Time to AC-t: When AC-transmission changes capacity expansion planning

Abstract

Capacity expansion models often dramatically simplify the physics of how power flows through a transmission network in order to preserve computational tractability. However, with the rapid growth of electrification, large loads, renewables, and storage, increased realism in planning future grids is imperative. We extend the open-source capacity expansion model GenX with AC optimal power flow physics, and analyze generation, storage, and transmission investment decisions for 20-zone, hourly models of Germany and Texas. We compare our results against those using two common approximations, the transport model and DC optimal power flow. We find that total costs and emissions largely agree across all formulations. However, we find substantial differences in outputs related to transmission, generation siting, storage, and reactive-power support between the approximate formulations and the true AC physics formulation. We identify where AC physics changes planning decisions and discuss how to potentially capture these effects without the full computational cost.