A circulating current control scheme for multiple three-level NPC inverter systems

In recent years, motor drive systems for steel rolling mills have been steadily increasing in capacity. In particular, the development of three-level NPC (Neutral Point Clamped) inverters is now focusing on the replacement of cycloconverters. In order to satisfy such a requirement, in this paper a multiple three-level NPC inverter system is proposed. The configuration consists of two three-level NPC inverters with the same configuration connected in parallel by current limiting reactors. In this case, the circulating current flowing between the two three-level NPC inverters must be controlled to prevent a decrease in the maximum output capacity. In particular, a new method for control of the circulating current is a major topic of discussion. The method is based on space voltage vector-based PWM (Pulse Width Modulation) control for the three-level NPC inverters and the circulating current is controlled by adjusting the output durations of the respective space voltage vectors. In this method the circulating current controller can be decoupled from the motor current controller. Therefore, it is possible to design the different controllers independently. In addition, the experimental results obtained using an induction motor show that the circulating current can be reduced to within 2% of the rated motor current when the motor current response exceeds 1000 rad/s. (C) 2001 Scripta Technica, Electr Eng Jpn, 136(2): 38-48, 2001.