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Modulation Methods Comparison for Neutral-Point-Clamped Three-Phase Inverters

Fig. 1. Three level voltage source inverter system.
The voltage-source pulse-width modulation (PWM) inverter has made a significant contribution in achieving energy conservation and improving system performance in many applications. Compared with two level voltage source inverters, neutral-point-clamped (NPC) inverters can achieve better performances on ripples, harmonics and EMI. The additional choices in switching the states of the NPC converter provide many opportunities for optimizing a system to reach a low differential mode harmonic performance and, minimum dc-link ripple current, also known as nearest three space vectors (NTSV) modulation. To reduce the EMI filter size, the additional choices in switching states of the NPC converter also provide the options to reduce the common mode (CM) output voltage generation known as common mode reduction (CMR) modu-lation. There is also a modulation method which can theoretically eliminate the common mode voltage referred as common mode elimination (CME) modulation to further reduce the EMI filter size.

Based on the detailed implementation of the three modulation methods, system performance on CM noise reduction, NP voltage ripple and semiconductor losses are analyzed and compared in detail. The results show that CMR modulation can help to reduce both DM and CM noise in the EMI range. CME modulation will increase system DM noise but it can help to reduce the CM noise, while the benefit of CME modulation is highly related to the DT in the system. The analytical calculation method of the NP voltage ripple is discussed in detail and both analytical and experimental results are shown to compare the NP voltage ripples using three different modulation method. Moreover, the loss calculation is also discussed in the paper, the analytical calculation result is shown, which shows that the losses are comparable between NTSV and CME modulation while CMR modulation can reduce the switching losses by about 30%.

Fig. 2. switching states in 3L VSI.
Fig. 3. CM current spectrum.

Fig. 4. NP voltage ripple comparison.
Fig. 5. Device loss comparison.
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