  This paper proposes a Clustered Unit Commitment (CUC) formulation to
accurately model flexibility requirements such as ramping, reserve, and
startup/shutdown constraints. The CUC is commonly applied in large and
long-term planning models to approximate the units' operational flexibility in
power systems due to its computational advantages. However, the classic CUC
intrinsically and hiddenly overestimates the individual unit's flexibility,
thus being unable to replicate the result of the individual UC. This paper then
present a set of constraints to correctly represent the units' hidden
flexibility within the cluster, mainly defined by the individual unit's ramping
and startup/shutdown capabilities, including up/down reserves. Different case
studies show that the proposed CUC replicates the objective function of the
individual UC while solving significantly faster, between 5 to 311 times
faster. Therefore, the proposed CUC correctly represents the individual unit's
ramping and reserve flexibility within the cluster and could be directly
applied to long-term planning models without significantly increasing their
computational burden.
