Design and Realization of a Discretized Photovoltaic System

In this paper, we propose an analysis of the structure and functioning in the Pspice simulation and experiment of a discretized photovoltaic (PV) system. This system is formed by two DC/DC Boost converter connected in series. Its role is to transfer electrical energy from two PV generators through two adaptation stages (upper and lower stages). We proposed a circuit control of the power switch of the DC/DC converter, specifically, the upper stage. The results show a good agreement between simulation and experience of electrical values (voltage, current, power) of each block of the PV system. The good performance of each stage (> 90%) and the complete PV system show that this architecture can provide an innovative solution in terms of reliability and performance improvement chain PV conversion.


Introduction
The future of the photovoltaic (PV) industry through lower costs of electricity generation and optimization of production is needed. These two issues require high performance PV systems with a very high efficiency. The classic PV systems (one or more PV panels with a single stage of adaptation provided with the MPPT command [1][2][3][4]) have a satisfactory efficiency; however, searching the maximum power point is more complex and requires a broader search to avoid operation around a false maximum power point (MPP). This could occur when there is an inhomogeneous sunlight or a dysfunction of the panels [5]. In addition, if one of the adaptation stages fails all the energy produced will be lost. To overcome this problem, the discretization of the PV panels, where each panel has its own adaptation stage seems an advantageous solution in order to increase reliability and the electrical production PV [1][2][3][4][5][6][7][8][9][10][11].
The works made in the literature show well the interest of the discretized systems realization for the autonomous installations [6][7][8][9][10][11] as well as for the injection on the distribution network [5]. Every PV panel (or field of PV panel) possesses its own adaptation system (Converter DC/DC, MPPT commands). However, we find few or no extensive studies on the realization and the functioning of a complete PV system formed by PV panels, DC/DC converters and commands of maximization of the powers supplied by panels (MPPT commands) [1][2][3][4]. This is due to a mutual dependence of the functioning of adaptation stages, in relation to each other, and particularly to control the power switch of each DC/DC converter of each stage [5][6][7][8][9][10][11]. In this context, in order to clarify and better understand the discretized systems functioning, we propose the study, in depth; the distributed architecture consists of two PV panels, where each panel has its own stage adaptation. Each stage consists of a DC-DC Boost (or buck) converter controlled by an MPPT command.
In this paper, we study the feasibility of such a discretized PV system in Pspice simulator and experiment. We present the results of the functioning of the complete PV system with two adaptation stages. Particular attention will be attached to the design of a power switch command circuit of Boost's DC/DC converters of every stage. The feasibility of the PV system will be deduced by analysing the efficiency of each stage and the complete discretized PV system. Figure 1 represents the synoptic diagram of the discretized photovoltaic system (PV), constituted by two adaptation stages. Every stage is constituted by:

Structure and Functioning of the Discretized PV System in Two Stages
• PV panels, in monocrystalline silicon, formed by 36 cells in series (Fig.2) [1][2][3][4][12][13]. As shown in the Figure 2, a PV cell is formed by the current generator I CC (short-circuit current), the diode (D), the shunt resistance (R Sh ), and series resistance (R S ). The current of the diode depends on the technological Where: VD: voltage at diode terminals, IS (T): saturation current, q: charge of the free electron, KB: Boltzmann constant. From the comparison of the results of simulations to those provided by the manufacturer, we have deduced various parameters from the diode and PV cell (R S and R Sh ), and dependence of the short-circuit current (I CC ) with solar radiation (Le (W/m 2 )) [12][13]. In Figure 3, we have represented the typical experimental and simulated power-voltage characteristics according to the illumination. These results show that the PV panel can provide in standard test conditions (STC) a power of 55 W, a current of 4.2 A and a voltage of 13V.
• A load which can be batteries or variable resistance.
In this last case, we fixed the values of the resistance superior to the optimal resistance of the PV panel for a given illumination and temperature [1][2][3][4]. From these equations, we can deduce the overall functioning of the PV system by the relations: In the discretized systems, the problem posed is the control of the upper stage switch since it has a reference voltage (Vref = Vs1) floating and dependent on the lower stage. To do this, we have developed a circuit coupled with the classic Driver to control the opening and the closure of the switch on the upper stage as a function of illumination and throughout the day of the functioning of the PV system. It should be noted that all components of control circuits and control are biased by the 12 V battery of the PV system. • A manual MPPT command ( Figure 5) formed by an oscillator which generates a saw tooth signal frequency of 10 kHz [2], a variable DC voltage generator and a comparator which generates a PWM signal of variable duty cycle (α) by comparing the saw tooth signal and the DC voltage [2]. In this work, we have adopted this type of control to analyze in depth the operation of two DC/DC converters in continuous operation and ensure proper functioning of the overall discretized PV system.

Functioning of DC / DC converters
We implemented the PV system of Figure 1 in the Pspice simulator for an illumination of 700 W/m², temperature of 30°C, a load of 50 Ω and duty cycle α1 = α2 = α= 0.5. Then, we plotted in Figure 6 the different signals of each DC/DC converter of the PV system. These results show that the functioning depends on the command of the switches of every DC/DC converter: • switch (MOSFET) of the lower stage has a source connected to ground. It is controlled by the manual MPPT command which generates a signal of frequency of 10 kHz, amplitude of 10 V and duty cycle of 0.5 ( Figure 6. A1). However, the source of the switch of the upper stage is floating. The output of the MPPT command is not enough to control the switch. To take account of this floating source, we inserted between the switch and the manual MPPT command a circuit that takes into account the floating source and improves the shape and the amplitude of the PWM signal controlling the switch. Under the conditions of our simulations, the shape of the PWM signal generated by this circuit is shown in Figure 6. A21 (it varies from 0 V to 35 V). This is more than enough to control the opening and closing switch the top stage (Vgs varies from -30V and 5V) ( Figure 6. A22). The overall results in the PSpice simulator shows both the validation of relations 2-5 and the good functioning of each component of the two converters in series when their switches are controlled by manual MPPT command and the circuit of shaping. Thus, we propose that this circuit is essential for discretized PV systems of several stages.

Influence of Load and Duty Cycle of the PWM Signal
To better understand the overall functioning of the PV system in Figure 1, we analyzed in the Pspice simulator the electrical quantities (voltage, current, power and efficiency) of the system as a function of load and duty cycle of the PWM signal. Typical results are shown in Figure 7 as a function: -The load for illumination of 1000 W/m2 and a temperature of 25 ° C, -The duty cycle of the PWM signal to an illumination of 1000 W/m2 and a load of 50 Ω. These results show a functioning depending on load and duty cycle: • The electrical input and output quantities (voltage, current, power and efficiency) of the two converters is practically identical. The floating voltage of the converter of the high stage does not exceed 35 V and the power switch of this stage is correctly controlled, in the opening and closing independently in the variations of load and duty cycle. • The output voltage of the overall system is the sum of output voltages of each converter, • The output current of the overall system is the current flowing to the output of each converter, • The efficiency of the global system (about 95%) is the same as those of the two converters (upper and lower). All results in this paragraph show the good functioning of each DC/DC converter and the global PV system. These results are checked for illumination between 300 W/m² and 1000 W/m². For each variation (load and duty cycle), the circuit of shaping played well its role. It takes into account the floating voltage (Vref) and generates a signal that accurately controls the switch on the upper stage. The global power generated by the two panels is supplied to the load via the two DC/DC converters in series with a high efficiency (above 95%).

Experimental Procedure
The bench of measure which allows us to validate all the results obtained in the previous paragraph is represented in figure 8 [1][2][3][4]. This bench, completely automated, is constituted: • Four PV modules oriented south of 42 ° according to the horizontal axis, • A meteorological station (pyranometer and temperature sensor) to accurately track the values of irradiance and temperature, • A multimeter (Keithley 2700) connected to a computer for data acquisition and real time trace of various electric quantities of the PV system (voltage, current, power, efficiency ). The DC/DC Boost converter, the MPPT commands, the circuit of shaping of the signal PWM and the system of polarization of the various active components are represented in figure 9.

Functioning of Converters
We realized and characterized the complete PV system in Figure 9 when the illumination is 700 W/m², the temperature of 30 °C, the load is a resistor of 50 Ω and manual MPPT command. The MPPT command was designed to generate a PWM signal of 10 kHz, and duty cycle of 0.5. Since, according to the simulation results, the output voltage of each DC/DC converter is of the order of 30 V, then we set the parameters of the circuit shaping of the high stage so that the PWM signal has amplitude of the order of 35 V. This amplitude is largely sufficient to control the switch of the DC/DC converter of this stage in the closure and in the opening. To confirm this and the simulation results in Pspice, we noted the different signals DC/DC converters of the PV system (Figure 1) To validate the functioning of the system, we analyze in the following paragraph the functioning of the complete PV system according to load and to duty cycle.

Functioning of the Global System
We analysed the functioning of our PV system ( Figure 9) as a function of load and duty cycle of PWM signal to an illumination of about 700W/m² and a temperature of 30 °C. We set the parameters of the circuit shaping of the upper stage ( Figure 1) to properly take into account the value of the floating voltage (Vref of about 30 V) and properly control the power switch of this stage at the closing and opening. The different electrical quantities (voltage, current, power and efficiency), experimental and simulated obtained are represented in Figure 11. These plots show: • a very good agreement between experiment and simulation in Pspice, • the output voltage of the PV system is equal to the sum of the voltages of each converter, • The output current of the PV system is the output current of each converter, • The power supplied by two panels is that one practically restored to the load through two inverters in series. During this transfer, the efficiency of every converter and on the discretized system is very satisfactory (Upper to 90 %). All the experimental results obtained in this paragraph shows the good functioning of discretized system and circuit shaping of the PWM signal that controls the power switch of the upper stage. Also, the good efficiency of the discretized PV system (> 90%), particularly the lower and upper DC/DC converter, shows the efficiency of the PV system designed and its use in PV systems. This allows overcoming the well problem of shading and the failure of PV cells and PV panels.

Conclusion
In this paper, we studied the feasibility of a discretized photovoltaic system (PV), constituted by two stages of adaptation. Every stage is formed by DC/DC boost converter controlled by a Manual MPPT command and an adequate circuit to control the opening-closing power switches of every DC/DC converter. The results obtained in the Pspice simulator and experimental show: • Good agreement in simulation and experience of the electrical quantities of each block of the PV system, • The good functioning of each stage of adaptation and complete PV system. • A good efficiency of each stage and particularly the complete system (greater than 90%). All these results allow us to conclude that the discretization of the PV installations is an advantageous solution with the aim of optimization and of increase of the PV electric production. So, it allows solving the problem of the efficiencies degradation of the PV installations during shades and failures of PV cells and panels.