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Published:Tuesday, July 30, 2019 2:31 PM

Security Constrained Economic Dispatch in Power Sector – A Step Closer to Optimization

Security Constrained Economic Dispatch in Power Sector – A Step Closer to Optimization

The Ministry of Power (MoP) had issued guidelines on Flexibility in Generation and Scheduling of Thermal Power Stations. It aimed to utilize un-dispatched surplus of cheaper generating stations in order to reduce power generation cost. Likewise, Power System Operation Corporation Limited (POSOCO) proposed procedures for a pilot project and circulated a consultation paper on Security Constrained Economic Dispatch of ISGS PAN India that envisaged an optimization technique of power generation at inter-state level. SCED appears as an excellent mechanism to achieve the objectives laid down my Ministry of Power beside numerous advantages.
 
This article give emphasis on scope of optimization, objectives & constraints and framework of SCED model. Security Constrained Economic Dispatch highlights enhancing the scope of optimization of Inter State thermal generating station schedules in India. The optimization exercise promises to minimize the total generation cost while honoring the technical constraints of grid and power plants.
 
Scope of optimization
 
To achieve an optimal solution to minimize the overall production cost of power, thermal ISGS are been considered whose tariff is regulated by CERC and those which honors the existing decentralized generation scheduling procedure. Out of the total installed thermal capacity in India, about 56 GW capacity comes under Reserve Regulation Ancillary Services (RRAS) mechanism and the same has been considered for optimization exercise by POSOCO.
 
  • Generation Allocation Procedure
The process to allocate shares in central sector generating stations to beneficiary states is cumbersome. There are instances when many ISGS had as many as fifteen beneficiaries spread across India or number of states had a share of more than 40 ISGS in their portfolio. Implementation of SCED will solve this complex system by judicious allocation of generators.
 
  • Coordinated Decentralized Scheduling
The scheduling process in practice currently for ISGS by NLDC and RLDCs is also known as coordinated multilateral scheduling. This mode of scheduling has many challenges such as inflexible contracts, non-trackable unscheduled generation (URS) and inability to utilize URS as Ancillary Services, Secondary Control through AGC etc.
 
  • Regulatory Interventions
So far, considerable efforts have been made at policy and regulatory level with the sole objective to utilize cheaper generating resources. Firstly, CERC’s January 2010 order regarding temporary re-allocation in case of any URS in central sector station where the other sets of beneficiaries in same power station would be eligible to procure power without any cost by making a request to concerned RLDC. Secondly, URS allocations in inter-region was availed by token allocations from power plants of one region to beneficiaries in another region. This eased the situation pertaining to congestion in interregional transmission corridors. Thirdly, the tariff policy dated 28th January 2016 mentioned that beneficiaries who wish to surrender the procured capacity or have an intent for un-requisitioning the power would inform 24 hours in advance with quantum of power and duration for efficient utilization of un-requisitioned capacity from power plants. In this way the generator would be free to sell such URS in market and the profit would be shared 50:50 between generator and beneficiary that has timely informed the generator about un-requisitioned.
 
 
Objective and Constraints
 
It is quite difficult to overhaul the entire power scheduling process and implement the centralized unit commitment model in India because of existing coordinated multilateral scheduling model. However, few constraints and variables can be taken under consideration that would meet the ultimate objective of minimization of fuel charge or variable cost.
 
  • Available Transmission Capability (ATC)
It is the limit for physical transmission network and a measure that bridge the technical characteristics to commercial requirements associated with power transmission. Presently, unit commitment of generation for each region is prepared in a way that resulting schedule into regional flows shall not exceed the ATC limit. The same ideology should be honored while preparing optimal schedule.
 
  • Technical Minimum
As per CERC, the technical minimum for operation of thermal ISGS units is 55% of maximum continuous rating (MCR) loading. To minimize the total production cost, the generation must be scheduled from cheapest to costliest variable cost and in doing so the technical minimum has to be followed during optimal schedule.
 
  • Ramp Rate
Likewise, the optimal generator schedule would have to honor the Up and Down ramp rate limits too with respect to previous schedule. As a matter of fact, every thermal power plant has its own ramp-up and ramp-down limits.
 
Framework for Implementation of Economic Dispatch in India
 
There are some changes that needs to be incorporated in the existing framework. Some of these changes are deliberated below:
 
  • Gate Closure
The “Scheduling and Dispatch Code” under IEGC enables revision of schedules. But presence of large number of participants creates continuous requests for revisions in schedule. It makes the system cumbersome. It also poses problems is real time assessment of available spinning and non-spinning reserves available in the system. According to the Gate Closure concept, flexibility of revising the schedules is with market participants prior to gate closure and post gate closure the system operators will take over and prepare for the pre-determined delivery period.
 
  • Spinning (Hot) and Non-Spinning (Cold) Reserves
Recent trends have shown that spinning reserves are depleted during the evening peak hour. This situation necessitates non-spinning reserves to come on bar under such circumstances. As the time required for thermal units to come on bar is high, non-spinning reserves should be kept running at technical minimum levels. Overall an enforcement of reserves is required in Security Constrained Economic Dispatch.
 
  • Ramping Requirements
As per Technical Standards for Construction of Electrical Plants and Electrical Lines Regulations 2010, the generating units shall have a minimum rate of loading or unloading of 3% per minute above the control load (i.e. 50% MCR). The Indian Electricity Grid Code (IEGC) provides that a minimum of 1% ramp rate shall be provided by the generators.
 
  • Inter-Regional Scheduling
To implement SCED, it is essential to change the scheduling methodology from corridor wise scheduling to net-injection/ net-drawl for each region.
 
  • National Deviation Settlement (DSM) Pool
With change in Inter-Regional scheduling procedure, there is a need for implementation of a “National Deviation Settlement Pool”. This would avoid transfer of payments from one regional DSM pool to other DSM pool as practiced in existing corridor wise scheduling procedure. Creating a national deviation settlement pool will enable all regional DSM pools to interact with National DSM Pool only.
 
  • Accounting and Settlement
Implementation of SCED would create incremental changes in existing schedules which would need to be settled. Settlement of incremental changes would include variable charges declared for the purpose of ancillary services, direct payment for scheduled energy by beneficiaries to the generator, payment of variable charges from National DSM pool to generator on account of increment in the injection schedule of a generator due to optimization and refund of variable charges by a generator to the National DSM pool on account of any decrement.
 
Conclusion
 

Security constrained Economic Dispatch majorly talks about optimization of schedules of central sector power plants in India with an objective to minimize total production cost. The novice technique certainly requires changes in regulatory framework to obtain meaningful performance and outcome. However, given the coordinated multilateral scheduling model the opportunity for SCED is relatively meager in India. 

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