Design and Testing of a SiC-based Solid-State Bypass Switch for 1 kV Power Electronics Building Blocks

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Virginia Tech


Over the past two decades, power consumption has increased exponentially worldwide, posing new challenges to power grids to meet the load requirements. With this growing power demand, the need for efficient high-density medium-voltage (MV) power converters has increased to support flexible power distribution grids. The modular multilevel converters (MMC) became the most typical MV power converters in applications from 2010. This topology has many advantages, such as voltage scalability, excellent output performance, and low voltage ratings for switching devices. However, without the excellent reliability of the MMC, applications cannot reap these benefits. The MMC topology comprises several series-connected submodules (typically a half-bridge or a full-bridge inverter). As a result of increased switching devices, the converter becomes vulnerable since a single device fault can disrupt the whole converter operation. Therefore, fault-tolerant strategies to replace faulty SM with a redundant SM are developed using additional bypass switches. Conventionally TRIACs and vacuum switches are employed as bypass switches that operate in the range of 2-10 microseconds. Despite having performance advantages, MMCs are still not fully employed in aerospace and naval industries due to their enormous size. Many Power Electronics Building Blocks (PEBB) are proposed, with size optimization, as submodules for modular converters. The PEBB1000, a 1000 V- PEBB proposed by Dr. Jun Wang, achieved a significant size reduction of 80% with a novel switching cycle control (SCC) scheme. This novel control scheme requires high switching frequency and high di/dt-currents for MMC operation. Due to di/dt-rate limitations, TRIAC-based switch cannot perform bypass operation. Therefore, research work has been conducted on bypass switches for PEBB1000 using wide-bandgap SiC devices. This thesis presents the design of a SiC MOSFET-based bypass switch for PEBB1000 in MMC application. A detailed fault case analysis is presented to show the feasibility of the bypass operation for 90% PEBB-level faults. Significant variations in PEBB1000 bypass requirements are observed through SCC-based MMC simulations. Accordingly, a 1700 V, 100 A bypass switch has been designed using the anti-series topology of MOSFETs. Various specifications, such as 142 nanoseconds operation time, 500 nanoseconds bypass commutation time, and 277A transient current conduction capability, are validated through practical tests. Results prove that SiC-MOSFETs work better than TRIACs in high di/dt-current conduction and operation times. For future work, false-triggering endurance has to be analyzed for 1000 V switching voltage.



SiC MOSFET, bypass switch, power electronics building blocks, submodules, protection, modular multilevel converters