UC Santa Cruz

In this thesis, bidirectional isolated dc–dc converters are analyzed for MVDC applications. Medium voltage dc (MVDC) is an attractive architecture for some power systems including all-electric ship systems based on numerous advantages including increased efficiency and reduced cost. The US Navy is investing in MVDC technology for future shipboard power systems. In this dissertation, an active 5-level T-type converter is used on the primary and secondary sides of a high-frequency transformer. The modified T-type converter features higher efficiency, fault tolerant capability, and smaller filter size compared to the common dual-active bridge dc–dc converters. The operation of the proposed converter is optimized based on minimizing power losses. For the optimization, the transformer core loss, reactive power, and rms current are considered. Wide bandgap devices are used for the minimization of semiconductor switching losses. A control method based on the Fourier series and decomposition theorem is proposed. A fault-tolerant analysis is carried out for this converter and post-fault switching methods are proposed for each faulty condition. To reduce voltage overshoot, a laminated dc-link is designed, and a two-level turn-off method is used for operating frequencies around 75 kHz. The steady state and dynamic operations of the proposed structure are verified through experiments. In addition, a neural network-based fault diagnostic model is developed. By applying feature selection methods, the model achieves an accuracy of over 95%. The developed model is implemented in real-time to detect and locate open-circuit failures of the switches. The detection duration for various types of single and double failures ranges from 10 to 60 cycles.