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Opportunities & Complexities in Solar Power Forecasting

Industry Insights
Typography

Background:   Solar power is one of the fastest growing industries in India. The country's solar grid has a cumulative capacity of 9.01 GW as of 31 December 2016. In January 2015, the Indian government expanded its solar plans, targeting 100 Billion Dollar of investment and 100 GW of solar capacity, including 40 GW's directly from rooftop solar, by 2022.

The deployment of solar based electricity generation, especially in the form of photovoltaics (PV), has increased in recent years due to a wide range of factors such as concern about greenhouse gas emissions, government policies and lower equipment costs. There are a number of challenges to the reliable and efficient integration of solar energy. Chief among them will be the development of new tools and practices that manage the variability and uncertainty of solar power.

The Indian grid is currently the 5th largest in the world. Maintaining grid stability and power quality is a tedious task with its own legacy of issues. Variable generations from renewable energy such as wind and solar plants together are posing significant technical difficulties of grid management. Guarding the future projections of higher share of renewables, it is required to have a good forecast and appropriate balancing action.

Regulatory Provisions in Place

CERC Regulation

The Central Electricity Regulatory Commission published the Framework on Forecasting, Scheduling and Imbalance Handling for Variable Renewable Energy Sources (Wind and Solar) on 7th August, 2015. This framework is applicable for solar and wind generators that are regional entities, that is, their scheduling and settlement is handled by the respective Regional Load Dispatch Centre (RLDC).

 This framework envisages that bulk of wind and solar capacity expected to come online over the next 7 years, in alignment with Government’s target of 100 GW solar and 60   GW wind by 2022, shall be inter-state in nature. That is, these generating stations shall sell power within as well as outside the host state, and connect directly to the CTU grid. This will become essential as a few states are rich in solar and wind resources, whereas the Renewable Purchase Obligations (RPOs) shall ensure that the whole country takes advantage of renewable power, while marching towards the national goal of universal electrification.

The proposed regulations seek to address the grid integration aspects related to such wind and solar generators directly connected to the State grid. Given below is the tabular representation of forecasting regulations as issued by various State Electricity Regulatory Commissions (SERCs)

The Impact of Aggregation in Scheduling

The Law of Large Numbers: The renewable energy actually becomes more predictable as the number of renewable generators connected to the grid increases thanks to the effect of geographic diversity and the Law of Large Numbers. It is a probability theorem, which states that the aggregate result of a large number of uncertain processes becomes more predictable as the total number of processes increases. Applied to renewable energy, the Law of Large Numbers dictates that the combined output of every wind turbine and solar panel connected to the grid is far less volatile than the output of an individual generator.

Due to the aggregation effect, forecasts for geographically diverse aggregates of solar generation facilities have smaller errors than the forecasts for individual facilities in the aggregate.   Local effects, which are more random and more difficult to forecast, tend    to average away when the aggregated forecast is looked upon. With aggregation, the impact of forecast errors on individual plants is not as severe because the aggregate fore- cast of all plants drives the generation scheduling.

Technical Complexity & Opportunity

Factors that affect forecast performance include forecast time-horizon, local weather conditions, geographic scope, data availability (e.g., plant size, location, components), and data quality (e.g., consistency, accuracy, resolution). Solar energy is inherently only available during daylight hours, so the grid operator must adjust the day-ahead plan to include generators that can quickly adjust their power output to compensate for the rise and fall in solar generation.

In addition to daily fluctuations caused by sunrise and sunset, the output from solar panels can also change suddenly due to clouds. Variability caused by clouds can make it more difficult for the grid operator to predict how much additional electric generation will be required during the next hour of the day, so it becomes difficult to calculate exactly what the output of each generator should be.

CERC Definition of Error %

‘Absolute Error’ - is the absolute value of the error in the actual generation of wind or solar generators with reference to the scheduled generation and the 'Available Capacity' (AvC), as calculated using the following formula for each 15 minute time block.

We have graphically represented the error% calculated at generator level (individually) and the error% calculated on basis of aggregated scheduling given below:

Note: In the aggregated level forecast, the deviations lies within the CERC error band of +/- 15% for most of the time blocks, thus significantly reducing the overall impact on the generator’s deviation settlement charges.

Note: In the aggregated level forecast, the deviations lies within the CERC error band of +/- 15% for most of the time blocks, thus significantly reducing the overall impact on the generator’s deviation settlement charges.

Conclusion: Forecasting of renewable energy is increasingly becoming a firm regulatory requirement. A total of 9 states along with the Central Electricity Regulatory Commission have come up with the regulatory mandate of having all the wind and solar power pro- ducers to forecast power generation on day ahead, week ahead and intra-day basis. The Karnataka and Gujarat Electricity Regulatory Commissions (in its Draft Regulation), have also acknowledged the importance of larger geographical aggregation and have permit- ted the agencies providing scheduling, to aggregate multiple wind/solar power genera- tion to help achieve better accuracy which allows for a reduced financial burden of devia- tion settlement charges on the generators. The fast evolving regulatory framework on renewable energy forecasting is also an opportunity for all the wind and solar generators to get started with the forecasting exercise in advance and limit their regulatory liabilities, when the actual mechanism is implemented - which appears to be a very near term reality.

Credits - Ms. Varteka Tripathi & Mr. Vishal Pandya, REConnect Energy

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