Dynamic Line Rating - Thermal Line Model and Control

Abstract: In times of increased environmental awareness, there exists a desire to not only build new environmentally friendly and renewable power generating units, but also to increase the efficiency of all existing technology. Wind power is such a renewable and environmentally friendly technology. Wind turbines and wind power have made giant leaps in the world in the past years, with an almost exponential growth in production and number of units. This increased utilization puts a lot of stress on the already existing grids, which were oftentimes built without this power increase in mind. To further add to these problems, wind power particularly is something that is not always readily available, neither is it stored. Dynamic line rating is a way of optimizing the power throughput through the distribution and transmission lines, by not only looking at the current through them as a way to determine its rating, but also by measuring how the weather affects the thermal system. In reality, several factors decide the line rating, which is how much current a line can carry. These weather impacting parameters include; wind speed, wind direction, solar irradiation and ambient temperature, not only the current through the conductor. Normally, the rating of the line is decided conservatively with a fixed value for the maximum current so as to make sure the surface temperature and resulting sag is always within acceptable boundaries. With these additional parameters however, a higher line rating can often be used. Considering the cooling effect of the wind means dynamic line rating is ideal for wind power, since in that case, the wind both produces power while allowing for a higher power throughput on the line. By continuously measuring all of these parameters, one can more effectively determine the rating of a line and the available amount of additional power it can carry, while still meeting regulations. This thesis explores dynamic line rating in connection with an E.ON offshore wind farm project at Kårehamn near Öland in Sweden. This is done by first building a Simulink computer model for the continuous determination of the conductor surface temperature – and based on this implementing an algorithm for controlling the output of Kårehamn wind farm. Rather than calculating the ampacity, the line rating, which is often done, this model will focus on explicitly controlling the temperature – since it is the temperature requirements that must be met in order to meet regulations regarding the line sag, the height of the lines above ground. A model and a control method were produced, that successfully controlled the surface temperature of the most critical conductor by sending a reference value to control the output of the wind farm. By building thermal models with input parameters such as current (converted from the power value), wind speed, wind direction and ambient temperatures, the surface temperature could be accurately calculated. The control algorithm developed in this work is compared to and found superior to an existing prototype solution used by E.ON. This thesis also successfully incorporated error handling and ways of controlling additional power generating units on the grid.