Constantin Balzer, M.Sc.
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In case of transmission of electrical energy by buried cables, the electrical losses will be dissi-pated to the environment in the form of heat. This leads to an increase of temperature of the conductor of the cable. As the latter is limited and directly proportional to the current, the so called thermal current rating represents one important limit of the cable capacity.
At present, the cable ampacity ratings are estab-lished based on standardized consumption patterns and conservative assumptions regarding the thermal properties of the bedding. Thereby, values below the actual limit are derived. This is particularly relevant for a high daily and seasonal dynamic in load patterns that is typical for power injection from photovoltaic generation. Moreover, the influence of natural water content changes of the surrounding soil on the thermal properties of the bedding is not taken into consideration.
The research project comprises primarily three distinct aspects: laboratory-based analyses under predefined boundary conditions; measurements under realistic conditions in a therefore built cable test site, and modelling the results using numerical simulations.
The results will finally be implemented into an online monitoring tool that provides the system operator with real time ampacity ratings that have been calculated with the help of measured or estimated data.
In order to quantify the thermophysical, hydraulic and geotechnical properties of soils and bedding materials, an evaporation test was modified among others to measure all relevant thermophysical and hydraulic properties of a sample simultaneously in comparatively short time.
To assess the dependency of these properties on the degree of compaction as well as the influence of the latent and convectional heat transport on the effec-tive heat conductivity, a special measurement device was developed and patented
Moreover, a true to scale experimental configura-tion was set up with the purpose of recreating the drying out of soil in the vicinity of the cable that has a significant negative impact on the ampacity rating.
Real-Scale Field Experiment Cable Test Site
In order to quantify the implact of environmental conditions, a large field experiment was built at TU Darmstadt in 2013. This experimental set-up ena-bles for investigations on various cables and cable arrangement in different bedding materials and soils under natural conditions. The overall dimen-sion of the test site is 14 m by 6 m, divided into four sections of 3.5 m length that are hydraulically and thermally decoupled by each other. The changes of the thermal and hydraulic condi-tions within the test site are monitored by numerous sensors.
The data that is collected by the laboratory experiments and the cable test side are finally used to validate numeric models. They implement all relevant physical processes and allow conducting parameter variations of all kind in order to derive general conclusions that may afterwards be implemented in the grid operation.