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 Pressure characteristics and chemical potentials of constrained LiFePO4/C6 cells(2018) Singer, Jan Patrick; Kropp, Timo; Kuehnemund, Martin; Birke, Kai PeterConstraining lithium-ion cells increases the cyclic lifetime. However, depending on an expected volume expansion during charge and discharge cycling, defining the optimal constraining pressure range is not straightforward. In this study, we investigate a lithium iron phosphate/graphite pouch cell at four initial constraining pressure levels. As a function of C-Rate, the thermodynamic principle of the non-monotonic pressure curve during full charge and discharge cycles is evaluated. Using the rubber balloon model to calculate the chemical potential of lithium iron phosphate and discussing the relationship between the chemical potential and pressure, we illustrate the pressure curve qualitatively. By applying differential pressure analysis, we evaluate the resulting pressure curves of a single graphite stage. Approaching a fundamental understanding of reduced cycling lifetime of full cells with unknown material composition, we allocate the stages and stage transitions of graphite as well as the phase transition of lithium iron phosphate. Local extreme values in the differential pressure analysis indicate phase and stage transitions. These values can identify critical operating conditions that should be considered for defining the optimum initial constraining pressure range.Item Open Access A comprehensive analysis of windings electrical and mechanical faults using a high-frequency model(2019) Tahir, Mehran; Tenbohlen, StefanThe measurement procedures for frequency response analysis (FRA) of power transformers are well documented in IEC and IEEE standards. However, the interpretation of FRA results is still far from reaching an accepted methodology and is limited to the analysis of the experts. The dilemma is that there are limited case studies available to understand the effect of different faults. Additionally, due to the destructive nature, it is not possible to apply the real mechanical deformations in the transformer windings to obtain the data. To solve these issues, in this contribution, the physical geometry of a three-phase transformer is simulated using 3D finite integration analysis to emulate the real transformer operation. The novelty of this model is that FRA traces are directly obtained from the 3D model of windings without estimating and solving lumped parameter circuit models. At first, the method is validated with a simple experimental setup. Afterwards, different mechanical and electrical faults are simulated, and their effects on FRA are discussed objectively. A key contribution of this paper is the winding assessment factor it introduces based on the standard deviation of difference (SDD) to detect and classify different electrical and mechanical faults. The results reveal that the proposed model provides the ability of precise and accurate fault simulation. By using SDD, different deviation patterns can be characterized for different faults, which makes fault classification possible. Thus, it provides a way forward towards the establishment of the standard algorithm for a reliable and automatic assessment of transformer FRA results.Item Open Access Measuring test bench with adjustable thermal connection of cells to their neighbors and a new model approach for parallel-connected cells(2019) Fill, Alexander; Mader, Tobias; Schmidt, Tobias; Llorente, Raphael; Birke, Kai PeterThis article presents a test bench with variable temperature control of the individual cells connected in parallel. This allows to reconstruct arising temperature gradients in a battery module and to investigate their effects on the current distribution. The influence of additional contact resistances induced by the test bench is determined and minimized. The contact resistances are reduced from 𝑅Tab+=81.18 μΩ to 𝑅Tab+=55.15 μΩ at the positive respectively from 𝑅Tab-=35.59 μΩ to 𝑅Tab-=28.2 μΩ at the negative tab by mechanical and chemical treating. An increase of the contact resistance at the positive tab is prevented by air seal of the contact. The resistance of the load cable must not be arbitrarily small, as the cable is used as a shunt for current measurement. In order to investigate their impacts, measurements with two parallel-connected cells and different load cables with a resistance of 𝑅Cab+=0.3 mΩ, 𝑅Cab+=1.6 mΩ and 𝑅Cab+=4.35 mΩ are conducted. A shift to lower current differences with decreasing cable resistance but qualitatively the same dynamic of the current distribution is found. An extended dual polarization model is introduced, considering the current distribution within the cells as well as the additional resistances induced by the test bench. The model shows a high correspondence to measurements with two parallel-connected cells, with a Root Mean Square Deviation (RMSD) of 𝜉RMSD=0.083 A.Item Open Access Fully integrated high quality factor GmC bandpass filter stage with highly linear operational transconductance amplifier(2017) Briem, Jochen; Mader, Marco; Reiter, Daniel; Amirpour, Raul; Grözing, Markus; Berroth, ManfredThis paper presents an electrical, fully integrated, high quality (Q) factor GmC bandpass filter (BPF) stage for a wireless 27 MHz direct conversion receiver for a bendable sensor system-in-foil (Briem, 2016). The core of 10 the BPF with a Q factor of more than 200 is an operational transconductance amplifier (OTA) with a high linearity at an input range of up to 300 mVpp,diff. The OTA’s signal-to-noise-and-distortion-ratio (SNDR) of more than 80 dB in the mentioned range is achieved by stabilizing its transconductance Gm with a respective feedback loop and a source degeneration resistance RDG. The filter stage can be tuned and is tolerant to global and local process variations due to offset and common-mode 15 feedback (CMFB) control circuits. The results are determined by periodic steady state (PSS) simulations at more than 200 global and local process variation parameter and temperature points and corner simulations. It is expected, that the parasitic elements of the layout have no significant influence on the filter behaviour. The current consumption of the whole filter stage is less than 600 μA.