  Distribution grid flexibility is discussed as a possible measure in curative
system operation, yielding a need for an efficient and robust coordination
mechanism for the joint flexibility provision by individual units to the
transmission grid. This paper introduces a method to coordinate distribution
grid level flexibility as a fast-responding curative measure based on Online
Feedback Optimization. We utilize an optimization algorithm in a closed loop
with the distribution grid to dispatch set points for active and reactive power
to flexibility providing units. The approach is evaluated in an experimental
setup, utilizing assets connected to an exemplary low voltage grid. Online
Feedback Optimization is found to be both a viable as well as a highly
effective approach to coordinate distributed energy resources in real-time
curative system operation.
