Loss Evaluation of Cell Capacitor with Quasi-unipolar Modulation for Modular Multilevel Cascade Converter-based Active Power Filter

This paper evaluates a reduction of cell capacitor loss with quasi-unipolar modulation for modular multilevel cascade converter-based active power filter (MMCC-APF). This modulation method uses the same PWM controller with a traditional bipolar modulation method. The generated PWM signals are interchanged among the bridge cells, resulting in a reduction of the cell capacitor ripple current without increasing the number of PWM channels. In a low-voltage application of MMCCs, this is important to reduce the cost of the control circuit and the size of the cell capacitors utilizing electrolytic capacitors. An experimental system of a 3.3 kVA MMCC-APF achieves a reduction of the power loss by 38% and the capacitor temperature rise by 18%.