  Flexibility requirements are becoming more relevant in power system planning
due to the integration of variable Renewable Energy Sources (vRES). In order to
consider these requirements Generation Expansion Planning (GEP) models have
recently incorporated Unit Commitment (UC) constraints, using traditional
energy-based formulations. However, recent studies have shown that energy-based
UC formulations overestimate the actual flexibility of the system. Instead,
power-based UC models overcome these problems by correctly modeling ramping
constraints and operating reserves. This paper proposes a power-based GEP-UC
model that improves the existing models. The proposed model optimizes
investment decisions on vRES, Energy Storage Systems (ESS), and thermal
technologies. In addition, it includes real-time flexibility requirements, and
the flexibility provided by ESS, as well as other UC constraints, e.g., minimum
up/down times, startup and shutdown power trajectories, network constraints.
The results show that power-based model uses the installed investments more
effectively than the energy-based models because it more accurately represents
flexibility capabilities and system requirements. For instance, the power-based
model obtains less investment (6-12%) and yet it uses more efficiently this
investment because operating cost is also lower (2-8%) in a real-time
validation. We also propose a semi-relaxed power-based GEP-UC model, which is
at least 10 times faster than its full-integer version and without
significantly losing accuracy in the results (less than 0.2% error).
