A Dynamic Electrical Scheme for the optimal reconfiguration of PV modules under non-homogeneous solar irradiation Roberto Candela1a, Eleonora Riva Sanseverino1b, Pietro Romano1c, Marzia Cardinale1d, Domenico Musso1e 1
University of Palermo, viale delle Scienze, 90128, Italy
a
[email protected],
[email protected],
[email protected], d e
[email protected],
[email protected]
Keywords: mismatch for photovoltaic modules, optimization, reconfiguration
Abstract. This paper presents a strategy for the maximization of the output power of photovoltaic (PV) systems under non homogeneous solar irradiation by means of automatic reconfiguration of the PV arrays layout. The innovation of the proposed approach is the employment of a simple Dynamic Electrical Scheme (DES), allowing a large number of possible modules interconnection, to be installed between the PV generator and the inverter. The models of the PV generator and of the DES have been realized and simulated with Simulink (Dynamic System Simulation for MATLAB). The attained experimental results appear to be quite interesting in terms of the attainable benefit in power and thus energy terms. The limited calculation times of the reconfiguration algorithm allows the application of the DES for the real time adaptation of the configuration to the changing weather conditions or other causes of non-uniform solar irradiation. Moreover, the results confirm that, in case of non uniform solar irradiation, this approach allows to attain considerably much better results than those attainable with a static configuration. Introduction The advantages of photovoltaic power generation for specific applications are a lot. It is enough to think at the limited environmental impact produced by this technology. Furthermore, photovoltaic modules are usually the lowest cost method to supply electricity to remote, off-grid, low-power applications. The literature on the subject mostly deal with the problem of catching the maximum output power from these systems. Most papers attain the objective of maximizing output power by means of suitable conditioning systems that adapt the load to the generation system. This is carried out by efficient Maximum Power Point Tracking, MPPT, algorithms [1-6]. Several MPPT methods force the operating point to oscillate have been presented in the past few decades. The ideal operation is to determine the maximum power point (MPP) of the photovoltaic (PV) array directly rather than tracking it by using the active operation of trial and error, which causes undesirable oscillations around the MPP (perturbation and observation). Other authors have proposed the PV array or modules reconfiguration technique to improve the PV generation system’s performance [7-11]. The fact that a higher efficiency can be attained by reconfiguration was proved in [12],[13]. In [12] the issue of non homogeneous solar irradiation was considered, but only a few configurations were explored and no automatic optimization algorithm was implemented for larger systems. In [13] a Matlab simulation of an automatic array reconfiguration technique based on irradiance equalization is presented. However the simulated architecture would only allow a few possible connections. The system is designed to improve the performance of the PV generator only taking into account fixed obstacles. The problem of mismatching and of the reconfiguration is also deeply studied in [7], where the effects of non-uniform solar irradiation distribution on energy output of different interconnected configurations in photovoltaic (PV) arrays is analyzed. In the paper, a suitable model taking into consideration the effects of bypass diodes and the variation of the equivalent circuit parameters with respect to operating conditions is proposed. Finally, in [8] an evaluation of the irradiation losses due
to partial shadowing is given. In this paper, the authors propose a new methodology for the maximization of the output power generated by the PV system by means of a suitable reconfiguration of the modules. It is well known that the optimal configuration for a given number of shaded modules is the one in which they are all connected in parallel. In this case, however, the output currents can exceed the allowable limits of currents of the system [14, 15, 16]. On the other hand, it was experimented that, in case of non-homogeneous irradiance, the series-connection of modules with similar irradiance produces a generated power increase. Starting from these comments, a fully reconfigurable Dynamic Electrical Scheme (DES) for PV generators is here proposed. The DES basic layout is composed by a series-connection of parallel-connected modules. It is well known that the output power of a PV system firstly depends on the different irradiation level, module temperature and shading. In particular, it was considered the common case of different states of shading of the modules and an optimal reconfiguration algorithm has been applied to suitably enable the DES. The control system uses the solar irradiances to determine the sub-optimal modules configuration following the irradiance equalization principle in the rows of the PV generator [4], and also pursuing the aim of the smaller number of panels switching operations compared to the starting configuration. The reconfiguration system is thus composed of a main control module which implements the optimal reconfiguration algorithm and of a DES which allows the implementation of the solutions outputted by the algorithm. The control module will be implemented on a micro-controller and will thus be adaptable to the physical system controlled. In the following sections, first the DES is described, then the optimization algorithm is detailed and finally in the application section the relevant results are reported. Dynamic Electrical Reconfiguration Scheme The DES depicted in Figure 1 allows the reconfiguration in a single string of parallel-connected modules, as an example, is shown in Figure 2. It is worth noting that this PV generator topology is absolutely dynamic as regards both the number of modules it can connect in parallel in a row and the number of rows it can connect in series. It also allows the two extreme cases in which either all modules are parallel-connected (all modules stay in a row) or all modules are series-connected (each module stays in a row).
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Figure 1 Dynamic Electrical Scheme structure 1 1
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