  Though considerable effort has been devoted to exploiting generation-side and
demand-side operational flexibility in order to cope with uncertain renewable
generations, grid-side operational flexibility has not been fully investigated.
In this review, we define grid-side flexibility as the ability of a power
network to deploy its flexibility resources to cope with the changes of power
system state, particularly due to variation of renewable generation. Starting
with a survey on the metrics of operational flexibility, we explain the
definition from both physical and mathematical point of views. Then conceptual
examples are presented to demonstrate the impacts of grid-side flexibility
graphically, providing a geometric interpretation for a better understanding of
the concepts. Afterwards the formulations and solution approaches in terms of
grid-side flexibility in power system operation and planning are reviewed,
based on which future research directions and challenges are outlined.
