OVERVIEW OF SOLAR POWER SYSTEMS
Introduction:
Harnessing of non-polluting renewable energy resources to control green house gases is receiving impetus from the government of India. The solar mission, which is part of the National Action Plan on Climate Change has been set up to promote the development and use of solar energy for power generation and other uses with the ultimate objective of making solar energy competitive with fossil-based energy options. The solar photovoltaic device systems for power generation had been deployed in the various parts in the country for electrification where the grid connectivity is either not feasible or not cost effective as also some times in conjunction with diesel based generating stations in isolated places and communication transmitters at remote locations. With the downward trend in the cost of solar energy and appreciation for the need for development of solar power, solar power projects have recently been implemented. A significant part of the large potential of solar energy in the country could be developed by promoting grid connected solar photovoltaic power systems of varying sizes as per the need and affordability coupled with ensuring adequate return on investment.
Solar Radiation is the crucial and undoubtedly the most important climatic measure for solar PV installation. Good solar profile leads to more generation.
Photovoltaic solar system use the light available from the sun to generate electricity and feed this into the main electricity grid or load as the case may be. The PV panels convert the light reaching them into DC power. PV modules have different technologies broadly naming Crystalline and Thin Film and have efficiency approximately of the order of 13-20 % and 12% respectively.
Power from PV modules is taken to inverter through DC cable. An interactive inverter is the obvious choice of the designer for most of the rooftop systems. An interactive inverter can manage two or more than two source of energy like-Solar, Grid, DG set and battery. However, availability of energy sources is very much depended on case to case. However, the basic functionality of the interactive inverter is quite easy to understand. First priority is always given to the solar system. This means, as long as solar PV system is generating electricity that shall be consumed on priority, if surplus electricity is available after meeting load requirement, that electricity shall be used to charge battery bank. And even now, if some power is left unused that shall be fed into the grid.
Second priority is generally given to grid. Grid power shall meet the load requirement and charge the batteries. Third and last priority is given to DG set, which will be used only if there is no solar and grid power available. Batteries shall also be getting charged through DG power.
Solar PV Panel:
Solar market considering some technological aspects, crystalline technology has two categories namely Mono-crystalline and Poly-crystalline. As the name indicates mono-crystalline cells are made up of single crystal structure and the poly crystalline cell are made up of multiple crystal structure. Efficiency of the mono-crystalline is higher than poly crystalline. In fact, it promises to give highest efficiency among all the available solar PV technologies.
Inverter:
Inverters are used for DC voltage to AC voltage conversion. Good inverter should have minimum value of Total Harmonic distortion (THD), so that output wave shape shall be closer to sine wave. Inverters connecting a PV system and the public grid are purposefully designed, allowing energy transfers to and from the public grid. For a solar PV plant there are three kinds of inverter available namely central inverter, String inverter and micro-inverter. Central inverter is generally available capacity of the order of hundreds of kW. On the other hand, string inverter is available in capacity of the order of few kW like 1 kW to 15 kW. Micro-inverters are used with every single module and their cost is very high.
An inverter has some more internal parameters like Master Slave, revolving master-slave, MPPT (maximum power point tracking), Multi-MPPT.
MPPT uses electronics and programming to vary the electrical operating point (usually the voltage) of the modules so that the modules are able to deliver maximum available power. This monitoring is informed by the knowledge that the Current (I)-Voltage (V) curve – also known as the IV curve – is knee shaped, the implication being that by slightly moving the voltage level a significantly higher current can be achieved. Additional power harvested from the modules is then made available to the conversion circuits, and ultimately results in more power going onto the grid/LT panel for captive use. Typically, in the field conditions, inverter efficiency is considered to be of the order of 96%.
Protection & Control:
i. It is proposed that the inverter will have islanding protection to isolate it from the grid power supply at the LT panel in case of no supply, under voltage and over voltage conditions so that in no case there is any chance of accident.
ii. The system will also have adequate rating fuses, fuses on inverter input side (DC) as well as output side (AC) side for overload and short circuit protection and disconnecting switches to isolate the DC and AC system for maintenances is needed.
iii. Fuses of adequate rating shall also be provided in each solar array module to protect them against short circuit
Monitoring Parameters:
Data logger system for the study of effect of various parameters on energy generated by the solar system is necessary. The critical parameters shall be measured, displayed and recorded/logged.
