Daily Times

At low penetration levels, renewable electricity generating systems do not pose any major problems for the grid, but their large-scale deployment, like what our government is presently considering (20 percent by 2025 and 30 percent by 2030), exerts considerable stress on the grid. That has both technical and financial implications. It is critical that our grid planners and operators recognise these potential implications and prepare the grid accordingly. Advance planning will certainly help in minimising any adverse effects of renewables on the grid, and deriving the maximum value from them.

Unlike conventional power plants for which, apart from cost and some locational issues, primary fuel supplies are virtually guaranteed. Renewable plants (solar and wind) are constrained, less by technology and more by the uncertain and variable nature of the primary source. Solar radiation, for instance, is available only during the daytime, and even during these periods, its availability is uncertain and variable. Similarly, even though wind is available 24 hours a day, but that availability is uncertain and variable too.

This intermittency and variability of renewable generating systems pose unique regulatory, technical, and economic challenges for the grid, each requiring careful study and response. The ensuing discussion, however, focuses only on the major technical issues associated with them.

Electricity, as an energy carrier, has some unique constraints. A major one is that it has to be produced and delivered the moment it is demanded. Its storage in any significant amount, if not impossible, is considerably difficult and expensive. Power system planners, therefore, use a number of techniques to ensure that electricity supply is always available at the beck and call of their consumers, both reliably and at a minimum cost.

The reliability is built into the system through two primary measures: adequacy and security. Adequacy is built in the system by ensuring that sufficient generating and transmission and distribution (T&D) capacities are always maintained in the system to serve consumer demand. Security of the system is ensured through proper design and configuration of the system to guard against loss of any major system element during power system operation.

Conventional power plants and T&D infrastructure that connects them are primarily designed to cater to only the technological issues associated with these plants. When renewable plants are added to the grid, two additional constraints come into play-uncertaintyin their availability and variation in their output even when these are available. For these reasons, electric utilities generally consider renewable plants as non-dispatchable because they cannot fully rely on these to serve the continuously varying consumer demand in the grid.

At small penetration levels in the grid, renewable plants do not pose any significant problems for grid operators as they are used to handling random variations in demand via a host of supply and demand management techniques. They treat output from renewable plants simply as a negative demand. After subtracting it from the overall consumer demand, what is left is served by them through traditional plants. The situation, however, changes when renewables’ share in the grid rises beyond a certain system-specific threshold.

The first major issue the system planners have to grapple with is what capacity value, if any, they should assign to these plants. Conventional wisdom suggests that owing to their intermittent and variable nature, renewables do not contribute any capacity value. Their benefit is restricted to only the cheap and sustainable energy they provide to the grid. This is not true, however, as numerous studies around the world have corroborated that, under favourable conditions, renewables indeed contribute a considerable capacity value to the grid, which should not be ignored. It may not be 100 percent but it is not zero either.

Capacity contribution of renewable plants is a complex issue, and involves considering of a host of factors relating to resource availability, technology, and demand patterns to figure it out. Our system planners will have to look into this aspect seriously as the nation cannot afford to ignore renewables’ capacity value as it will imply maintaining almost 13,000 MW of conventional capacity in the system just to back up the 30 percent renewable generating capacity that the government is contemplating by 2030. Financial consequences for such an approach would be devastating.

A critical enabling component of the new scheme (large-scale integration of renewable plants) will be the system and operational planning capability at the NTDC

On the operational side, the large-scale presence of renewable plants in the grid will impact three critical aspects of the grid: frequency regulation, ramping (load following), and cycling (startup/shutdown) capability, each having its own technical constraints and financial implications.

In the traditional grid, these duties are routinely performed by mostly fast response gas-fired power plants. Large-scale presence of renewable plants will necessitate additional duties from conventional plants for which these may not have the capability at present. Furthermore, the extra cycling and ramping will increase these plants’ wear and tear and maintenance requirements, consequently shortening their useful life. These capabilities will have to be provided through appropriate retrofits in the existing plants or through new plants, which could be difficult as well as costly.

The very low operating cost of renewable plants will dictate that these be placed at top of the merit order to derive economic benefits from renewable electricity whenever it is available. As such, renewable plants will be competing with base load coal, nuclear, and similar plants, necessitating curtailment in the latter’s output. This may not be always possible due to technical constraints. Besides, part-load operation of these conventional plants will deteriorate their efficiency and increase their costs.

Renewable resource rich areas and corridors will probably be located far away from major load centres, thus requiring new T&D networks to connect these plants to the main grid. The T&D networks will need to be augmented and strengthened in terms of their capacity and capability to make a successful integration and operation of renewable generation possible. This will necessitate deployment of advanced telecommunication and control technologies in the grid to continuously ensure its integrity, stability, and security.

An effective and beneficial deployment, integration, and operation of renewable plants will also demand a thorough re-assessment of the present T&D grid codes and associated procedures and protocols (including the planning criteria) to make them suitable for these newer technologies. Similarly, the capacity at the national and regional power control centres will need to be enhanced to equip them with the requisite expertise and tools to accurately forecast renewable plants’ availability, and blend them accordingly in their dispatch portfolios.

A critical enabling component of the new scheme (large-scale integration of renewable plants) will be the system and operational planning capability at the National Transmission &Despatch Company (NTDC). The staff there, at present, may not be sufficiently trained, or may not have the requisite tools to study the complex technical and economic aspects of large-scale deployment, integration, and operation of renewable power plants in the grid. Their proper training and equipping with the necessary tools to handle these newer generation technologies and projects is an investment that will provide invaluable payoffs to the nation in the years to come, and as such must be pursued on top priority.

The writer is a freelance consultant specialising in sustainable energy and power system planning and development