Multilevel converters have a number of capacitors as sources of multilevel voltages. The volume of the capacitors should be minimized to realize a high-power-density converter. In this paper, the selection criteria of the capacitors in a flying capacitor converter are investigated. In this type of converter, the required capacitance, consequently, volume of the capacitors are decreased when the PWM carrier frequency is higher. However, there is a limitation on the reduction in the volume due to the temperature increase caused by the power losses in the capacitors when the volume becomes small during high-PWM-frequency operation. On the basis of an investigation including the temperature increase, it is clarified that a capacitor with a high capacitance density is not necessarily the best for the flying capacitor depending on the operating conditions.
In motion control systems, detailed characteristics of power converters are usually not considered. Practically, voltage ripple, harmonics, and electromagnetic interferences (EMI) generated in widely used 2-level inverters become concerns to realize a high-performance control. Compared with 2-level inverters, multi-level (ML) inverters essentially reduce these problems. Furthermore, the equivalent carrier frequency of N-Ievel ML inverters is expected to be N-l times higher than that of the 2-level inverters in case that a carrier phase-shifted modulation is utilized. This paper focuses on the equivalent carrier frequency and studies the performance and stability of current control systems by using ML inverters.
A power flow controller (PFC) that realizes flexible power flow control in DC power networks is investigated herein. In previous studies, an additional node called a compensation node was introduced in PFC. This compensation node is necessary for regulating the link voltage that decreases owing to circuit loss. However, as the compensation node is not directly related to the power flow control, adding the compensation node may increase the cost. In this paper, we propose a link voltage control method for PFC without the compensation node. Adding the current command for the link voltage control to the current command for the power flow control makes it possible to realize flexible power flow control and constant link voltage control without the compensation node. The effectiveness of the proposed method is verified experimentally.
