悬浮电容H桥级联七电平逆变器的混合载波频率调制策略

The Hybrid Carrier Frequency Modulation Strategy of Seven-Level Flying-Capacitor Cascaded H-Bridge Inverter

多电平逆变器具有输出电压谐波含量少、开关器件电压应力低、等效开关频率高等优点,广泛应用于新能源发电场合。悬浮电容H桥级联七电平逆变器,因开关频率低、器件数量少、输出电平数量多得到广泛的关注。为了降低损耗,使H桥逆变单元开关管以低开关频率动作,悬浮电容逆变单元开关管以高开关频率动作,但是会引起过调制和悬浮电容电压失控的问题。为解决过调制问题,该文提出基于调制波分解的混合载波频率调制策略,增加过调制区间的调制过程,分解悬浮电容逆变单元超出载波部分的调制波,使H桥逆变单元开关管以低开关频率运行;同时提出基于工作模式切换的有功功率分解控制,详细分析两种工作模式下的稳压控制原理,根据电容电压的状态改变悬浮电容逆变单元有功功率大小,实现不同工况下悬浮电容电压均衡。最后通过仿真和实验验证了所提调制策略的正确性。

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.