  As the decarbonization of power systems accelerates, there has been
increasing interest in capacity expansion models for their role in guiding this
transition. Representative period selection is an important component of
capacity expansion modeling, enabling computational tractability of
optimization while ensuring fidelity between the representative periods and the
full year. However, little attention has been devoted to selecting
representative periods longer than a single day. This prevents the capacity
expansion model from directly simulating interday energy sharing, which is of
key importance as energy generation becomes more variable and storage more
important. To this end, we propose a novel method for selecting representative
periods of any length. The method is validated using a capacity expansion model
and production cost model based on California's decarbonization goals. We
demonstrate that the representative period length has a substantial impact in
the results of the capacity expansion investment plan.
