Direct generalized modulation of electrical conversions including self stabilization of the DC-link for a single phase multilevel inverter based AC grid interface

摘要

The output voltage waveform of a multilevel inverter is composed of several intermediary voltage levels, which are obtained from capacitor voltage sources. If the number of levels reaches infinity, the total harmonic distortion of the modulated voltage becomes practically zero. However, the numbers of the achievable voltage levels are limited by voltage unbalance problems, voltage clamping requirement, circuit layout, and packaging constraints. This paper presents a modulation strategy that enables to copy directly a modulated multilevel waveform on to output voltages of a (n+1) level inverter. The redundancies of different switch configurations are used to perform the voltage balancing of the DC link. The obtained modelling shows that high voltage multi levels are obtained by combination of n different three-level functions, which are called modulation functions. Therefore, a novel PWM strategy is designed to generate n fictive modulated voltage systems via the direct width modulation and the position setting of modulation functions. Simulation results are presented for a five-level inverter. Experimental results are given for a three-level inverter.