Transmission Network Expansion Planning: Bridging the gap between AC heuristics and DC approximations

It was recently observed that a significant gap exists between the costs of Transmission Network Expansion Planning (TNEP) solutions produced by the DC power flow approximation and AC power flow heuristics. This paper confirms the existence of that gap and shows that DC-based TNEP solutions exhibit significant constraint violations when converted into AC power flows. The paper then demonstrates how to bridge this gap with the LPAC power flow model, a linear-programming approximation of the power flow that captures reactive power and voltage magnitudes. Indeed, LPAC-based TNEP solutions have minimal violations in AC power flows and provide high-quality solutions. The strength of the LPAC formulation is further demonstrated on the joint optimization of line expansion and VAr compensation, as well as a competitive market study. These studies suggest that the underling TNEP formulation has significant impacts on the proposed expansion plans.