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High-voltage cascaded H-bridge multilevel (CHBML) inverters usually include many isolated dc voltage sources. Some dc source faults result in a drop in the dc voltage, thereby leading to unequal cell dc voltages. On the other hand, the differences in cell dc source parameters result in unequal dc voltages too. At present, riding through the faults of dc sources and operating under the condition of unequal dc voltages are required to improve the reliability of CHBML inverters. Unfortunately, the conventional phase-shifted carrier pulse width modulation (PSCPWM), which is widely used for CHBML inverters, cannot eliminate low-frequency sideband harmonics when cell dc voltages are not equal. This paper analyzes the principle of sideband harmonic elimination, and proposes an improved PSCPWM based on the particle swarm optimization algorithm. This modulation technique eliminates low-frequency sideband harmonics by calculating and regulating the carrier phases according to different cell dc voltages. The proposed PSCPWM enhances the reliability of the CHBML inverter and extends the range of its application. OPEN ACCESSEnergies 2015, 8 9671Simulation and experimental results obtained from the prototype of the CHBML inverter verify the theoretical analysis and the achievements made in this paper.

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Good fault-tolerant ability is one of the advantages of cascaded H-bridge (CHB) inverters. When the faulty cell is bypassed, it is necessary to inject zero-sequence (ZS) voltage into three-phase output voltages to balance the line-to-line voltages. Conventional methods injecting ZS voltage is aimed at maximizing the achievable amplitude of the output line-toline voltages. However, the injection of ZS voltage may lead to real power back flow in one or two phase. For CHB inverters which have no regeneration ability, the DC voltage will rise to an intolerable level. This paper analyzes the requirements for the ZS voltage to both achieve maximum output voltage and avoid real power back flow. Further, a method to suppress real power back flow by reducing the fundamental component in ZS voltage is proposed. Compared with conventional methods, the proposed method can allow the inverter to drive a load with lower power factor without leading real power back flow or sacrificing the voltage output capability. Thus the reliability of the CHB inverter is enhanced. Simulation results verify the validity and the advantage of the proposed method.