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 units 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 units 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 units ramping and reserve flexibility within the cluster and could be directly applied to long-term planning models without significantly increasing their computational burden.
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