The Loadability of Overhead Transmission Lines—Part I: Analysis of Single Circuits

This paper provides a methodology for a thorough technical comparison between traditional three-phase ac lines, on the one hand, and possible (in some cases unconventional) alternative solutions that include HVDC lines, four-phase ac lines, and combined ac-dc lines, on the other. The comparison is based on the loadability curves, which are calculated here for all of the considered lines taking into account thermal limits, voltage drop, power losses, and-when feasible-steady-state and transient stability. The focus is on four-phase ac and combined ac-dc lines, for which the loadability curves are derived for the first time, whereas the loadability curves of three-phase ac lines-well-known in the literature-are derived and used here just as the natural reference for the comparison. Due to the great deal of theoretical and numerical developments, the study is split in two. Part I analyzes single-circuit lines of comparable loadability at the thermal limit (three phase, four phase, and monopolar HVDC). Part II broadens the analysis to double-circuit lines of comparable loadability at the thermal limit (three-phase, bipolar HVDC, and combined ac-dc) and performs an overall comparison among the different systems in single-circuit and double-circuit configuration.