Power Electronics in Hybrid AC/DC Grids and Microgrids

Dear Colleagues,

Power electronic converters, particularly voltage source converters (VSCs), are fundamental components of hybrid AC/DC grids and microgrids. Converters are used to interface renewable energy sources like solar panels and wind turbines with the grid, ensuring optimal power extraction and grid integration. In hybrid AC/DC grids, multi-terminal High Voltage Direct Current (HVDC) systems are employed to connect AC and DC networks, enabling efficient long-distance transmission and interconnection of different grid systems. Furthermore, power electronic converters facilitate the integration of energy storage systems (ESS) into microgrids, enabling the storage of excess energy during periods of low demand and supplying it during peak demand or when renewable energy sources are unavailable. Battery energy storage systems (BESS), supercapacitors, and flywheel energy storage systems are commonly used in microgrids for energy arbitrage, frequency regulation, and grid support functions.

As the proportion of converter-interfaced generation (CIG) in the AC grid increases, we are encountering new challenges, including reduced system strength (including lack of inertia), subsynchronous oscillations (SSOs), and issues related to small- and large-signal stability and protection. These challenges arise when using legacy grid-following (GFL) technology. This demands a new approach to dynamic modeling frameworks, stability analysis tools, fault analysis, and protection design and motivates the research community to propose innovative controls, including the application of grid-forming (GFM) technology. Moreover, research on protection against DC faults in hybrid AC/DC grids with high CIG penetration is in its nascent stage.

This Special Issue aims to focus on the existing and emerging challenges in hybrid AC/DC grids and microgrids pertaining to modeling, benchmarking, stability analysis, short-circuit fault analysis and protection, and controls. Specific topics of interest are as follows:

1. Modeling of such systems with very high penetration of CIG that can capture recently observed phenomena like SSOs and validation of models through electromagnetic transient (EMT) simulations.
2. Hybrid AC/DC grid and microgrid fault analysis and protection with very high CIG penetration.
3. Small- and large-signal stability analysis of the system in the presence of a significant CIG penetration.
4. New control approaches to solve challenges posed by traditional GFL technology in weak grid conditions as applied to the DC grid converters and CIGs.
5. Application of grid forming (GFM) technology in DC grids and CIGs when interfaced with weak AC grids.
6. Provision of ancillary support (like primary frequency support) through the DC grid and CIGs to the AC system including in microgrids.

Dr. Nilanjan Ray Chaudhuri
Dr. Ali Mehrizi-Sani
Guest Editors

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• grid forming converter
• grid following converter
• PLL
• HVDC
• multiterminal DC
• converter-interfaced generation (CIG)