Technical Basics — Static Ratings
Transmission lines are typically rated based on thermal capacity limits or mechanical deformation limits (sag). Static ratings are the limits set to the electric current on the line so to not exceed the lines designed thermal or mechanical limits. However, heat transfer from electric current is always compensated by wind and ambient temperature. Conservative static assumptions on weather is necessary to establish static ratings because weather conditions are not always adequately available to the operator or engineer.
Technical Basics — Real-Time Line Ratings
When extra capacity is needed on the line, system operators may consider applying ambient-adjusted ratings. While still using the static assumptions for wind, ambient-adjusted ratings are dynamic because real-time ambient temperature is used to momentarily uprate the lines. An assumption that gives some level of confidence in ambient-adjusted ratings is that temperature is easier to predict than wind; therefore, it is safer to use. Extra capacity is obtained on the line because ambient temperatures are usually lower than the static temperature assumptions, thus some cooling on the lines can be expected and beneficial.
Dynamc Line Ratings Compared with Static and Ambient Adjusted Ratings
Generally, the goal of the static rating is to set a limit on the amount of current for a line that should not be exceeded. When static ratings are compared with dynamic ratings, one finds that the dynamic ratings can be lower than the static ratings. For those periods where the static rating is above the dynamic rating, the static rating should be considered unsafe. Ideally, this situation should only occur about 5% of the time by design. Having an operational dynamic-line rating system can allow planners to test their conservative assumptions. Moreover, dynamic-line ratings can be applied on historical weather data to generate the static ratings from the beginning, thereby alleviating the system planner from assuming conservative weather conditions.
A similar situation occurs where the ambient-adjusted rating is higher than the dynamic-line rating. In these situations, more is not necessarily better as dynamic-line ratings should be close to the thermal reality occurring on the line. For this reason, we highly recommend using dynamic-line ratings in parallel with ambient-adjusted ratings.