Dual-Stage Control Structure for Multilevel Voltage Source Inverters

Abstract

This work proposes an alternative for total harmonic distortion (THD) attenuation in power inverters by combining two different circuit stages. The Macro stage comprises of a Neutral-Point-Clamped (NPC) multilevel inverter operating in high voltage and low switching frequency. The Micro stage is a two-level three-phase inverter, which is faster and more accurate due to its higher frequency and limited voltage. In this dual-stage approach, the opposite characteristics of the converters lead the Micro stage to correct the distortions of the Macro. It is possible to work with small values of filtering components, lower than the ones needed in a one-stage approach. This way, the presented proposal aims to reduce the physical size of the converter and its costs. The NPC inverter works in open loop with its switching signals generated by optimized pulse patterns (OPP). For correct THD filtering, a resonant control strategy with a Notch filter is designed in the frequency domain, as well as a feedforward structure designed in time domain. Simulation results show that the proposed system keeps voltage THD in acceptable levels for IEEE Standards.