New Delhi: Coal-fired units cannot achieve the ramp rate they are financially measured against once they operate in the lower half of their range, the Central Electricity Authority (CEA) has concluded, and it has asked for the scheduling and settlement framework to be amended to reflect that. Without control-system retrofits or tuning, a unit held to a machine ramp rate of 2 percent per minute between 55 and 70 percent load will deliver an average of only about 0.8 percent per minute across a 15-minute scheduling block.
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Between 40 and 55 percent load, the report puts the achievable average at not more than 0.5 percent per minute, and between 70 and 100 percent load at not more than 1.2 percent per minute.
The benchmark those figures fall short of is 1 percent per minute. Under the Central Electricity Regulatory Commission's (CERC) tariff regulations, failure to achieve it cuts a station's rate of return on equity by 0.25 percentage points. The report states that restricting the machine rate in the 55 to 70 percent band "will expose the generator to financial implications arising from non-achievement of the prescribed ramp rate, in addition to the deviation related charges under the applicable DSM Regulations."
The report follows three meetings chaired by BC Mallick, Principal Chief Engineer-II, CEA, held on March 11, May 8 and June 18, and attended by NTPC Limited, Damodar Valley Corporation (DVC), Bharat Heavy Electricals Limited (BHEL), Rajasthan Rajya Vidyut Utpadan Nigam Limited, Adani Power Limited and Grid Controller of India Limited (Grid-India). It has been circulated to all generating companies, the National Load Despatch Centre (NLDC), the regional load despatch centres and the regional power committees. Surata Ram, Chief Engineer (Thermal Project Renovation and Modernisation), CEA, wrote in the covering letter that the report "consolidates the technical deliberations, key observations and recommendations emerging from these discussions," and is circulated for "wider dissemination and appropriate consideration by all thermal generating companies in their operational planning and future flexibility initiatives."
The report locates the source of equipment stress in the frequency and size of load changes rather than in the depth of the load itself. "Frequent, high-magnitude ramping — rather than low-load operation — is the primary cause of severe wear and tear in coal-fired power plants," it concludes. "Because coal units are increasingly being used for fine grid balancing, they ramp up and down constantly. When these load ramps occur consecutively, the unit lacks sufficient time to stabilise."
An immediate reversal makes it worse. "Furthermore, an immediate counter-ramp in the next scheduling block compounds this mechanical stress," the report states. Mallick noted during the third meeting that repeated high boiler ramp rates on the same machine primarily elevate the risk of tube leakage.
Ramp rate compliance is assessed on the change in average generation between two consecutive 15-minute blocks. Generators told the meetings that this measure does not capture how a boiler actually behaves, because output barely moves during the boiler response and stabilisation phases at either end of a ramp.
NTPC submitted that "to achieve average ramp rate of 1 percent per min in a time block, generators need to ramp at the rate of 2.5 to 3 percent per min during active ramping phase of around 5 to 7 minutes to compensate for negligible change in load during boiler response time and stabilisation time." All the other generating companies present said the same, putting the required machine rate at 2.5 percent per minute to deliver a 1 percent block average.
The report accepts the point and records where it leads. "To compensate the deviation in generation, utilities are going for higher quantum of load change in the same time block (even more than 3 percent per minute) to achieve the generation target/ avoid DSM," it states. "This results in actual load ramp rates exceeding recommended limits, putting additional stress on the thermal sets by surpassing their design conditions."
It also warns that the practice scales. In the 55 to 70 percent band, it says, "the resultant is often uncontrolled or uneven higher ramp rates between 0.5 percent to 2.5 percent." The conclusion is blunt: "This approach for ramping is technically undesirable, because when higher ramp rates, say 3 percent will be required, utilities would target 4 percent to 5 percent ramp rate."
Because a thermal unit cannot make a step change, the gap between scheduled and actual generation persists through every ramping block and closes only in a block with no ramp. That gap attracts charges under the deviation settlement mechanism (DSM). The report concludes that "generators may be exposed to DSM charges during ramping even when the unit is operating within its technical ramping capability."
A partial relief already exists, but only at the margin. Grid-India has treated a block-wise 0.5 percent per minute ramp rate as equivalent to 1 percent per minute in the first ramping block, which protects the return on equity. NTPC asked that the same treatment be extended to deviation charges. Grid-India informed the meeting that the relief has been considered only for the first ramping block and is not applicable to subsequent ramping blocks.
The report's central settlement proposal is a new category of scheduling block. "As thermal sets ramp up in diagonal manner, they cannot give a step ramp/ change in load," it says. "Therefore, while ramping up/down, a new concept called ramping block should be introduced. In such a block, the target load will be achieved at the end of the block & generation in the block will be about 50 percent less/ more than scheduled generation."
For settlement, it recommends multiplying the scheduled generation of a 15-minute block by 0.5 while a unit is ramping, so that under-generation on the way up and over-generation on the way down do not attract deviation charges. The report separately puts the inherent gap at the same order, stating that "there is a possibility of difference of fifty percent between schedule and actual generation, if steady ramp rate is applied."
It also recommends stable-load blocks between ramping blocks to let a unit settle before the next instruction.
The report asks that ramping duty be spread across the fleet rather than allocated by cost. "Avoid consecutive ramp-up and ramp-down of the same units or plants; instead, select a group of units for ramp-up and another group of units for ramp-down," it recommends. "Ramping schedules should be rotated among a pool of units or plants rather than assigned strictly according to the Merit Order." It adds that merit order despatch may be reviewed if necessary to adopt the recommendation.
BHEL stated at the third meeting that merit order despatch needs to change, "although, consumer can get relatively costlier power but O&M cost will be reduced and machine life will be extended." DVC said the machine should run sustainably rather than gaining DSM pool charges.
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NLDC set out a different position on several of the proposals. On rotation, it observed that "schedules are prepared on a plant-wise basis rather than unit-wise. Accordingly, if rotation of ramping among individual units is desired, the same may be managed by the respective generating stations."
On stabilisation, after Rajasthan Rajya Vidyut Utpadan Nigam pointed to the 30-minute stabilisation period mandated between two ramp steps in pre-commercial-operation tests under the grid code, NLDC said that clause applies only to those tests. It added that "neither the CEA Flexible Operation Regulations nor the CERC Regulations presently provide for any stabilization period while scheduling generating stations. If such a requirement is considered necessary, it may be introduced through an appropriate regulatory amendment."
NLDC's own suggested workaround was examined and rejected. It had proposed that a plant begin changing generation one block in advance so that its actual energy lands close to the scheduled energy in the ramping block. The report concludes that "this proactive ramping may result in excess or less generation (MWh) in the previous block, which could attract charges under the Deviation Settlement Mechanism, thus not recommended."
The two sides also gave different accounts of how much warning a plant gets. Generating companies said ramp instructions arrive 12 to 15 minutes before the block, and Adani Power put it at 8 to 10 minutes. NLDC said plants participating in security constrained economic despatch receive their final schedule 30 minutes before the start of the block, and those despatched under ancillary services 15 minutes before.
Beyond the scheduling changes, the report recommends shifting the balancing task itself. "Instead of using the thermal fleet for fine grid balancing, the same may be done from other sources, such as hydro power plants, gas power plants, pump storage plants, and, BESS," it states, referring to battery energy storage systems. It adds a recommendation for sufficient battery storage at specific locations.
BHEL told the second meeting that the additional load cycles being imposed on units come from grid stabilising mechanisms including automatic generation control, security constrained unit commitment and security constrained economic despatch. "Ultimately, these cycles are not part of flexible operation and are detrimental for thermal units," it said.
The report's closing recommendation is that it is "also recommended to carryout the requisite modification or changes in the respective regulatory framework to give effect to the recommendations."
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