Cooling the Lines: A Weather-forced Thermal Line Rating

In The Netherlands a fixed climatological value is used to estimate the maximum transport capacity of power lines. The reference capacity is based on a very conservative approach assuming relatively extreme conditions for the Netherlands with an air temperature of 30 C and a 0.6 m/s wind speed. This conservative approach results in a very safe mode of operation for the grid, however, it means that a great deal of capacity goes unused.

We are developing an operational forecast model that calculates the relative transport capacity up to 3 days ahead, expressed in probabilities. The model uses the Cigre algorithms, which allows us to calculate a cooling capacity of a power line under certain weather circumstances. Based on Ohms law the relative cooling capacity is then translated into a relative transport capacity.

The climatological value corresponding to the conservative approach is regarded as a 100% transport capacity. The model will calculate a relative transport capacity based on the weather forecast which will in most cases exceed the 100%. And, during windy conditions the value could reach over 150%. Based on the uncertainty of the weather forecast (using ensembles and an estimate of the forecast error) the relative transport capacity probability values are also calculated. For example an 80% probability value will say that there is a chance of 80% that a certain relative transport capacity will be exceeded.

The model will make an hourly forecast for each span of a power line - a span is the power line between two pylons. The weather for each span is calculated by weighting forecast data of nearby meteorological stations. Special attention is given to wind speed since this is by far the most important parameter. Initial results will be presented.