The Scientific World Journal

Volume 2016 (2016), Article ID 2708075, 20 pages

http://dx.doi.org/10.1155/2016/2708075

## PLL Based Energy Efficient PV System with Fuzzy Logic Based Power Tracker for Smart Grid Applications

Department of Electrical and Electronics Engineering, Jerusalem College of Engineering, Anna University, Chennai 600100, India

Received 24 December 2015; Revised 23 February 2016; Accepted 15 March 2016

Academic Editor: Ying-Yi Hong

Copyright © 2016 G. Rohini and V. Jamuna. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This work aims at improving the dynamic performance of the available photovoltaic (PV) system and maximizing the power obtained from it by the use of cascaded converters with intelligent control techniques. Fuzzy logic based maximum power point technique is embedded on the first conversion stage to obtain the maximum power from the available PV array. The cascading of second converter is needed to maintain the terminal voltage at grid potential. The soft-switching region of three-stage converter is increased with the proposed phase-locked loop based control strategy. The proposed strategy leads to reduction in the ripple content, rating of components, and switching losses. The PV array is mathematically modeled and the system is simulated and the results are analyzed. The performance of the system is compared with the existing maximum power point tracking algorithms. The authors have endeavored to accomplish maximum power and improved reliability for the same insolation of the PV system. Hardware results of the system are also discussed to prove the validity of the simulation results.

#### 1. Introduction

The steady increase in demand for power supply with overexploited decreasing conventional sources increases the necessity for alternative energy sources. Photovoltaic (PV) array, using solar energy as a source, is a better option as it provides a pollution-free, easily available, clean energy source [1]. For a smart grid application, PV array can be used as a source of energy in conjunction with the existing conventional source and wind energy. A photovoltaic system converts photon energy from sunlight into electricity. With PV panel prices dropping and the desire to “go green” expanding, demand for PV power is improving. As the efficiency of a PV array is much less, it becomes imperative to extract the maximum available power out of it. Various maximum power point tracking (MPPT) techniques are available to extract maximum power available from the solar energy [2–6]. According to maximum power transfer theorem, if the source impedance is equal to the load impedance, maximum energy can be transferred from the source to the load. The inclusion of DC-to-DC converter with MPPT technique varies the equivalent impedance, thereby matching the load resistance to achieve maximum power transfer. The perturb and observe (P&O) method is the simplest method. Maximum power control is achieved by forcing the derivative of the power to be equal to zero under power feedback control [7, 8]. But this method produces oscillations closer to the maximum power point, thereby raising the response time. Incremental conductance (INC) process traces the maximum power by observing the conductance. It uses and compares it with , thereby adjusting the step size to track the maximum power point of PV array [9, 10]. Incremental resistance (INR) is similar to INC except that it compares with for tracking maximum power point [11]. Fuzzy logic control is an artificial intelligence technique which can be used to track the maximum power point to improve the performance of the system. As there is fuzziness in the input for PV array like change in insolation, temperature, and shading effect during the entire day, this logic is more suitable than the other methods for tracking maximum power point. In addition, this system does not require the knowledge of the exact model [12–17]. To transmit the power at the required voltage and current level from the PV array to the load, boost converter is used [18]. The interleaved soft-switching boost converter (ISSBC) shares the input current all along each phase, thereby decreasing the current rating of the switching device. The ripple content present in the input current and the output voltage and the size of the passive components are also reduced in this converter [19, 20]. The problems caused due to EMI, switching loss, and diode recovery loss can be overcome by using the resonant soft-switching technique [21–24]. This technique does have certain limitations. The fidelity of the PWM signal to the analog input is determined by the linearity of the ramp-wave signal. The performance of the comparator determines speed, accuracy, and jitter. Phase-locked loops (PLLs) synchronize a local voltage-controlled oscillator (VCO) to an external frequency input by means of an electronic servo loop [25]. It is used with converter to achieve constant operating frequency over a wide output voltage range, eliminating the dependence of switching frequency on duty cycle or voltage conversion range [26].

In the present work, to improve the dynamic performance of the PV system, cascaded converter topology is used with intelligent power tracker and modified control strategy. The first conversion stage uses the fuzzy logic controller to track the maximum power. The simulation is performed for three algorithms in comparison with fuzzy based MPPT whereas in the literature comparison of fuzzy logic with any one algorithm is available. The phase-locked loop is used in the cascaded three-stage converter to extend the soft-switching region to minimize the losses. The interleaved soft-switching technique available in the literature is extended in the present work for three-stage converter for improved efficiency. In addition, the power handling capability is improved and the ripple content at the output is reduced.

The PLL control scheme prevents the operating frequency of the circuit from being lower than the resonant frequency.* In this work*, phase-locked loop is implemented to maintain the required frequency of operation with reduced ripple and to increase the soft-switching range of operation. The PV array was mathematically modeled using the basic equations to match the actual parameters of the nonlinear model. The performance of the system is compared with the other conventional algorithms. The simulation is performed using the Simulink software and the hardware results are found to be in line with the simulation results.

This paper is organized as follows. Section 2 presents system description. Mathematical model of solar PV module is discussed in Section 3. In Section 4, fuzzy logic based maximum power tracker used in the present work is discussed. Section 5 presents design of three-stage soft-switching boost converter. Section 6 describes the phase-locked loop for extending the soft-switching region. Section 7 deals with results and discussions. Finally, Section 8 summarizes the work done as conclusions.

#### 2. System Description

Block diagrammatic representation of the PLL based energy efficient PV system with intelligent power tracker and cascaded converter is shown in Figure 1. The system consists of PV array acting as the source which converts light energy into electrical energy. In this work, both feedforward and feedback control are performed. Maximum power point tracking is performed as feedforward control by using fuzzy logic controller for the first converter in the cascaded network. Driving pulses to the switches are generated using the fuzzy based power tracking algorithm. Three-stage soft-switching boost converter (TSSSBC) is used for the second stage of cascaded converter to match the load requirements. Pulses for the TSSSBC are 120° phase shifted from each other. A three-stage boost converter is designed, developed, and modified from a normal boost converter. As the number of stages in the boost converter is increased, the size of the inductor is reduced, so that it could be made compact and reduction in ripple content could also be achieved. The phase delay for each stage is given by , where is the number of stages. As the number of stages increases, the quality of regulated output increases, but, at the same time, the complexity of the circuit also increases. To ensure reduced complexity with reduced ripple content, a three-stage boost converter is used in the present work. To ensure accurate terminal voltage with lower ripple content, feedback control with phase-locked loop is proposed in the present work.