The Hybrid Carrier Frequency Modulation Strategy of Seven-Level Flying-Capacitor Cascaded H-Bridge Inverter
Liu Yize1, Wu Xuezhi1, Liu Jingdou1, Xu Wenzheng1, Cheng Hao2
1. National Active Distribution Network Technology Research Center Beijing Jiaotong University Beijing 100044 China; 2. CSSC Systems Engineering Research Institute Beijing 100094 China
Abstract：Multilevel inverters have the advantages of low harmonic content of output voltage, low voltage stress of switching devices, and high equivalent switching frequency, widely used in new energy power generation. Compared to other hybrid seven-level topologies, the seven-level flying-capacitor cascaded H-bridge inverter has attracted attention because of its low switching frequency, small number of devices, and many output levels. For the seven-level flying-capacitor cascaded H-bridge inverter, the existing modulation strategies, such as low-frequency modulation, have high harmonic content in output voltage. The high-frequency modulation has less harmonic content but a high switching frequency. Hybrid modulation has the problem of over-modulation or narrow modulation range. This paper proposes a hybrid frequency carrier modulation strategy based on modulation wave decomposition. In this strategy, the switching tubes of the H-bridge inverter and the flying capacitor inverter operate at low and high switching frequencies, respectively. However, it causes over- modulation. Thus, the modulation strategy increases the modulation process of the over-modulation interval and decomposes the modulation wave of the flying capacitor inverter beyond the carrier. At the same time, the over-modulating waves of the H-bridge inverter and the flying-capacitor inverter are adjusted. As a result, the switching tube of the H-bridge inverter operates at a low switching frequency. An active power decomposition control strategy based on working mode switching is proposed to control the voltage equalization of flying capacitors. This method realizes the voltage equalization of the flying capacitor through the control mode without additional hardware circuit. Two working modes of the inverter are obtained according to the influence of current direction and magnitude on voltage equalization of the flying capacitor when the voltage crosses zero. The voltage regulating control principle of the two working modes is obtained by analyzing capacitor charge and discharge. The closed-loop control scheme of the capacitor voltage regulation system based on working mode switching is designed. Capacitor voltage equalization can be realized by adjusting the active power injected into the system by the H-bridge inverter. Simulation and experimental platforms are established for the seven-level flying-capacitor cascaded H-bridge inverter. The simulation and experimental results indicate that: (1) The proposed method solves the over-modulation problem of inverter output and reduces the switching loss of the H-bridge inverter. The maximum switching frequency of the H-bridge inverter is 0.32 times the carrier frequency, and the minimum power frequency is 50 Hz. (2) The stability control of the flying capacitor voltage under different working modes is realized. (3) The scheme uses fewer devices to realize the seven-level output of the inverter. Under the modulation, the THD value of the output voltage waveform is low, and the inverter has 2 times frequency characteristics.
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