  Most of the new technological changes in power systems are expected to take
place in distribution grids. The enormous potential for distribution
flexibility could meet the transmission system's needs, changing the paradigm
of generator-centric energy and ancillary services provided to a demand-centric
one, by placing more importance on smaller resources, such as flexible demands
and electric vehicles. For unlocking such capabilities, it is essential to
understand the aggregated flexibility that can be harvested from the large
population of new technologies located in distribution grids. Distribution
grids, therefore, could provide aggregated flexibility at the transmission
level. To date, most computational methods for estimating the aggregated
flexibility at the interface between distribution grids and transmission grids
have the drawback of requiring significant computational time, which hinders
their applicability. This paper presents a new algorithm, coined as QuickFlex}
for constructing the flexibility domain of distribution grids. Contrary to
previous methods, a priory flexibility domain accuracy can be selected. Our
method requires few iterations for constructing the flexibility region. The
number of iterations needed is mainly independent of the distribution grid's
input size and flexible elements. Numerical experiments are performed in four
grids ranging from 5 nodes to 123 nodes. It is shown that QuickFlex outperforms
existing proposals in the literature in both speed and accuracy.
