A Circulating Current Control Scheme for Multiple 3-Level NPC Inverter Systems

Abstract

In recent years, a motor drive system for a steel rolling mill is being a larger and larger capacity. Especially the development of the 3-level NPC (Neutral Point Clamped) inverters is focused on to replace the cycloconverters nowadays. In order to satisfy such a requirement, in this paper, a multiple 3-level NPC inverter system is proposed. The configuration is that two 3-level NPC inverters with the same configuration are connnected in parallel by current limiting reactors. In the case, a circulating current flowing between the two 3-level NPC inverters must be controlled to prevent the decrease of the maximum output capacity. Particularly, a new control method of the circulating current is mainly discussed. The method is on the basis of the space voltage vector PWM control for 3-level NPC inverters and the circulating current is controlled by re-adjusting each output period of the respective space voltage vector. According to the method, the circulating current controller can be decoupled with the motor current controller. Therefore, it is possible to design the different controllers independently. In addition, the experimental results show that the circulating current is limited under 2% of the rated motor current at the same time when the motor current response records more than 1000rad/s.