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New Modulation Scheme for 3-level Active Neutral Point Clamped Converter with Loss and Stress Reduction

Fig. 1. Four quadrant operation and switching loops for the new modulation for ANPC phase leg
In this paper, the modulation schemes for a 3-level NPC phase leg is thoroughly investigated using phase leg loss and stress distribution analysis. Since the conventional modulation schemes have some issues when using a SVM modulation strategy with a NP voltage balance, an improved modulation scheme for a 3-level ANPC phase leg is proposed. Taking advantage of the flexibility of the neutral current path configuration for the ANPC phase leg, this modulation scheme uses both neutral current paths to conduct neutral current. Thus, the conduction loss is significantly reduced. Compared with the conventional modulation schemes for ANPC, which have a different neutral state for the positive and negative half line cycle, the proposed modulation scheme has a unified neutral state for both the positive and negative half line cycle. With the new modulation scheme, new switching loops for the 3-level ANPC phase leg are generated in the four-quadrant operation. The new switching loops have two pairs of complimentary switches and two pairs of synchronized switches. The two inner switches share the high frequency loss and stress for the two clamping switches instead of doing line frequency switching with the conventional method. In the new switching loops, two paralleled switching devices commutate simultaneously with another switching device. So the load current is shared by the two switches, and the switching stress is reduced. The double pulse tests measures the switch-ing characteristics for the new loops. It shows a largely reduced switching stress, and basically an unchanged switching loss for the new loop. The phase leg loss distribution shows the more evenly distributed phase leg loss and stress. Also, the system loss breakdown shows a 15% conduction loss reduction. With this modulation scheme, the 200kVA 3-level ANPC power con-verter operating at 20kHz can reach up to 98.5% efficiency. Another benefit for the proposed method is that only 2 pairs of PWM channels are needed for each phase leg with the proposed method. So the three-phase system can save 6 PWM channels. The proposed method is also verified on a 200kVA, 20kHz 3-level ANPC converter hardware.

Fig. 2. Inverter side current ripple waveform
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