Harmonics and Voltage Quality in Post-Fault Reconfigured Multi-Level Inverters

The main focus of this paper is to investigate the effect of post-fault reconfiguration on the inverter voltage total harmonic distortion in multilevel inverters (MLI). Three different operating conditions are compared: 1) Healthy, 2) Faulty, and 3) Reconfigured using different methods that utilize inherent inverter redundancy. Literature mostly addresses balancing line- to-line voltage RMS when reconfiguring the inverter after fault occurrence, and line-to-neutral voltage and harmonic content after reconfiguration is ignored. However, line-to-neutral voltage quality is essential for per-phase analysis of Y-connected three-phase loads, neutral point power conditioning when the neutral exists, unbalanced loading conditions, and harmonic content analysis required by some grid interconnection standards such as AS4777 and IEEE1547. This paper first introduces an accurate analytical model to analyze the harmonic content of the line-to neutral voltage. Then, two fault conditions are introduced into the model and verified in experiments. Three popular reconfiguration methods to mitigate these faults are then evaluated to analyze and compare harmonic content and RMS values of the achieved line to-neutral voltages. Simulation and experimental results of healthy, faulty, and reconfigured conditions show that the predicted RMS and total harmonic distortion of the line-to-neutral voltage using the proposed model are accurate. Results also show that the line-to-neutral voltage is severely impacted by existing reconfiguration methods.