Soft-switching solid-state transformer for traction applications
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Conventionally, power converters in traction systems interface with the medium-voltage (MV) catenary through a low-frequency transformer (LFT) operating at 16.7 Hz/50 Hz/60 Hz. The LFT converts the MV AC to low-voltage (LV) AC where low-voltage-rated power electronics are used to convert the LV AC to DC to interface with traction motor drives. However, the LFTs are heavy and less efficient. The drawbacks of the conventional traction converters can be overcome by the next-generation traction converters. In the next-generation traction converters, instead of using an LFT to interface with MV AC grid, MFT-based converters are used. Due to their attractive features, the MFT-based traction converters have attracted significant research interest and multiple topologies have been investigated. With the advent of SiC devices, the performance of those converters can be further improved. However, simple drop-in replacement of Si devices with SiC ones can result in issues such as electromagnetic interference (EMI) caused by large dv/dt (30-50 kV/µs). A novel soft-switching solid-state transformer (S4T) has been proposed in previous work to offer significant benefits including absence of switching losses, reduced and controlled dv/dt, and benign fault modes for MV applications. Because of the superior advantages of the S4T over the traditional voltage-source converters (VSCs), modular-S4T (M-S4T) is a promising candidate for the next-generation traction converters. Reverse-blocking (RB) devices are used as the main switching and resonant devices in the S4T. However, very little is known about the detailed behavior of such RB devices, especially when used in current-source converters (CSCs) with zero-voltage switching (ZVS). To control the M-S4T and maintaining the voltage sharing between series-connected S4T modules, a novel fast dynamic control algorithm called model predictive priority-based switching (MPPS) has been proposed in previous work. Although the fast dynamic control algorithm is developed, due to deadbeat control feature of the MPPS control, any delay in the communication between the S4T modules will have an adverse impact on the converter performance. Therefore, a control architecture using the MPPS is required to operate the M-S4T to achieve stable operation and fast transient performance. In addition, despite significant work in HIL simulation of VSCs, there is very little knowledge about HIL simulation for CSCs with small energy storage element using fast dynamic control. Considering the lack of knowledge of the M-S4T in the aforementioned areas, this dissertation presents deep investigation of the M-S4T for traction applications with clear understanding, solution and improvement to the aforementioned shortcomings including investigation of the power devices and design/ verification of control architecture as well as optimization of the converter.