Comprehensive Design of Power Electronics Building Block Based Modular Medium-Voltage Direct Current Solid-State Circuit Breakers
Authors:   , Zhao Shuyan , Lu Fei 
Published: May 2024

Compared to traditional mechanical circuit breakers, solid-state circuit breakers (SSCBs) adopt semiconductor devices in both main conduction and fault handling branches, which are advantageous in terms of arc-free tripping, ultrafast response speed, and extended lifetime. The rapid development of wide-bandgap semiconductor technologies in the past two decades provides new opportunities to develop next generation ultrafast high power SSCB technologies. However, wide bandgap based SSCBs still face challenges in incomplete power electronics system design methodology and inevitable power loss, and limited power capability. This dissertation aims to advance the fundamental theoretical research and practical engineering implementation of wide bandgap SSCBs and provide a comprehensive design methodology for achieving high power, low loss, and high efficiency SSCBs with ultrafast response speed and high reliability. The main work of this dissertation mainly includes four aspects. First, this dissertation proposes a universal methodology for designing metal-oxide-varistor-resistor-capacitor-diode (MOV-RCD) passive voltage clamping circuit for unidirectional SSCBs based on main switches' thermal profiles. Then a mutated design of MOV resistor-capacitor-varistor (MOV-RCV) passive voltage clamping circuit is proposed to handle bidirectional fault interruption for SSCBs. When it comes to stacking up multiple main switches to realize medium-voltage direct current (MVDC) level, a novel resonant wireless converter with series-series parallel (S-SP) compensations is proposed to provide isolated constant-voltage outputs to power gate drive units of main switches in series. Thereafter, an economic implementation of high efficiency in natural cooling modular SSCB rated at 4kV/100A and 400kW is demonstrated, which features cell-disk-tower modular configuration and circular symmetrical layout. A record-breaking 99.98% efficiency at the thermal steady state is achieved on the implemented SSCB design. Second, complete modularity realization is further investigated for SSCB designs. A power electronics building block (PEBB) concept is introduced as the basic functional unit for modular SSCBs, which consists of auxiliary power supply, main switch device, gate drive unit, voltage clamping circuit, cooling unit, and other functional units. Each PEBB has full functions to operate as an individual SSCB, which provides flexibility to fit various applications by simply putting multiple PEBBs in series or parallel. A 1kV/40A and 40kW PEBB submodule is demonstrated, which features a resonant wireless energy pick-up auxiliary power supply system coupled gate drive and an active MOV based voltage clamping circuit. Finally, a 4kV/400A/1.6MW SSCB is implemented based on 40 identical PEBBs, in which 10 submodules are in parallel in each layer to support 400A nominal and 2kA fault current magnitudes, and 4 layers are in series to support up to 4kV DC nominal voltage. A high thermal steady state efficiency of 99.97% is recorded. Third, the limitations of conventional parallel-type passive voltage clamping based DC SSCBs are investigated in terms of main switch gate ringing issues and conflicts between main switch dv/dt, switching energy stress, and voltage overshot build-up time. Main switch gate oscillation issue with fault current bypass (FCB) voltage clamping based SSCBs is also investigated, whose driving factors of di/dt and dv/dt are analyzed. A novel soft turn-off SSCB with flexible dual-tripping schemes is proposed to solve this issue, which greatly enhances the operation safety of main switch when handling high magnitude fault. Lastly, a novel machine learning enabled cluster grouping of varistors for parallel structured SSCBs is proposed. It is common to parallel multiple varistors in modular parallel structured SSCBs to enhance fault handling capability. However, commercial MOVs are not originally designed for parallel connections. A huge discrepancy among commercial MOVs is revealed first, which causes unbalanced fault currents for paralleled MOVs, thus affecting their lifetime. Then a K-means algorithm-based machine learning clustering method is proposed to group MOVs before installing them into SSCBs, which significantly improves parallel MOVs lifetime from ~200 times to ~5000 times of operations. To sum up, this dissertation explicates the comprehensive modular design of MVDC SSCBs, focusing on voltage clamping circuit, main switch thermal, auxiliary power supply designs, and reliability and lifetime enhancement. In future work, the developed modular MVDC SSCB technology will be deployed in real MVDC power systems to evaluate its practicality and coordination performance.