As global concerns regarding climate change are increasing worldwide, the
transition towards clean energy sources has accelerated. Accounting for a large
share of energy consumption, the electricity sector is experiencing a
significant shift towards renewable energy sources. To accommodate this rapid
shift, the transmission system requires major upgrades. Although enhancing grid
capacity through transmission system expansion is always a solution, this
solution is very costly and requires a protracted permitting process. The
concept of flexible transmission encompasses a broad range of technologies and
market tools that enable effective reconfiguration and manipulation of the
power grid for leveraged dispatch of renewable energy resources. The
proliferation of such technologies allows for enhanced transfer capability over
the current transmission network, thus reducing the need for grid expansion
projects. This paper comprehensively reviews flexible transmission technologies
and their role in achieving a net-zero carbon emission grid vision. Flexible
transmission definitions from different viewpoints are discussed, and
mathematical measures to quantify grid flexibility are reviewed. An extensive
range of technologies enhancing flexibility across the grid is introduced and
explored in detail. The environmental impacts of flexible transmission,
including renewable energy utilization and carbon emission reduction, are
presented. Finally, market models required for creating proper incentives for
the deployment of flexible transmission and regulatory barriers and challenges
are discussed