R. Polansky

Aging of petroleum and synthetic based insulation oils from the view of structural analyses

Insulating system of power transformers is mostly composed of two basic components: insulation oil and cellulose paper (oil-paper system). These components are of an organic nature and thatpsilas why they become very sensitive to operation degradation conditions (temperature, oxidation, moisture etc.). Aging of these components can endanger the failure free operation of the transformers. The main objective of the paper was to analyze the aging process of liquid part of oil-paper insulation system by selected structural analyses. Three petroleum and three synthetic based insulation oils were chosen for the purpose of this experiment. The commercially available cellulose paper formed the solid part of the insulation. All insulation systems were exposed to accelerated thermal aging at increased temperature of 90 degC for the time range of 50 - 3000 hours. The degradation of oils inner structure was analyzed by Fourier transform infrared spectroscopy (specifically by Attenuated Total Reflectance technique). The analysis of insulation oil behaviour during thermal decomposition was the other aim of our experiment. Thermogravimetry and derivative thermogravimetry were applied as the proper tools for this purpose. Results of structural analyses clarify very well the differences between the behaviour of mineral and synthetic insulation liquids at common temperatures as well as during the thermal decomposition.

Influence of oil type on the oil-paper insulation properties

The behavior of the oil-paper insulating systems of transformers is primary determined by paper and oil basic properties. Recent studies are concerned to study of using synthetic oils instead of traditionally used petroleum based oils. This study monitors the influence of oil type on the final insulating system properties. The main degradation factor -temperature -is respected and properties under thermal stress are studied considering the degradation knowledge importance. The thermal analyses as well as the phenomenological test methods were used to perform the test results.

Property Changes of Transformer Oil during Limiting States of Transformer

Power transformer service availability is influenced by a condition of insulating and cooling medium - by transformer oil properties. The transformer is subjected to many untypical situations arising casually or due to changed operation conditions. Between the casual phenomena belongs e.g. an unbalanced loading, which causes phases overload etc. These situations reflect on the properties of cooling medium. Parameters such as an excessive formation of gasses in transformer oil, an increase of water presence in the oil or changes of temperature distribution along the winding of transformer can be changed during the operation. Monitoring of these changes is performed in our experimental department, which was developed directly for this purpose. Tested transformer is equipped with sensors for mentioned parameters monitoring. Information obtained from laboratory testing of the experimental transformer will be useful for assessment of processes proceeding in common operation of these devices.

Thermomechanical analysis of electrically conductive adhesives

Mechanical properties of isotropic electrically conductive adhesives (ICAs) are mainly influenced by their formulation and thermal processing. Thermomechanical analysis of ICAs was used with the goal to find influence of addition of two types of nanoparticles, silver nanoballs and carbon nanotubes, and two types of curing processes, on air and in vacuum, on the glass transition temperature and storage modulus of bisphenol epoxy adhesives filled with silver flakes (70 to 80 % by weight). Specimens were formed as blocks. Three groups of specimens were fabricated: specimens of adhesives used as received and cured according to the recommendation of a supplier in air; specimens of adhesives used as received and cured according to the recommendation of a supplier in vacuum, specimens of the same adhesives added with low fraction (0,3 to 10 % by weight) of silver nanoballs, colloidal dispersion of silver, or carbon nanotubes. There were found differences in the glass transition temperatures and storage and loss moduli in dependence on the type of particles added into adhesive. It was also found that the adhesive cured in vacuum had lower thermal expansion coefficient and higher the storage and loss modulus in comparison with adhesive cured in air.

Behaviour of poly(ethylene:vinyl acetate) and polyether urethane-urea during thermal decomposition

Poly(ethylene:vinyl acetate) (pEVA) and polyether urethane-urea (PEUU) are widely used i.a. as materials for electric cable insulation, hence they contain various fiame-retardants for this purpose. Two flame-retardant filled pEVA samples are compared with PEUU sample without flame-retardant. Thermal effects accompanying the burning of these materials are examined by differential scanning calorimetry (DSC), performed together with Fourier transform infrared spectroscopy (FTIR), which records and identifies gas products evolved during the combustion reactions. Quantification of mass loss is simultaneously assessed by thermogravimetry (TGA). Obtained data indicates that various processes proceed in these materials in the area of critical temperature range.

Dielectric changes of PET and Glass-fiber based epoxy composites during thermal aging

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.

Dielectric spectroscopy of thermally aged insulation

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.

Partial discharges of thermally and electrically aged insulation

The working life of electrical machines is primary affected by the insulation system quality. Diagnostics methods help to understand the momentary state of insulation and to avoid the possible damage or electrical breakdown of machines through the repair or insulation rewinding. Partial discharges testing belongs to one of the high applicable test method of insulating materials within electrical machines. The described experiment consists of laboratory thermal and electrical aging of main wall insulation and consequently testing of partial discharges. The test material consists of resin rich mica composites based on glass fabric and epoxy resin. The flat specimens of insulation were cured and thermally and electrically aged and the partial discharges were measured accordingly. The thermal aging was performed at four different temperatures (170, 175, 180, 186°C) at different times. The electrical aging was completed at two different voltage levels (10, 12 kV) at different times. The flat specimens were tested at special electrode setup. The conductive rubber electrodes were used to avoid the gliding discharges on the surface of specimen. The trends of partial discharge main parameters (ignition voltage, extinguish voltage, pulse count, average discharge current, peak charge level) are studied and described in dependence on exposure time, temperature and applied voltage.

A study on the usage of nonwoven nanofibers in electrical insulating materials

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.

Influence of Temperature Aging on Oxidation Stability and Activation Energy of Insulating Liquids

Insulation liquids are mainly used in oil filled transformers for their cooling and insulating properties. Besides a testing of electrical properties, there is also important to monitor oxidation stability and activation energy as the factors which can decide on an application possibility of the insulating liquid. The main aim of this paper is to describe the influence of temperature aging on the oxidation stability and activation energy of tested insulating liquids (common petroleum based product and dioctyladipate - di(2-ethylhexyl) ester of adipic acid - synthetic based liquid). Both liquids were exposed to a longterm thermal aging at the temperature of 90degC for 3000 hours. A thermogravimetry was applied to determine the activation energy, oxidation stability tests were performed by differential scanning calorimetry.