Comprehensive review of transmission system operators–Distribution system operators collaboration for flexible grid operations

The digitalization transforms grids into a much more complex grid structure than it is today. Based on this transformation, commitments to renewable energy sources (RESs), energy integration at large and small scales, bidirectional energy flows, and storage resources are increasing. Because many of today's electrical grids are obsolete, these developments require radical changes to our existing power systems. The flexibility of the market is essential for the increasing number of small and large-scale producers, consumers, and prosumers. Transmission System Operators (TSOs) and Distribution System Operators (DSOs) will be key buyers from this market, but their effectiveness depends on efficient collaboration and use distributed flexibility resources at scale. In particular, Distributed Energy Resources (DERs) play an essential role in ensuring market flexibility and balancing flexible networks. The use of DERs at both the transmission and distribution level enables collaboration between TSOs and DSOs. This article discusses the individual roles of TSOs and DSOs and the challenges they face in the power system. Coordination schemes and key stakeholders are presented. It comprehensively examines optimization-based solution and blockchain-based applications developed for TSO and DSO coordination challenges and provides a roadmap for developing a more efficient, reliable, and sustainable power system in the future.