Cooling of overhead power lines due to the natural convection

Abstract

The development of Dynamic Thermal Rating (DTR) systems for transmission lines in recent years has led to a better and safer utilization of the existing transmission network, as it, for most cases, enables transmission capacity of lines to be increased above the traditionally used static thermal rating value, when weather conditions are favourable. Yet, on-site measurements in the Slovenian system have also shown that many sites are regularly subject to conditions of very limited or even no winds, causing the line rating to fall well below the static value. In those conditions, the natural convective cooling can surpass forced convective cooling and correctly assessing natural convective cooling becomes crucial for the Transmission System Operators (TSO). Different mathematical models deal with natural convective cooling in different ways. Moreover, references have emerged that claim that cooling due to the natural convection gives the same results as with forced convection at 0.6 m/s crossing wind. To clarify the thermal behaviour of power lines when no external wind is present, this paper employs thermo-fluid simulation of natural convection and compares it with laboratory measurements for two different conductors. The results are also compared with CIGRE, IEEE and IEC guidelines.