Capacitance Minimization for A Medium-Voltage Motor Drive Based on Cascaded H-Bridges

Cascaded H-bridges (CHB) converter equipped with pulse width modulated (PWM) rectifiers has been widely adopted in medium-voltage motor drive applications for the ability of power regeneration. But capacitor in each submodule suffers severe issue of voltage fluctuation at twice the inverter frequency, which can be worse when motor works at low speed and heavy load. The causes of voltage fluctuation are studied in this paper, including low running speed and fluctuating instantaneous power of inductors in front-end rectifiers. The fluctuating power of inductors will cause large voltage fluctuation on capacitors, and this issue is ignored in previous studies. This paper also proposes an improved power feedforward method to tackle the issue of fluctuating power of inductors, in which the input power and output power of submodules are totally counteracted. By counteracting power of input and output totally, voltage fluctuation decreases dramatically and capacitance can be minimized. Therefore, electrolytic capacitors can be replaced by film capacitors with small size and long lifetime. Simulation results based on 6kV, 710kW motor verify the effectiveness and validity of the proposed method.