05 Fakultät Informatik, Elektrotechnik und Informationstechnik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/6
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Item Open Access Einfluss der Abschlussimpedanz von Hochvoltkabeln auf Funkstörgrößen in elektrisch angetriebenen Kraftfahrzeugen(2012) Reuter, Martin; Waible, Manuel; Tenbohlen, Stefan; Köhler, WolfgangIn diesem Beitrag wird die Frage untersucht, welche Auswirkung die Fehlanpassung von Kfz-Hochvoltkabeln auf EMV-Störgrößen in der Komponentenmessung nach CISPR 25 hat.Item Open Access Assessment of overload capabilities of power transformers by thermal modelling(2011) Schmidt, Nicolas; Tenbohlen, Stefan; Skrzypek, Raimund; Dolata, BartekThis contribution presents an approach to determine the overload capabilities of oil-cooled power transformers depending on the ambient temperature. For this purpose the investigated method introduces a simplified, empirical based thermal model that predicts changes in oil temperature with high accuracy. This model considers the entire transformer as a single, homogenous tempered body with a certain thermal capacity. All electrical losses are perceived as an input of equally distributed heat and assumed to be the sum of the load and no-load losses given by the transformer design. In contrary to earlier approaches the heat exchange with the ambience is modelled as a complex function depending first of all on the temperature difference between the transformer and its surroundings. Furthermore, the loading rate, material properties, levels of temperatures and emerging temperature gradients are taken into account as influencing factors determining the heat exchange. To display the behaviour of a specific transformer, the model employs several empirical factors. For determination of these empirical factors an evaluation time of two to four representative weeks of transformer operation is found to be sufficient. To validate the created model and test its operational reliability, measuring data from several ONAN- and ONAF-transformers are consulted. These data sets comprise the top oil and ambient temperature as well as the loading rate and the status of the cooling system. Furthermore, the corresponding name plate data is integrated. Subsequently to the calculation of the top oil temperature, the maximum constant loading rate resulting in a hot-spot temperature below critical level is determined based upon the remarks of IEC 60076 - 7 [1]. Finally, a characteristic linear function for each investigated transformer displaying the maximum loading rate depending solely on the ambient temperature is derived. In case of the investigated ONAN- and ONAF-transformers within a power range of 31.5 - 63 MVA, significant overload potentials could be disclosed.Item Open Access Suitability of ultra high frequency partial discharge measurement for quality assurance and testing of power transformers(2013) Tenbohlen, Stefan; Siegel, Martin; Beltle, Michael; Reuter, MartinWell known reasons for local failures in power transformers are caused by partial discharges (PD) in the electric insulation. Continuous deterioration over time increases the defect which finally can lead to a breakdown of the entire insulation. The importance of PD measurement is accommodated by standardized electrical measurement according to IEC 60270 [1] which is required for acceptance certificates at routine testing. Therefore, the apparent charge QIEC has become an important value for transformer quality. Since a couple of years, alternative measurement methods for PD are used. Originally developed for gas insulated systems [2], [3], ultra high frequency (UHF) measurement found its way into transformer diagnosis over the last years [4]. To become an accepted quality factor, UHF has to be proven a reliable testing method, which can bear up against electrical measurements. Therefore, the general physics of UHF PD has to be considered at first. Ultra-high-frequency antennas measure electromagnetic emissions of PD directly in-oil inside a transformer. It becomes apparent, that UHF measurement usually is advantageous concerning external disturbances. Compared to the electric measurement, the UHF method is robust against external signals [5], which makes it suitable for both, offsite measurement at routine testing under laboratory conditions with low ambient noise and onsite, e.g. after transportation and installation of transformers with usually high noise levels. These considerations make the UHF method interesting as supplement for transformer routine tests. Therefore, a sensor calibration or at least a validation of its sensitivity is required [6] comparable to the electrical measurement. To provide profound knowledge of the equipment, the antenna factor (AF) of the UHF sensor needs to be determined under inside-transformer conditions. This contribution shows the determination of the UHF sensor’s AF using a Gigahertz-Transversal-Electro-Magnetic Setup (GTEM cell). To meet inside-transformer conditions, an oil-filled GTEM cell is required for correct permittivity. Correction factors can then be introduced to minimize measurement errors and to establish better comparability of different UHF sensors. Hence, a standard test setup can be defined.Item Open Access Influence of termination impedance on conducted emissions in automotive high voltage networks(2012) Reuter, Martin; Tenbohlen, Stefan; Köhler, WolfgangThis contribution investigates the effect of changed network topologies within HV systems on component level EMC tests. Therefore the recent state of art in component level testing is presented and the differences of LV and HV network topologies are discussed. An adaption of the component level test setup for HV components is introduced and a minimized HV system investigated. Results of a case study on an inverter for a hybrid car are presented and the impact of the measurements in the design of HV systems will be discussed.Item Open Access Water saturation limits and moisture equilibrium curves of alternative insulation systems(2011) Tenbohlen, Stefan; Jovalekic, Mark; Bates, Lisa; Szewczyk, RadoslawA method developed for establishing moisture equilibrium curves for any combination of liquid and solid insulation is presented in this paper. Moisture saturation curves for natural and synthetic esters have been presented in the temperature range up to 140°C together with curve for mineral oil as a reference. Sorption isotherms have been established for cellulose based and aramid fiber based materials. Eventually, the moisture equilibrium diagrams have been created for given combinations of solids and liquids. Moisture equilibrium curves have been created for combinations of mineral oil and ester fluids with aramid fiber based papers and boards, as they are commonly used in alternative insulation systems. The new curves give information on moisture distribution within the alternative insulation systems and may be critical for setting the material choices, design rules and maintenance guidelines for equipment using these combinations. Only then the materials could be used optimally and their specific characteristics could bring full range of benefits to the equipment. Also the condition monitoring and diagnostics for the purpose of asset management will be more reliable when these new characteristics are used. It has been observed that insulation components made of aramid insulation may have lower water content comparing to cellulose based conventional materials at the same water content measured in dielectric liquid. As a result, the performance of aramid insulation components may be less sensitive to moisture in oil (aging processes, dielectric strength, partial discharge performance) comparing to conventional systems based on cellulose.Item Open Access New mitigation methods for transient overvoltages in gas insulated substations(2012) Burow, Simon; Riechert, Uwe; Köhler, Wolfgang; Tenbohlen, StefanDuring switching of disconnectors (DS) in GIS a varying number of pre-strikes and re-strikes occur. Due to the very short duration of the voltage collapse, traveling surges are generated in the busbar duct. These very fast transient overvoltages (VFTO) can become the limiting dielectric stress which defines the dimensions at UHV voltage levels. The decision shall be based on the maximum VFTO peak value that occurs with reference to the rated lightning impulse withstand voltage (LIWV) of the equipment. If the maximum VFTO is higher than the LIWV, it is necessary to consider the VFTO level as dimensioning criteria or to suppress VFTO by suitable measures. The main challenges are the reduction in VFTO amplitudes and finally the reduction of the effects of VFTO on the equipment. For the different sources of VFTO and for the different equipment different mitigation methods are known. The damping of VFTO by integration of a damping resistor is a well proven technology. The way to overcome the drawback of such unwieldy designs is to use other internal damping measures. Several methods have been proposed and examined in the past, such as ferrite material or high frequency (RF) resonators. The VFTO damping solution utilizing ferrite rings has been analysed and tested and will be described here. The measurements show that a damping effect can be achieved, but with an important drawback: the magnetic material goes easily into saturation, which complicates the design and reduces its general applicability and robustness. A new approach for damping is to implement compact electromagnetic high-frequency resonators with low quality factor specially designed to cover a wider frequency range. The novelty of this idea is not only to design the resonators but also to dissipate the VFTO energy. The VFTO damping effect of the developed RF resonator tuned to the dominant harmonic component was confirmed by experiments. Rings of a nanocrystalline alloy placed around the GIS conductor were also investigated. Depending on number, material and size of the rings a good mitigation could be achieved.Item Open Access Impedance analysis of automotive high voltage networks for EMC measurements(2011) Reuter, Martin; Tenbohlen, Stefan; Köhler, Wolfgang; Ludwig, A.This paper deals with a method of determining the high-frequency impedances of automotive HV power networks. A Vector Network Analyzer (VNA) is used to measure Scattering parameters of different HV power cables and an automotive Li-Ion accumulator battery. Matrix conversions allow calculating an impedance network, which is able to represent an automotive HV networks.Item Open Access Characterization of automotive high voltage networks for EMI measurements(2010) Reuter, Martin; Tenbohlen, Stefan; Köhler, WolfgangThis paper deals with a method of determining the high-frequency impedances of automotive HV power networks (300 kHz - 200 MHz). A Vector Network Analyzer (VNA) is used to measure Scattering-Parameters of different HV power cables. Matrix conversions allow calculating an impedance network, which is able to represent automotive HV networks.