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 Quantitative analysis of the sensitivity of UHF sensor positions on a 420 kV power transformer based on electromagnetic simulation(2019) Beura, Chandra Prakash; Beltle, Michael; Tenbohlen, Stefan; Siegel, MartinWith an increasing interest in ultra-high frequency (UHF) partial discharge (PD) measurements for the continuous monitoring of power transformers, it is necessary to know where to place the UHF sensors on the tank wall. Placing a sensor in an area with many obstructions may lead to a decrease in sensitivity to the UHF signals. In this contribution, a previously validated simulation model of a three-phase 300 MVA, 420 kV power transformer is used to perform a sensitivity analysis to determine the most sensitive sensor positions on the tank wall when PD activity occurs inside the windings. A matrix of UHF sensors located on the transformer tank is used to perform the sensitivity analysis. Some of the windings are designed as layer windings, thus preventing the UHF signals from traveling through them and creating a realistic situation with very indirect propagation from source to sensor. Based on these findings, sensor configurations optimized for UHF signal sensitivity, which is also required for PD source localization, are recommended for localization purposes. Additionally, the propagation and attenuation of the UHF signals inside the windings and the tank are discussed in both oil and air.Item Open Access Classification of superimposed partial discharge patterns(2021) Adam, Benjamin; Tenbohlen, StefanPhase resolved partial discharge patterns (PRPD) are routinely used to assess the condition of power transformers. In the past, classification systems have been developed in order to automate the fault identification task. Most of those systems work with the assumption that only one source is active. In reality, however, multiple PD sources can be active at the same time. Hence, PRPD patterns can overlap and cannot be separated easily, e.g., by visual inspection. Multiple PD sources in a single PRPD represent a multi-label classification problem. We present a system based on long short-term memory (LSTM) neural networks to resolve this task. The system is generally able to classify multiple overlapping PRPD by while only being trained by single class PD sources. The system achieves a single class accuracy of 99% and a mean multi-label accuracy of 43% for an imbalanced dataset. This method can be used with overlapping PRPD patterns to identify the main PD source and, depending on the data, also classify the second source. The method works with conventional electrical measuring devices. Within a detailed discussion of the presented approach, both its benefits but also its problems regarding different repetition rates of different PD sources are being evaluated.Item Open Access Compatibility study of silicone rubber and mineral oil(2021) Karambar, Smitha; Tenbohlen, StefanIn this study, three types of silicone rubbers, namely, insulative silicone rubber, conductive silicone rubber and silicone rubber with conductive as well as insulative layers are investigated for their compatibility with mineral oil. Mineral oil with different silicone rubber samples is thermally aged at 130 °C for 360 h, 720 h and 1080 h and at 23 °C, 98 °C and 130 °C for 360 h. At the end of each ageing interval, mineral oil and oil-impregnated silicone rubbers are investigated for their dielectric properties. Aged mineral oil samples are investigated for their moisture content, breakdown voltage, colour number, dissolved gases and total acid number, whereas solid insulation samples are investigated for their moisture content. Additionally, pressboard samples in mineral oil and mineral oil without any solid insulation materials are also aged under the same conditions and are investigated for their dielectric properties. From the obtained results, it can be assessed that the presence of carbon particles in conductive silicone rubber negatively impacts the dielectric properties of mineral oil. Among the investigated silicone rubbers, the insulative silicone rubber exhibits good compatibility with mineral oil and a strong potential for being used in mineral oil.Item Open Access Frequency range of UHF PD measurements in power transformers(2023) Tenbohlen, Stefan; Beura, Chandra Prakash; Sikorski, Wojciech; Albarracín Sánchez, Ricardo; Albuquerque de Castro, Bruno; Beltle, Michael; Fehlmann, Pascal; Judd, Martin; Werner, Falk; Siegel, MartinAlthough partial discharge (PD) measurement is a well-accepted technology to assess the quality of the insulation system of power transformers, there are still uncertainties about which frequency range PDs radiate and which frequency range should be evaluated in a measurement. This paper discusses both a UHF PD frequency range obtained from studies investigating laboratory experiments and a frequency range from numerous practical use cases with online and on-site measurements. The literature review reveals a frequency spectrum of ultrahigh-frequency (UHF) PD measurements in the range of 200 MHz to 1 GHz for most publications. Newer publications extend this range from 3 to 6 GHz. The use cases present UHF PD measurements at transformers with power ratings up to 1000 MVA to determine frequency ranges which are considered effective for practical applications. The “common” frequency range, where measurements from all use cases provide signal power, is from approximately 400 MHz to 900 MHz, but it is noted that the individual frequency range, as well as the peak UHF signal power, strongly varies from case to case. We conclude from the discussed laboratory experiments and practical observations that UHF PD measurements in power transformers using either valve or window antennas, according to Cigré, are feasible methods to detect PD.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 Transformer winding condition assessment using feedforward artificial neural network and frequency response measurements(2021) Tahir, Mehran; Tenbohlen, StefanFrequency response analysis (FRA) is a well-known method to assess the mechanical integrity of the active parts of the power transformer. The measurement procedures of FRA are standardized as described in the IEEE and IEC standards. However, the interpretation of FRA results is far from reaching an accepted and definitive methodology as there is no reliable code available in the standard. As a contribution to this necessity, this paper presents an intelligent fault detection and classification algorithm using FRA results. The algorithm is based on a multilayer, feedforward, backpropagation artificial neural network (ANN). First, the adaptive frequency division algorithm is developed and various numerical indicators are used to quantify the differences between FRA traces and obtain feature sets for ANN. Finally, the classification model of ANN is developed to detect and classify different transformer conditions, i.e., healthy windings, healthy windings with saturated core, mechanical deformations, electrical faults, and reproducibility issues due to different test conditions. The database used in this study consists of FRA measurements from 80 power transformers of different designs, ratings, and different manufacturers. The results obtained give evidence of the effectiveness of the proposed classification model for power transformer fault diagnosis using FRA.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.