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Recently, polyurethanes (PURs) have become a very promising group of materials with considerable utilization and innovation potential. This work presents a comprehensive analysis of the changes in material properties important for PUR applications in the electrical industry due to the incorporation of magnesium oxide (MgO) nanoparticles at different weight ratios. From the results of the investigations carried out, it is evident that the incorporation of MgO improves the volume (by up to +0.5 order of magnitude) and surface (+1 order of magnitude) resistivities, reduces the dielectric losses at higher temperatures (−62%), improves the thermal stability of the material, and slows the decomposition reaction of polyurethane at specific temperatures (+30 °C). In contrast, the incorporation of MgO results in a slight decrease in the dielectric strength (−15%) and a significant decrease in the mechanical strength (−37%).

Partial discharges suppression in slot part of rotating machines is an issue to be solved using the different attempts. Since decades the conductive layers of main wall slot insulation are used. Besides their resistivity / conductivity which allows proper contact with stator core and partial discharges / vibrational sparking reduction, their dielectric properties shouldn’t affect the losses in stator insulation. Thus, the dielectric properties of composites used as main wall insulation of rotating machines are presented and discussed. The comparison of final dielectric properties of a composite concerning three different types of conductive tapes are presented. One type of epoxy resin was used as matrix and influence of conductive tapes based on a) PET-non-woven with additional polyester threads, b) polyester / glass fibres and c) electrically conducting glass fabric on whole composite dielectric spectra were investigated. The methodology of investigation is based on broadband dielectric measurement in frequency spectra. The test results show significant differences of composite dielectric behaviour dependent on tape type used as filler.

Volume resistivity is one of basic material properties determined for electrical insulating materials. In many cases, the measurement process can affect the accuracy of the measured data. Therefore, this paper analyzes the measurement process while measuring this parameter. A necessary step to perform the analysis is the analysis of the variability that may arise in the whole measurement process. A controlled experiment is then designed to investigate the factors assumed to influence the measurement result based on this analysis. This controlled experiment is followed by experiment evaluation, results analysis, and a suggestion of possible process or organizational changes that can be made to the measurement system to improve the monitoring of material properties using this diagnostic method. The tests performed showed an expected result: the shorting book and the electrostatic bracelet affect the measured value of the volume resistivity but every parameter to a different degree.

Composites with the epoxy matrix are still the most common composites used in high voltage electrical insulation systems. Polyimides are materials with excellent heat resistance and high electrical strength for application, where high overall endurance and reliability even under harsh operating conditions is needed. Polymer composites with very good dielectric parameters may be composed of base polymer composite and additional dielectric barrier. The combination of the polyimide film and the epoxy-based composite, e.g., in the form of a prepreg, leading to the formation of a multicomponent sandwich composite, is relatively simple and the resulting material properties can be very advantageous. However, the detailed analysis of polyimide films is necessary for a suitable oversizing of the mentioned composites. The study of selected dielectric properties, e.g., volume resistivity, permittivity, and dissipation factor, of several types of polyimides, are presented in this contribution in the first step. Epoxy resin without filler was also analyzed as a base material for the tested composite. Two-component composites (epoxy resin with polyimide film) as a fundamental variant of the mentioned multicomponent sandwich composite were tested in the second step. The results show the impact of polyimide type on the final composite properties from the dielectric point of view. The polyimides incorporated in the composite generally decrease the volume resistivity but increase the dielectric strength.

Potting compounds are an integral part of a large number of electrical devices, in which play the role of mainly mechanical or photosensitive protection. So-called cold-curing resins are one of the subgroups of potting mixtures. This article focuses on this type of potting compound based on epoxy (EPs) and polyurethane (PUs). In addition to the study of their electrical properties, structural properties are also introduced. The main focus of research is on the incorporation of nanometric magnesium oxide (MgO) in different weight ratios and the study of its effect on the properties of synthesized composites. The electrical properties analyses include the changes in relative permittivity, dielectric losses in the wide frequency range, and volume resistivity estimated at DC conditions. In terms of structural analyses, the effect of MgO incorporation on glass transition temperature changes was studied. From the performed investigations it is evident that the selected basic materials can be partially modified to improve their properties, especially in lower levels of filling, where there is a positive decrease of relative permittivity close to industrial frequency (≈3 % for EP+1MgO; ≈11 % for PU+1MgO). For investigated composites is also evident a trend in the growth of dielectric losses. MgO affects also the volume resistivity and glass transition temperature of EPs-MgO and PUs-MgO composites.

This article presents research focused on the reliability of soldered joints. The quality of solder joints is formed already during the soldering process. The first parameter that determines the quality of the connection is solderability. Good solderability is essential for the subsequent reliability of the joint. The second parameter, which affects reliability and is also determined during the soldering process, is the thickness of intermetallic compounds. However, this parameter is also affected by the operating conditions. It increases over time. These two parameters were selected for our experiment. The object of the experiment is to compare the standard lead-free solder alloy SAC 305 with solders that have been enriched with cobalt or germanium. The comparison and full results from measurement will be discussed in the full paper. This article presents research focused on the reliability of soldered joints. The quality of solder joints is formed already during the soldering process. The first parameter that determines the quality of the connection is solderability. Good solderability is essential for the subsequent reliability of the joint. The second parameter, which affects reliability and is also determined during the soldering process, is the thickness of intermetallic compounds. However, this parameter is also affected by the operating conditions. It increases over time. These two parameters were selected for our experiment. The object of the experiment is to compare the standard lead-free solder alloy SAC 305 with solders that have been enriched with cobalt or germanium. The comparison and full results from measurement will be discussed in the full paper.

This study presents the influence of various types of nanoparticle (NP) fillers incorporated into a polyurethane (PU) (VUKOL 022) matrix and its subsequent changes in complex permittivity. Two types of surface modification of SiO2 fillers were investigated. The frequency dependence of the real and imaginary parts of complex permittivity was measured within the frequency range of 1 mHz to 1 MHz using the capacitance method. The 1 wt.% NPs in PU caused an increase (MgO, TiO2, n-SiO2, and f-SiO2) or a decrease (d-SiO2) in the real permittivity. The $\alpha $ -relaxation and intermediate dipolar effect were observed at the temperature dependence of the imaginary permittivity. The change in permittivity by various surface modifications of SiO2 and other nanofillers was discussed based on the multi-core model. Moreover, the NPs caused a shift in the local maximum of the permittivity, which was a result of the interfacial polarisation and a charge multiplication of the $\alpha $ -relaxation process.

Biodegradable ester-based electro insulating fluids have been tested for many dielectric properties and have been proposed and already used in electric devices as the substituent for mineral oil. Some properties are advantageous as high flash and fire point (> 250°), low impact of the water content on dielectric properties (H 2 O saturation hundreds ppm), positive impact on cellulose aging compared to mineral oil, readily biodegradability (OECD 301), etc. However, some properties are contradictory as high viscosity, relatively low oxidation stability, high pour point (eg. −17 ‐ ‐ 23°C rapeseed oil), etc. When employing such as fluid in the e.g. power transformer one needs to face also to the problems with material compatibility. Interactions of some materials in the transformer, such as structural elements, seals, inner part of bushings, separating plane, internal corrosion protection, etc. can interact with the organic ester to produce undesirable chemical reactions that can negatively affect the electrical and other properties of this liquids and oil leaks, failures etc. The paper describes the compatibility problems of such rapeseed based insulating liquid with other materials used in the transformer.

The breakdown phenomena and determination of dielectric strength is one of the essential topics in the field of insulating materials for high-voltage applications. In the case of basic thermosetting plastics, including epoxy resins, dielectric breakdown analysis has been performed many times, and the mechanism of conducting channel formation is already well known. However, the situation becomes more complicated if other material components are combined with the base polymer. The application of composites in electrical insulation systems is currently becoming more common. Of all available electrical insulating types of composites, the attention is paid to sandwich composite in this contribution. Specifically, a sandwich structure, which is an interconnection of a heat-resistant thermoplastic (polyimide film) and a conventional thermoset for high-voltage applications (epoxy resin layer), is analyzed. Tested structures were prepared as the composition of each one of the three different types of polyimide films and always the same conventional epoxy resin which is cured at elevated temperature after the amine curing agent addition. In the case of performed testing of sandwich composites, the conductive channel is formed rapidly through the two polymer layers, which are very different in chemical structure. In addition, an interface is formed between the material layers when these layers are interconnected during the epoxy resin curing process, and the effect of this interface must also be considered in the analysis of dielectric breakdown. This contribution is focused primarily on visual description of differences in a breakdown that occurs when different polyimide films without or even with a certain surface treatment are tested. The values of dielectric strength determined by the dynamic method for tested structures are also mentioned. It appears that the choice of a suitable polyimide film can be quite significant in the sandwich structure of the electrical insulation system in terms of dielectric strength.

This paper is focused on the analysis of selected dielectric parameters stability under the conditions of selected accelerated climatic aging. This stability is analyzed for glass fiber reinforced composites with different thermosetting matrix suitable for the production of electrical insulating and also construction elements. Different types of thermosetting resins (epoxy and polyurethane resins) were chosen for the preparation of composites, which are the subject of the presented experiment. Tested composites were firstly characterized in the delivered state and then after the accelerated climatic aging. Important dielectric parameters were evaluated, which are the volume resistivity and the surface resistivity. Two different tests of accelerated climatic aging were realized. The first one was Temperature/humidity cyclic test, which consists of alternated intervals (i) under the temperature of 55 °C and the humidity of 93 %RH and (ii) under the temperature of 25 °C and the humidity of 97.5 %RH. The second one was Damp heat steady state test when samples were exposed to the temperature of 85 °C and the humidity of 85 %RH continuously for 1000 hours. Based on the measured data of resistivities, prospective materials were selected for further activities leading to the production of construction elements with also good electrical insulating properties and with their minimal affection by simulated climatic conditions.

The influence of electrospun nanofibres made from polyamide 6 (PA6) on the gas bubble size distribution by volume in fibre/thermoset-matrix systems cured without a vacuum was comprehensively studied via various methods. An analysis of the microstructural, thermal and dielectric properties proved that the unique structure of PA6 electrospun nanofibres can withstand the curing conditions required for fibre/thermoset-matrix systems and that the overall dielectric behaviour of manufactured composites is not negatively influenced by the addition of nanofibres. Visual and numerical analyses of the tomograms obtained via X-ray computed microtomography (micro-CT) suggested that a nanofibrous structure can act as a solid grid limiting the growth of large gas bubbles or their random agglomeration during curing. The initial observations were independently supported by partial discharge analysis (PDA), which confirmed that the addition of nanofibrous layers leads to the presence of bubbles with generally smaller dimensions and that the nanofibrous layers significantly suppress the partial discharge (PD) activity inside a composite structure.

Nonwoven polyamide nanofibers have extensive applications in a wide range of areas. However, a general problem of polyamide nanofiber is a poor adhesion to other materials or to microorganisms. In this work, we exposed nonwoven polyamide nanofibers to ultraviolet, long-wave (UV-A) radiation for different times and studied the changes in surface and bulk properties by available techniques. Obtained results confirmed the changes in surface chemistry, polarity, charge, roughness and morphology. Also changes in crystallinity portion and alpha and gamma phases were observed. We confirmed that the UV-A radiation can be used for surface activation and adhesion improvement. These changes are more visible and significant after short radiation time (till 10 min).

The effect of catalysts in the form of a transformer copper winding with Kraft paper insulation on the lifetime of transformer oil was studied via ultraviolet-visible (UV-Vis) spectroscopy and Fourier transform infrared (FT-IR) spectroscopy. The tested naphthenic mineral oil was exposed to accelerated thermo-oxidative aging with limited access to air at temperatures of 110, 120, and 130 &deg, C for times ranging from 24 to 2424 h in the presence and absence of catalysts. The UV-Vis technique focused on assessing the change in the color of the oil and quantified a 35%&ndash, 45% reduction in oil lifetime, depending on the aging temperature, due to the influence of catalysts. The FT-IR analysis focused on the decrease in the spectral band at 3650 cm&minus, 1, which corresponds to the low-temperature antioxidant present in the oil, and quantified the 35% reduction in the oil lifetime independent of the aging temperature. This high rate of reduction in the transformer oil lifetime points to the dominant catalytic role of the Kraft paper and copper during the aging process of the transformer insulation system.

This article adds more information to the problem of natural ester use as an electroinsulating fluid in cellulose paper-oil systems, e.g., for use in power transformers. An accelerated thermal aging (120, 140 and 160 °C) of samples consisting of cellulose paper (transformerboard), a treated natural ester (filtered, basic Al2O3, 0.53% DBPC), a copper plate and moisture was studied. Research was especially focused on the impact of moisture on the aging of the above system. Samples with three different oil moisture levels (80, 143 and 305 ppm water content) were subjected to thermal aging to display the change in a selection of properties (tanδ, acid number, water content). For a study of the polarization effects, broadband dielectric spectroscopy (BDS) was performed to observe the complex permittivity and conductivity dependencies on the temperature and frequency (±30 °C, 0.01-107 Hz). Havriliak-Negami (H-N) diagrams were used to further understand the results. The degradation products during thermal aging were recognized by Fourier transform infrared (FT-IR) spectroscopy. Hydrolysis and hydrogenation occurred in the insulating system with a natural ester aged by elevated temperatures, which is in contrast to CO2 formation in the case of electric aging. The results showed an increase in the acid number, dissipation factor, and number of hydrolysis and hydrogenation products. Furthermore, changes in polarization were observed as a consequence of the thermal aging of paper-natural ester insulation system with the presence of copper and moisture.

This article deals with the issue of selected polymer composites consisting of a thermoset matrix and reinforcing glass fibers. These composites are designed for high voltage applications as an electrical insulating material with high mechanical durability. Three composites with a significantly matrix, specifically, epoxy, polyurethane and polyester resin, are selected with an emphasis on usability in the technical practice. Key properties of tested composites are dielectric and mechanical properties and from this point of view the homogeneity of prepared specimens is very important. An important influencing factor, which is a frequent cause of deterioration of material properties, is a higher lev-el of moisture or even direct contact with water leading to the diffusion of water into the material structure. The main subject of presenting research is therefore the assessment of the water diffusion impact on processed material in the relation to the resistance of the tested specimens to the applied high voltage. Tested composite structures are observed by optical microscopy and the computed tomography is used for analysis of the material volume. Purely surface dielectric properties in the delivered state were evaluated based on the measurement of surface current at a constant direct or alternating test voltage. Results of surface current measurement for all composites in the delivered state show minimal differences between composites. The minimal dependence, primarily for composite with polyurethane, of the water diffusion test results on the internal structure and on the character of voids within tested specimens is determined. The lowest level of voids was found for epoxy matrix composite, but this material did not pass the water diffusion test. Differences of the HV resistance obtained from the water diffusion test are most likely caused mainly by different chemi-cal resistance of resins against boiling slightly alkaline aqueous solution.

Paper is focused on the investigation of the possibility of using statistical methods as discriminant analysis and principal component analysis for a classification of the aging of insulating material. The Statistical method was used in combination with Fourier Transform Infrared Spectroscopy results (FT-IR). The investigation was carried out on thermally aged mineral oil samples. The tested oil was exposed to thermal treatment at a temperature of 130°C for times ranging from 24 to 572 hours. Infrared spectra were measured at predefined times of aging using transmission technique in the middle area of the infrared electromagnetic spectrum. Program TQ Analyst was used for other evaluation of changes in measured infrared spectra and the possibility of their separation according to aging times. Based on the experiment performed, discriminability of individual levels of aging based on infrared spectra (IR) measurements was identified even in cases of minimum spectra differences. Combination of FT-IR measurement together with the application of multivariate statistical methods seems to be promising in possible application to aging level identification and automatization of the process of evaluation.

The polymeric nanofibrous layers as a new material for possible improving dielectric properties and suppression of partial discharges in fiber/thermoset-matrix composites are introduced in this paper. Electrospun nanofibers made from polybenzimidazole (PBI) and polyimide (PI) were incorporated into the structure of the commonly used fiber/thermoset-matrix composites to enhance their dielectric behavior. PBI and PI were electrospun using a Nanospider laboratory scale machine. The spinning process was set to produce nanofibrous layers with two different areal weights (1 and 3 g.m−2 for composites with PBI and 3 and 5 g.m−2 for composites with PI). Control composites without nanofibers as well as composites containing electrospun PBI and PI nanofibrous layers were manufactured by compression molding in a laboratory press without any previous vacuum debulking. To verify the positive or negative influence of the incorporated nanofibrous layers on the overall dielectric behavior of the composites, the volume resistivity p (Ωm) and dielectric strength E d (kV.mm−1) were comprehensively measured. Initial results revealed that the volume resistivity of modified composites increased (of about 126 % for PBI and 217 % for PI) as well as the dielectric strength (of about 11 % for PBI and 53 % for PI). Obtained results were subsequently supported by partial discharge analysis which confirmed that the nanofibrous layers are capable to significantly suppress the partial discharge activity inside the composite structure.

In this study, improved observables and new endpoint criteria were developed to assess the remaining useful life of mineral transformer oil using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR). The tested oil was exposed to thermal ageing at temperatures of 110, 120 and 130 °C for times ranging from 100 to 2400 h. The intensity of the infrared spectral band of the low-temperature inhibitor and the temperature of the thermo-oxidative DSC peak were considered for evaluation. The obtained results were correlated with standardized dielectric dissipation factor (tan δ) measurements. The results indicated that the analysed observables changed continuously during the applied thermal ageing. The endpoint criteria were subsequently determined and Arrhenius diagrams were constructed. The results demonstrated that the determination of the actual state or the lifetime of the transformer oil via DSC or FT-IR may be beneficial in terms of reducing the measurement time, the influence of moisture on the measurement results and the oil volume required for the tests. Thus, DSC and FT-IR represent promising alternatives to dissipation factor measurements.

A unique portable measuring system using an impedance spectroscopy method with a self-adapting frequency of measurement is introduced. The system is intended for the on-line in situ monitoring of composite materials curing under industrial conditions. The capabilities of the developed system are demonstrated through the results obtained from on-line in situ measurements of unreinforced thermosetting resin, as well as of composites under real manufacturing conditions. Observations are supported by the results of other established methods for determining the degree of curing: temperature-modulated differential scanning calorimetry (MDSC), Fourier transform infrared spectroscopy (FT-IR) and broadband dielectric spectroscopy (BDS). Compressive and bending tests were also carried out on manufactured composites removed at different stages of the post-curing phase. Due to the self-adapting frequency, the system has enhanced sensitivity in the post-cure phase when the diffusion-controlled reactions proceed and, therefore, is suitable also for the analysis of hard post-cure samples.

The paper is focused on an investigation of the cause of different processing behaviors of the PVC granules of one batch delivered by different transport modes (air, shipping). While airborne transported granules (delivery time approximately 14 days) did not show any production problems, the surface of the final cables produced from granules transported by shipping (delivery time approximately 2 months) contains in some cases a surface defect in the form of swelling. Both samples of granules were analyzed using methods of structural analysis to investigate mutual differences between samples. Simultaneous Thermal Analysis (STA), Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared Spectroscopy (FT-IR) were used for the samples comparison. As an additional test, analysis of dissolved gases evolved during thermal decomposition of the sample from shipping transport was recorded. Results of the performed analyses showed different behavior of tested samples. Typical decomposition behavior of PVC corresponds especially to air transport granules while the sample transported by shipping contained an additional weight reduction at around 200 °C. The composition of evolved gases at this temperature contains only water and carbon dioxide, which are probably the decomposition products of hydrotalcite contained in the sample of PVC granule, whose structure could contain up to 12 % of the water of crystallization. The obtained results demonstrated that probable cause of the different processing behavior of samples represents the higher content of the water of crystallization in granules transported by shipping.

Composites based on a linear low-density polyethylene (LLDPE) and containing clay filler halloysite nanotubes (HNT) are perspective due to advantageous fire retardancy. This paper is focused on an impact assessment of an accelerated aging on dielectric properties of these composites (LLDPE/HNT), more specifically, accelerated aging by intensive ultraviolet (UV) radiation is taken into account. Evaluation of this type of aging is important in every practical situation when electrically insulating layers are exposed to sunlight. Composites were prepared with three different weight percentage of clay filler and pure LLDPE is used as the reference material. Samples were exposed to UV radiation for 50 h, 100 h, and 150 h. Emphasis is placed primarily on the characterization of changes in dielectric strength in a harmonic electric field and volume resistivity of tested materials. Furthermore, tested materials were also analyzed by broadband dielectric spectroscopy to describe an influence of the exposure to UV radiation on the frequency and temperature dependent relative permittivity. Detection of a minimal deterioration and, in some cases, even a slight improvement of selected dielectric parameters of LLDPE/HNT caused by UV radiation, is very positive. Results obtained by broadband dielectric spectroscopy prove changes of polarization processes in tested composites which occurred as an effect of UV irradiation.

Changes in the activation energy of thermal decomposition of mineral oil during accelerated thermal ageing were studied utilizing thermogravimetry. The main aim was to investigate possibilities of such approach for precise diagnostics of structural changes in mineral oil along the transformer lifetime. The examined oil was a commercially available Nytro Lyra X manufactured by Nynas AB. The accelerated thermal ageing was performed in a laboratory oven under a limited access of air at a temperature of 110, 120 and 130°C. The Fourier transform infrared spectroscopy (FT-IR) was also used to support and interpret observations obtained from thermogravimetry. Values of activation energy were calculated based on standard ASTM E1641. Results of FT-IR revealed that the intensity of the infrared spectral band of the low-temperature inhibitor decreases along thermal ageing within all of the applied temperatures. Evolution of activation energy values over ageing times and temperatures contains three linear regions separated by local minimum and maximum points. Observed changes in activation energy accompanying the ageing process of mineral oil, especially achieving its local maxima are very promising. Together with results of our previous experiments, the activation energy values can present potential observables and its maxima can potentially be considered as a new endpoint criterion for determining the residual lifetime of mineral transformer oil.

This study evaluated new experimental parameters and end-point criteria for determining the thermal endurance of fibreglass-reinforced epoxy laminates via thermal analysis. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA) were used in these experiments. The composite was exposed to temperatures ranging from 170 to 200 °C for times ranging from 10 to 480 h in accord with IEC 60216. The maximum heat flow temperature (DSC T max ) for the first thermo-oxidative reaction, the maximum weight loss temperature (DTGA T max ), and the maximum decomposition rate were investigated. The change in the glass transition temperature after the thermal ageing was also determined using DMA. The structural changes in the samples were examined via Fourier transform infrared spectroscopy and microscopic analysis. Oxidative degradation at the surface accompanied by pyrolytic degradation in the bulk of the sample was observed. The end-point criteria were derived for all applied methods based on the time required for the composite structure to delaminate considerably. The obtained data were used to construct Arrhenius diagrams, and cross-correlation was sought among all experimental parameters. These measurements demonstrated that the DTGA T max , DMA T g , and DSC T max characterizes the ageing process sufficiently well within the applied temperature interval and satisfy the IEC 60216 requirements.

The mechanical behavior and the thermal stability of an encapsulant based on ethylene-vinyl acetate (EVA) were studied. The EVA properties were verified at temperatures ranging from −70 ◦ C to 500 ◦ C. Thermogravimetry, differential scanning calorimetry and dynamic mechanical analysis were used in this study. It has been shown that the encapsulant has a good weight stability; however, the encapsulant passes through both a glass transition and a melting phase in the range of operating temperatures. The kinetic parameters of crosslinking were also analyzed. It is possible to achieve 65% crosslinking at a temperature of 150 ◦ C and a time of 5 minutes. The activation energy of crosslinking is 95.6 kJ/mol. K e y w o r d s: renewable energy resources, photovoltaic, EVA, encapsulant, glass transition, thermal analysis

The online monitoring of resin curing process is described in the presented article. There exists several monitoring technique useful for resin curing monitoring and consequently for the curing technology improving, at the present. The advantage of the described monitoring system and monitoring method is in the using a unique sensors directly inserted into the tested resin or technology part which allows observing the resin characteristics as well as the local temperature directly at the sensor area. The data acquisition is processed by the in-house developed portable curing monitoring device and wirelessly transfered to the PC for further analysis. The study describes the experimental work of using the monitoring system and sensors and also the curing process of resins and composites using electrical impedance magnitude.

Analysis of infrared spectra (IR) can serve to the identification of material properties, changes during aging or quantitative analysis of compounds in composite materials. For the proper quantification, the decision about the technique of measurement and method of evaluation must be performed. Peak intensity or area is possible to use for this purposes. Both methods have their advantage and disadvantages. From this reason paper is focused on the example of different methods of spectral evaluation based on the analysis of aging process of mineral oil. The spectral band (height and peak area) of low-temperature inhibitor (3650 cm−1) was evaluated by means of Fourier Transform Infrared Spectroscopy (FTIR).

Reliable operation of transformers largely depends on a lifetime of insulating system. For this reason the paper deals with a description of our experience with developed monitoring system, which is focused mainly on a control of oil filling of experimental transformer. Relative humidity, amount of dissolved gases and increased temperature are observed, as they have decisive influence on insulating system of transformer. Monitoring of these characteristics is completed with analysis of oil samples by infrared spectroscopy in mode of Attenuated Total Reflection technique.

The influence of accelerated thermal treatment of thermosetting epoxy laminate on its glass transition temperature was studied. Lamplex® FR-4 glass fibre-reinforced epoxy laminate (used for printed circuit board manufacturing) was used in these experiments. The composite was exposed to thermal treatments at temperatures ranging from 170 °C to 200 °C for times ranging from 10 to 480 h. The glass transition temperature (Tg) was analysed via dynamic mechanical analysis (DMA). It has been proven that the glass transition temperature rapidly decreases in reaction to thermal stress. The obtained Tg data were used for Arrhenius plots for different critical temperatures (Tg-crit. = 105–120 °C). From their slopes (−Ea/R), the activation energy of the thermal degradation process was calculated as 75.5 kJ/mol. In addition to this main relaxation mechanism, DMA also recorded one smaller relaxation process in the most aged samples. Microscopic analysis of the sample structure showed the presence of pronounced small regions of degradation both on the surface and in the inner structure, which are probably the causes of microscopic delamination and the smaller relaxation process.

Suitable (biodegradable) substitution of classic mineral petroleum-based oils used in power transformers is studied. Paper presents test results of six paper-oil insulation systems varying by different type of oil. Three common transformer oils and three new synthetic oils (never used in power transformers before) were selected for the purpose of experiment. The oils have been exposed to accelerated thermal aging (at the temperature of 90 °C) and their properties were measured in time intervals of 0, 50, 125, 225, 500, 1000 and 2000 hours.

The main objective of the paper is to describe the experience with newly developed monitoring system for transformers with oil-paper insulation system. From the view of operation reliability, it is necessary for operator s to provide a monitoring of transformer's failure free operation. The application of monitoring system enables to warn ti mely of the rising failure and also decrease of degradation of single subsystems of machine. When failure is detected, it is important to support this detection by other measurements tha t would lead to exact determination of the failure. SFRA analysis (Sweep Frequency Response Analysis) is such additional method in our monitoring.

Nonwoven nanofibers as a new material for possible improving properties of electrical insulation are considered in this paper. Polyamide nanofibers are used for modification of commonly used three-component mica-based high voltage electrical insulating material (epoxy resin, glass cloth and mica). Influence of different volume ratio and density of the nanofibers on the properties of resulting composites was analyzed. Melting behavior of nonwoven nanofibers and structure of resulting composites were examined by differential scanning calorimetry (DSC) and by scanning electron microscopy (SEM). Dielectric strength and voltage dependence of dissipation factor were subsequently tested. Mechanical properties were analyzed via Charpy impact test. The obtained results demonstrated that voltage dependence of dissipation factor showed interesting behavior especially at electric field intensity greater than 5kV/mm when the partial discharge activity starts to occur. Under these circumstances some of the modified composites are characterized by notable lower values of dissipation factor. Moreover, while the impact strength of some of the modified composite increased (of about 17%), dielectric strength stayed the same or better than expected.

Halloysite nanotubes (HNT) represent a type of aluminosilicate clay of natural origin with nanotubular structure. HNT have been started to use in many industrial fields, including the application in the electrical engineering. This paper is focused on the determination of dielectric properties of pure halloysite nanotubes, as well as, a polyethylene (LLDPE and HDPE) based HNT nanocomposite (blends). Individual samples in each series had contained different amount of HNT filler. The impact of the HNT content and the impact of the type of polyethylene on the dielectric properties were observed and evaluated. Measurements were carried out under the AC field via a broadband dielectric spectroscopy. Dielectric properties were evaluated through real and imaginary parts of the complex relative permittivity. Results are displayed as surface visualizations of frequency and temperature dependence. The relaxation times of selected relaxation processes were analyzed according to the theory of relaxation polarization.

Low-fire-hazard cable (LFHC) intended for low voltage applications with a fire barrier made from mica tape was tested under conditions similar to those of the fire test specified in IEC 60331-21. In addition to the other standard requirements, the measurement setup was modified to enable insulation resistance measurement throughout the entire fire test. In this manner, cable insulation as a whole can be tested under fire conditions. Results from insulation resistance measurement are supported by thermogravimetric analysis and by the data obtained from two thermocouples placed near the cable sheath and inside the cable core during the test. As results show, insulation resistance is markedly affected by the melting of the organic components of insulation, by decomposition of the flame retardant and by ignition of the core insulation along the first period of the test (10 minutes). The stable value of the insulation resistance from 10th to 90th minutes of testing, together with the insulation's behavior during cooling suggests the important role played by conductivity of the flame itself. The suggested procedure can assist in the comparison of the fire-proof functionalities of different cable designs, in the analysis of their failure mechanisms and in cable design optimization.

Paper deals with analysis and description of the curing process of the samples made of the epoxy resin using method of Fourier transform infrared spectroscopy (FT-IR) in technique of Attenuated Total Reflectance (ATR). Infrared spectra were measured on samples cured in two steps with isothermals at 93 °C and 150 °C when samples in delivered state and cured for 20, 42, 52, 60, 70, 90 and 110 minutes were analysed. Infrared spectra were recorded on Nicolet 380 spectrometer (Nicolet Instrument) using Attenuated Total Reflectance technique (ATR) with ZnSe crystal in the range of wavenumbers of 4000 – 600 cm−1. Spectral bands of anhydride and epoxy group were used for the curing identification. The glass transition temperature was measured as a comparative parameter for the curing level investigation. On the basis of results, good applicability of the method of Fourier transform infrared spectroscopy for identification of un-cured samples is evident.

In this paper measuring and establishing electrical and mechanical properties of foam epoxy resin Foam 128 as a material for stator winding bracing in large rotary machines are described. The material is considered in the light of partial discharges, loss factor, flexural strength and strength in compression. The aim of this paper is to decide whether the foam epoxy resin could be used when repairing damaged winding bracing in large rotary machines.

This paper is focused on the comparison of the behavior of two low fire hazard cables of different construction under fire conditions. The fire barrier of the first one was created by glass and glass mica tape while the second one was protected by ceramifible silicone insulation. The insulation resistance between insulated cores of cables was measured and periodically recorded. The fire test was done according to IEC 60331, the whole test took 90 minutes of a burning and 15 minutes of cooling. The results show interesting changes in insulation properties of the cable in the first few minutes when the insulation resistance decreases by several orders of magnitude. On the other hand, during cooling the insulation, the state of cables is almost restored to its initial values.

The main objective of this paper is to investigate the possibility of application of Fourier transform infrared spectroscopy and method of discriminant analysis for classification of a solid dielectric. Classification method was determined by applying accelerated thermal aging on glass-mica-based insulating system material. Based on previous experience and on the thermal class of the insulating system, the aging temperature was set to 170 °C. Infrared spectra were measured after 192, 384, and 600 hours of aging using technique of Attenuated Total Reflectance in the middle area of the infrared electromagnetic spectrum (4000-600 cm-1). Regarding the results, it is possible to state that the method of discriminant analysis in connection with Fourier transform infrared spectroscopy can be regarded as a proper diagnostic indicator of aging for material of insulating systems.

The paper deals with the problem of thermal aging of PET and Glass-fibre based composites. These electro-insulating materials are used in high voltage engineering as parts of insulation systems of large rotating machines like turbo or hydro generators. These materials were subjected to accelerated thermal aging to provide more information about the aging effects and allow us to estimate the lifetime of the materials at operating temperatures. The laboratory method used to compare the dielectric properties (relative permittivity, loss factor, parallel capacitance, parallel resistance and impedance) during the accelerated thermal aging was the dielectric spectroscopy. Thermal aging of the samples was performed in the laboratory thermal oven and the changes of their physical and chemical properties were measured and evaluated at given exposure times. Different temperatures (170-194 °C) were chosen for the accelerating of the aging process. The aging time was determined for each temperature value. Specimens of tested material were flat plates 100×100 mm. The measurement of these specimens was carried out by means of a special electrode test setup at frequencies from 200 Hz to 2 MHz. For comparison of the aging process of the investigated materials, the trends of measured parameters were studied and described in dependence on exposure times, temperatures and frequencies applied during the measurement. The measurement results show that the properties of the measured insulation materials change depending on the internal structure, temperature and the time of exposition.

There is difficult to imagine modern manufacturing of electrical devices without components containing (mainly for technological reasons) organic substances. This fact is mainly concerned to composite materials, in which the bond component is made of organic substance. Most of the electro-insulating materials used in common applications are composite materials. The possibility of the determination of dielectric properties of these systems is a problem of theoretical as well as practical importance. The paper is focused on thermal aging and accompanied dielectric spectroscopy diagnostics of two mainly used materials, which are utilized as a part of insulation system of electrical devices. The first tested specimen was mica composite material based on glass fibre and epoxy resin and the second one was two component composite based on epoxy resin as well. The specimens were tested under laboratory conditions. The materials were thermally aged and the changes of its physical and chemical properties were measured and evaluated. For accelerating the aging process different temperature values (170–200°C) were chosen. The aging time was determined for each temperature value. Specimens of tested material were performed and cured as flat plate 100×100 mm. The measuring of these specimens was carrying out in frequency domain from 200 Hz to 2MHz at special electrode test setup. For comparing the aging process of the investigated material the trends of measured parameters were studied and described in dependence on exposure time, temperature and applied frequency during measurement.