Renewable energy plays a significant role in the actual electricity production due to their environmental friendly features, absence of CO2 during the production, and availability even in remote areas. Actually, there are six widely known renewable energy sources, which are solar, wind, hydraulic, biomass, geothermal and marine. Nevertheless, the most widely explored are wind and solar energy due to their abundance in large geographical areas on the earth 1 2 3 4. In this paper, we focus on exploring DC micro-grid using solar energy only. Solar energy is transformed to electrical energy by means of solar panels, which is based on semiconductor materials such as silicon. Due to the photovoltaic effect, free electrons and holes are produced at positive and negative junctions respectively, providing direct electricity 5.This later has non-linear voltage-current (IPV(VPV) and power-voltage (PPV(VPV) characteristics. Besides, it can be influenced by the atmospheric conditions of temperature and irradiance which reduces the energy produced 6.Thus a control technique is necessary to be applied for an optimal use of the power in all operation conditions. In fact, in order to harvest the maximum available power from the generators and ensure large lifetime of panels, a good choice of maximum power point regulator is necessary 7.
DC-DC converters are electronic devices that convert a DC voltage to another form of DC voltage by stepping it up or down depending on the system design. They may be divided into three large families, which are buck converters, boost converters and buck-boost converters. They differ in the way the magnetic elements are operated. The choice of the converter topology depends on the load requirement. For instance, buck converters are useful for installations that need a high current. For high voltage needs, boost converter is chosen. Nevertheless, if the installation needs to step-up and step-down voltage automatically, a buck-boost converter is necessary 8 9.In the PV installations, the duty cycle of these converters represents the relation between the input (V_pv) and output voltage (V_dc) of the DC-DC converter, which is acquired using Maximum Power Point Tracking algorithms (MPPT). The MPPT can be applied to both of these converters depending on the system design to correct the variations detected in the characteristics of solar panel 6. The cluster of panel, DC converter, load and controller constitute DC micro-grid.
In fact, the MPPT is required to operate the PV system at maximum power point (MPP) in order to increase its efficiency and protect the panel. Many MPPT algorithms have been studied and developed in the literature; they differ in the number of sensors used, complexity, dependence on the system parameters, the technique of implementation and cost 8 10. MPPT algorithms are classified in two categories: direct methods that do not require prior evaluation of the PV modules characteristics such as perturb and observe (P&O), Incremental conductance (Inc-Cond), Fuzzy logic based MPPT and neural networks. However, indirect methods depend on the PV generator parameters like fractional open circuit voltage and fractional short circuit current. These methods are well reviewed in 7 10 11. Both fuzzy logic control (FLC)and incremental conductance have shown high tracking precision and proved their efficiency compared to the others existing MPPT12 13. However these MPPT circuits do not use the variation of both the output and the input voltage to extract the maximum power from the PV curve under fast varying weather conditions. Accordingly, the present paper proposes a new MPPT algorithm to benefit from the advantages of the previous methods through the investigation of the relationship between the output voltage and PV voltage, and the variation on the power generated by the PV generator in order to improve the power quality and minimize faults during the transfer to the load. For this, a FLC is implemented to get the variation on the duty cycle and then an INC-Cond method is used to get the exact value of the duty cycle. The details of this method are given in Section IV.
The objects of developing this MPPT are:
Improving the response time so that the MPPT tracks maximum power faster.
When the sudden variations occur in irradiance, the controller response is faster to this change and the loss of power will be minimum.
Improve power quality by reducing ripple in the power yield
Get better energy conversion efficiency .
The rest of this article is divided as follows: firstly, we describe the system used, in section 1 and the design of micro-grid parameters is done in Section II. State of the art of the MPPT algorithms adopted to constitute the proposed controller is reported in Section III. The description of the proposed strategy is done in Section IV. Simulations results and comparisons are discussed in Section V. And conclusions are presented on Section VI.