Abstract
Due to nonlinear behavior of power production of photovoltaic (PV) systems, it is necessary to apply the maximum power point tracking (MPPT) techniques to generate the maximum power. The conventional MPPT methods do not function properly in rapidly changing atmospheric conditions. In this study, a fuzzy logic controller (FLC) optimized by a combination of particle swarm optimization (PSO) and genetic algorithm (GA) is proposed to obtain the maximum power point (MPP). The proposed FLC uses the ratio of power variations to voltage variations and the derivative of power variations to voltage variations as inputs and uses the duty cycle as the output. The range of changes in fuzzy membership functions and fuzzy rules are proposed as an optimization problem optimized by the PSO-GA. The proposed design is validated for MPPT of a PV system using MATLAB/Simulink software. The results indicate a better performance of the proposed FLC compared to the common methods.
ACCORDING to the U.S. Department of Energy, the demand for electricity is expected to increase 30% by 2035 as a result of new consumption models, e.g. smart plug-in electric vehicles and smart homes [
Among all the renewable energy resources, solar power is worldwide fastest-growing energy source, which is renewable and clean with affordable availability [
There are two main limiting issues associated with PV systems, i.e., high installation cost and low energy conversion efficiency [
The perturbation and observation (P&O) method is a typical technique used for MPPT due to its simplicity and easy implementation [
Recently, the design of a suitable controller for MPPT has attracted lots of attention [
The application of artificial intelligence techniques for MPPT such as neural networks [
Heuristic methods such as genetic algorithm (GA) [
To overcome the above-mentioned problems, this paper proposes a combination of PSO and GA (PSO-GA) to optimize the fuzzy system of MPPT controller. This combination covers the weaknesses of each individual algorithm, leads to optimized parameters in FLCs, and improves the speed and accuracy of the system. The main scheme is to optimize the shape, membership functions and fuzzy rules of fuzzy method using PSO-GA. The GA is used to find an approximate solution using mutation and crossover operators. The PSO is used to reach the exact solution. The performance of the proposed MPPT controller is compared with P&O method, INC method, FLC-GA, and FLC-PSO with rapid changes of radiation and temperature. The proposed method can reduce the steady-state oscillation and increase the response speed and accuracy compared with other methods.
The rest of this paper is organized as follows. The PV system model is described in Section II. Section III describes different MPPT methods such as P&O, INC, and FLC. The proposed PSO-GA fuzzy controller is explained in Section IV. The simulation results are included in Section V followed by the conclusion in Section VI.
A PV cell is modelled using the single diode equivalent circuit. In this model, the open-circuit voltage and short-circuit current are considered as two important parameters. The short-circuit current depends on the irradiance, while the open-circuit voltage is affected by the type of cell material and temperature. More details about single-diode equivalent circuit of PV cell and its equations can be found in [
Figures

Fig. 1 I-V and P-V characteristics in variable irradiances. (a) I-V characteristics. (b) P-V characteristics.

Fig. 2 I-V and P-V characteristics in variable temperatures. (a) I-V characteristics. (b) P-V characteristics.
Different MPPT methods such as P&O, INC and fuzzy systems are used to track the MPP of PV system. A brief overview of those methods is presented in this section.
The P&O method compares the previously delivered power with the one after disturbance by periodically varying the voltage of panel to reduce the oscillation around the MPP [
The basis of this method is to measure the derivative of PV output power with its voltage [
(1) |
where , , and are the output power, voltage, and current of MPP, respectively.At the MPP where the slope of curve (dP/dV) is zero, differentiating (1) with respect to the voltage, we can obtain:
(2) |
(3) |
Therefore, the basic equations of this method can be written as:
(4) |
(5) |
(6) |
The FLC consists of three main parts. The first part is a fuzzy maker that converts input variables which contain true values into a fuzzy or linguistic set. The second part is the fuzzy inference that combines if-then fuzzy rules based on the principles of fuzzy logic. In the third part, the fuzzy variables are converted back to real values by using defuzzification layer to apply them to the main control system.
Unlike conventional MPPT controllers, intelligent controllers such as FLCs are robust with sudden atmospheric changes. In this section, the MPPT controller is optimized using the fuzzy system and PSO-GA. The proposed controller is an off-line controller and the computation costs are not investigated. The design phase of this controller includes fuzzy design because we want to set the duty cycle (D) of DC-DC converter using fuzzy rules. For this purpose, the fuzzy inputs and outputs must be defined and their definition scope and membership functions must be determined. In this paper, the ratio of power variations to voltage variations and the variation ratio of power variations to voltage variations (the derivative of the first one) are considered as the inputs of the fuzzy system. Also, since the goal of controller design is to set D, it is considered as the output of the fuzzy system. Thus, the fuzzy system consists of two inputs and one output. The membership functions considered for the input and output are triangular membership functions. The range of variations of these variables is covered by 3 membership functions for the inputs and 9 membership members for the outputs. It should be noted that in all the above cases, the range of variables defined in the phase system is symmetrically covered by triangular membership functions.
The most of MPPT methods operate based on the P-V characteristic of PV module. In FLC, the controller inputs E and dE given by (7) and (8) are the rate of power variations to voltage variations and the variation of E at time t, respectively. The output of the controller is also the duty cycle.
(7) |
(8) |
where and are the output power and voltage of PV module, respectively.
In the first step, the definition range and membership functions of all fuzzy inputs and outputs are identified. There are 5 variables defined for each input membership function and 17 variables defined for the output membership function, all of which are identified through optimization process.

Fig. 3 Sample of FLC membership function.
In the second step which is the most important stage of the fuzzy controller design, the fuzzy rules should be designed. Since the fuzzy inputs are divided into three membership functions, fuzzy rules will contain nine rules, as listed in
The last step is the definition of objective function. The goal is to optimize (minimize) the error level of fuzzy inputs E and dE. In this paper, the integral square error (ISE) criterion is used as the cost function.
(9) |
where is the simulation time; and and are the ratio of power mismatch to voltage mismatch and its derivation which should be optimized, respectively.
In this section, PSO-GA is used to optimize the fuzzy system for MPPT controller in PV system. The GA cannot provide a precise solution for the problem due to its random nature. It requires complex and time-consuming calculations for convergence. In contrast, the PSO can reach the exact solution by comparing its position with surrounding positions, and the global positions of all the particles. However, it may fall into the local optimum in high-dimension space, if used inappropriately. The GA results in various solutions using crossovers and mutations, which can cover the weakness of the PSO. Also, the PSO can cover the weakness of the GA by accelerating the computation speed and increasing the accuracy. For this reason, PSO-GA is proposed, which has the speed and accuracy of PSO and the diversity of GA.
In the first step, the initial solution is randomly generated over the search space. The initial position of the particle is also generated from a uniform distribution in the range , where and are the lower and upper bounds of the variables, respectively.
In the second step, the PSO is applied on the initial population. The position and velocity of each particle are determined based on individual particle experiences and other particle experiences. The algorithm ranks the results and saves the best and worst solutions to be used for the fast convergence. The initial population is evaluated and the population is ranked based on the values of and . The is the best solution in every iteration, and is the best solution in all iterations.
In the third step, the GA is applied on the remaining particles with low rank. In each iteration, the GA generates a new population using crossover and mutation operations. Finally, the population generated by GA and PSO is combined for the next iteration. This combination is used as initial solution for the next iteration. The algorithm will stop after specific number of iterations. The flowchart of the proposed algorithm is depicted in

Fig. 4 Flowchart of PSO-GA.
In this study, the PSO-GA is used to optimize the performance of FLCs. This section focuses on optimizing the objective function using the PSO-GA. In this paper, the parameters of the fuzzy system are set using the initial values. The input values of the fuzzy system are obtained using (7) and (8). Using (9), the objective function value is obtained during the simulation. In the next step, the operators of PSO-GA are applied to the fuzzy system parameters. Then, the objective function value is redefined using (9). In each step, the objective function value is compared with the previous value of the objective function and the better (lower) one is considered as the output of this step. These steps will continue until the stopping criteria are met. The objective function is plotted in terms of the number of iterations in

Fig. 5 Results of PSO-GA.
After the optimization by PSO-GA, the optimal parameters are obtained for the design of fuzzy system. Given the values of the optimal parameters for the input and output membership functions, the optimal results of membership functions are obtained as shown in

Fig. 6 Optimal results of membership functions of FLC with parameter E, dE or D. (a) E. (b) dE. (c) D.
Using the variables x(28) to x(36), the fuzzy rules are obtained as listed in
The simulation of three-phase PV system is carried out in various scenarios. The P&O method based controller, INC method based controller, PSO-based optimized FLC, and GA-based optimized FLC are compared with the PSO-GA-based optimized FLC.

Fig. 7 PV system under study.
In this case, at s, the temperature increases from 25 ℃ to 30 ℃ and at s, it changes to 35 ℃ and finally returns to initial temperature 25 ℃ at s.

Fig. 8 Voltage variations with variable temperatures and constant irradiance.

Fig. 9 Power variations with variable temperatures and constant irradiance.
According to
In this case, the variable irradiances and constant temperature are considered. At s, the irradiance changes from 1000 W/

Fig. 10 Voltage outputs with variable irradiances and constant temperature.

Fig. 11 Power outputs with variable irradiances and constant temperature.
In this case, the irradiance and temperature change simultaneously and the proposed controller is compared with other controllers. At the beginning of the simulation, the irradiance is 700 W/

Fig. 12 Power outputs with variable irradiances and temperatures.
The output power of the PV system for Case 3 is shown in
The selection of the type of fuzzy inference system, the shape and interval of changes in fuzzy membership functions and fuzzy rules have a significant impact on the controller performance. In this paper, a new FLC has been proposed for MPPT. The parameters of the FLC have been optimized using the PSO-GA. To investigate the performance of the proposed PSO-GA-based optimized FLC, the system has been tested with rapid changes of irradiance and temperature. The simulation results verify that the proposed controller outperforms the P&O method based controller, INC method based controller, GA-based optimized FLC, and PSO-based optimized FLC under different operation conditions. The proposed controller has a faster response rate and higher accuracy compared to other controllers. In addition, in terms of the accuracy, the proposed controller increases 2%-8% of the output power of the PV system compared to other controllers with different irradiances and temperatures, which results in better MPPT.
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