Absorbing Variability

LDCs tackle operational complexities in solar power integration

While solar power capacity installation and generation has improved significantly, its integration into the existing grid system continues to be a challenge. In this regard, the role of the National Load Despatch Centre (NLDC) and state load despatch centres (SLDCs) becomes critical. They are responsible for facilitating the transfer of electricity across the country as well as to cross-border entities. These load despatch centres (LDCs) facilitate competitive wholesale power market trade and settlement.

The Indian grid receives power from 900 generating stations. The power is transported with the help of 7,000 substations, 3,100 transformers and transmission lines at the 765 kV, 400 kV and 220 kV levels. In May 2019, it catered to a peak demand of 182,610 MW and energy requirement of 4,145 MUs. Meanwhile, a peak hydro generation of 816 MUs was recorded in September 2019, a peak solar generation of 147 MUs in May 2019 and peak wind of 467 MUs in June 2019.

The variability, uncertainty and decentralised nature of solar power make its integration and despatch a challenge. The LDCs have evolved to enhance their systems to optimally absorb the power thus produced into the existing system. As the transmission network in the country is being strengthened, the NLDC and SLDCs will have to create innovative mechanisms that can manage the transfer of intermittent power as opposed to the steady coal and hydro power generation.

Generation variation

According to Alok Kumar, deputy general manager, Northern Regional Load Despatch Centre (NRLDC), Power System Operation Corporation (POSOCO), the Indian grid has a typical ramp rate of around 250 MW per minute, which may go up to 500 MW per minute on special days. The country needs about 148 GW of steady power for 25 per cent of the time to be able to meet the base demand. The ramping requirement in the morning and evening is usually met by about 32 GW of hydropower generation. With the integration of renewable energy into the grid, the thermal and hydro power units are being made increasingly flexible. Flexibility makes the power system agile to respond to any change in demand and supply. The services required for flexible and reliable system operation are ramping, part load operation, spinning reserves, energy storage, black start and multiple start-stop operations. During the high hydro period, 10-20 per cent of flexing is carried out while during low hydro generation months 30-35 per cent of flexing is required across the country.

S.L. Agrawal, executive engineer, SLDC, Gujarat Electricity Development Corporation (GETCO), suggests that the operational complexity of solar energy generation can be attributed to a high bell curve variation displayed during the day. On sunny days, Gujarat solar power plants can generate 1,329 MUs of electricity. On other days, peak generation may be to around 500 MUs, requiring high ramp-up and ramp-down rates during morning and evening operations.

The Indian grid faces hourly load changes of 12-15 GW. The plant load factor of thermal power plants has reduced significantly to around 60 per cent owing to increasing renewable energy generation, making grid balancing even more challenging. Considering that 160 GW of wind and solar power capacity is likely to be added by 2022, the balancing requirement will be even greater. The Central Electricity Authority has estimated the pan-Indian balancing requirement to be as high as 24,500 MW per hour by 2022.

Key challenges

The operational issues associated with solar power generation include variability in the daily generation pattern, unpredictability, high system inertia, steep ramping curve of net load, low voltage ride through (LVRT), lack of monitoring capability of distributed energy resources, and reactive power management. For system operators, it is important to balance the generation and demand, and thus forecasting assumes immense importance.

LVRT is also becoming a challenge owing to the many different makes of inverters being deployed in solar power plants. Two LVRT non-compliance events may be due to different reasons. In the past year, an event where the LVRT was not functioning led to the outage of solar generation in the Bikaner-Bhadla 400 kV circuit. The NLDC and solar power developers are trying to resolve LVRT issues at the regional power committee level.

Solar power plants are usually set up in remote areas with barren land where power demand is typically low. The power is transmitted to areas with a high demand load. Agrawal is of the view that the long transmission lines used to transfer power from remote generating areas to demand areas cause overvoltage in the grid during off-peak periods. Also, a solar plant is a static generator, which does not respond to grid frequency variations and produces power at a constant frequency or power factor. For system operators, this poses a challenge during high demand and supply variations as the solar generation system is unable to respond to quick changes in voltage.

For balancing the load variability, a high ramp-up and ramp-down facility is required, which is absent in the existing thermal generation units as most of them are fairly old. Ageing units have slow response to pickup and backdown requests made by system operators based on demand and supply variations. While coal accounts for the majority of the balancing load, gas is the other option for operators. Gas power plants have a high ramp-up capacity, but are low on the merit order owing to high gas prices and sensitivity to global gas markets. Hydropower also has a high ramp-up capacity but the seasonal availability during monsoons makes it a poor option for baseload operations. While pumped-hydro may be a good option, non-operating existing stations are unable to create a big difference.

Measures taken

Accurate forecasting and scheduling is important to optimally absorb solar power into the grid system on a day-ahead basis with an error margin of less than 4 per cent. Agrawal informs that the Gujarat SLDC has an in-house forecasting methodology and also relies on a forecasting service provider. Meanwhile, the scheduling for renewable energy pooling stations is software integrated for accounting with advanced technology. Real-time data connectivity is established between the generating station and GETCO with a mix of available technologies such as radio, GSM, lease line and fibre.

Various agencies, including the NLDC, have taken a series of initiatives to facilitate the integration of renewable energy into the grid. These include the development of green energy corridors to facilitate the integration of large-scale renewables, strengthening of the existing transmission network, dynamic reactive compensation, and inclusion of energy storage in tenders. The establishment of renewable energy management centres to enable forecasting of renewable generation and real-time monitoring is seen as a huge step towards ensuring smooth grid operations in light of increasing renewable energy penetration. Further, the automatic generation control initiative is expected to be implemented by February 2020 on interstate generation stations.

The way forward

The future of the Indian grid system will be marked by a higher degree of solar power penetration, requiring flexible generation, transmission and distribution systems. Also, flexible power markets will be needed to allow minute-based supply allocation. A policy and regulatory framework is required to incentivise generators for greater flexibilisation. Meanwhile, a standardised measure to assess the performance of the grid system needs to be put in place.

It is important to monitor and forecast the generation from distributed rooftop solar power systems to enable better grid coordination. Also, there is a need for primary and secondary response instruments in conventional generation, such as pumped storage solutions, to increase the ramp-up and ramp-down capabilities of plants.

Based on presentations by Alok Kumar, Deputy General Manager, NRLDC, POSOCO, and A.P. Shah and S.L. Agrawal, Executive Engineers, SLDC, GETCO at a Renewable Watch conference on “Solar Plant Performance”

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