Abstract : This paper analyzes the working mode of a multi-level series inverter unit and introduces several commonly used PWM control methods. Taking a six-unit series multi-level inverter as an example, three of the multi-level PWM methods are compared and studied. It is concluded that the multi-level carrier PWM control method has a smaller output voltage waveform dv/dt and a lower total harmonic content, making it most suitable for PWM control of unit-series multi-level inverters. Keywords : Multi-level inverter; Unit series; PWM method; Comparative study 1 Introduction Traditional medium- and high-voltage high-power converters use semi-controlled or fully controlled thyristors, which are current-controlled components. In recent years, self-turn-off devices such as IGBTs have also developed towards high voltage and high power, and even some new devices such as IG have emerged. Currently, IGBTs have a withstand voltage of 6kV, and the withstand voltage level of practical components is expected to stabilize at 6kV in the future. However, for medium-voltage 6kV and 10kV drives, a single inverter bridge with 6kV components is still not feasible, and the series connection of components or the use of a multi-level structure remains unavoidable. Series connection of components is difficult to implement. Multilevel structures can solve the component withstand voltage problem without series connection, making them easier to implement. Furthermore, they offer many advantages, such as multiple voltage vectors, better waveform control, and improved motor insulation. Therefore, multilevel structures are very promising for medium and high voltage frequency conversion. Even when component withstand voltages become very high in the future, their application will still be necessary. Multilevel structures will be the preferred structure for medium-voltage frequency conversion main circuits for some time to come. Series-connected multilevel inverters are a type of multilevel structure. This structure is easy to implement with multiple series connections, has no midpoint potential fluctuation problem, can bypass in case of inverter unit failure, has high reliability, low harmonic content, and is inexpensive. This article introduces several commonly used PWM control methods and conducts a comparative study for design reference. 2. Inverter Unit Circuit Operating Mode Series -connected multilevel inverters are a type of multilevel structure. The rectifier inverter unit circuit is shown in Figure 1, and the one-phase structure of the frequency converter main circuit is shown in Figure 2. [b][align=center]For more details, please click: Analysis and Research on PWM Methods for Series-Connected Multilevel Inverters[/align][/b]