Raphael Färber

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Färber

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Raphael

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0000-0001-8511-8670

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Publications1 - 10 of 33

Mixed-frequency medium-voltage aging analysis of epoxy in the absence of partial discharges and dielectric heating
Küchler, Florian; Färber, Raphael; Bill, Fabian; et al. (2023)
Journal of Physics D: Applied Physics
Premature failures of polymeric insulation under inverter-type electrical stress are predominantly associated with partial discharge (PD) erosion or dielectric heating. In the present contribution, an approach for aging analysis in the absence of the aforementioned mechanisms is proposed and applied to anhydride-cured epoxy samples, which are designed with a recessed shape to achieve PD-free aging. Dielectric heating was found to be negligible under all applied experimental conditions. Aging of samples was performed with a specialized setup for the generation of mixed-frequency medium-voltage (MF-MV) waveforms under controlled temperature and humidity conditions. The health state of samples was evaluated before and after different aging sequences by analysis of potential aging markers, namely the short-term AC breakdown strength, the complex dielectric permittivity (real and imaginary part), the volume resistivity, the glass transition temperature and the characteristic absorbance peaks obtained by Fourier-transform infrared spectroscopy (FTIR). Of these, only the breakdown strength exhibited significant aging effects under hygroelectric stress, which is hypothesized to be attributed to localized microcracking caused by electromechanical stress. Pure electrical MF-MV stress (i.e. at room temperature and dry conditions) was not found to be critical under the applied experimental conditions. By means of FTIR, hydrolysis was excluded as a possible aging mechanisms. In summary, the proposed aging analysis approach was found to be suitable to reveal aging effects empirically as well as to give indications about the underlying aging mechanisms without the need for excessively long or accelerated lifetime testing.
On Surface Resistivity Measurements of Thin Insulation Materials
Küchler, Florian; Färber, Raphael; Chen, Bo; et al. (2023)
IEEE Transactions on Dielectrics and Electrical Insulation
Surface resistivity measurements are frequently used for material characterization. However, the influence of sample thickness is not covered sufficiently in ASTM D257 and IEC 62631-3-2 standards. In the present work, electric field simulations and polarization depolarization current (PDC) measurements are used to study the thickness dependence of such measurements. It is found that measurements according to the standards are not reasonable for low thickness (< 1 mm) and high surface resistivity (> 1013…1014 Ω). The dependence on thickness is shown to be mainly due to the different tangential electric field stress along the measured surface. Furthermore, indications for charge injection at high electric field stress inside the sample volume are found by a positive polarity of the depolarization currents. A novel electrode configuration for more accurate (more than one order of magnitude) and much less thickness-dependent surface resistivity measurements is proposed in this work and its benefits are demonstrated. It is also shown how the parasitic influence of an electrode holder can be effectively reduced by more than one order of magnitude when measuring surface resistivities. In addition, it is found that polarization times for steady-state surface resistivity values should be at least several minutes instead of the suggested 60 s from standards.
Humidity and Temperature Effects on the Dielectric Properties of PET Film
Küchler, Florian; Färber, Raphael; Franck, Christian (2020)
2020 IEEE Electrical Insulation Conference (EIC)
The aim of the present manuscript is to contribute to the understanding of material performance of machine insulation film at high temperatures and changing relative humidity (RH). This represents a common stress under normal operation and may lower the material insulation performance. PET film is used as the example material and the complex dielectric permittivity is used as a marker to investigate changes in the dielectric properties. Measurements are carried out with a high precision dielectric spectrometer in a range of 10(-2)..10(5) Hz at different temperatures (10..125 degrees C) and RH levels (0..80 %). In addition, evaluation of absorbed water at different RH levels (0..80 %) is done gravimetrically and exhibits a diffusion character according to Fick's law. Moreover, the proportionality of maximum absorption and RH level becomes apparent. Variation of temperature shows altered alpha and beta loss mechanisms as well as changed DC conductivity proportions. The continuous permittivity increase during water absorption indicates the higher (moisture-induced) polar content in the polymer. However, a small portion of water seems to be bound to terminal -OH groups of PET, thus leading to an altered beta relaxation. The observed increase of DC conductivity, not only with higher temperature but also with higher water content, points out the humidity-driven charge transport through the dielectric. A further, frequency-dependent loss mechanism is only visible above a certain RH level that indicates pure water formation within the polymer. This study shows that even though the level of absorbed water for PET is below 0.4 %, the dielectric spectra reveal significant changes.
Pressure scaling laws for partial discharges in wedge-shaped, dielectric-bounded gas gaps
Färber, Raphael; Sefl, Ondrej; Franck, Christian (2024)
Journal of Physics D: Applied Physics
A pressure scaling law for the partial discharge inception voltage (PDIV) of wedge-shaped, dielectric-bounded gas gaps is derived and experimentally validated. The investigated prototypical electrode geometry is of relevance in a number of practical applications, such as contacting enameled wires in electric motors or transformers. The derived pressure scaling law is of particular interest for electric propulsion in aviation systems. The results show that the PDIV can be accurately parametrized from first principles as a function of the scaling parameter ps/ɛr, where p is the gas pressure, s is the thickness of the insulating coating and ɛr its relative dielectric permittivity. Previously published empirical relationships between the PDIV and pressure are shown to be local approximations of the presented general scaling law. In particular, the often assumed linear relation of PDIV with pressure is shown to not be generally valid.
Effect of Humidity on Lifetime and Dielectric Properties of PET Film
Küchler, Florian; Färber, Raphael; Franck, Christian (2020)
2020 IEEE 3rd International Conference on Dielectrics (ICD)
Due to recent developments in semiconductor switching technology, the associated insulation systems are exposed to higher pulse voltage amplitudes and frequencies as well as shorter rise times, which might accelerate material degradation. Often, the insulation is also subjected to humidity changes which affect the dielectric properties and possibly the insulation lifetime. This motivates the study of humidity effects under DC-biased pulse-width modulated (PWM) voltage stress. The aim of this contribution is to provide an experimental basis as well as explanations for humidity-induced variations in the dielectric properties and changed lifetime behavior of PET film. Throughout all experiments, the relative humidity (RH) of air is varied. The lifetime is evaluated as time-to-failure (TTF) by applying DC voltages with and without superimposed PWM voltages below and above the partial discharge inception voltage (PDIV). In order to quantify changes in the dielectric properties, the complex dielectric permittivity is evaluated by means of broadband dielectric spectroscopy (BDS). The water absorption is measured gravimetrically. Although the observed relative water absorption of the PET specimens is always below 0.37 %, the lifetime is drastically decreased by more humid conditions under DC stress (below PDIV). Above PDIV, PD erosion dominates at low RH, whereas DC breakdown processes become more significant at high RH. The BDS results reveal humidity-induced changes in the relaxation and conduction behavior. Finally, the breakdown mechanisms, which are altered by water absorption, are discussed in terms of thermal and space-chargedriven breakdown.
Measurement and Evaluation of Partial Discharges at Higher-Frequency Sinusoidal and Impulse Test Voltages
Sefl, Ondrej; Färber, Raphael; Wieser, Yves; et al. (2023)
Proceedings of the 23rd International Symposium on High Voltage Engineering
In this work, four instruments are investigated and compared concerning the measurement of partial discharges under high-frequency sinusoidal and impulse test voltages in a laboratory environment. The chosen instruments, typically employed for detecting partial discharges under voltages with high dV/dt-ratios, are a high-frequency current transformer, a D-dot probe, a detection capacitor connected in series to the test object, and a photomultiplier. Initially, the sensitivity of the instruments is assessed for discharges taking place over a single period of a sinusoidal test voltage (1 kHz) and during a positive, unipolar, slow-rise impulse. Next, individual instrument signals in response to either a single weak (units of pC) or a moderate (hundreds of pC) partial discharge are analysed in detail, and the suitability of the instruments for the construction of a phase-resolved partial discharge pattern is evaluated (sine, 10 kHz). Ultimately, the performance of the instruments for steep-flank (~375 kV/μs) impulse test voltages is studied. Analog filters are employed to improve the instruments’ detection capabilities, and an overview of the instruments’ strengths and weaknesses is presented. The detection series capacitor instrument is shown to have the best overall performance for the studied aspects of partial discharge measurement.
Effects of Mixed-Frequency Voltage Stress on Dry-Type Insulation Systems
Guillod, Thomas; Färber, Raphael; Franck, Christian; et al. (2015)
Streamer inception thresholds derived from a statistical electron transport model
Färber, Raphael; Franck, Christian (2021)
Journal of Physics D: Applied Physics
The aim of this article is to review a statistical description of electron transport in gases based on transport parameter theory, and to derive expressions for streamer inception thresholds in quasi-homogeneous electrical fields within this description. The critical average number of electrons $n_{\mathrm{crit}}$ required to induce avalanche-to-streamer transition is derived for arbitrary gases as a function of the gap electric field and the relevant gas parameters. The model is then evaluated for synthetic air. Significant deviations from the traditional approximation $n_{\mathrm{crit,cl}}=10^8$ are found for small gaps ($d < 1 $mm). Moreover, the minimal necessary number of electrons for avalanche-to-streamer transition is found to scale approximately with the inverse of gas pressure. The application of the adapted streamer inception criterion to the calculation of the partial discharge inception voltage for twisted enamelled wires, where discharges are incepted in gaps with $d\ll 1$mm, is discussed. Our results support the conclusion that space charges generated by sub-critical avalanche processes play a determining role for streamer onset in such configurations, i.e. that their partial discharge inception cannot be reliably quantified by applying the streamer criterion in the Laplacian electrode field. Finally, the streamer inception probability is compared to the deterministic streamer inception criterion. In particular, the problem of non-exponential growth in the early stages of the electron avalanche development is argued to be properly addressed within the applied statistical interpretation of transport parameters.
Zero-Dimensional Modeling of a Nanosecond Pulsed Discharge
Bleiner, Davide; Balmelli, Michelangelo; Färber, Raphael; et al. (2024)
IEEE Access
This article proposes a 0D model for Nanosecond Pulsed Discharges (NPD). The model incorporates the high-frequency transmission line, a lumped equivalent circuit for the load, a two-temperature model for heavy particles and electrons, and an ionization scheme. The load impedance is modeled as a stray capacitance in parallel with a stray inductance and a time-varying electrical resistance, which depends on the plasma radius and electron number density. The ionization mechanism used to simulate the electron number density includes the impact ionization of N2 and O2 and two- and three-body attachments on O2, all dependent on the applied electric field and gas temperature. The temperature variation is calculated using the energy conservation equation, with electrical power as the source. The model is tested against current and voltage measurements of NPDs in sub-mm gaps at pressures ranging from 2 to 8 bar. The comparison of simulation results with experimental data shows that the plasma’s electrical resistance rapidly drops to low values within approximately 1-2 ns after breakdown. This drop is attributed to the formation of a fully ionized micrometer-sized thermal spark, a conclusion supported by optical emission spectroscopy measurements. This model is intended for experimental plasma researchers seeking a simple tool to understand plasma states through basic electrical measurements and for electrical engineers needing insights into varying load impedance, a crucial parameter for pulse generator design.
Influence of the stoichiometric ratio on the material properties and the aging behavior of anhydride-cured epoxy systems
Küchler, Florian; Färber, Raphael; Franck, Christian (2023)
Journal of Physics D: Applied Physics
When partial discharge (PD) and dielectric heating effects can be excluded, comparatively little is known about the aging behavior of polymeric insulation materials under combined environmental and electric field stress. Since it was found in an earlier study that hygroelectrical stress leads to a strong reduction in the residual breakdown strength of anhydride-cured epoxy samples, the goal of this contribution is to determine if a material modification could enhance the insulation performance under hygroelectrical stress. For this purpose, epoxy samples of different stoichiometric ratios (SRs) were manufactured and hygroelectrically stressed. The material properties were evaluated by AC breakdown strength measurements as well as by Fourier-transform infrared (FTIR) spectroscopy before and after each aging sequence. Gravimetric analysis quantified the water diffusion behavior and the (steadystate) water absorption during exposure to different relative humidity (RH) levels at room temperature. The breakdown strength decreases for both increasing and decreasing SR compared to SR = 95 %. Moreover, FTIR measurements revealed that an increase of the SR leads to a higher amount of unreacted anhydride which correlates with an increasing amount of absorbed water. Thus, the mutual presence of water in the atmosphere and of unreacted anhydride in the epoxy material is identified as the main risk factor for epoxy degradation under the influence of electrical stress, since unreacted anhydride favors stronger water absorption and the presence of water inside the material is attributed to a lowering of the potential barrier for molecular bond breaking. Remarkably, the breakdown strength of non-aged samples is lower at SR = 80 % compared to SR = 95 %, but the long-term performance with respect to insulation aging is enhanced, which correlates with the non-/less-existing unreacted anhydride in the material.

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Raphael Färber

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