Novel control strategies for voltage balance of DC capacitors in NPC three-level inverter

The advantage of simple design on DC side makes the Neutral-Point-Clamped three-level converters being researched and applied widely, especially in those application area with requirement of high power and/or high performance. The balance of DC capacitors´ voltage becomes one of pivotal problems in putting this topology into use. Inspired by the general formula of normal carriers in PWM process, two control strategies are proposed in this paper to give a good management to DC capacitors´ voltage, which are so-called hysteresis zoom of capacitors´ voltage strategy and hysteresis shift of capacitors´ voltage strategy. Furthermore the flexible balance of capacitors´ Voltage strategy is presented to enhance the output performance of the foregoing two strategies, obeying the rule that output performance holds a position prior to DC capacitors´ voltage balance. Simulation and experimental results verify the feasibility of strategies above.