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Solar Today Cover Story | October 2018

Distributed Generation for Solar PV

One of the key challenges that India faces today in the installation of GW-scale solar projects in the construction of transmission lines to dispense power at diverse locations of the country. This approach of system planning referred to as LCPV (Large Centralised Solar PV generation) is driven by the old benchmarks which are meant for large thermal or hydro projects

Possible solutions to address the issues are being discussed at various forums these days. One such discussion in on the Distributed Solar PV Generation (DGPV). The strengths and limitations of LCPV and DGPV needs to be analysed to reach a reasonable and optimal solution. Analysis of these approaches requires understanding economies of scale of DGPV as compared to LCPV which requires analysis of factors like planning, execution and operational ease. The need of the hour is to look for sustainability and predictive algorithm that precisely measures the trade-off between LCPV and DGPV for the renewable sector.

LCPV is a legacy approach and was suitable for the thermal power projects which had PLFs (Plant Load Factors) varying from 60-90 per cent. As compared to this, solar PV plants are much smaller in design and size. Any solar PV plant is primarily designed with generation load factors varying from 17-20 per cent of installed PV capacity. The only gain is in administrative cost which is less than 1 per cent of the operating cost.

DGPV is not a new approach of system planning in India and growth in the solar sector since 2010 has been driven on this philosophy. It has matured over a period and is presently at par with conventional sources of power. DGPV approach is primarily driven by the establishment of infrastructure contiguous to the consumers.

A properly planned and operated DGPV can offer the consumers and the society significant benefits. These include economic savings and improved environmental performance. DGPV can reduce the demand for traditional utility ancillary services and does not involve extra-electrical elements whuch are required for managing electrical grids. A typical infrastructure mandatory for DGPV is the transmission line, varying from 5-20 km at sub-transmission voltage level.

Central Generation or LCPV approach is based on power generation by a centrally situated plant or a group of plants that provide power in bulk. Historically, large fossil-fired coal boilers or nuclear boilers or very large hydro plants were the source of power. These plants required massive investments and management of large transmission infrastructures. Following this, LCPV plants in solar are being developed at Badhla in Rajasthan and Pavagada in Karnataka. The capex incurred for the evacuation infrastructure for 4,000 MW solar capacity in Bhadla is approximately Rs 20 billion cumulatively for PGCIL, STU and Solar Park Development Agency on Transmission infrastructure.

Under the DGPV approach of planning, generating systems located at geographically distant places but in the vicinity of consumers. This scheme curtails losses and the cost of transmission and distribution largely. Cost of transmission of power for LCPV makes it greatly non-economical in case new generating station is introduced nearby. The typical utilisation of any solar PV is 20-22 per cent. If we add another renewable source like wind turbines to make the plant hybrid, the utilisation factor may increase to 35 per cent to its best. The utilisation of transmission infrastructure in conventional energy sources ranges from 60-90 per cent. Thus, the same transmission network may be employed for transmission of non-intermittent renewable in DGPV scheme. This is because DGPV system does not need a bulk network for transmission; except a radial connection as system management and maintenance is very simple and limited. Further introduction of the hybrid scheme in conjunction with integration of storage technology, DGPV can create small islands of mini and micro grids thus, increasing the availability of power system.

The network security shall also be better with DGPV mechanism. A wide range of control mechanism for future electricity market like arbitrage, peak support, peak shaving, etc. shall be more feasible at lower budget. A simpler system of voltage control at radial distribution scheme improves this economy further. The LCPV system does not facilitate any support to the local voltage control and security system, entailing separate electrical elements for this functions which incur additional budget for sub-transmission and transmission system.

Driving growth
India is blessed with abundant sunshine across most of the states which is a primary requirement of solar PV plants. In India, solar radiation varies between 1,650 to 2,000 kwh/m2/yr that translates into variation of 18 per cent in generation across the country.

The government is actively promoting policies for the construction of multi-GW solar parks and all the forthcoming tenders are reflective of the same. We believe that this approach is not appropriate and needs to be re-examined. Developing small-mid size capacity plants across the country is healthier for equitable development, along with the creation of better employment. As per estimates, 100 MW solar power project generates direct employment of 2,570 with numerous indirect jobs. Hence, a more decentralised approach towards solar is desirable from an economic perspective.

The construction of solar parks requires massive land. (1 GW solar park will need 5,000 acre of land). The acquisition of such large land at a single location is not easy and invariably ends up in disputes with farmers who lose their livelihood. The risk of project delay due to land can be significant. Quantum of land required for 100-250 MW projects is relatively less, thus, limiting challenges for land acquisition and impediments in execution.

Losses are higher for LCPV plants
The lower capacity utilisation factor for solar plants, ranging from 20-22 per cent results in the inefficient and prodigal building of transmission lines, which would have been avoided by using existing networks. The LCPV approach promotes plants located away from the load centre which leads to the construction of dedicated transmission network resulting in sub-optimal utilisation of resources and time. The low utilisation will ultimately translate to end users paying 4-5 times the charges for transmission infrastructure as compared to thermal power plant or if the plant was located near the load centre.

The evacuation of power over long distance leads to higher electrical losses of 3-5 per cent, ultimately borne by end consumers. In contrast, if generation capacity is restricted to 100-250 MW and is installed near to the load centre, losses reduce substantially. Sample Case Studies: 1,000 MW centralised generation Vs 1,000 MW distributed generation:

Western Region
A case study was developed wherein 1,000 MW capacity solar projects were evenly distributed in the state of Gujarat, Haryana, Madhya Pradesh, Maharashtra and Uttar Pradesh with an approximate project size of 60-70 MW capacity.

For decentralised systems, average CUF of 18.05 per cent which if translated in terms of energy (kWh) is equivalent to 1,582 MUs. This energy, when delivered at STUs at 132 KV level considering 15 km of transmission line, will result into total losses of 0.538 per cent which is 9.207 MUs. Considering average tariff of Rs 2.9 per kWh, the total transmission losses will be Rs 0.24 million. In comparison to the above, if a similar plant of 1,000 MW at single location (LCPV) and at an extremely good radiation zone, say in Bhadla, Rajasthan will result in better energy yield of 19.3 per cent CUF which if translated in terms of energy (kWh) is equivalent to 1,690 MUs yearly. This energy will be collected at a medium voltage (33 kV, 66 kV) and stepped up to 400 kV at site by CTU substation and transmitted to the load centres located in Gujarat, Haryana, Madhya Pradesh, Maharashtra and Uttar Pradesh and considering the same to be delivered at 132 kV STU bus will have cumulative transmission losses of 130 MUs. Considering average tariff of Rs 2.62 per kWh, the total transmission losses will be Rs 3.41 million.

The incremental transmission losses because of the location with better irradiance will be equivalent to Rs 3.16 million. In per unit terms, in Bhadla (LCPV system) transmission charges are Rs 1.31/kWh as compared to only Rs 0.19 paisa/kWh in case of decentralised system. The benefit of higher CUF gets negated completely because of transmission charges and losses.

Southern Region
A similar case study was done for 1,000 MW solar projects were evenly distributed in the southern states of Tamil Nadu, Andhra Pradesh and Karnataka with an approximate project size of 60-70 MW capacity. This will result an average CUF of 18.12 per cent which if translated in terms of energy (kWh) is equivalent to 1,587 MUs. This energy when delivered at STUs at 110 KV level considering 15 km of transmission line, will result into total losses of 0.694 per cent which is 11.01 MUs. Considering average tariff of Rs 2.70 per kWh, the total transmission losses will be Rs 0.297 million. If bulk generation of 1,000 MW at single location (LCPV) of good radiation like Pavagada, in Karnataka is considered, it will result in better energy yield of 18.38 per cent CUF which if translated in terms of energy (kWh) is equivalent to 1,610 MUs yearly.

Considering the average tariff of Rs 2.70 per kWh, the total transmission losses and PoC charges will be approximately Rs 1.40 per kWh as compared to only Rs 0.20/kWh for decentralised STU connected plant.

For any business and market to survive, it is essential to examine the options. All the options must be carefully examined in terms of cost, technical feasibility, ease of execution and operation. From our case study, it is quite clear that the large-scale Central PV Plants (LGPV), the cost of power at sub-transmission level is significantly higher than the same power when delivered at sub-transmission level by DGPV. Further, the EHV transmission system and voltage management requires complex switching systems and intricate grid operation for grid operators. The policymakers should take into cognisance that the future of grids is mini and micro grids with storage and islanding operations and one should move in that direction.

"The policymakers should take into cognisance that the future of grids is mini and micro grids with storage and islanding operations, and one should move in that direction."

Authored by DK Saxena, Senior Executive Vice President (Electrical); Deepesh Gupta, Deputy General Manager (Electrical); and Vipul Singhal, Manager (Solar DC), Avaada Energy.

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