Natural DC‐link voltage balance in a single‐phase NPC inverter with four‐level legs and novel modulation method

This study presents a concept of modulation of a single‐phase seven‐level neutral point clamped (NPC) converter with natural DC‐link voltage balance and symmetrical DC‐link capacitors discharge. The method is analysed in a single‐phase full‐bridge inverter composed of four‐level diode‐clamped legs (4L‐FBCLD) which utilises three capacitors in a DC‐link. Under the classic pulse width modulation (PWM) technique, the discharge of DC‐link capacitors is unequal in a converter operating with active power. The proposed modulation assures symmetrical discharge of the DC‐link capacitors and is achieved in a two‐stage design. In the first stage, the carrier‐based phase disposition PWM technique with modified carriers is used. In the second stage, the pulses are directed to suitable switches with the use of a state‐machine decoder. The switching patterns are different in each level of modulation and are used to trigger natural balancing in the converter on the first and third level of modulation. This study presents the concepts of switching patterns, PWM generation, as well as the simulation and results of the experiment. The latter demonstrates the operation and feasibility of the 4L‐FBCLD, symmetrical DC‐link capacitor discharge and natural balancing in the converter.