The government has increased its focus on promoting energy sustainability and increasing energy access through clean, cheap and reliable sources. To this end, it has set a target of developing 175 GW of renewable energy capacity in the country by 2022. This includes 100 GW from solar, 60 GW from wind and 25 MW from small-hydro and biomass sources. To meet the 100 GW solar target, significant capacity is envisaged to be developed through solar parks. A total of 34 solar parks aggregating 20 GW (500-1,000 MW each) have been planned by the government.
In order to facilitate the integration of such large-scale solar generation, adequate transmission infrastructure must be in place. To this end, Power Grid Corporation of India Limited (Powergrid) has released a report, Green Energy Corridors (GEC)-II Part A, focusing on the development of evacuation infrastructure for the upcoming ultra mega solar power parks. It envisages integrating solar power with the grid while ensuring grid security and reliability, and facilitating the transfer of solar power to the load centres.
The GEC-II Part A report covers the plan for grid integration of solar power parks at the inter-state and intra-state levels. At the inter-state level, the government has identified 13 solar parks with a cumulative capacity of 9,220 MW. These will be spread across Gujarat, Madhya Pradesh, Andhra Pradesh, Karnataka, Rajasthan, Arunachal Pradesh and Himachal Pradesh. At the intra-state level, 21 solar parks with a cumulative capacity of 10,780 MW have been identified.
In order to facilitate the transfer of power from the envisaged ultra mega solar power parks, Powergrid has proposed interstate and intra-state transmission schemes. While the interstate transmission system is estimated to cost about Rs 80.41 billion, the intra-state transmission system for solar parks will be developed at an estimated cost of Rs 47.45 billion.
While developing a dedicated transmission system to facilitate solar power evacuation, certain key factors need to be considered. The foremost requirement is having facilities to balance renewable power, which, in turn, requires a flexible power system. Given the variable and uncertain nature of renewable power, the power system must be able to maintain supply security when rapid changes occur in generation and/or demand. Therefore, a suitable market mechanism needs to be developed for encouraging the participation of flexible reserves to meet short-and-medium-term power requirements in case of volatility in generation. Given that hydro generation and pumped storage hydro have a long gestation period, the report suggests that the existing hydro capacity, especially the reservoir type, must be solely allocated for balancing variable renewable generation.
The report also identifies forecasting and real-time monitoring of generation as important tools to address the variability and uncertainty aspect of grid integration of solar energy. State-of-the-art forecasting tools help the grid operator maintain the power system balance for economic, reliable and secure grid operations even in case of high renewable energy penetration. The establishment of renewable energy management centres (REMCs) for forecasting and real-time renewable energy monitoring has already been conceptualised under the earlier GEC scheme. Powergrid has proposed that the ultra mega solar parks should also be integrated with these REMCs for monitoring, scheduling and forecasting purposes.
Strategy framework for solar transmission development
Implementation strategy
Transmission systems for solar parks are typically categorised as internal and external. An external transmission system is developed by the central or the state transmission utility (CTU/STU), or the transmission licensee. It includes the setting up of 220/400 kV pooling stations contiguous to the solar park and 220 kV interconnections in the park as well as offtake transmission arrangements at the 220/400 kV level for grid integration. An internal transmission system includes the interconnection from the solar module level to the pooling station level through inverters. The development of an internal transmission facility along with communication, supervisory control and data acquisition (SCADA) and control infrastructure within the park typically comes under the purview of the developers or the implementing agency.
Since the gestation period of transmission projects is longer vis-à-vis solar generation projects, it is imperative that the transmission system is developed in time to avoid generation bottlenecks. For this, land for pooling stations for an external transmission system should be contiguous to the solar park and be handed over by the implementing agency to the CTU/licensee at the earliest. In addition, the implementing agency should immediately apply for the Grant of Connectivity and Long Term Access (LTA) licence as per the Central Electricity Regulatory Commission’s (CERC) regulations , since this is a prerequisite to start the implementation work.
Financing strategy
The capacity utilisation factor of renewable energy sources, particularly solar, is quite low at about 18-22 per cent as compared to 75-80 per cent for thermal generation. As a result, the per unit cost of renewable energy transmission is very high. It is estimated that the cost of setting up a transmission system for renewable energy generation is about double the cost for a conventional hydro-thermal mix generation. Further, the generation tariff for solar energy itself is higher than the average pooled power costs; therefore, consumers should not be burdened with higher transmission tariffs.
In order to rationalise the transmission tariff for solar generation, there is a need to develop transmission systems through soft concessional loans and partial grants. For intra-state system strengthening, 40 per cent grant can be secured from the National Clean Energy Fund, and 40 per cent concessional loans from multilateral funding agencies. As per the Ministry of New and Renewable Energy’s scheme for ultra mega solar power parks, a central financial assistance of Rs 2 million per MW is to be provided for the development of solar parks and for external transmission systems. This will be apportioned in the ratio of 60:40, that is, Rs 1.2 million per MW or 30 per cent of the project cost (whichever is lower) will be provided to the solar power park developers towards the development of solar parks and Rs 800,000 per MW or 30 per cent of the project cost (whichever is lower) will be provided to the CTU or the STU towards the development of the external transmission system. This scheme is expected to rationalise transmission tariffs to some extent.
Further, the funding for transmission schemes should be arranged through soft concessional loans from multilateral or bilateral funding agencies. At the same time, due to compressed time schedules for the development of transmission schemes, provisions should be devised for expeditious approval of loans/procurement, etc. from multilateral/bilateral funding agencies.
Challenges and solutions
The report identifies several challenges associated with the integration of ultra mega solar power parks into the grid and offers solutions for them. These are discussed below.
Different gestation periods
As mentioned earlier, the gestation period of solar projects is quite short as compared to that of transmission projects. As per the prevailing regulations, the interstate transmission system for generation projects is developed based on the long-term access (LTA)/connectivity application. However, considering the short gestation period of solar projects, the report proposes that transmission system development should be undertaken much ahead of generation without waiting for the LTA/connectivity application. Moreover, the location of generation projects and their capacity need to be firmed up in advance so that transmission planning can lead generation and its implementation can match solar generation development. Overall, the government should evolve a “transmission-leaing-generation” approach. In addition, a single-window clearance system for all regulatory approvals needs to be in place to speed up the process.
Establishment of REMCs
Currently, a decentralised approach towards forecasting is being practised in India, under which individual solar and wind energy generators are required to submit their schedules individually. However, centralised forecasting would be better suited in the case of solar parks as it offers several benefits such as a reduction in forecast errors due to aggregation and the use of advanced forecasting techniques. To leverage these benefits, REMCs need to be set up based on a control area/zonal aggregation concept for monitoring and forecasting of solar power generation.
Grid connectivity standards
While grid connectivity standards have been clearly defined for wind power generation by the Central Electricity Authority (CEA), this is not the case for solar energy. It is, therefore, essential to set grid connectivity standards for solar power generation given the government’s huge capacity addition targets. It is also prudent to introduce various technical standards for solar photovoltaic generation to make it grid friendly and to meet requirements such as voltage (fault) ride-through.
Data sharing by renewable energy generators
For centralised forecasting and scheduling, the availability of renewable energy data is of utmost importance. Forecast service providers need static, historical and real-time data from the SCADA system to generate accurate forecasts. However, most developers have a non-disclosure policy for data sharing, critically limiting the ability of forecast service providers. As such, there is a need to incorporate a mandatory data-sharing clause in the Indian Electricity Grid Code.
Forecasting and ancillary service regulations
A high renewable penetration scenario requires increased balancing and system flexibility. In such a scenario, ancillary services would be needed by the system operator to achieve system balancing in real time, which would ensure power system reliability and maintain the quality of electricity. While this has been operationalised at the central level, there is an urgent need to introduce regulations for ancillary services and renewable energy forecasting at the state level.
Research in forecasting technologies
In India, limited work has been carried out in the renewable energy forecasting domain. Forecasting rapid solar ramp rates is also garnering attention among electricity system operators in India but has not received significant attention from the research community. Thus, indigenous research needs to be promoted in this regard.
Roles and responsibilities of statutory bodies
Lastly, there is a need to clearly define the roles and responsibilities of all statutory bodies that are involved in the transmission system. For instance, the CEA should evolve technical standards for grid connectivity while the ministries and the state governments should release policies and regulations for transmission system development, put in place institutional arrangements for data sharing by developers, and take steps towards capacity building. The central and state electricity regulatory authorities should operationalise a framework for forecasting and ancillary services.
