Evaluation of alternative modulation schemes for three-level neutral-point-clamped three-phase inverters

This paper presents a comprehensive evaluation of modulation techniques for three-level neutral-point-clamped (NPC) inverters, focusing on the following system aspects: EMI performance, neutral-point voltage ripple, and switching losses. The modulation techniques considered are: nearest three space vectors (NTSV), common-mode reduction (CMR), and common mode elimination (CME). After describing in detail their implementation, an in-depth theoretical analysis on EMI performance is presented based on the space vector sequences employed and the respective space vector dwelling times. Simulation and experimental results in both time and frequency domains are used to verify the techniques and show their effectiveness in terms of EMI noise reduction. An analytical calculation method for the NP voltage ripple is then presented and used to compare the voltage ripple generated by these modulation schemes; experimental results are used to verify the analysis. Lastly, this paper also presents an analytical loss calculation method, which shows that NTSV and CME are comparable in these terms, while CMR can reduce switching losses significantly.