Editorial of the special issue: Interactions between AC and DC power systems, and related considerations

Across the world, in order to meet environmental targets, electricity power networks are transitioning from conventional, predominantly fossil-fuelled synchronous power generation towards renewable and other low-carbon alternatives. These resources, particularly wind and solar, are often not located in areas for which historic transmission networks were originally designed. These resources also interface that network based on inverter connections to the grid, whose performance is wholly driven by the nature of their control & protection (C&P) logic, rather than inherently responding based on their physics, as would a synchronous machine. In addition, the need for connection, network extension, and AC network reinforcement is increasingly driving the global adoption of HVDC systems, whether as standalone point-to-point designs or more complex multi-terminal DC networks serving multiple purposes. HVDC is again an inverter/rectifying power electronic converter interface to AC grids, both existing and new (e.g. offshore) whose performance is defined by the nature of their C&P. In all cases, these HVDC systems and inverter-based resources (IBR) must be interfaced with the existing AC grid, ensuring that the performance required to maintain AC network stability and security for consumers is upheld. Additionally, care must be taken to prevent interactions between AC-connected resources and between the AC and DC systems, and within the DC system that could lead to instability. This represents a need for new tools, and techniques for managing these new systems, alongside approaches able to manage the black-boxed nature of the C&P being considered, where its effect can be observed, but not the underlying structural or tuning detail that causes it.

Electrical power networks are currently undergoing a significant transition towards IBR and HVDC dominance. There are several challenges across this transition, including dependency on the availability of renewable energy resources, low inertia, lack of spinning reserve, insufficient fault current for the correct operation of AC protection, and the inability to source or sink large amounts of active power in weak or remote areas of the AC networks. As a result, future power networks with increased renewable generation will face two major challenges: reliability and stable operation.

Grid-forming converters and HVDC systems have recently been proposed as some of the key approaches that may address these challenges. In order to achieve this objective further investigation and consideration of innovative solutions are required to best tackle key technical issues such as interactions between AC and DC power systems, coordination between grid-forming converters, grid-following converters, and conventional power plants, stability concerns in weak AC grids, multi-terminal HVDC operation, DC network stability and its capacity to support AC stability, multi-terminal multi-vendor interoperability, and the development of efficient and reliable DC circuit breakers to ensure the safe and dependable operation of future power systems.

The Special Issue editors would like to extend their gratitude to all the authors for their valuable contributions, the reviewers for their insightful feedback on the papers, and the IET staff for their administrative support, without which this special issue would not have been possible.