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Lookup NU author(s): Professor Vladimir TerzijaORCiD, Dr Shahab DehghanORCiD
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© 2026 Elsevier Ltd. The increase in electrical energy produced from renewable energy sources integrated into transmission and distribution systems, along with the displacement of dispatchable synchronous generators, indicates an impending shift away from the conventional provision of ancillary services. These services, crucial for maintaining a reliable and resilient system, are transitioning from being predominantly provided by transmission-connected synchronous generators to a mix of synchronous and non-synchronous generations, including resources connected to distribution networks. As the range of available resources accessible to power system operators expands continuously, there is an opportunity to enhance the flexibility in operating the system. To avail this opportunity, it is necessary to enhance the cooperation between transmission system operators and distribution system operators. In addition, the provision of ancillary services from distributed energy resources to the transmission network without compromising the power system security needs to be further explored. This critical review comprehensively examines the provision of ancillary services by aggregated distributed energy resources in active distribution networks to upstream transmission grids, covering DER types and aggregation mechanisms, optimization formulations and solution approaches, test systems and validation methods, and technical, regulatory, and financial barriers with mitigation strategies. Finally, conclusions and recommendations for future research are provided.
Author(s): Miah MS, Shah R, Aslam MU, Amjady N, Walker S, Terzija V, Dehghan S, Mithulananthan N
Publication type: Review
Publication status: Published
Journal: Renewable and Sustainable Energy Reviews
Year: 2026
Volume: 233
Print publication date: 01/06/2026
Online publication date: 02/03/2026
Acceptance date: 23/02/2026
ISSN (print): 1364-0321
ISSN (electronic): 1879-0690
Publisher: Elsevier Ltd
URL: https://doi.org/10.1016/j.rser.2026.116858
DOI: 10.1016/j.rser.2026.116858
