Balzer, Constantin (2020)
Calculation of ampacity ratings of power cable systems by the use of thermal quadrupoles under consideration of soil properties.
Technische Universität Darmstadt
doi: 10.25534/tuprints-00017220
Dissertation, Erstveröffentlichung, Verlagsversion

Kurzbeschreibung (Abstract)

The so-called ampacity rating, i.e. the maximum admissible conductor current due to thermal restrictions, represents the major constraint for the power transmission by cable systems. In order to determine this rating, two methodologies can be employed: On the one hand side, the calculation on the basis of analytical expressions allows a fast computation “with pen and paper”. Yet, complex configurations or non-linear material properties of the bedding can only be approximated. In contrast to this, numerical simulations can implement any sort of configurations or material properties, but on the expense of a high demand in computing capacity. However, the requirements on the operation of power grids are changing: The load patterns become more and more fluctuating due to power injection from dispersed generation. Also, enhanced measurement and control techniques in combination with incentive regulation encourage system operators to make the best use of their existing infrastructure and investments. Against this background, the need for a calculation method that combines both advantages of the aforementioned methods, i.e. the rapidity of analytical calculations with the universal possibilities of numerical simulations, is rising. Only with such methods, real time capable ampacity ratings can be included into an intelligent system operation, providing flexible thermal limits adopted to the actual properties of the load flow and bedding materials. As the hitherto existing methods for such real time capable algorithms rely on analytical approximations that entail restrictions to their application, the presented work aims at deriving an exact analytical method. In order to do so, solutions for all elements of cable systems have been brought together or derived by the author himself. Furthermore, it is shown that with the help of thermal quadrupoles, the separated solutions can be subsumed into one single network for the total cable system. Moreover, for elements that cannot be calculated analytically, such as the thermal coupling of multiple cables inside a non-homogenous bedding, numerically calculated spectra can be incorporated into the network of thermal quadrupoles. All dependencies in time, i.e. stationary, periodic and transient, can be calculated with the help of the methodology. Furthermore, an enhanced determination of cable ampacity ratings cannot be performed without reviewing the heat and mass transfer mechanisms inside the bedding. With the help of numerical simulations that have been validated through laboratory experiments, the factors of influence on the soil drying-out as well as the ambient water content are examined for a large variety of natural soils. Also, the dynamic of soil-drying out under transient heat injection is investigated. The presented results have been achieved in the framework of an interdisciplinary research project between the High Voltage Laboratories and the Geothermal Science and Technology of TU Darmstadt as well as a German distribution system operator. They have been incorporated into an application tool that has successfully been put into operation.

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