The electrification of aircraft requires insulating polymers capable of reliable operation under high-voltage DC stress. This study investigates the current-voltage behavior of glass-fiber-reinforced LCP and PEEK under controlled temperature and humidity. Results show that LCP exhibits mainly ohmic conduction at moderate temperature and only slight super-linear behavior at elevated temperature. In contrast, reinforced PEEK shows stronger field-dependent conduction, including humidityinduced transitions toward space-charge-limited current (SCLC) at $\mathbf{8 0}^{\circ} \mathbf{C}$ under 50%RH $\left(\mathbf{E}_{\text{Th }} \approx \mathbf{3 ~ k V} / \mathbf{m m}\right)$, and possible Schottky-type injection mechanisms at $\mathbf{5 0}^{\boldsymbol{\circ}} \mathbf{C}$. The results highlight the significant role of charge trapping and fiber-matrix interfaces in PEEK. These differences should be considered when selecting high-performance insulation materials for future HVDC aeronautical electrical systems.

Silicone gel encapsulants used in high-voltage SiC power modules operate under a wide range of hygrothermal conditions, where dielectric stability is essential for insulation reliability. This study investigates the temperature and humidity dependence of the dielectric response of a two-component silicone gel widely employed as a power module encapsulant. Dielectric spectroscopy $\left(\mathbf{1 0}^{\mathbf{- 2}}-\mathbf{1 0}^{\mathbf{6}} \mathbf{~ H z}\right)$ was performed over the temperature range 20-80 °C and relative humidity levels from 40% to 80% RH. The real permittivity exhibits negligible frequency and humidity dependence, indicating stable short-time polarization mechanisms. In contrast, low-frequency dielectric losses display a -1 slope of the imaginary permittivity, characteristic of conduction-dominated behavior. The dc conductivity follows an Arrhenius law with an activation energy of approximately 0.21 eV and remains nearly independent of humidity. Although moisture increases the magnitude of conductivity, it does not modify the underlying thermally activated ionic hopping mechanism, confirming the intrinsic transport stability of the material under hygrothermal stress.

The increase in voltage within photovoltaic networks enables significant reductions in CAPEX (Capital Expenditures) due to the use of smaller cable cross-sections. However, this voltage increase leads to an intensification of PID (Potential Induced Degradation) effects, which are already present at voltages below 1.5 kV. This work focuses on the electrical conduction and field distribution within PV modules through an electrical study of glass-polymer assemblies. Current measurements were carried out in a plane-to-plane configuration (with a guard electrode) on $10 ~\text{cm}^{2}$ assemblies of a 3 mm thick glass sheet with a $500 \mu ~\mathrm{m}$ laminated polymer film (encapsulant). Several assemblies of glasses and polymers were compared at different temperature and relative humidities. Monolayers of glass and polymers were also characterized. Glass-encapsulant assemblies were submitted to HVDC voltages at both polarities. The measured current is different in the two cases. Considering a soda-lime glass and a type of polyolefin, the current is at least four times larger when the glass is connected to the positive polarity than when it is connected to the negative one. Therefore, the two dielectrics do not behave like two resistors in series. In the glass, sodium ions ($\text{Na}^{+}$) are known to migrate under the effect of the electric field. They can also enter the polymer and migrate through. If they are numerous enough, these charge migrations may change the field distribution in the dielectric. Tentative explanations of the observed differences in the steady-state current magnitudes are proposed considering these changes in the electric field distribution.

Rising voltage towards medium voltage (MV), paired with high performant wide band-gap (WBG) semiconductors, lead to a significant increase of power electronics converters performances. Printed circuit boards (PCBs) are facing today higher electrical stress, resulting in early degradations due to partial discharges (PDs). This study focuses on the PD localization and the measurement of partial discharge inception voltage (PDIV) of typical metal geometries in PCBs. The considered objects are: pads, which are metallic layers on the PCB surface in contact with air, face-to-face copper strips and parallel covered tracks. PDs were observed thanks to a camera coupled with an image intensifier. Hence, pads demonstrated that corners are favorable to PD activity. A voltage gain of +25% can be obtained by a curving the corners to 1.5 mm. For the interlayer strips, the PDIV is increased by +103% when the insulation distance rises from 200 μm to 1.3 mm within the PCB. However, for the largest insulation gap (1.6 mm), the close distance with air leads to the appearance of early PDs. Covered tracks illustrated that almost doubling the coating thickness results in a voltage gain of +17%, and +28% when it is tripled.

Partial discharges (PDs) monitoring is a common tool for diagnosing electrical systems. An innovative, low-cost sensor for distance detection and positioning of partial PDs has been developed. An ambitious challenge for this PD monitoring system is to predict dielectric breakdowns on a grid and particularly in MVAC cables. The first part of the article demonstrates the proper functioning of this sensor on a test bench. In a second part, monitoring measurements of PD activity issued from artificial defects are presented. PD initiated in submillimetric cavities were collected. Parameters and PD patterns have been interpreted with the aim to correlate with the electrical treeing propagation until breakdown. The final section is dedicated to a concise review of electrical treeing monitoring. Analysis of existing approaches are presented, as well as the interesting opportunities offered by the sensor.

The literature diverges about the understanding of DC conduction in elastomeric materials, as silicone rubbers, attributing the conduction whether to a solid or to a liquid one. In this study, three different liquid silicone rubbers (LSR) free of fillers are fabricated from the same bi-component raw material, but with different curing times and component proportions. They were submitted to current-voltage measurements at 40°C, with 1h polarization and depolarization cycles, from 1 to 10 kV. The current-voltage characteristics of a low cured sample presented a sub-ohmic profile. The sample cured and post-cured for a longer time presented an ohmic conduction. The non-stoichiometric material presented also an ohmic conduction, but with a deviation to a sub-ohmic at higher voltages. Moreover, its reversal currents in the depolarization profile were characteristic of blocking activity. Analysis of conduction in rubbers is proposed to explain the liquid-like behavior for elastomers, and the solid-like appearance in certain cases.

High-temperature vulcanized silicone rubbers (HTV SiR) have been using for a long time as insulation of MVAC components. This work aims to investigate the suitability under DC of such HTV SiR. Measurements of current were carried out with control of electric field (1, 2, 3, 4, and 5 kV/mm), temperature (20, 40, 60, and 80°C), relative humidity (1%, 50%, and 90%), and electrode system (gold, silver and copper sputtering, and conductive silicone tape). Regarding the electric field, a non-linear current density was observed above values of fields between 1 and 2 kV/mm. An approach by Schottky injection presented a good fit with data points, and clarified the impact of electrode systems, but proved less accurate when material properties were calculated from the model. The influence of temperature followed an Arrhenius-like behavior, which is in accordance with many others conduction mechanisms. Moisture increased current in at least 1 order of magnitude, but the correlation with the water uptake is still under examination. In conclusion, the current density showed a strong dependence on all parameters selected for the study.

Partial Discharges affect insulating materials, especially when the operating voltage exceeds the nominal voltage for short periods. Hence, there is a need for improving the surface erosion resistance resulting from partial discharge. The present work aims to characterize the erosion (depth, shape) of PEEK samples, either unfilled, filled with mineral fillers or glass fibers. PD resistance was evaluated using a rod-to-plane electrodes test cell with an air gap of 0.2 mm. The thickness of the samples tested was 1 mm. The material sheets were subjected to PD under 50 Hz AC voltage from one to several weeks. After PD exposures, the erosion was characterized by analysis of the surface's roughness and erosion depth using a profilometer. EDX measurements also helped to evaluate the filler quantity on the surface of the specimen exposed to PD. The evolutions of erosion with time and as a function of applied voltage are presented. Comparisons are made on the effect of various filler contents. The possibility of enhancing PD resistance through proper selection of filler contents is discussed.

This paper presents simulation and calculation results of electric field enhancement due to various cavity shapes in polymeric high voltage direct current (HVDC) cables. Two aspects are considered: field enhancement factor under DC and partial discharges (PD) in microcavities. PD can occur within defects already existing inside crosslinked polyethylene (XLPE) matrix under various conditions. These conditions are well known under alternative current (AC) but not so well under DC and DC with harmonics. Charges accumulation at XLPE/void interfaces are responsible for local field enhancement and discharges. Aim of this investigation is to identify if these effects, once combined, can increase microvoids size.

Three sets of 10 motorettes were characterized along aging. Thermal aging consisted of isotherms at 240°C. Electrical aging consisted of the application of an AC voltage just above the Partial Discharge Inception Voltage (PDIV). Aging sequence was a series of 6 cycles of 72 hours. In between aging cycles, the evolution of PDIV and Insulation Resistance on Coil-Coil and Coil-Ground configurations were monitored. Polarisation Index provided by the ratio of measured resistance after 10 min and 1 min were calculated. As concerns thermal aging, PDIV decrease was of 10 to 22%. This decrease occurred mainly after the first aging. Electrical aging under partial discharge activity can be revealed by a significant increase of the Polarisation Index. Electrical aging led to 90% failure during the third aging cycle with typical PD-free EIS while none of the motorettes with strengthened EIS were affected during six cycles. PD analysis revealed with the strengthened paper a specific behaviour consisting in a discontinuous regime of discharges.

This paper presents results carried out on polymeric high voltage direct current (HVDC) model cable. Two aspects are considered: partial discharges (PD) and current measurements, with the aim of investigating possible growth of voids of micrometer size. PD can occur within defects already existing inside crosslinked polyethylene (XLPE) matrix under various conditions. These conditions are well known under alternative current (AC) but not so well under DC with harmonics. Currents are at the origin of charges accumulation at XLPE/void interfaces, which may in turn be responsible for local field enhancement, discharges and possibly into void growth. Model cables have been subjected to (70°C) only or to HVDC field with harmonics under 60 kV/mm DC and 8 kV/mm 1800 Hz AC at nominal temperature (70°C). PD measurements on unaged model cable suggest the existence of cavities whose diameters can be estimated around $10~\mu \mathrm {m}$. Current densities are compared between thermal and electro-thermal ageing, effect of ageing will be highlighted.

The determination of the Partial Discharges Inception Voltage (PDIV) of the different configurations in electrical motors is essential to ensure the sizing and to choose the right electrical insulation system. For that, the standard IEEE 117-1974 suggests the using of the motorette. Measurements performed on this model show the good reproducibility of the results for non-impregnated and impregnated motorettes. The determinations of the PDIV have also been done using the Paschen's law modified by Dunbar. PDIV resulting from experimental measurements are in good accordance with those obtained by the modeling.

ABSTRACTThis work reports on the relationship between structure and dielectric properties of biaxially oriented polypropylene. The morphology of semicrystalline bioriented isotactic polypropylene films is investigated using wide angle X‐ray diffraction and Polarized Optical Microscopy. A β‐orthorhombic structure, with a crystallinity ratio of about 46%, and “Crater” morphology of the β‐form is identified. Dielectric properties are measured by Broadband Dielectric Spectroscopy over a wide temperature range (−150 to 125°C). Since the dissipation factor of the PP is very low, special care was taken to obtain valid data. Two main relaxation processes are observed: a α‐relaxation peak associated to the glass transition temperature (Tg) at temperature about −7°C, and a broad β*‐relaxation at about −60°C, partly attributed to CH orientation. The variation of the dissipation factor versus sample thickness (from 3.8 to 11.8 µm) is correlated and partly explained by the increase of crystallinity ratio and lamella size at larger thicknesses. It comes out that the thinnest film seems perfectly meet the application requesting, namely lowest dissipation factor and highest permittivity. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42602.

Composites of epoxy resin with high percentage of silica fillers (66%) are designed to improve mechanical and electrical properties of transformers used in railway application. FTIR, (DRX and FE-SEM) and absorption/desorption phenomena are used to investigate the structure, the morphology and the diffusion of water in the microcomposites, respectively. Good dispersion of silica filler with size less than 10 mu m was assessed by SEM, although some clustering (agglomerates) of greater than 1 mu m was observed. The absorption water in the microcomposite obeys to the first Fickian law and shows saturation water of 0.6%. The calculation of the diffusion coefficient of water leads to a value of 2.9*10(-12) m(2) s(-1) in the studied system. Relaxation times of alpha-relaxation and ionic conduction relaxation processes are determined. A correlation is observed between the ionic conductivity and dielectric relaxation processes. The dc-current behavior shows a change in the conduction mechanism from electronic conduction below the T-g to ionic conduction above the T-g. Shallow traps of 0.54 eV and deep traps of 2.21 eV are determined below and above the T-g, respectively. The TSC analysis confirms the VFT behavior of the alpha-relaxation of the microcomposite as obtained by the dielectric spectroscopy. (C) 2015 Elsevier Ltd. All rights reserved.

The dielectric behaviour of nano-SiO2 filled low density polyethylene is investigated over a frequency range of 10 mHz - 10 MHz and for different temperatures from 250 K to 350 K. It is shown that the presence of nanoparticles change significantly the dielectric behaviour of the polymer system. The frequency variations of the permittivity and of the tan delta emphasize a alpha-relaxation process, for each of the nanocomposite samples. The relaxation is more important and occurs at higher frequencies with the increase of the filler content. The increase of the temperature leads to a shift of the relaxation frequency to higher values. Results from chemiluminescence measurements and from X-ray diffraction analysis are discussed in connection with the dielectric behaviour.

Samples of epoxy filled with silica have been subjected to hygrothermal aging at 80°C and 80% relative humidity for several months. These samples were characterized by the measurements of resistances, partial discharges and breakdown voltages. Results for both silanized and non-silanized fillers were obtained. After ageing, large differences were observed in both breakdown voltage and resistance. The breakdown voltage was reduced by a factor of 10 after 74 days of ageing without silanization whereas the decrease was a factor of 2 with silanized fillers. The resistance was reduced by more than 4 decades in the former case and one decade in the second case. The breakdown was preceded by the occurrence of partial discharges. Partial Discharges Inception Voltage (PDIV) dropped by a factor of 10 between 32 days and 74 days of ageing. The PDIV was sensitive to the sample temperature: it was lower at larger temperatures. Partial Discharges (PD) patterns suggest that the discharges occurred in gaseous cavities. Based upon these measurements, a model of breakdown phenomenon based upon cavities filled of water vapour is proposed and discussed.