DC microgrids have attracted significant attention over the last decade in both academia and industry. DC microgrids have demonstrated superiority over AC microgrids with respect to reliability, efficiency, control simplicity, integration of renewable energy sources, and connection of dc loads. Despite these numerous advantages, designing and implementing an appropriate protection system for dc microgrids remains a significant challenge. The challenge stems from the rapid rise of dc fault current which must be extinguished in the absence of naturally occurring zero crossings, potentially leading to sustained arcs. In this paper, the challenges of DC microgrid protection are investigated from various aspects including, dc fault current characteristics, ground systems, fault detection methods, protective devices, and fault location methods. In each part, a comprehensive review has been carried out. Finally, future trends in the protection of DC microgrids are briefly discussed.

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DC Microgrid Protection: A Comprehensive Review

Gallium Nitride High Electron Mobility Transistors (GaN HEMTs) enable higher efficiency, higher power density, and smaller passive components resulting in lighter, smaller and more efficient electrical systems as opposed to conventional Silicon (Si) based devices. This paper investigates the detailed benefits of using GaN devices in transportation electrification applications. The material properties of GaN including the applications of GaN HEMTs at different switch ratings are presented. The challenges currently facing the transportation industry are introduced and possible solutions are presented. A detailed review of the use of GaN in the Electric Vehicle (EV) powertrain is discussed. The implementation of GaN devices in aircraft, ships, rail vehicles and heavy-duty vehicles is briefly covered. Future trends of GaN devices in terms of cost, voltage level, gate driver design, thermal management and packaging are investigated.

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Current Status and Future Trends of GaN HEMTs in Electrified Transportation

GaN field effect power transistors based on Si substrate show low on-state resistance and very small Cgs capacitance. Therefore these devices are good candidates for high-frequency switching operation. In this paper, we first focus on reverse conduction and transistors behavior during dead times in an inverter leg structure. Then we present an approach by calorimetric method, dedicated to transistors losses evaluation during operation. Using this method, we evaluate in a single measurement the transistors temperature and losses versus a chosen dead time or versus frequency. At least, we conclude on good practices regarding the drive of these components.

Optimization of the Driver of GaN Power Transistors Through Measurement of Their Thermal Behavior

Using wideband gap semiconductors (SiC, GaN) appears as possible solution to the growing demand for the development of high temperature, high frequencies power electronics This will help downsizing the current Power Electronics systems and extend their range of operation conditions The aim of this paper is to present the results obtained concerning the packaging of a full-wave rectifying bridge using SiC Schottky Barrier Diodes, capable to operate above 300°C The diodes are rated 1200 V and 50 A The proposed packaging is based on an insulating-gas encapsulation and stacking of the components The rectifying function was tested by feeding a current trough the bridge at room temperature, and while applying high voltage in an insulating gas-filled heated chamber Results show that the type of interconnection proposed can withstand at least 10 A and 200 V in rectifying conditions Gas encapsulation allowed for an operation of the diodes under high voltage conditions up to 350°C even if at this temperature diode leakage current is too high to perform an appropriate rectification

Packaging of SiC-SBD for high temperature operation

Aluminium oxide (Al2O3), Aluminium Nitride (AlN) and Silicon Nitride (Si3N4) are ceramics commonly used in power electronics modules, and are the current candidates for high temperature applications. A first study has previously shown different charge displacement behaviors at high temperature (up to 400°C). Surface potential measurements revealed a fast decay for AlN and Al2O3 with the increase of the temperature while on Si3N4 charges remain on surface at 400°C. However, these charges may move through the surface or be injected in the volume of the ceramics. - Surface potential results are correlated with broadband impedance spectroscopy and to current-voltage measurements of the ceramics. AlN and Al2O3 have a resistive-like behavior at high temperature, characterized by a space charge limited conduction current mechanism at high fields. For Si3N4, despite its resistivity decreases with temperature, it shows a dielectric behavior and an ohmic conduction mechanism over the studied temperature and field ranges.

High temperature behavior of ceramic substrates for power electronics applications

Using wideband gap semiconductors (SiC, GaN) appear as possible solution to the growing demand for the development of high temperature power electronics. These devices are devoted to work in harsh environments mainly for "on board applications" like automobile, aircraft and space exploration. In some cases, high temperature is associated to the environment itself (when the converter is located in the vicinity of a thermal engine for example) but in other, it is only related to the increase of the power density associated to power integration. However, the opportunities given by these wideband gap semiconductors devices may be reduced since their packages are inadequate as regards insulation, interconnection and thermal management under high-temperature operations and environments. The aim of this paper is to present the results obtained concerning the insulating parts of this packaging, mainly secondary passivation and encapsulation in a 200 -450 °C temperature range. Gas insulation is used and this original solution is examined for different gases and conditions.

Hiroki Kawano

Papers

High temperature high voltage packaging of wideband gap semiconductors using gas insulating medium