Effective utilization of solar power for isolated water pumping system

Abstract

The imbalance between energy demand and the power generation has led the world to look for alternative solution called Renewable energy sources. Amongst the available energy sources, Photovoltaics (PV) has gained a lot of prominence in the present days due to its reliability and less maintenance. Therefore, the applications of PV have also become multitudiouos ranging from street lights, roof top lighting till satellites. However the major drawback are its high initial costs and low energy conversion efficiency. Therefore, one of the major focus of research lie in development of efficient power electronic converters and controllers. Due to the abundance of solar energy, and adequate requirement of water, research focus has been more on development of PV fed water pumping systems. Over the years of developments in the field of PV pump systems and power electronics technology, several topologies and control strategies have been developed. However, all these studies mentioned above assumes the PV to be receiving uniform irradiance. In an array consisting of a number of panels, it is very much sure that not all panels receive same irradiation. Often some panels are exposed to shades of buildings, clouds, etc., causing multiple peaks in the characteristics of PV array, and is termed as partial shading condition (PSC). Extracting maximum power (MPP) under such conditions becomes more challenging and complex. The non linear behavior of PV, power electronic converters and the load makes the analysis even more difficult. This study on the impact of PSC in PV systems is of high importance especially in the field of water pumping systems because PV fed water pumps are often employed in agricultural sectors where panels are exposed to soiling, shades of trees and clouds. In case of uniform shading, the MPP for varying irradiance occurs near the open circuit voltage region of the PV characteristics. In case of partial shading, the MPP occurs anywhere from short circuit current to open circuit voltage regions. The occurrence of MPP anywhere over such wide range is found to have considerable effects on water pump systems. In case of a two-stage conversion system with an intermediate DC-DC converter, the occurrence of MPP at constant current region will impose iiiproblems such as high voltage stress on MOSFET/IGBT switches due to a higher duty cycle. It may also cause transitions from CCM (continuous conduction mode) to DCM (discontinuous conduction mode) as a result the efficiency falls drastically due to which the pump may even fail to operate though sufficient power is available at the PV terminals. Therefore, this thesis presents the aforementioned effects of partial shading on PV fed motor pump systems. Since majority of the pump systems use induction motor, the pump system considered here for analysis is also an induction motor based. The thesis aims in studying the effects of PSC on double stage water pumping systems by developing mathematical models for PV, MPPT, DC–DC converter and induction motor system. Since the systems are non-linear, the equations governing the systems are also nonlinear and implicit by nature. Therefore, advanced numerical techniques are to be employed to converge at the solution. Therefore, importance is also given to the solution techniques of the developed mathematical equations. To understand, the effect of shading on the converters stability, small signal models are also developed and several interesting observations are reccorded. Finally, the effect of filters connected between the inverter and motor, to mitigate transient over voltages is studied and a cost effective solution is presented. The entire simulation is carried out in MATLAB and verified on a laboratory setup. The results from the mathematical model and experiments are found to agree with each other. In nutshell, this thesis addresses the key points of stand alone water pumping systems and can also be used as a computer aided tool to study the PV fed pumping systems and thus helping in effective utilization of PV power.