Characteristics of Solar PV Array under Partial Shaded Conditions R.Ramaprabha and B.L.Mathur Abstract:- Increasing prices of fossil fuels and awareness of global warming have drawn attention towards the use of Solar Photo Voltaic Cells as an alternative source of electric power. Experimental determination of voltage-current characteristic of a solar panel is required for its simulation and setting the parameters of the circuit for extracting maximum power. An attempt to determine such a characteristic by connecting a variable resistance across the panel and measuring the voltage and current by meters is not successful all the times due to varying cloud positions and temperature during the experiment. A method to quickly draw the characteristics and recording the result using electronic load has been presented in this paper. As a single cell produces voltage around 0.7V only, a number of cells are connected in series forming a Solar Photo Voltaic Array (SPVA) to produce voltage levels of practical use. For a practical SPVA spread over large area, it is difficult to maintain uniform insolation on all the cells at all times. Such a problem may arise due to passing clouds, shadow caused by neighboring buildings, trees etc. In this paper, characteristics of three solar panels receiving different insolations and connected in series have been drawn using fast electronic load. A method to add the characteristics of individual panels to obtain the combined characteristics has also been presented. Key Words: - Solar PV array, Partial shading, Electronic Load, PSpice software.
I. NOMENCLATURE G1, G2, G3 - Insolation at the first, second and the third panel respectively. IL1, IL2, IL3 -Short circuit currents of the first, second and the third panel respectively. D1, D2, D3 – Diodes used in the equivalent circuit. ID1, ID2, ID3 –Currents through the diodes D1, D2 and D3 respectively. Rse, Rsh -Series and shunt resistances of the equivalent circuit of a SPVA. RLoad – Load resistance in the circuit. Db1, Db2 and Db3 – Bypass diodes across first, second and the third panel respectively1 R.Ramaprabha, Senior Lecturer, Department of EEE, SSN College of Engineering, Rajiv Gandhi Road (OMR), Kalavakkam- 603 110, Chennai, Tamilnadu, India,.(e-mail:
[email protected]) Dr.B.L.Mathur, Professor & Head of EEE Department, SSN College of Engineering, Chennai (e-mail:
[email protected])
II.INTRODUCTION
R
ecent awareness to protect the global environment has emphasized the need for effective use of non-renewable energy sources. Solar photo voltaic produces clean energy and is currently considered to be an important energy source. A great deal of research has been conducted in this field over the last few decades. Solar PV panel is a power source having non linear internal resistance. The panel output power varies with temperature and insolation. It is desired to operate Solar Photo Voltaic (SPV) panel at its maximum power output for economic reasons. To extract maximum power from the panel, its internal resistance should be equal to the load resistance. Chopper circuit is interposed between SPV and the load resistance to adjust the load resistance seen by SPV equal to its internal resistance by varying the duty cycle of the chopper. A few among several maximum power point tracking methods available in the literature [1] are used in practice A major challenge in using a SPV source containing a number of cells in series is to deal with its non-linear internal resistance. The problem gets all the more complex when the array receives non-uniform insolation. Cells under shade absorb a large amount of electric power generated by cells receiving high insolation and convert it into heat. This heat may damage the low illuminated cells under certain conditions. To relieve the stress on shaded cells, bypass diodes are added across the modules. In such a case multiple peaks in power-voltage characteristics are observed under non uniform illumination. Classical Maximum Power Point Tracking (MPPT) methods are not effective due to their inability to discriminate between local and global maxima. Nevertheless, it is very important to understand the characteristics of SPV under partial shaded conditions to use PV installations effectively under all conditions. Analog models of SPV sources at varying temperature, insolation and partial shaded conditions were simulated using PSpice software. The developed model was validated with experimental results. To capture the characteristics under constant insolation and temperature conditions, the experimental characteristics were obtained using linear MOSFET as an electronic load. It was shown that, the combined characteristics can be obtained by graphically adding the characteristics of the individual panels.
III. CHARACTERISTICS OF SPVA UNDER PARIAL SHADED CONDITIONS The Voltage Vs Current characteristics of a PV module vary with solar insolation and temperature. Researchers have analog and mathematical models of SPV cells for varying insolation and temperature [2]-[7]. In this paper, a PSpice model of SPVA consisting of three SPV panels receiving non uniform insolation and temperature is presented. Characteristics of three panels connected in series are also obtained by graphically adding the characteristics of the individual panels. A single SPV module is modeled as a constant current source in parallel with a forward biased diode along with a series and a shunt resistance as shown in Fig.1. [8]. In the SPVA, if all the three panels are receiving the same insolation and temperature, there is a single equivalent circuit (Fig.1) can be used to represent the entire array. The current IL is proportional to the insolation received by the panel and it is also approximately equal to the short circuit current of the panel. Rsh is the slope of the I-V characteristics near short circuit and Rse is the slope of its I-V characteristics near open circuit (Fig.2). Parameters of diode D are set in the parameter model of the diode in the PSpice software. Parameters like number of cells in series and temperature are set. Simulated I-V and P-V characteristics are presented in Fig.2.
Fig.1 PSpice model of SPVA with uniform insolation.
Fig.3 shows PSpice simulation of three identical SPV having unequal insolation connected in series. Diodes Db1, Db2 and Db3 are bypass diodes. In the first case let us consider that the bypass diodes are absent. Let the insolation on the first panel be high, on the second panel be medium and on the third panel be low. It means IL1>IL2>IL3. Now if the current IL through RLoad is lower than IL3 then ID3=IL3-IL, ID2=IL2-IL and ID1=IL1-IL. Now let IL>IL3 but IL
