A simplified PWM strategy for NPC three-level converter with unbalanced dc-links is proposed in this paper to achieve high quality line-to-line output voltages and to maximize the linear modulation range. The simplified strategy takes the direct output voltage modulation by calculating the special solutions of the voltage-second balance equations without detecting the position of the reference vector in the asymmetrical and complicated space voltage vector diagrams to reduce the calculation time. A novel solution based on the state transition is proposed to extend the maximum linear modulation index to 1.15. Furthermore, the asymmetric control of the split dc-link by the proposed PWM is implemented by adjusting the special solutions. Difference between the conventional SVPWM and the proposed strategy is conducted to illustrate the advantages of the simplified strategy. The effectiveness of the proposed modulation strategy is verified by simulation and experiment results.Index Terms-linear modulation range, NPC inverter, PWM, unbalanced dc-link.

In this paper, two new optimized common-mode voltage reduction PWM (CMVRPWM) strategies based on solving the established constrained nonlinear programming models in the time domain are proposed and analyzed. The proposed current ripple losses optimized CMVRPWM (CRLO-CMVRPWM) minimizes the mean-square values of the three-phase current ripples by calculating the optimized special solutions of the voltage-second balance equations under the designed switching sequences. CRLO-CMVRPWM can achieve better output waveform quality than the existing methods. The proposed switching losses optimized CMVRPWM (SLO-CMVRPWM) on-line optimizes the bus-clamping styles according to the phase currents to minimize the switching losses under different load power factors. Compared to the near-state PWM (NSPWM) with fixed bus-clamping styles, SLO-CMVRPWM can reduce more switching losses in broader range of the modulation index. Simulation and experiment results verify the superiority of the proposed strategies to the conventional ones. Index Terms-Common-mode voltage (CMV), constrained nonlinear programming, current ripple, linear modulation range, pulsewidth modulation (PWM), switching losses, voltage-source inverter (VSI). in 2009 and 2012, respectively.He is currently working toward the Ph.D. degree in power electronics and drives in China University of Mining and Technology.His current research interests include high-power three-level explosion-proof inverter modeling, fault-tolerance control of inverters.

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A novel simplified pulse width modulation(PWM) strategy for neutral point clamped (NPC) three-level converter is proposed in this paper.The direct output voltage modulation is applied to reduce the calculation time. Based on this strategy, several optimized control methods are proposed. The neutral point potential balancing algorithm is discussed and a fine neutral point potential balancing scheme is introduced. Moreover, the minimum pulse width compensation and switching losses reduction can be easily achieved using this modulation strategy. This strategy also gains good results even with the unequal DC link capacitor. The modulation principle is studied in detail and the validity of this simplified PWM strategy is experimentally verified in this paper. The experiment results indicated that the proposed PWM strategy has excellent performance, and the neutral point potential can be balanced well with unequal DC link captaincies.

Fault detection can increase the reliability and efficiency of power electronic converters employed in power systems. Among the converters in the power system, a Neutral Point Clamped (NPC) three-level inverter is most commonly used to drive electric motors. In this paper, a new approach for open-circuit fault detection and location of the NPC three-level inverter for a shifting process using a constant voltage-to-frequency ratio is proposed. In order to diagnose open-circuit fault in as short a time as possible, an adaptive electrical period partition (AEPP) algorithm is proposed to pick single electrical periods from real-time three-phase current signals. The Maximal Overlap Discrete Wavelet Transformation (MODWT) and Park’s Vector Modulus (PVM) are used for feature analysis and normalization of electrical period signals. The statistical characteristics of the electrical period signals are extracted, and a random forest model is constructed to realize the state classification. Compared with the traditional fault diagnosis method, the proposed algorithm finds fault locations quickly and accurately. The effectiveness and accuracy of the proposed algorithm are verified by experiments.