Unified MPPT Controller for Partially Shaded Panels in a Photovoltaic Array

International Journal of Automation and Computing 11(5), October 2014, 536-542 DOI: 10.1007/s11633-014-0828-z Unified MPPT Controller for Partially Shaded Panels in a Photovoltaic Array R. Sridhar1 1 2 S. Jeevananthan2 S. S. Dash1 N. T. Selvan1 Department of Electrical and Electronics Engineering, SRM University Tamil Nadu 603203, India Department of Electrical and Electronics Engineering, Pondicherry Engineering College, Pondicherry 605014, India Abstract: The power output of the photovoltaic (PV) system having multiple arrays gets reduced to a great extent when it is partially shaded due to environmental hindrances. The maximum power trackers which are conventionally used may not be competent enough to find the maximum power point (MPP) during partially shaded conditions. The sensible reason for the failure of conventional trackers is during partial shaded conditions the PV arrays exhibit multi peak power curves, thereby making simple maximum power point tracking (MPPT) algorithms like perturb and observe (P&O) to get stuck with local maxima instead of capturing global maxima. Therefore, global search MPPT aided by evolutionary and swarm intelligence algorithms will be conducive to find global power point during partially shaded conditions. This work suggests a unified controller which feeds control signal to its power electronic conditioner placed at each module. The evolutionary algorithm which is taken into consideration in this work is differential evolution (DE). The performance of the proposed method is compared to the classical un-dimensional search controller and it is evident from the Matlab/Simulink results that the unified controller prevails over the distributed counterpart. Keywords: Maximum power point tracking (MPPT), solar energy, photovoltaic (PV), differential evolution (DE), solar power generation, evolutionary algorithm, soft computing. 1 Introduction The power demand in the whole world is increasing day by day and serious efforts have been made to meet the demand by increasing the power production[1] . The depleting nature of the conventional sources and the polluting contents in them is a great concern to deal with. In recent past, much emphasis has been given to use renewable sources like wind, solar, biomass etc., for power production. Among these renewable sources, solar photo voltaic system is the most promising one. Though there are many advantages in photovoltaic (PV), there are few potential barriers that hinder its growth. The two main barriers are low conversion efficiency and its inconsistent nature of power output[2−4] . Maximum power point tracking (MPPT) is a technique which is adopted to improve the conversion efficiency by making the PV system to operate at its maximum power[5−7] . When the PV panels are exposed to uniform insolation, it is rather easy for the MPPT tracker to find the peak power in non linear P-V curve as there will be a single peak power. There are numerous MPPT techniques available in market among them all the perturb and observe, incremental conductance etc., methods are quite familiar[8−12] . But these MPPT techniques bound to struggle when the panels are exposed to sun s irradiation inhomogenously. Several research articles have been archived on the impact of partial shading on PV panels and the failure of the conventional MPPT techniques during partial shading[13−15] . The core reason for the failure of mundane MPPT algorithms is during partial shading, PV array exhibits P-V curves having multiple power peaks. Researches expressed their innovativeness by suggesting soft Regular paper Manuscript received October 4, 2013; revised February 24, 2014 computing techniques based global search algorithm to find global maxima during partially shaded conditions. Among all the soft computing based global search algorithm, particle swarm optimization (PSO) has been dealt elaborately[16−21] . Another meta-heuristic algorithm differential evolution has also rendered its part in global maximum power point (GMPP) tracking[22−25] . Several other soft computing techniques like genetic algorithm (GA) and ant colony algorithm (ACO) have also been considered[23−25] . The conventional controllers are either distributed controllers or uni-dimensional type controllers which result in poor performance and more complex. To overcome the drawbacks of existing controllers, a unified MPPT controller which adopts multi dimensional differential evolution (DE) algorithm is proposed. The whole setup is realized in Matlab/Simulink environment. The paper has been arranged in the following manner, Section 2 gives analysis of PV system modeling and Section 3 deals with multidimensional DE MPPT. Results are analyzed in Section 4 and the conclusive part of the work is dealt in Section 5. 2 Characteristics of photovoltaic array 2.1 Uniform or homogenous insolation The circuit of a photovoltaic cell is represented in Fig. 1. The output PV current depends on the ambient temperature and solar irradiation. PV s output current is represented as:   q(Vpv +Ipv Rs ) −1 − Ipv =Iph − Io1 e AKb Tk (Vpv + Ipv Rs ) Rp (1) 537 R. Sridhar et al. / Unified MPPT Controller for Partially Shaded Panels in a Photovoltaic Array panel s characteristics whereas the fourth panel shows the array characteristics. The characteristics are simulated in Matlab/Simulink GUI environment assuming that the PV system is exposed to uniform irradiation. 2.2 Fig. 1 Electrical circuit model of a PV cell  q(Vpv +Ipv Rs )  Ns AKTk F (Ipv , Vpv , Tk , G) = Iph −Ipv −Io1 e −1 − (Vpv + Ipv Rs Ns ) =0 Rp Ns (2) where Ipv is output current, Iph denotes the generated PV current, I01 is the reverse saturation current, A is ideality factor, Rs is the series resistance and Rp is the parallel resistance, Tk is the temperature constant. Equation (1) indicates the output PV current for a single array. In order to have higher capacity, several PV arrays are to be connected in series or in parallel. Thus, the output of the entire module can be represented as shown in (2), where Ns is the number of series connected cells and Np is the number of parallel connected cells. The I-V & P-V characteristics of a PV array are shown in Fig. 2, where three individual panels are connected to form an array and the first three panels show the respective Fig. 2 Under partially shaded conditions The PV array is normally shaded by disturbances like branches of trees, passing clouds, poles and buildings, etc., which result in partial shading of PV systems as shown in Fig. 3. In a PV array, the cells configured in series will render constant current whereas the shaded cells will operate with a reverse bias resulting reverse power polarity leading to net power drop and thereby reducing the net power conversion efficiency[26] . Partially shaded module is shown in Fig. 4. The bypass diode paves the way for current when it is partially shaded and the shaded region will act as a reverse biasing circuit[26] (...truncated)