The Ministry of Power (MoP) has recently issued a draft proposal to expand the scope of security constrained economic despatch (SCED) to include more plants and run day-ahead operation of security constrained economic despatch (DA-SCED). The scope of national-level merit order scheduling under SCED is proposed to be expanded to include all regional entity thermal power plants (which can declare a compensation charge such as energy charge on a monthly basis) and intra-state thermal generators.
The DA-SCED schedules will provide a look-ahead visibility to the power stations regarding their expected schedule over 96 time-blocks. This would enable them to better manage ramping up or down in real time. The SCED run in real time (two-time blocks ahead) will further optimise the schedules after incorporating real-time market (RTM) results subject to security constraints. The unrequisitioned surplus power could be traded by the generators in the RTM and the ancillary market. Besides, the states can rebalance their portfolio through RTM.
The SCED pilot project has been operational since April 2019. Fifty regional thermal interstate generating stations (ISGSs) with a total capacity of 59,840 MW, whose tariff is determined or adopted by the Central Electricity Regulatory Commission, are included in SCED optimisation. An integrated software application has been developed in-house by Grid Controller of India Limited. It runs in real time to optimise the total variable cost of generation pan-India, while meeting technical and grid security constraints. The SCED pilot has resulted in the reduction of variable cost to around Rs 23 billion on a pan-Indian basis and these benefits are being shared with generating stations and their beneficiary states.
SCED helps in maintaining resource adequacy in an optimal manner, even in cases when the reserve situation after the day-ahead market (DAM) does not guarantee minimum reserves in the system. SCED has a transparent and simple settlement process for maintaining additional reserves, in line with the draft IEGC-2022. It builds upon the existing real-time SCED despatch that takes place on a block-by-block basis, running two blocks before the actual time of operation. The benefits obtained from the SCED process will be accumulated in the national SCED pool account. The benefits will be shared with generating stations and their beneficiary states.
Proposed operation of DA-SCED
In order to operationalise DA-SCED, the generating stations will submit their declared capability along with the ramp rate and minimum turndown level for the next day on D-1 at 0600 hours. The energy charge rate/compensation charge to be considered for SCED will be submitted by the generating stations on a monthly basis. The regional load despatch centres (RLDCs) will prepare the entitlements and declare the share of each beneficiary on D-1 by 0700 hours. The beneficiaries will submit their requisitions/schedules from the ISGS on D-1 by 0800 hours.
Based on station availability and schedules submitted by beneficiaries/procurers, RLDCs will prepare the injection and drawal schedules by 0930 hours. The first run of DA-SCED will be carried out on D-1 at 0945 hours before opening of the bidding window for DAM in the power exchanges. The look-ahead schedules obtained after the SCED run will be published on the scheduling portal. NLDC/ RLDCs will refer to the output of DA-SCED to assess the availability of reserves for the next day. The DAM will be cleared by 1300 hours and the power exchanges will convey the DAM results to the NLDC after the market has been cleared.
In order to operationalise the day-ahead ancillary services market (AS-DAM), the NLDC will determine the requirement of tertiary reserves to be procured on a time-block basis. The analysis obtained from the output of DA-SCED would be one of the inputs. The bids for tertiary reserve ancillary services (TRAS)-Up and TRAS-Down collected by the power exchanges will be consolidated and cleared by the NLDC by 1300 hours. The time blocks with total cleared megawatt quantum less than the reserve requirement will be identified along with the quantum of the shortfall. The shortfall in the quantum of reserves will be compared with the indicative reserves from the DA-SCED run.
With regard to the unit commitment to ensure adequate reserves, the injection and drawal schedules by the regional entities and the results of the DAM will be available with the NLDC by 1700 hours. The above inputs will be incorporated in the second run of DA-SCED at NLDC for 96 time blocks by 1730 hours. The available up and down reserves in the ISGS will be reassessed based on the SCED schedules for the next day and the committed reserve capacity available through DAM-AS. If the available reserves are greater than the quantum of the shortfall during all time blocks, no action is required to commit additional units. In case the available reserves are less, additional units will be instructed to come on bar. Additional up reserve will be committed from units under reserve shutdown by 1800 hours, in merit order of variable cost.
The newly committed units will be included in the third run of DA-SCED for 96 time blocks at the NLDC at 2200 hours. SCED will ensure that the stations committed for reserve adequacy are scheduled up to a minimum turndown level. To maintain the load generation balance, commensurate reduction will be done in generation from the on-bar generating stations under SCED stack, subject to technical constraints, following merit order. The resulting increase and decrease in schedule quantum would be marked and maintained under a separate head in the scheduling software.
With regard to settlement for DA-SCED, payments to and from the generating stations on account of SCED schedules will flow from the national SCED pool account. Payments for stations where incremental power is scheduled as well as for beneficiaries of those stations will be made to/from the deviation and ancillary services pool account to ensure resource adequacy. Any deployment of ancillary services from the additionally committed generating station under secondary reserves ancillary service or TRAS will be settled in accordance with the CERC Ancillary Services Regulations, 2022.
Key enablers required to expand SCED
One of the key enablers for DA-SCED is to operationalise a suitable regulatory mechanism under Clause 46 of Draft IEGC 2022 for streamlined implementation. A minimum of three months post notification of the revised IEGC will be needed for the execution of DA-SCED. In order to operationalise DA-SCED, a customised software will be required at NLDC that would be developed in-house like the software for the real-time SCED pilot. This software will be used to solve an optimisation problem consisting of the technical parameters and the merit order of energy charges/compensation charges. Furthermore, changes in hardware, interacting interface, web-based energy scheduling software and website/APIs, among other things, would be needed.
Apart from this, to implement SCED at the intra-state level, upgradation of the system and processes will be needed at the SLDCs and at the level of intra-state generators. This will include improving/streamlining the scheduling, metering, accounting, and settlement systems at the state level in line with the Scheduling, Accounting, Metering, and Settlement of Transactions in electricity (SAMAST) recommendations. In addition, SLDCs must adhere to the scheduling timelines that are similar to those of the interstate power plants. They should also undertake the automation of processes and systems to enable more efficient and rapid data exchange.
Net, net, the implementation of the SCED pilot has yielded positive results, leading to increased utilisation of available cheaper generation within and across regions. Similar results are expected in the future following the proposed expansion of the SCED scope.