Compared with the traditional cascaded H-bridge multi-level inverters, the hybrid cascaded multi-level inverters have been receiving attention because they can generate more levels with the same number of power cells. However, with the general hybrid pulse width modulation (PWM), the output power distribution between the high-voltage and low-voltage H-bridge cells is extremely uneven in low amplitude modulation, and it may appear that the high-voltage cell feeds power into the lowvoltage cell in some modulation ratio intervals causing the low-voltage cell capacitor voltage boost. To avoid this problem, a method of a modified hybrid PWM strategy with power balance control is proposed. It has achieved the output power balance of H-bridge cells in full amplitude modulation, the occurrence of the phenomena of extremely uneven output power distribution between the high-and low-voltage cells in low amplitude modulation is avoided, and the performance of the inverter is improved. Simulation and experimental results verify the correctness and feasibility of the proposed strategy.

In response to the deficiencies that the Carrier Phase Shift (CPS-PWM) and Carrier In-Phase Disposition (IPD-PWM) modulation techniques are applied to Cascaded H-bridge (CHB) multilevel inverters, combined with the advantages of the two techniques, a Hybrid Multi-carrier PWM technique based on carrier reconstruction is proposed. The aforementioned technique uses the carrier segment in the half carrier period of the IPD-PWM technique as the basic unit and optimizes the modulation performance by periodically adjusting its arrangement in the vertical direction. When CHB multilevel inverter adopts Hybrid Multi-carrier PWM technique, as with CPS-PWM technique, the output power balance between the cascaded H-bridge cells is achieved naturally; at the same time, the harmonic spectrum of the output voltage is identical with that of the IPD-PWM technique, that is to say, the harmonic characteristics of the output line voltage of the inverters are effectively improved. In this paper, the CHB seven-level inverter is taken as an example, and the correctness of the relevant conclusions is verified by simulation and experimental results. INDEX TERMS Hybrid multi-carrier PWM, cascaded H-bridge inverter, power balance, harmonics analysis.

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