@article{SchoberKuechlerKrause2013, author = {Schober, Fabian and K{\"u}chler, Andreas and Krause, Christoph}, title = {Oil conductivity - an important quantity for the design and the condition assessment of HVDC insulation systems}, series = {FHWS Science Journal}, volume = {1}, journal = {FHWS Science Journal}, number = {2}, address = {W{\"u}rzburg}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:863-opus-463}, pages = {59 -- 78}, year = {2013}, abstract = {HVDC (high voltage direct current) transmission is a very effective way of power transmission and it is necessary to supply the worlds growing amount of energy. In HVDC equipment the major insulating materials are oil and pressboard. Oil conductivity is an important quantity for the design, the diagnosis and the condition assessment of HVDC insulation systems. Unfortunately, insulating oil has a strongly nonlinear conductivity which depends on various parameters (field strength, temperature, water content and ageing) and is time-dependent because of charge carrier drift processes. Measuring methods are required to provide the information necessary for the condition assessment of HVDC systems. Therefore, measurements of new and aged oils are compared and taken into account for a diagnosis of oil-filled HVDC insulation systems. Furthermore, today's standards are analyzed and compared to the so called PDC method. Relations between initial and steady-state conductivities at different temperatures, field strengths and for various oil types are presented and consequences for the condition assessment of insulation systems are discussed.}, language = {en} } @inproceedings{KriegLalaSchoberetal., author = {Krieg, Andreas and Lala, Gjergj and Schober, Fabian and Wiener, Johannes and Paulus, Johannes and K{\"u}chler, Andreas and Zink, Markus H. and Liebschner, Marcus}, title = {FEM model for describing the dielectric behavior of oil-impregnated pressboard under DC stresses}, series = {Papers of 33rd International Scientific Conference "Science in Practice"}, booktitle = {Papers of 33rd International Scientific Conference "Science in Practice"}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:863-opus-2609}, pages = {62 -- 66}, abstract = {According to the worldwide growing demand for electric energy, it is necessary to construct safe and efficient power systems. HVDC (high voltage direct current) applications become more and more important due to larger distances for the energy transportation and the increasing demand for reactive power transportation. Appropriate insulation materials, i.e. mineral oil and pressboard, are widely used in HVDC components, mainly in power transformers though the knowledge about dielectric behavior of oil-impregnated pressboard under DC stresses in time domain is not sufficient. In this paper, a Finite Element Method (FEM) model is presented to describe the electrical conduction behavior of the material by separate consideration of mineral oil and pressboard. With this model the electric potential distribution under DC stress was simulated to analyze polarization and electrical conduction. The model parameters volume ratio as well as angle variations of FEM model are investigated and their influences on the current through the model are evaluated. Furthermore, geometrical and physical parameters are estimated and the results are compared to currents that are measured on oil-impregnated pressboard samples in time domain.}, language = {en} }