With India gearing up to meet its 175 GW renewable energy target by 2022, the share of renewables in the total energy demand is expected to increase to 21 per cent. While shifting to renewable energy generation is necessary for the decarbonisation of the economy, it poses huge challenges to the grid due to the infirm nature of renewable sources. Thus, the large scale integration of renewables into the grid without destabilising it requires the urgent implementation of grid balancing mechanisms. For efficient grid operations, it is necessary to know how much energy will be generated by each generating plant throughout the day. This is fairly simple for conventional energy plants where the generation is controlled by the plant operator. For solar and wind plants, it is more challenging to determine generation owing to the intermittent availability of resources. Wind can suddenly stop blowing and lower the generation output of a wind power plant or clouds can decrease the generation from a solar power plant. This is where forecasting techniques become important. With accurate forecasting, energy generation from solar and wind sources is scheduled for every specified interval of time (normally 15 minutes), and grid operators accordingly manage the available generation capacity.
Renewable energy sources face uncertainty in terms of generation, and hence, reserves are needed to manage them. There are three levels of responses to manage the uncertainty – primary, secondary and tertiary. Primary response is an automatic response, which is carried out locally by measuring the frequency in seconds. The secondary response is carried out after getting a signal from a central control centre and takes a few minutes. Tertiary response is a higher level of reliability service and takes about 15 minutes. Primary, secondary and tertiary response mechanisms are continuously evolving in systems. Renewable energy plants in India had no secondary control for several decades, but now automatic generation controls have become common to cater to the larger balancing requirements. Tertiary response requires energy reserves to address the scheduling and forecasting errors.
Ancillary services utilise energy reserves, that is, the available generation capacity that is not being utilised. It also encompasses how and when a reserve is triggered, performance assessment of the reserve and financial accounting for that reserve. In ancillary services, merit order despatch is followed, which means the cheapest source is used first. Since every generator can only despatch a specific capacity, the declared capacity of a generator, energy charges and rate of discharge also need to be taken into consideration for ancillary services. For every trade and transaction a record is maintained. It is like a financial interbanking transaction and settlement system, which keeps track of every purchase made in 15-minute blocks. This includes record of capacity charge, energy charge, transmission charge, reactive charge, deviation charge and other surcharges. For proper grid management, fast response ancillary services are required. The generator needs to be incentivised for sudden ramping up or ramping down of generation. Currently, incentives are provided only for increasing generation.
Essential reliability services
The word “ancillary services” has been replaced with “essential reliability services” or ERS. Ancillary means extra, which does not explain the absolute necessity of these services. For instance, a vertically integrated utility operator has full control over energy supply as the quantum of generation depends on its despatch instruction, and ancillary, security and reliability related transactions are all done at one place. However, with multiple generators using different energy sources, transmission systems and distribution grids, there are many contracts involved with multiple energy and financial transactions. Hence, ERS becomes important in real–time grid operations. ERS is not planned in advance unlike power procurement, which is decided one month in advance, or even earlier. Realtime ERS helps bridge the gap between the planned and actual demand and supply.
As the country’s grid prepares to integrate a large quantum of infirm renewable power, the industry, policymakers and grid operators need to bring renewables at par with conventional energy. It is true that renewable energy has constraints, but so does conventional energy. Moreover, solar and wind power have crossed grid parity and become cheaper than thermal power. They have now become mature technologies. Though infirm, renewable energy plants use the latest technologies and generate faster responses. Going ahead, for the sustainable development of renewable energy capacity in the country, assuring the balance of grid through ERS is of utmost importance. n
Based on the remarks by S.K. Soonee, Adviser, Power Systems Operation Corporation at a recent Renewable Watch conference