International Journal of Engineering and Technical Research (IJETR) ISSN: 2321-0869, Volume-3, Issue-5, May 2015 310 www.erpublication.org Abstract—The problem of harmonics existing in multilevel converters has been tackled in past by various techniques. Optimized PWM techniques for reduction of harmonics have been successful for cascaded H-bridge fed from equal voltages however, this paper presents Selective Harmonic Mitigation technique implemented on series connected cascaded H-bridge converter working on equal as well as unequal voltages. The presented technique is capable of reducing selective harmonics, to an extend acceptable by grid codes, even when converter is fed from unequal voltages. The bulky and costly grid connected tuned filters of power system can be avoided and reduced by implementation of Harmonic Mitigation Techniques. The simulation results are being presented to validate the proposed method. Index Terms—Harmonic distortion, Multilevel systems, Optimization technique, Pulse width modulation converters. I. INTRODUCTION The Cascaded H-bridge converter topology presented in this paper enables the converter to produce high quality, high voltages waveforms by making use of the low or medium voltage switching devices. It also makes the converter an attractive option for grid connected applications. But the converter working at high power levels generally requires low switching frequency, this is due to the imperfect behavior of the switching devices which reduces the efficiency gets effected hence this results in distorted output[1].Also for such Cascaded H-bridge (CHB) inverter applications, it may be desired that each cell of the converter draws equal energy from the dc source that it is connected to. Though this can be achieved over a single or several fundamental cycles. But it ensures that these sources discharge at the same rate and that each cell of the cascade is utilized evenly. In applications where the dc sources are not exactly equal, distortion may be present in the converter waveformSelective Harmonic Mitigation – Pulse Width Modulation (SHM-PWM) orSelective HarmonicElimination –Pulse Width Modulation (SHE-PWM) are the methods known to generate waveforms with low switching frequency without making a compromise with the waveforms quality. The waveforms objective includes reduction of particular harmonics in the generated waveform or an optimization of Ajay chavan , The Oxford College Of Engineering (TOCE), Bangalore, india,9739968358. Dr. Puneet Kaur, The Oxford College Of Engineering (TOCE), Bangalore, india. this waveform in order to meet a certain harmonic code for a particular application[2]. Several three-level power cells, formed using full H-bridges, can be series connected to build a converter with a higher number of levels. In general, if npower cells are connected in series to build the converter and all the cells have the same dc voltage, the number of levels that can be achieved is 2n + 1. This topology is named the n-cell CHB converter, and it presents a high level of modularity and redundancy as well as an ability to produce high quality output voltage waveforms [3]. A Cascaded H-bridge converter fed from two unequal dc sources’ and has been shown in fig .1. [1]. The SHM-PWM implemented in this paper is based on the dynamic calculation of the switching angles for unequal voltages. Fig.1. Five- level cascaded H-bridge converter based on the series connection of the two Three- level power cells II. SHM-PWM PRINCIPLE The basic principle of SHM reveals that calculating the values of the switching angles according to input DC voltages. The amplitude of fundamental harmonic can be set and hence set of specific harmonics can be cancelled. Consider the Fourier analysis of a typical waveform shown in Fig.2. →ith switching angles αi (i = o . . . k − 1)[4] Implementation Of Selective Harmonic Reduction Approach for Series Cascaded H-bridge Converters Fed By Unequal DC Voltages Ajay Chavan, Dr.Puneet Kaur