Real-time Analysis and Simulation of Multi-String Grid Connected Photovoltaic Inverter using FPGA Satabdy Jena Dept. of Electrical Engineering NIT Meghalaya Email:satabdy@gmail.com Gayadhar Panda Dept. of Electrical Engineering NIT Meghalaya Email:gayadharpanda@gmail.com Rangababu Peesapati Dept. of Electronics & Communication Engineering NIT Meghalaya Email:p.rangababu@nitm.ac.in Abstract—The state-of-art in research practices across the globe emphasizes the real-time implementation of the control techniques. This paper investigates the performance of an Field Programmable Gate Array (FPGA)-based real-time implementa- tion of a three-phase three-level Voltage Source Inverter (VSI) for grid integration of multi-string PV array. FPGA-based prototyping of control circuits enables the study of sensitivity on parameters variations, increases the safety and reduces the time and costs of implementation. The proposed system consists of a centralized multilevel inverter that aids in the conversion of DC power obtained from a multi-string PV array to AC power, which is fed to the utility grid as well as the three phase local loads connected at the Point of Common Coupling (PCC). The multilevel structure finds use in handling large amount of power, reducing voltage stress across the semi-conductor devices and reducing the harmonic distortion. The system is developed in Matlab/Simulink environment and the control algorithm of the VSI is implemented in Virtex-6 ML605 Evaluation-Kit with Xilinx System Generator providing the effective grid-interfacing environment. Keywords—Field Programmable Gate Array (FPGA), Grid Connected,Multilevel, Multistring,Photovotaic (PV). I. I NTRODUCTION With the increase in demand of power, the pressure on the existing natural resources, which are limited and are exhaustive,have seen a rising trend. In order to bridge the gap, tapping energy from the non-conventional resources has emerged as the most feasible solution[1]. Due to abundance in availability of solar energy, photovoltaic systems have gained importance. There are generally two configurations of PV systems : stand-alone and grid connected. The grid-connected PV systems are however most widely used as they do not require any energy storage systems and the excess of energy can be delivered to the grid [2-4]. In this manner , the customers too get the opportunity to become sellers and also seves as a source of livelihood. Many control techniques like proportional-resonant (PR), deadbeat, hysteresis, etc. have been discussed in literature [5,6]. However proportional-integral (PI) are the most widely used due to their ease of implementation [7]. In order to validate the effectiveness of the control techniques, they need to be tested in real time or by developing a hardware prototype. Field Programmable Gate Array (FPGA) is a powerful embedded technology that provides hardware-in-loop implementation for precise control and high speed processing.FPGAs are semiconductor devices based around a matrix of Configurable Logic Blocks connected through programmable interconnects synchronized through a top-level clock. Its limitless flexibility, parallel computation environment, usage of different sampling rates and fast operation has made it a widely-used tool for deployment of control circuits in hardware [8,9]. They have also considerably reduced power consumption and costs. Recent developments in FPGA technology have led to faster computation ability, higher degree of flexibility, high sampling frequencies and parallel computations thereby ensuring better performance, less execution time and low power consumption. Hardware- in-loop is a feature which uses the hardware in the simulation loop, i.e. a part of the loop is implemented in the hardware and the response is received from hardware back in the software. This leads to easy-testing and the possibility to see how the plant is behaving in hardware. In this paper, a grid connected multi-string PV system has been considered with a central inverter. The centralized inverter is of multi-level structure to yield high power output from medium voltage sources. It also provides the advantage of operation at both lower and higher switching frequencies. Low switching frequencies provide higher efficiency and lower switching losses. The proposed control algorithm and the grid synchronization technique of the system has been implemented on FPGA. This paper is divided into the following sections: Section II briefs the details of the proposed system topology, Section III gives an idea of the control strategy employed, Section IV details the Simulation results, Section V imparts the method of implementation in FPGA and Section VI concludes the study. II. SYSTEM DESCRIPTION The proposed system consists of two photovoltaic arrays with their individual MPPT, boost converter, a centralized three phase three-level inverter, VSI control loops and an isolation transformer providing galvanic isolation between the renewable energy source and the utility grid. The structure of the proposed system is shown in Figure 1 and the system terminology is explained in Table 1. The voltage of the PV array is boosted to a sufficient higher level by a boost converter with the aid of an MPPT algorithm. The centralized multilevel inverter is connected across a DC link capacitor whose voltage is rendered constant by the voltage control loop. Figure 2 shows the configuration of the three phase three level inverter.The LCL filter is a combination of the inverter side LC filter and the grid side inductance. It