Ancillary services in active distribution networks: A review of technological trends from operational and online analysis perspective

Originally, ancillary services (ASs) were provided by large-scale power plants to ensure the secure and reliable operation of power systems. However, with the advent of renewable energy sources either in large-scale or as smaller distributed generation units, there is a strong need to revisit the structure and aim of traditional ASs and reconsider them under two main concepts; ASs provided to distribution networks (DNs), aiming to solve local issues at the distribution level, and ASs offered to transmission system (TS), targeting to solve issues having a wider impact on the power system. The former can be considered as DN-oriented ASs, while the latter as TS-oriented ASs. In this changing environment, the increasing installation of measuring infrastructure has created new sources of data, providing the opportunity to improve the performance of several ASs by applying measurement-based analysis. This review paper gives an insight into the most important DN- and TS-oriented ASs foreseen to apply in future power systems. Additionally, state-of-the-art on measurement-based analysis techniques that can be used to facilitate the provision of several ASs by providing in real-time critical information regarding system dynamic performance is presented. •Detailed review on future ancillary services (ASs) in transmission systems.•Detailed review on future ASs in distribution systems.•Overview of measurement-based techniques to facilitate the provision of ASs.•Discussion on future research areas regarding ASs provision.