Under the ambit of the Grid Code and regulatory purview, Power Generation Company needs to confirm on their generation capacity available and scheduled generation; Power Transmission Company needs to confirm on the availability of the transmission line; and similarly, Electricity Distribution Company needs to declare the power requirement for the coming 24 hrs. Such declarations are to be made for the next day (24 Hours, in 15 min slots). Thus, the entire volume of electricity being generated and demanded is balanced via the availability of the transmission corridor. Assuming adequate availability of the transmission lines, if generation exceeds the demand raised by the DISCOM, LDCs ask particular power plants to back down and likewise, if demand is more than the scheduled generation, LDCs ask DISCOMs either to procure power from the open market or to curtail consumption.
But there are certain instances wherein neither generation, nor consumption remains in the control of the respective authority. Such phenomena bring sudden variations in the grid frequency, which is very critical to be maintained. Grid frequency goes up when (i) power injected into the grid exceeds the demand, it; and (ii) demand suddenly lapses below the scheduled demand. On the other side, grid frequency goes down, when (i) grid consumption increases than the scheduled consumption; and (ii) generation lapses suddenly and so power injection reduces as compared to what was scheduled. Any deviation in the grid frequency invites a lot of troubles for the electrical and electronic appliances. Failure and mal-function of them result in the unwanted outcome. Such phenomena even caused grid outage in 2012. Grid outage (or power outage or blackout) causes huge economic losses to the stakeholders.
Below kind of happenings may contribute to the grid frequency imbalance:
- a sudden breakdown in the power plant,
- changes in the weather (causes unforeseen changes in wind and solar power generation),
- fault in the transmission system,
- delay in maintenance job completion,
- damage or theft in the distribution network, etc.
LDCs suggest any of below kinds of actions:
- when demand increases/generation decreases
- asks a unit or multiple units of power plants to start
- asks respective DISCOM (where such phenomena got observed) to curb the consumption
- when demand decreases / generation increases
- asks a unit or multiple of power plants to back down or run idle
- asks DISCOM to (where such phenomena got observed) divert the power to higher power consumption zones
In order to avoid such mishap and make the generation, transmission and distribution companies work optimally – it’s very important to penalize and incentivize those who cause the grid disturbed and those who help in balancing the grid, respectively. This gave birth to the “Availability Based Tariff” (ABT) that is a frequency based electricity pricing mechanism for the unscheduled power transactions. India adopted ABT mechanism in 2000, though restricted to the central power generation units (NTPC, NHPC, etc.). Later on, CERC made it mandatory for all, including the inter-state power systems.
From ABT perspective, it means readiness of the generating station to deliver ex-bus output, expressed as a % of its ex-bus rated capacity (MCR – Maximum Continuous Rating).
- It’s done for each day of 24 hrs starting from 00.00 hours be divided into 96 time-stamped blocks of 15 minutes each.
- Advanced declaration of its capacity for the generation in terms of MWh delivery ex-bus for each time block of the next day. In addition, the total ex-bus MWh which can actually be delivered during the day will also be declared in case of hydro stations. These shall constitute the basis of generation scheduling.
- While declaring the capability, the generator should ensure that the capability during peak hours is not less than that during other hours.
- The Scheduling as referred to above should be in accordance with the operating procedures in force.
- Advanced declaration of its power drawl in terms of MWh requirement for each of the receiving point-buses for each time block of the next day. Also, it needs to give total MWh drawl for each of the receiving point-buses. These shall constitute the basis for balancing the requirement with the generation schedule.
- Based on the above declaration, the Regional Load Dispatch Centre (RLDC) shall communicate to the various beneficiaries their respective shares of the available capacity.
- The schedule of actual generation shall be quantified on an ex-bus basis, whereas for beneficiaries (DISCOMs), scheduled drawals shall be quantified at their respective receiving points.
- For calculating the drawl schedule for beneficiaries, the transmission losses shall be apportioned in proportion to their drawls.
- In case of any forced outage of a unit, or in case of any transmission bottleneck, RLDC will revise the schedules. The revised schedules will become effective from the 4th time block, counting the time block in which the revision is advised by the generator, to be the 1st
- It is also permissible for the generators and the beneficiaries to revise their schedules during a day, but any such revisions shall be effective only from the 6th time block reckoned in the manner as already stated.
Largely we can summarise ABT mechanism as below:
- Brings in strict grid discipline towards responsible generation and consumption management, adhering the stakeholder to its commitment (following the merit order dispatch)
- Economically viable electricity
- Technologically and non-technologically – efficient player would only survive in a long run
- Ease of addressing the issues related grid disturbances and point out the culprits
- It requires special meters with features of remote data monitoring using open protocols
- Dynamic and customizable software that can handle the huge amount of calculations as per defined rules by the regulators
Further, ABT extends to the world of ABT metering, calculations, and related technologies.