IJETS: International Journal of Engineering Technology & Science ISSN: Volume: 01 Issue: 01 | Jan-2014 _______________________________________________________________________________________ Copyright © 2014, IJETS 1 THD REDUCTION IN RENEWABLE ENERGY SYSTEM WITH MULTILEVEL INVERTER K.Venkata Kishore 1 , T.Vijay Muni 2 , N.Sesha Reddy 3 1 Assistant Professor, Department of EEE, NRI Institute of Technology, Andhra Pradesh, India. 2 Associate Professor, Department of EEE, NRI Institute of Technology, Andhra Pradesh, India. 3 Assistant Professor, Department of EEE, NRI Institute of Technology, Andhra Pradesh, India. Abstract This paper presents the control of a multilevel inverter supplied by a Photovoltaic (PV) panel, wind and a batteries bank. It is well known that the power quality of multilevel inverter signals depends on their number of levels. However, the question that arises is whether there is a limit beyond which it is not necessary to increase the number of level. This question is addressed in this paper by studying seven-level and nine - level diode clamped converters. The harmonics content of the output signals are analyzed. Comparison between the seven level and nine level diode clamped converter is shown. A simplified Pulse Width Modulation (SPWM) method for a multilevel inverter is developed. The controller equations are such that the SPWM pulses are generated automatically for any number of levels. The effectiveness of the propose method is evaluated in simulation. Matlab®/Simulink is used to implement the control algorithm and simulate the system. Index Terms: Multilevel inverter, Multilevel SPWM, THD --------------------------------------------------------------------***---------------------------------------------------------------------- 1. INTRODUCTION Nowadays, the industry requires power equipment increasingly high, in the megawatt range. The rapid evolution of semiconductor devices manufacturing technologies and the designer‟s orientation have enabled the development of new structures of converters (inverters) with a great performance compared to conventional structures. So, these new technologies of semiconductor are more suited to high power applications and they enable the design of multilevel inverters. The constraints due to commutation phenomena are also reduced and each component supports a much smaller fraction of the DC-bus voltage when the number of levels is higher. For this reason, the switches support more high reverse voltages in high-power applications and the converter output signals are with good spectral qualities. Thus, the using of this type of inverter, associated with a judicious control of power components, allows deleting some harmonics [1]. Among the control algorithms proposed in the literature in this field [2-3-4], the SPWM, appears most promising. It offers great flexibility in optimizing the design and it is well suited for digital implementation. It also helps to maximize the available power. The main advantage of multilevel inverters is that the output voltage can be generated with a low harmonics. Thus it is admitted that the harmonics decrease proportionately to the inverter level. For these reasons, the multilevel inverters are preferred for high power applications. However, there is no shortage of disadvantages. Their control is much more complex and the techniques are still not widely used in industry [7-8]. In this paper, modelling and simulation of a multilevel inverter using Neutral-Point-Clamped (NPC) inverters have been performed with motor load using Simulink/ MATLAB program. In the first section multilevel inverter control strategies are presented before to detail a study of seven- level inverter in the second section. Total Harmonic Distortion (THD) is discussed in the third section. The aim is to highlight the limit at which the multilevel inverters are no longer effective in reducing output voltage harmonics. 2. SYSTEM DESCRIPTION The system consists of a PV-FC-WIND hybrid source. The photovoltaic [3], [4], wind mill and the PEMFC [5],[6] are modeled as nonlinear voltage sources. These sources are connected to dc–dc converters which are coupled at the dc side of a dc/ac diode clamped multilevel inverter. Photovoltaic (PV) systems are stand-alone power generators that have good environmental footprints. The modeling and the Maximum Power Point Tracking (MPPT) control strategy for a PV system are developed in [9]. In the latter, the control strategy that is presented is based only on the measurement of the PV current to track the maximum power. A batteries bank and the energy from the wind mill is added to the DC-bus to ensure the energetic autonomy of the system. A Proportional-Integral (PI) controller is used to regulate the DC-bus voltage at a constant value. As a consequence the batteries and the wind energy compensate