As fast and ultrafast charging stations (FCS/UFCS) increase their degree of penetration in the grid, it becomes beneficial to connect them directly at the medium-voltage (MV) level, as this reduces volume, losses, and cost. Recent solutions propose the replacement of the typical combination of the low-frequency transformer plus low-voltage rectifier by a solid-state transformer (SST), which can be composed, among other topologies, by a modular multilevel converter. This article proposes and discusses the application of the modular multilevel converter (MMC) in FCS infrastructures, utilized as an active front-end rectifier, either as part of an SST or for direct connection to the second stage, comprised of isolated dc/dc converters. Focus is given on the advantages and limitations of the converter operating as a step-down rectifier, where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$U_\text{DC}&lt; \hat{U}_\text{AC}$</tex-math></inline-formula> . As different architectures are presented, first the MMC step-down constraints and optimum operation points are explored and discussed, then the architectures are evaluated in terms of key performance indicators, such as size, efficiency, and reliability. At the end, experimental results for a full-scale MV MMC are presented, validating the analysis carried throughout this article. 

Analysis, Limitations, and Opportunities of Modular Multilevel Converter-Based Architectures in Fast Charging Stations Infrastructures

As fast and ultrafast charging stations (FCS/UFCS) increase their degree of penetration in the grid, it becomes beneficial to connect them directly at the medium-voltage (MV) level, as this reduces volume, losses, and cost. Recent solutions propose the replacement of the typical combination of the low-frequency transformer plus low-voltage rectifier by a solid-state transformer (SST), which can be composed, among other topologies, by a modular multilevel converter. This article proposes and discusses the application of the modular multilevel converter (MMC) in FCS infrastructures, utilized as an active front-end rectifier, either as part of an SST or for direct connection to the second stage, comprised of isolated dc/dc converters. Focus is given on the advantages and limitations of the converter operating as a step-down rectifier, where $U_\text{DC}< \hat{U}_\text{AC}$. As different architectures are presented, first the MMC step-down constraints and optimum operation points are explored and discussed, then the architectures are evaluated in terms of key performance indicators, such as size, efficiency, and reliability. At the end, experimental results for a full-scale MV MMC are presented, validating the analysis carried throughout this article.

The increasing penetration of offshore wind power generation promotes the revolution of wind turbine toward high-power application. The development of high-power wind turbine undoubtedly poses new technical challenges. This article presents a comprehensive overview for high-power wind energy conversion system (WECS) from key technique aspects, including topologies, stability, reliability, and ancillary service capability, and further investigates the key challenges and potential solutions. The various topologies of power converter applied in high-power wind turbine are first reviewed and discussed. The different semiconductor technology, including recent wide bandgap devices, and their potential contributions for offshore high-power converters are discussed. Furthermore, the potential stability issues of high-power WECS are investigated and reviewed. Also, the stabilization control strategies are discussed. Further, the reliability issue and enhancement strategies of high-power WECS are discussed, including condition monitoring, active thermal control strategy, and fault-tolerant operation method. In addition, the ancillary service capability of high-power WECS, including frequency-active power control capability and voltage-reactive power control capability, is reviewed. Finally, the future trends and potential solutions are discussed in this article.

A Comprehensive Overview of Power Converter Applied in High-Power Wind Turbine: Key Challenges and Potential Solutions

The increasing penetration of offshore wind power generation promotes the revolution of wind turbine toward high-power application. The development of high-power wind turbine undoubtedly poses new technical challenges. This article presents a comprehensive overview for high-power wind energy conversion system (WECS) from key technique aspects, including topologies, stability, reliability, and ancillary service capability, and further investigates the key challenges and potential solutions. The various topologies of power converter applied in high-power wind turbine are first reviewed and discussed. The different semiconductor technology, including recent wide bandgap devices, and their potential contributions for offshore high-power converters are discussed. Furthermore, the potential stability issues of high-power WECS are investigated and reviewed. Also, the stabilization control strategies are discussed. Further, the reliability issue and enhancement strategies of high-power WECS are discussed, including condition monitoring, active thermal control strategy, and fault-tolerant operation method. In addition, the ancillary service capability of high-power WECS, including frequency-active power control capability and voltage-reactive power control capability, is reviewed. Finally, the future trends and potential solutions are discussed in this article. 

IGCTs are attractive power semiconductor devices for HVDC applications. However, their switching speed at turn-on can only be controlled by the means of an external snubber circuit, which adds complexity and cost. This paper investigates experimentally the possibility to downsize and even completely remove this turn-on snubber/clamp in the case of an MMC submodule based on 6.5 kV IGCTs, using fast silicon diode modules (rated at up to $13\ \text{kA}\ \mu \mathrm{s}^{-1}$). The removal is found to be possible, although limiting phenomena (dynamic avalanche and snap-off) appear, reducing the actual operating range of the submodule.

Experimental study of the reduction and removal of turn-on snubber for IGCT based MMC submodule using fast silicon diodes

IGCTs and IGBTs are compared in the case of a HVDC MMC. Specific figures of merit, and a current metric providing simple means to compare them, are introduced and discussed. Simulation results of a MMC model and figures of merit are shown to provide consistent results, proving that the proposed figures of merit are a very simple and fast way to select the best semiconductor switch. Furthermore, our analysis supports the growing interest in IGCTs for MMCs, as they are found to produce the lowest level of losses.

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Figures-of-Merit and current metric for the comparison of IGCTs and IGBTs in Modular Multilevel Converters

A custom package for SiC diodes is presented. It is designed to accommodate “research-grade” devices (available in small quantities, with large variations in characteristics, and, in our case, poor topside metallisation). It allows testing of the diodes at high voltage (> 5 kV) and high current (600A). The packaged diodes described here are used as a drop-in replacement for Si diode modules in a double-pulse setup. The packaging design is based on a “sub-package” approach, to achieve high production yield and facilitate exchange of devices failing during the tests. Packaging technologies include silver sintering for the die attach and the use of an original die topside pressing system with spring probes. Experiments validate the concept of the package.

Arthur Boutry

Papers

Figures-of-Merit and current metric for the comparison of IGCTs and IGBTs in Modular Multilevel Converters

Experimental study of the reduction and removal of turn-on snubber for IGCT based MMC submodule using fast silicon diodes

Design of a test package for high voltage SiC diodes