Popularity of DC distribution systems is increasing for many residential and industrial applications such as data centres, commercial and residential buildings, telecommunication systems, and transport power networks etc. Compared to AC systems, they have demonstrated higher power efficiency, less complexity, and more readiness of integrating with various local power sources and DC electronic loads. However, one of the major technical issues hindering this trend is the lack of effective DC fault protection devices/circuits. Although conventional electromechanical circuit breakers work well in AC systems, they are not suitable for DC systems due to their long response time (ranging from tens of milliseconds to hundreds of milliseconds). Such a long response time is far beyond the withstand time (typically tens of microseconds) of most power electronic devices in short-circuit operating conditions. In contrast, Solid-State Circuit Breakers (SSCBs) are able to offer ultrafast switching speed thanks to the modern power semiconductor devices which can turn off in microseconds or even in tens of nanoseconds. Furthermore, the ever-increasing fault current level in DC systems poses a significant mechanical and thermal stress on the whole DC system. Therefore, the desire for the protection devices with the feature of fast switching speed along with the current-limiting capability has prompted intensive research in this area over the last decade in both academia and industry. However, the relatively high conduction losses and limited short-circuit capability are two of the major drawbacks of SSCBs. With the growing maturity and increasingly commercial availability of Wide-Bandgap (WBG) semiconductor devices, a SSCB based-on WBG devices is a promising solution to alleviate the issues since WBG semiconductors have demonstrated superior material properties over the conventional silicon material such as lower specific on-resistance, higher junction temperatures and higher breakdown voltage.
This research aims to design and develop a WBG-based solid-state circuit breaker for a 400V DC microgrid application. To accomplish this task, this work starts with a comprehensive review of DC microgrid technology followed by an extensive review of the state-of-the-art DC circuit breakers. Then, to develop a circuit topology for the proposed SSCB, a practical current limiter is analysed, simulated, and evaluated. Based on this topology, the proposed SSCB is configured with a high-voltage normally-on Silicon Carbide Junction Field Effect Transistors (SiC-JFETs) cascading a low-voltage normally-off power MOSFET. This solution offers several advantages. For example, it does not require any additional sensing and tripping circuitry for short-circuit protection and therefore has a fast response speed. Meanwhile, the use of power SiC JFETs tends to reduce the conduction losses and enhance the short-circuit robustness of SSCBs. In addition, it offers the feature of current limiting which could ease the thermal and mechanical stresses on the whole DC system. The operating process of the proposed SSCB is analysed and the analytical results are compared with the simulated results; In the end, a prototype SSCB has been built and evaluated for short-circuit protection in a 400V DC system. In addition, to effectively suppress the overvoltage at the turn-off of SSCBs, a novel hybrid snubber circuit has been proposed by taking into account the advantages offered by both conventional Resistor-Capacitor-Diode (RCD) snubbers and Metal-Oxide Varistors (MOVs). Finally, other functions of the proposed SSCBs including overload protection, over temperature protection and protection coordination have been investigated and some operating issues such as false tripping and SSCB reset have been addressed.
