CONCEPTION OF MULTILEVEL INVERTER APPLIED TO UPS SYSTEM Lincoln Moura Oliveira, Levy Ferreira Costa, Deivid Marins, Sergio Daher, Fernando Luiz Marcelo Antunes GPEC DEPARTAMENTO ENGENHARIA ELETRICA UNIVERSIDADE FEDERAL DO CEARA BRASIL

Abstract This paper presents the grid connected mode of a 3 kVA single-phase uniterruptible power supply (UPS), with features of low frequency operation, high efficiency and unit power factor of the input. The input stage of the proposed UPS is composed by a power factor correction rectifier based on classical boost converter, which is used to supply the dc bus. The output stage is composed by a multilevel inverter, which is based on a low frequency full-bridge converter associate to a multiple winding transformer with one primary coil and several secondary coils. Each secondary coil is used to produces the partial alternating voltage and the series combination of the output coils are used to generating the complete output voltage. The theoretical analysis, design example, as well as experimental results of the proposed UPS system are presented in this paper.
 LINK
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A Review of Multilevel Inverter Based Active Power Filter Madhukar Waware and Pramod Agarwal

A Review of Multilevel Inverter Based Active Power Filter
Madhukar Waware and Pramod Agarwal

Abstract—Use of nonlinear loads at high voltages has increased tremendously in industry. These non-linear loads distort power quality by generating harmonics. Conventional active filters have limitations in high power high voltage systems. Use of multilevel inverter in active filters effectively reduces harmonics in high voltage systems eliminating need of high cost and bulky transformer. This paper presents the state of art of different topologies of multilevel inverter (MLI) used for active power filters (APF), the different control techniques and advanced modulation methods. It is aimed to provide a broad perspective on the status of MLI based APF technology to researchers and application engineers dealing with power quality issues in high voltage systems.
FULL PAPER HERE :http://www.ijcee.org/papers/314-E336.pdf