Linear Power Balance Strategy for Asymmetric Cascaded H-Bridge Nine-Level Inverter
Ye Manyuan1, Yu Shengming2, Liu Wenfang2, Chen Yinbo1, Shao Yunpeng1
1. School of Electrical and Automation Engineering East China Jiaotong University Nanchang 330013 China; 2. State Grid Anhui Electric Power Co. Ltd Huaining County Power Supply Company Anqing 246121 China
Abstract：Compared to traditional two-level inverters, cascaded H-bridge (CHB) multi-level inverters based on pulse width modulation (PWM) strategy are widely used in medium and high voltage power applications, such as DC transmission and photovoltaic grid connection, due to their excellent harmonic performance, high efficiency, and low dv/dt. According to different voltage values on the DC side of the inverter, CHB multi-level inverters can be divided into symmetrical and asymmetric cascaded H-bridge multi-level inverters. Asymmetric cascaded inverters output higher voltage levels under the same number of power switches and DC voltage sources, which have high research value. However, the traditional Type II asymmetric CHB inverter has the problem of current reversal under partial amplitude modulation. Therefore, the paper adds a low-voltage unit to the traditional type II asymmetric CHB inverter topology, adjusting the DC side voltage ratio to 2:1:1, and effectively avoiding current reversal by selecting switch states at different levels. For the asymmetric cascaded nine-level inverter topology, the traditional hybrid PWM (H-PWM) strategy has the advantage of optimizing the harmonic performance of the output voltage. However, under this modulation strategy, only low-voltage unit H3 operates in the high-frequency state, and high-voltage unit H1 and low-voltage unit H2 both operate in the low-frequency state. The imbalanced output power and uneven distribution of switching losses exist among the cascaded units of the inverter, which seriously affects the lifespan of the inverter topology. Accordingly, the paper proposes an improved hybrid PWM (IH-PWM) modulation strategy based on the H-PWM modulation strategy. The proposed strategy improves the output voltage waveform but does not address the issue of the imbalanced output power between cascaded units. This paper further optimizes the IH-PWM strategy and proposes a hybrid modulation strategy based on power units redistribution (PRH-PWM). For the high-voltage unit H1, the PRH-PWM strategy controls the conduction angle of the switch to control the amplitude of the fundamental wave of the unit output voltage. Thus, the average output power of the unit is always half of the average output power of the inverter under the full amplitude modulation. At the same time, it increases the modulation range for high-voltage unit H1 to participate in inverter operation. For low-voltage units H2 and H3, the PRH-PWM strategy combines the advantages of phase- disposition and phase-shift PWM modulation strategies. By recombining the power units, the inverter achieves output power balance and uniform distribution of switching losses between low-voltage units while outputting high-quality voltage waveforms. Simulation and experimental platforms are established for asymmetric cascaded H-bridge nine-level inverters. The results indicate that the high-voltage unit H1 participates in the operation of the inverter under full amplitude modulation, which improves the utilization rate of the cascaded units and extends the service life of the inverter. Under the full amplitude modulation, the cascaded units' average output power ratio of the nine-level inverter remains at PH1: PH2 : PH3 =2: 1 : 1. The PRH-PWM modulation strategy achieves a linear balance of output power between the cascaded units of the nine-level inverter. The THD value of the nine-level inverter's output phase voltage waveform is low under the high amplitude modulation, and the quality of the inverter voltage waveform is better under the PRH-PWM modulation strategy.
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2024, Vol. 39 
