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Case Study: Towards 25 years of carbon-free power

Case Study: Towards 25 years of carbon-free power

Lanco Solar recently integrated its 5 MW operating solar PV plant, out of 35 MW to be commissioned by end of 2011, to the grid in Gujarat. V Saibaba describes why this project fits into our landmark category.

After the Gujarat government invited proposals for setting up solar farms in the state, Lanco Infratech was allocated 35 MW in Phase I. The first 5 MW in Gujarat was commissioned in December 2010, feeding AC output to an 11/66 KV switchyard, which, in turn, is connected to GETCO Bhadrada 66/11kV substation. The entire power off-take is carried out by Gujarat Urja Vikas Nigam Limited (GUVNL) based on preferential tariff declared by the state. This photovoltaic (PV) farm is spread across 27 acre in Bhadrada, with a site perimeter of 2.8 kms.

Solar photovoltaic (PV) has 70-80 per cent of the global market share. The PV industry is further classified into two technology options, crystalline silicon and thin film. For the first 5 MW, Lanco Solar chose to go with crystalline silicon technology.

The major challenge in solar projects is obtaining adequate land acquisition, as the land requirement for solar projects is fairly large, around six acre per MW
for solar PV. To further reduce levellised cost of electricity (LCOE), economies of scale can be achieved by developing single-location large scale plant. However, acquiring single patch of large piece of land becomes harder.

Site selection is a critical consideration as well. The historical generation data for Indian sites is not readily available for past 30 years (minimum sample size required to fit in a probabilistic distribution curve) as the industry being nascent in India, there are not enough ground mounted insulation monitoring system. The satellite data recorded by Homer, NASA, and three-tier show a fair amount of deviation among them. To get more visibility on irradiation levels, Lanco Solar has installed its own weather mast insulation measurement system in different parts of the country to obtain real time insulation data trends.

The major challenge faced in the installation of solar PV plants is site preparation. Power is produced from PV panels aligned with high precision and accuracy to each other and mounted on fixed steel structures. Solar power generation is very sensitive to module alignment, uneven field or impact of external shading on the solar panels, so engineering and design stage play a vital role. Project Specification: There are several ways of mounting modules on structures, which could be fixed, seasonal tilt, or tracking. In seasonal tilt or tracking structures, modules track the sun, either seasonally (seasonal tilt) or continuously through the day(tracking), thus increasing the generation from the plant. However, selection of the type of mounting structure is based on trade off between incremental higher generation vis-à-vis higher cost in fabricating movable structures, along with the increase in technology risk associated with it.

For Lanco's first 5 MW plant in Bhadrada, fixed tilt mounting structures were chosen to install the panels. Each structure was large enough to place 24 modules on
it. The vast area of mounting structures, 908 in number, was made of hot dip galvanised steel, and the array layout was made with such accuracy and precision that there was no need for further realignment of modules once they were installed.

The multi crystalline modules, with a rated capacity of 230 Wp and an efficiency of 14.10 per cent generates DC output. All the modules are connected using DC cables, selection of which forms a very important part as well from safety point of view. The cables at the plant have been selected on the basis of their flexible installation and safety features. The cables are UV protected, halogen free, flame retardant, self extinguishing, and can withstand shortcircuit temperature of 250OC and operating temperatures up to 90OC for normal operation for 25 years.

The power plant is also equipped with adequate junction boxes, switch gears, fuses, other overload and lightning protection arrangements, etc. to comply with the best in class safety practices. The junction box contains bypass diodes that protect the module from reverse current during hours of darkness, shade or when covered by leaves or other debris. The junction box assembly houses the diodes and DC cable in the “removable” top cover of the housing to facilitate ease of field repair or replacement in the event of damage or wear. The top cover and base module are secured together with a locking mechanism to prevent accidental exposure of high-voltage contacts. The modules are linked in a photovoltaic array via fuses or circuit breakers and an EMC-filter (to filter out the harmonics generated during DC-to-AC conversion). The current in each string (fuse) can be measured separately. DC current being generated by modules is fed into a containerized inverter, which converts incoming DC power into AC power.

Containerised inverters come in compact form and have plug-and-play capability, which significantly reduces the project timeline, and generally because of their large capacity and in built transformer, reduce power losses as well. The container is connected to the grid via the internal medium-voltage switchgear. Four inverters of similar capacity, 1.25 MW each, have been arranged in a decentralised fashion across the solar power plant. The decentralised inverter arrangement is ideal as the site is in a Y-shaped layout, thus reducing significantly the cable losses and additional cable cost.

All the a All the activity (on the DC side or AC side) is monitored and controlled from the control room. The control room features HT-LT switchgears, auxiliary and emergency power system, power evacuation metering, on line provision of data transfer and a MS Windows 2000 based Human Machine Interface (HMI) SCADA system and a viewers' gallery. The SCADA system in solar power plant enables remote monitoring of string health, generation trends for each inverter on daily, weekly and monthly basis, alarm, and other standard real time remote controlling features.

Putting all this equipment together will result in 80 lakh units of carbon free electricity for 25 years.

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