Weather-based loading of overhead lines — Consideration of conductor's heat capacity

Power transmission capacity of overhead lines is limited by the maximal conductor temperature: should this maximal temperature be exceeded, unacceptable line sag as well as increased ageing of the conductor itself could result. This is especially the case for the most common steel reinforced conductors, where grease - used for protection against corrosion - may leak at high temperatures. Aside from the carried current, conductor temperature largely depends on the actual weather conditions: wind speed, ambient temperature as well as solar radiation may influence the thermal behaviour of a conductor at different rates. According to actual standards, adverse weather conditions are taken as a basis when determining the maximal current for normal operation, which is then characteristic for an overhead line. However, if precise knowledge of actual weather conditions is available, the current constraint may be adapted leading to higher utilisation of the overhead line. Usually, such information is given by weather forecasts and features an immanent forecast uncertainty. Moreover, it might be advantageous from an operational point of view to adapt the current constraint for a defined time interval in advance (e.g. for the next one hour), as this information has to be implemented into the settings of the protection system. In order to account for these uncertainties, the volatility of weather parameters depending on the forecast horizon should be examined. Moreover, the question, whether short-term variations of weather conditions could lead to unacceptable conductor temperature rise, has to be answered. Dynamic investigation of the thermal behaviour of overhead lines is therefore performed in this work, with special focus on the conductor's heat capacity. Additionally, steady-state calculations are performed and the results of both approaches (dynamic and steady-state) are compared. It is then possible to evaluate a safety margin for the calculated current constraint. A detailed thermal investigation of an Aluminum Conductor Steel Reinforced (ACSR) is given as a case study.