Isolated Modular Multilevel DC–DC Converter With DC Fault Current Control Capability Based on Current-Fed Dual Active Bridge for MVDC Application

In this paper, a current-fed modular multilevel dual-active-bridge (CF-MDAB) dc–dc converter is proposed for medium-voltage dc (MVDC) application. The proposed converter inherits favorite characteristics of DAB circuits including soft switching and small passive components. Thereby, high efficiency and high power density can be achieved. Moreover, with direct input and output dc current control, the CF-MDAB is suitable for a breakerless MVDC system since it can realize dc fault ride-though operation. In addition, the dv/dt in the converter is mitigated with the quasi-three-level modulation. In this paper, the proposed converter is applied to integrate the battery energy storage to a MVDC grid as an example to illustrate its operation principles and fault current control capability. The operation principles are presented for both normal and dc fault conditions; the dynamic models are also derived not only under normal operation mode but under dc fault operation mode as well. The control systems under different operation modes are designed, respectively, based on the developed mathematical models. A downscaled 40-kHz 3-kW CF-MDAB prototype was built in the laboratory. The experimental results under both normal condition and dc fault condition verified the analysis as well as the control performance of the proposed converter.