基于开关电容的混合级联多电平系统研究

Switched-capacitor multilevel inverters (SCMLIs) have many advantages such as flexible structure and automatically balanced capacitors' voltages. However, as voltage ripples of capacitors and current spikes occurred at switching instants, it is difficult to apply SCMLIs for high power conversion. Hence, this project studies a hybrid cascaded multilevel inverter system, of which a switched capacitor as the low voltage auxiliary power unit and a traditional three-level circuit as the high voltage main power unit. This not only enables SCMLIs to be applied to high power conversion, but also improves the output voltage quality of hybrid cascaded multilevel systems by utilizing flexible structure of SCMLIs. The project carried out research from the three aspects of hybrid multilevel system structures, switched-capacitor circuits and system modulation algorithms. Firstly, high-voltage low-speed, low-voltage high-speed semiconductors and low-voltage big capacitors are used as basic components to study hybrid multilevel system structures having simple structure, high efficiency, and excellent system performance. Secondly, the switching current spike is suppressed by optimizing parameters of switched-capacitor unit, and a mathematical model of SCMLIs is built to analyze capacitor's voltage ripple and its influence on system efficiency, fundamental and harmonics of the output voltage. Finally, the modulation algorithm is optimized for the hybrid system, enabling it to operate with high efficiency and to provide high quality AC output.

开关电容型多电平逆变器具有结构灵活和电容电压自动平衡等诸多优点，但由于电容电压纹波和开关切换瞬间电流尖峰的影响，使其很难单独应用于大功率场合。为此，项目研究一种以开关电容为低压辅助功率单元、以传统三电平电路为高压主功率单元的混合级联多电平逆变系统，既避免了开关电容型多电平逆变电路因功率传输能力不足而应用受限的问题，又充分利用其结构灵活的优势来提高混合多电平系统的输出特性。项目从混合多电平系统结构、开关电容多电平逆变电路、系统调制算法三方面开展研究。首先，以高压低速、低压高速半导体开关加低压大电容为基本元件，研究结构简单、转换效率高、输出特性好的混合多电平系统结构。其次，通过优化开关电容电路参数来抑制电流尖峰，通过建立数学模型来分析电容电压纹波及其对系统效率、输出基波和谐波的影响。最后，针对开关电容与不同半导体开关的混合结构，优化调制算法，使混合级联多电平系统高效运行并提供高质量交流电输出。

常规的多电平逆变技术中，中性点钳位型（NPC）和飞跨电容型（FC）多电平系统在输出电平数较多时电容电压平衡控制非常复杂，而级联H桥（CHB）需要较多的直流输入电源才能实现电平数的增加。相比之下，近几年备受关注的开关电容多电平逆变技术具有结构简单灵活、自主升压、只要一个输入电源和电容电压自动平衡等有点。然而，原有开关电容多电平相关研究也存在一些问题，比如拓扑结构杂乱、调制策略不够好、开关电容充电浪涌电流大，系统功率传输能力不足等。. 鉴于此，项目首先对开关电容多电平逆变器拓扑开展深入研究，利用拓扑推演策略设计了多种高新能开关电容多电平逆变器拓扑，包括双全桥开关电容多电平逆变器、双翼半桥型开关电容多电平逆变器、单电源级联型开关电容多电平逆变器、三相四电平开关电容多电平逆变器等，从结构和运行特性上优化了开关电容多电平逆变器拓扑。其次，在深入分析开关电容多电平逆变电路运行特性的基础上，创新性提出了基于载波移相和载波层叠的混合脉宽调制策略，充分利用开关电容电路的冗余状态，在逆变器输出高频多电平PWM电压波形的同时，使电容器高频交替运行在充放电状态，有效减少了电容电压纹波和输出电压谐波。然后，将开关电容与传统的两电平H桥和三电平NPC结合，研究了混合级联多电平逆变器，在使用较少级联单元的情况下有效增加了输出电平数，还避免了开关电容功率转换能力不足的问题。最后，项目提出了基于阻抗源的开关电容多电平逆变技术，成功将阻抗源和开关电容技术的优势有机同一，成功抑制开关电容充电浪涌电流，还有效提升了逆变器的自主升压能力。并基于这一技术进一步提出了双接地逆变器，有效消除了逆变器的漏电流。项目通过仿真分析和样机实验，对所提出的多电平逆变器拓扑和调制策略的正确性进行了有效验证。