Active Flying Capacitor Voltage Balancing in Single-Phase 5L-ANPC Inverter

<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> Flying capacitor (FC) voltage balancing is critical in medium-voltage hybrid multi-level inverters to ensure safe operation and reduce harmonics and disturbances. Due to compact designs with small film capacitors for extended lifetimes, FC voltage balancing faces challenges such as measurement delays in sensing and control, particularly under high load currents, which introduce large loop gains, disturb FC voltages, and may cause instability. This paper addresses these issues by analyzing the active FC voltage balancing process in a 5-level active neutral-point-clamped (5L-ANPC) inverter through a closed-loop control perspective. A dynamic FC voltage model is derived based on inverter topology and level-shifting modulation, and a nonlinear dynamic model is derived to study direct FC voltage balancing. To mitigate disturbances caused by load current overcharging and discharging, a finite repeating charging mode sequencing (FRCMS) method is proposed, which reduces loop gain by integrating active balancing states into repetitive switching patterns. The intermediate value of the active state insertion further improves dynamic performance by compensating for the nonlinear load current, balancing interval duration, measurement, and loop delay. Simulation results and experimental validations verify the effectiveness of the proposed method in improving FC voltage stability under both steady-state and transient conditions.