CEA) requires a ramp rate of 3% per minute for thermal generators operating above the 50% Maximum Continuous Rating (CEA 2010). However, analysis by the Power System Operation Corporation Limited (POSOCO) found that only 35% of thermal generators in India are providing at least 1% per minute ramping capability, with the majority of coal-fired central generating stations declaring a ramping capability of only 0.5%–0.7% per minute (POSOCO 2019b). Similarly, for minimum loading, the Central Electricity Regulatory Commission (CERC) requires a minimum generation level of 55% for interstate generators, but recent reports reveal that only 6% of central generating stations are reaching a minimum generation level below 60% (CERC 2016; POSOCO 2020d). On an all-India basis, approximately 60% of units are flexing their output by 20%–30% of installed capacity (POSOCO 2020d).
To address the grid variability issues and improve flexibility in the generation mix, India needs to focus on ramp up and ramp down rates for solar and wind power. Solar and wind power are variable energy sources that depend on weather conditions, and their output can change rapidly. Therefore, it is essential to have appropriate ramp up and ramp down rates to ensure smooth integration of solar and wind power into the grid and maintain grid stability.
Ramp up refers to the rate at which the output of a power plant increases, while ramp down refers to the rate at which the output decreases. For solar and wind power plants, ramp up and ramp down rates are crucial to match the fluctuations in their generation with the demand in the grid. If the ramp up and ramp down rates are too slow, it can result in grid instability and affect the quality of electricity supply. On the other hand, if the ramp up and ramp down rates are too fast, it can cause overloading of the grid and damage to the equipment.
In India, solar and wind power plants are typically connected to the grid through power electronic converters, which can provide fast ramp up and ramp down rates. These power electronic converters can control the output of the renewable power plants and enable them to respond quickly to changes in weather conditions or grid demand. However, the ramp up and ramp down rates of solar and wind power plants also depend on various factors, such as the type of technology used, the size of the power plant, and the grid infrastructure.
The National Renewable Energy Laboratory (NREL) has conducted studies to determine the ramp up and ramp down rates required for solar and wind power plants in India. According to NREL’s report on energy storage in India, during the “Lights Off” event on April 5, 2020, system operators in India managed unprecedented ramp rates on the order of 3 to 4 GW per minute (NREL 2020).
During the “Lights Off” event that took place on April 5, 2020, where people across India switched off their lights for nine minutes to show solidarity during the COVID-19 lockdown, the Indian grid faced unique challenges in managing the sudden demand and ramp rates trend. The event resulted in an unprecedented change in electricity demand and ramp rates, and the grid operators had to respond quickly to maintain grid stability.
To address the challenges during the “Lights Off” event, system operators in India took several measures to manage the demand and ramp rates trend. One of the key measures was adjusting the droop settings of some hydro generators to 1%–2% from the normal range of 4%–5%. This allowed for faster response to frequency changes, helping to stabilize the grid during the event.
Fast-ramping hydropower and gas units also played a crucial role in meeting the ramping needs during the event. These units were able to respond quickly to the sudden changes in demand, providing the necessary flexibility to the grid to maintain stability. The system operators managed unprecedented ramp rates on the order of 3 to 4 GW per minute during the event, showcasing the capability of the fast-ramping units in addressing the sudden changes in demand.
Additionally, the Indian grid operators had to closely monitor the generation mix and ensure that the thermal generators were operating within the required ramping and minimum loading capabilities. While the Central Electricity Authority (CEA) requires a ramp rate of 3% per minute for thermal generators operating above the 50% Maximum Continuous Rating, analysis by the Power System Operation Corporation Limited (POSOCO) found that only 35% of thermal generators in India were providing at least 1% per minute ramping capability, with the majority of coal-fired central generating stations having a ramping capability of 0.5%–0.7% per minute. This highlights the need for improving the ramping capabilities of thermal generators in India to effectively manage sudden changes in demand.
Despite the challenges, the Indian grid operators were able to successfully manage the demand and ramp rates trend during the “Lights Off” event and maintain grid stability. The experience from this event highlighted the importance of grid flexibility and the need to improve the ramping capabilities of thermal generators, while also leveraging fast-ramping hydropower and gas units to address grid variability issues.
Looking ahead, as India continues to increase its renewable energy capacity, the grid operators will need to further enhance the flexibility of the grid to accommodate the variable nature of solar and wind power. This may involve investing in energy storage technologies, upgrading transmission infrastructure, and improving the ramping capabilities of thermal generators. A combination of these measures, along with effective demand-side management strategies, will be crucial in ensuring a reliable and resilient grid as India transitions towards a more renewable energy-based power system.
Based on the data from Palchak et al. (2019), the load profile in India is expected to change significantly by 2030, with notable changes in the load and net load curves for the day with the highest ramping requirements and highest renewable generation.
For the year 2022, the maximum 1-hour ramp-up requirement across India is projected to reach 32 GW, while the 90th percentile 1-hour net load requirement for ramping up and down is also expected to increase. However, by 2030, these values are anticipated to increase further, with the maximum 1-hour ramp-up requirement reaching 44 GW. This indicates a substantial increase in the ramping needs of the grid, reflecting the increasing penetration of renewable energy sources in India’s power system.
The Southern and Western regions of India are expected to experience the largest increases in net-load ramping requirements, as investments in new renewable energy capacity are likely to be concentrated in these areas. This suggests that these regions will face significant challenges in managing the variable nature of renewable generation and ensuring grid stability.
To address the increasing ramping requirements and maintain system flexibility, India’s power system will need additional fast-responding resources. Refurbishing thermal units to operate more flexibly has been estimated to incur additional costs of 5% to 10% of the total project cost. However, relying solely on fast response from hydropower plants, as demonstrated during the “Lights Off” event, may not be a sustainable long-term solution, as it requires advanced knowledge of events and running the plants outside of their normal operating range.
Energy storage, particularly battery storage, has been identified as a cost-effective solution to meet the increasing needs for system flexibility in India’s power grid. Unlike hydropower plants, battery storage is not subject to droop setting limits and can provide fast response when needed. Implementing energy storage technologies alongside other measures such as upgrading transmission infrastructure and improving the ramping capabilities of thermal generators could help address the challenges posed by the changing load profile and increasing renewable energy penetration in India’s power system.