By Ashay Abbhi
The transmission network is the largest casualty of renewable energy integration. Given the mandate to generate and procure a greater amount of renewable energy, states face multiple challenges, in terms of renewable penetration and transmission, owing to limited evacuation capacity. This has also led to the curtailment of renewable energy plants, which have a must-run status. Thus, to overcome the intermittency and variability issues associated with renewable generation, the grid network has to be strengthened significantly. In this regard, the state transmission companies are taking significant measures to create a robust state transmission grid network.
According to B.B. Mehta, chief engineer, state load despatch centre (SLDC), Gujarat Energy Transmission Corporation (GETCO), network planning is one of the critical components of grid strengthening and expansion. This is done by taking into account the existing operational conditions (generation, transmission and distribution), the expected improvements and the future expansion plans. It also includes the identification of new transmission network elements and upgradation of existing infrastructure in order to provide quality power supply to consumers. Meanwhile, it is important to understand the future system requirements to provide adequate transmission capacity during outage contingencies in any network element. This will help determine the investment requirement and the timeline for network strengthening. The primary purpose of strengthening the core grid network is to optimally utilise the existing infrastructure, reduce transmission losses and ensure the seamless integration of renewable power.
The key issues pertain to the transmission network and grid operations as per S.B. Chandrashekharaiah, executive engineer, supervisory control and data acquisition (SCADA), SLDC, Karnataka Power Transmission Corporation Limited (KPTCL). The expansion of transmission infrastructure needs to be undertaken in parallel with renewable capacity addition to ensure smooth integration. For this, it is important to understand the growth pattern and determine a date of completion of mega renewable energy projects. The key factors that need to be considered in transmission network planning are load demand, variations in renewable energy injection, path of load flow, reactive power and voltage issues along the transmission line.
Grid operations pertain to the management of variable renewable energy generation. For instance, to manage the unpredictable wind generation pattern, or the effect of cloud packets on solar generation, it is important to determine the ramp-up and ramp-down rates. Therefore, fast-governable generation commensurate with the additional renewable energy capacity is a necessity for effective grid management.
The objective of transmission planning is to enable the smooth integration of the upcoming renewable energy into the grid network. To this end, it is critical to identify the type, location and phase of the new transmission system. According to Mehta, these can be determined through power system studies that analyse the load forecast, capacity addition programmes of the state, central and private sectors, and expected import from and export to the interstate transmission system (ISTS) network. The adequacy criteria for transmission networks have to be determined station-wise. The key feature of a new transmission system is its scalability. This system is effective in the long term, and also open for development in stages for optimal utilisation.
The technical requirements of transmission system planning pertain to redundancy and contingency criteria, radial feeders, load growth and nature of load, reactive power management, power quality, the downstream network (which is dependent on geographical and socio-economic conditions), load diversity and seasonal variations in the demand pattern for renewable energy integration.
Keeping in view the key factors, GETCO has devised a comprehensive transmission network plan for 2018-22. The network planning is divided into three broad categories – transmission lines, substations and transformation capacity, across four voltage classes of 400 kV, 220 kV, 132 kV and 66 kV. GETCO’s cumulative line length stands at 61,056 ckt. km as of 2017-18. The 400 kV lines account for 9 per cent of the total length (5,497 ckt. km), 220 kV for 31.9 per cent, 132 kV for 8.9 per cent and 66 kV for about 50 per cent. During 2018-22, GETCO is planning to add another 3,800 ckt. km to the 400 kV voltage class, 4,500 ckt. km to 220 kV, 250 ckt. km to 132 kV and 4,800 ckt. km to 66 kV. Meanwhile, 322 substations across the four voltages have been planned. This will take the total number of substations from 1,869 in 2017-18 to 2,191 in 2021-22. About 90 per cent, or 266 substations, have been planned for the 66 kV voltage class. GETCO’s total transformation capacity in Gujarat stood at 110,047 MVA as of 2017-18. Of this, 46.4 per cent of the capacity was at 66 kV voltage, 30 per cent at 220 kV, 15.2 per cent at 400 kV and 8.4 per cent at 132 kV.
Around 37,460 MVA is expected to be added across all voltage categories over the next four years to take the total to 147,507 MVA. Much of the planned capacity, that is, 17,280 MVA, or 46 per cent, is expected to come up in the 220 kV voltage class. Another 11,000 MVA (about 30 per cent) will be added in the 400 kV category.
Under the Green Energy Corridors (GEC) scheme, two 400 kV and four 220 kV substations have been planned along with 860 ckt. km of 400 kV lines, 1,000 ckt. km of 220 kV lines, and 50 ckt. km of 132 kV lines, as part of Phase I of the intra-state transmission network for renewable energy integration. Phase I is expected to be completed by 2021-22. Under Phase II, three 400 kV and seven 220 kV substations have been planned. In addition, about 2,160 ckt. km of 400 kV and 2,700 ckt. km of 220 kV transmission lines have been envisaged for this phase.
Apart from this, GETCO will undertake a system study based on the anticipated renewable capacity addition and other network topology parameters during the given time frame. According to Mehta, grid strengthening and expansion are primarily based on the operational feedback and real-time constraints in grid operation. The system study is necessary to determine the best location, and the optimal capacity of the proposed substations and new transmission lines. It is also important to prioritise the investment requirement. In addition, the study helps determine the optimal size and locations of capacitors and rectors to ensure proper voltage control and minimise transmission losses. With the study, transmission network congestion can be avoided.
Telangana has an installed solar power capacity of about 3,613 MW as of February 2019, which is one of the highest in the country. The state will add another 180 MW by the end of 2018-19. Telangana has planned to reach the target solar capacity of 5,000 MW by 2020-21. Besides solar, the state has 100 MW of wind, 13.66 MW of mini hydro, 74.2 MW of bagasse, 67 MW of biomass and 25.1 MW of industrial and municipal waste power capacity.
Telangana has been at the forefront of promoting the distributed renewable energy model in the country. When the land acquired for setting up a 500 MW solar park in Mahaboobnagar district was cancelled by the high court, the state set up the capacity through the distributed solar generation model. According to T. Jagath Reddy, director, transmission, Telangana State Transmission Corporation (TSTransco), the high solar capacity growth in the state has largely been achieved on the back of the distributed generation model. The benefits of such a model include local generation, and the absorption of solar power by agricultural and other loads. Meanwhile, it helps save nearly Rs 400 million per year with injection at the 33 kV level, and about Rs 5 billion in system strengthening investments. Also, since developers have already declared their choice of substation for solar power injection, it helps plan the transmission system better and avoid any delays in commissioning. It is estimated that distributed generation helps save around 122 MUs of energy annually and reduces transmission losses by 1.64 per cent.
Telangana’s transmission network has 4,320 ckt. km of lines at the 400 kV voltage level, 7,564 ckt. km at 220 kV and 11,089 ckt. km at the 132 kV level as of January 2019. The state has plans of adding 1,874 ckt. km at the 400 kV level, 1,208 ckt. km at 220 kV and 1,783.8 ckt. km at 132 kV by 2023-24. Meanwhile, it has 17 substations at 400 kV, 81 substations at 220 kV and 233 substations at the 132 kV level. The state plans to add another 64 substations by 2023-24, of which four substations will be implemented at the 400 kV level, 17 at 220 kV and 43 at the 132 kV level. Its transformation capacity currently stands at 14,390 MVA, 20,063 MVA and 27,765 MVA at the 400 kV, 220 kV and 132 kV levels respectively. TSTransco plans to add 5,140 MVA at the 400 kV voltage level, 3,970 MVA at 220 kV and 3,731 MVA at 132 kV. A total investment of Rs 87.3 billion has been earmarked for strengthening the entire transmission network in the state over the next four years.
According to Chandrashekharaiah, under Phase I of the GEC intra-state transmission scheme, five substations have been planned to be implemented at various voltage levels. Along with this, 342 ckt. km of transmission lines will be laid across all voltages. Built at a cost of Rs 12 billion, these transmission lines will facilitate the evacuation of an additional 850 MW of wind, 150 MW of solar and 205 MW of mini- hydro power in the state. In Phase II, five substations will be added with 816 ckt. km of transmission lines. This will help evacuate the existing 200 MW of renewable energy along with 714 MW of wind and 140 MW of solar power. Phase II is expected to be built at around Rs 14.5 billion.
Transmission availability for power evacuation has been one of the biggest challenges of renewable energy integration into the country’s grid network. It has resulted in the curtailment of renewable energy plants, despite their must-run status. It has also led to the cancellation of auctions, thereby delaying capacity addition. Also, with the lack of transmission capacity, the risk profile of new renewable energy projects becomes high, resulting in a higher cost of capital and wary investors. Given that renewable energy capacity will be continuously added to the country’s power sector, strengthening of the grid assumes high priority.
The GEC project as well as initiatives by renewable-rich states such as Gujarat, Karnataka and Telangana will enable greater evacuation of power from renewable energy projects. Meanwhile, a robust grid network will be more resilient towards the variable and intermittent nature of renewable power. For the smooth integration of the upcoming renewable energy capacity, the plans of the state and central governments must be in sync with the pace of capacity addition to avoid stranded capacity or power curtailment.
Based on presentations by B.B. Mehta, Chief Engineer, SLDC, GETCO; S.B. Chandrashekharaiah, Executive Engineer, SCADA, SLDC, KPTCL; and T. Jagath Reddy, Director, Transmission, TSTransco, at the Renewable Watch conference “Evacuation and Integration of Renewable Energy”