The solid-state transformer (SST) is a developing technology which is being integrated in low and high-voltage power systems, supporting the integration of renewable energy in smart grid applications and fast charging stations for electric vehicles (EV). SST is becoming more critical in certain applications of advanced power distribution systems as it provides several advantages over the conventional transformer including its controllability, small weight and size, potential low cost, and simplicity of connection for offshore wind farms. However, some challenges arise in modular SST design and implementation including control architecture for equal power balancing, protection, and communication compatibility that need to be addressed. This paper provides an overview of different SST communication protocols and offers prospects, applications, and future directions for research and development. A control architecture for input and output voltage balancing is presented for a direct power conversion (DPC) type SST system. A detailed SPI communication protocol is also provided which allows the implementation of decentralized operation of the modular SST system.

Wide band-gap (e.g. silicon carbide) power semiconductor devices find a good fit for solid state circuit breaker (SSCB) applications due to the superior performances like low conduction resistance and high blocking voltages. Although the SiC power devices could help reduce the SSCB conduction losses, the thermal stress is still one of the major challenges, especially during overload conditions, where a great amount of heat is generated in a short period. This paper investigates the use of phase change material (PCM) embedded heat sink as the cooling solution for wide band-gap devices in SSCBs to address the transient overload thermal stresses. Laboratory experiments for PCM filled heatsinks have been conducted and compared with the non-PCM filled heatsinks in this paper to explore its potential advantages in managing overload thermal requirement for wide band-gap devices. The results revealed that after the PCM is activated, the temperature rise speed under transient overload is greatly reduced within a timeframe which means the PCM based heatsink is effective to address the overload thermal stress and can also be a thermal reservoir temporarily in case any fan cooling fails.

Power semiconductor devices play an important role in circuit breakers (CB) to provide fast, arc-less, fault current interruption in dc distribution systems. Series connection of power devices is often used to increase the breaker voltage rating for application in medium voltage dc systems. This paper presents a low-cost gate driver circuit based on passive components which can achieve gate control of the series power device stack using only a single isolated gate driver IC. The circuit provides reliable coupling of the CB logic control signal while achieving a balanced device voltage distribution during the fault turn-off event. Transient voltage suppression (TVS) diodes are utilized to limit the turn-off voltage of the power devices and provide the current return path for the gate capacitor discharge of the upper device(s). Simulation and experimental results are provided to verify circuit operation in a dc solid state circuit breaker system.

Three generations of medium voltage fault isolation devices (FID) for a power electronics based distribution system, the FREEDM System, are reported in this paper. In the Gen-I FID, three 6.5 kV silicon IGBTs are series connected to achieve 15 kV class distribution voltage blocking capability. Whereas in the Gen-II and the Gen-III FIDs, 15 kV Silicon Carbide (SiC) ETO devices are used. Since one single such device can meet the voltage withstand requirement, the operation losses as well as dimensions are significantly reduced. Future improvements in the Gen-III FID include the use of hybrid schemes and the development of symmetrical blocking high voltage SiC devices to further minimize the operation losses.