With the increasing penetration of renewable energy in the power system, real-time system operation is faced with the challenge of balancing this variable generation. Wind and solar generation leads to steeper ramps, deeper turndowns, and shorter peaks in system operation. Therefore, in the emerging power sector scenario, flexible power generation, in which conventional power plants can ramp up and down quickly and efficiently while maintaining optimal operations at low output levels, is important for grid stability. The standards and guidelines of the Central Electricity Authority (CEA) for thermal power plants prescribe a ramp capability of 3-5 per cent per minute. Meanwhile, the Indian Electricity Grid Code stipulates that the recommended rate for changing the governor setting (supplementary control for increasing or decreasing the output at the generation level) for all generating units, irrespective of their type and size, would be 1 per cent per minute, or as per the manufacturer’s limits.
Power System Operation Corporation Limited (POSOCO) has recently issued draft guidelines for the assessment and evaluation of ramping capabilities of thermal interstate generating stations (ISGSs). The objective of the guidelines is to provide a detailed procedure for the assessment of ramping capabilities to reduce or increase the applicable rate of return on equity (RoE), depending on the reduction or increment in the ramp rate of the ISGS. The guidelines will be applicable to all regional coal- and lignite-based generators whose tariffs are being determined by the Central Electricity Regulatory Commission (CERC). The guidelines have been formulated in compliance with the CERC (Terms and Conditions of Tariff) Regulations, 2019, which state that effective April 1, 2020, the rate of RoE for thermal generating stations will be reduced by 0.25 per cent in case of failure to achieve a ramp rate of 1 per cent per minute, while an additional rate of RoE of 0.25 per cent (up to a ceiling of 1 per cent) will be allowed for every 1 per cent per minute increment in the ramp rate over and above the base ramp rate of 1 per cent per minute.
The primary objective of POSOCO’s guidelines is to determine the ramping capability of coal-based power plants so that they can perform load following duties efficiently as the system-wide ramp requirements increase. Further, once a generator exhibits reasonable ramp rates during load following on a day-to-day basis, it would be able to effectively participate in secondary regulation through automatic generation control as well as tertiary control, and thus help in frequency control.
Ramping performance evaluation methodology and assessment procedure
As per the guidelines, for evaluating ramping performance, a generator would need to declare its ramp-up rate (MW per block) and ramp-down rate (MW per block) for each of the 96 time blocks to the concerned regional load despatch centre (RLDC) for the purpose of scheduling on a day-ahead basis, and with subsequent revisions during the course of the day. These will be referred to as the “declared ramp-up rate” and the “declared ramp-down rate”. The number of blocks in which the declared ramp-up rate and the declared ramp-down rate is 1 per cent per minute or greater (that is, 15 per cent of ex-bus generation corresponding to the maximum continuous rating per time block) will be counted as “Td”, while the total number of blocks in the period of computation will be counted as “Tm”. These ramp rates will be considered while providing final schedules to the generator. The actual injection will be measured by interface meters or special energy meters for each of the 15-minute time blocks, as scheduling and metering are also being done on a 15-minute basis.
The key factors for the assessment of the overall ramp performance of a power plant are summarised below:
- Proportion of blocks (Td) out of total blocks (Tm) in the period in which the ramp-up/down rate is declared by the ISGS is 1 per cent per minute or more, that is, (Td/Tm).
This proportion (Td/Tm) signifies the readiness of the generator to support the grid in terms of ramping performance and represents the ability to ramp up or down except during times of exigencies, breakdowns, startups/shutdowns, and other conditions.
- Number of blocks (D) out of declared blocks (Td), in which an ISGS is scheduled by an RLDC with a ramp-up/down at the rate of ³1per cent per minute.
- Proportion of blocks (E) out of scheduled blocks (D), where an ISGS has achieved an actual ramp-up/down rate ³scheduled ramp rate (when it is ³1 per cent per minute), that is, (E/D).
The proportion (E/D) signifies the percentage in terms of number of blocks in which an ISGS has achieved its scheduled ramp rate out of the total blocks, when it is scheduled to ramp at more than 1 per cent per minute by an RLDC. Here, D is the number of time blocks for which the scheduled ramp rate is greater than or equal to 1 per cent per minute, while E is the number of blocks out of D in which the ISGS has achieved a ramp rate greater than or equal to the scheduled ramp rate. As the actual generation is unlikely to exactly match the scheduled generation in each block, because of the inherent randomness of physical systems, a tolerance of 5 per cent in the ramp rate is allowed while measuring the actual generation and counting each of the blocks E (out of D). This proportion is used while assessing if an ISGS qualifies for additional RoE as per the Tariff Regulations.
- Proportion of blocks (F) out of scheduled blocks (D) where an ISGS has achieved an actual ramp-up/down rate of ³1 per cent per minute when the scheduled ramp rate is ³1 per cent per minute, that is, (F/D).
The proportion (F/D) signifies the percentage in terms of number of blocks in which the ISGS has achieved a ramp rate of at least 1 per cent per minute, out of the total blocks where it is scheduled to ramp at more than 1 per cent per minute by an RLDC. Here, F is the number of blocks out of D in which the ISGS has achieved a ramp rate greater than or equal to 1 per cent per minute. As the actual generation is unlikely to exactly match the scheduled generation in each block, because of the inherent randomness of physical systems, a tolerance of 5 per cent in ramp rate is allowed while measuring the actual generation and counting each of the blocks F (out of D). This proportion is used to check for failure to meet the mandatory 1 per cent ramp rate, in which case the RoE is reduced by 0.25 per cent as per the Tariff Regulations.
- The AARR in the blocks where the scheduled ramp rate is 1 per cent per minute or more.
The AAAR in the blocks where the scheduled rate is 1 per cent per minute or greater (D) is expressed in per cent per minute.
The determination of addition or reduction in RoE is to be done at the end of each month of the financial year on a cumulative basis by the RLDCs, with a period of calculation “M” taken suitably in the methodology. Here, “M” is the number of months in the period of calculation. Furthermore, a statement of the summary of calculations is to be prepared by the respective RLDCs at the end of each month of the financial year on a cumulative basis, which will indicate the ramping performance evaluation of each coal/lignite-based ISGS based on the aforementioned metrics specified in the guidelines, along with recommendations for a change in the rate of RoE. Based on the ramping performance evaluation, each ISGS will bill or adjust the RoE percentage separately on a monthly basis from beneficiaries according to the revised rate of the RoE.
The determination of change in the RoE percentage will be done as follows:
- If Td/Tm is <0.85, the RoE will be reduced by 0.25 per cent
- If Td/Tm is ³0.85
- Check for additional RoE
– If the number of blocks D <60*M, no additional RoE
– If D is ³60*M and E/D is <0.75, no additional RoE
– If D is ³60*M and the proportion E/D is ³0.75, then
Additional RoE (per cent) = (Integer (AARR) –1)*0.25 per cent, subject to a ceiling of 1 per cent
- Check for reduction in RoE
– If the number of blocks D <90*M, no reduction in RoE
– If D is ³90*M and F/D is ³0.75, no reduction in RoE
– If D is ³90*M and F/D is <0.75, a reduction in RoE by 0.25 per cent
To conclude, the guidelines for assessing the ramping capabilities of thermal power plants are essential in the emerging power sector scenario and will go a long way in managing the flexible operation of thermal power plants and maintaining grid stability amid the increasing penetration of renewable energy sources.