Vaclav Mentlik

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.

Electroinsulating fluids — New insulating mixtures

Technological progress creates new challenges, especially in the areas of energy savings, environmentally friendly materials and sustainable development. These challenges exist for electroinsulating materials, including electroinsulating fluids. Finding substitutes for the petroleum-based fluids used in electrotechnics is not a simple task. It involves the geopolitical location of the manufacturer and the user of the machine, fluid prices and fluid parameters. The electrical parameters, along with various other properties, of newly proposed fluids need to be evaluated over a long period of simulated operation in order to determine their suitability for use in power transformers. New international standards are being developed to address this issue. Many laboratories around the world are searching for an acceptable new insulating fluid. This paper presents the results of a study of newly proposed and prepared mixtures of insulating fluids with higher flash points and suitable electrical parameters.

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.

Aging phenomena of paper-oil insulating system under different voltage stress

Until now not much attention was paid to behavior of insulating materials under different voltage stresses. For decades, the applied research was done generally under AC voltage. As power electronics was involved in drives and dominated instead of DC drives, new troubles with lifetime occurred. The great influence of higher harmonics does not affect operating parameters only but also it limits the lifetime of the insulation system. Steep voltage rises are generated by power electronics, significant overvoltage occur. As economic aspects play its role, often not sufficient filters are utilized to reduce switching overvoltage, which leads to additive voltage stressing. Recently also DC transmission is planned or even build. DC transmission is especially interesting at very high voltage levels, which brings the space charge phenomena together. Space charge is causing electric field distribution with unexpected stresses affecting the insulation material. There is also the effect of voltage polarity, which has significant influence on material endurance. The paper describes the behavior of paper oil insulation system, which is widely used in field and affected by all voltage mechanisms. For smooth and trouble-free operation a precise lifetime test must be done to determine material limits and recommend the range of voltage stress for reliable operation.

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.

Partial Discharge Potential Free Test Methods

During the last few decades, partial discharge measurement and analysis have been considered one of the most important test methods for evaluating the quality of electrical machines and insulating systems. Test methods based on potential free capacitive and/or inductive couplers have taken place as well as methods based on the galvanic circuit with measuring impedance. These potential free couplers detect the electromagnetic field that is generated by partial discharge activity. The partial discharge detection, with using of the capacitive coupler, was the main reason to perform the measurements. The conclusions of the test are presented in this paper. Furthermore, the analysis of partial discharge of several physical models was performed because of the couplers calibration and sensitivity evaluation. The different methods and apparatuses were used for partial discharge detection with the aim of the accuracy and sensitivity of the potential free method determination. The diagnostic equipment consists of a wideband analog apparatus, a digital apparatus, couplers and measuring impedance. The measurement was done according to IEC 270 standard. The capacitive couplers have been used as well for performing the potential free test method measurement.

Natural esters as a part of HV transformer insulation system

Natural esters are candidate element of transformer insulation system. Recently most common insulation in power transformers is composed of mineral oil and transformer paper. As there are efforts to preserve sustainable development, crude oil products should be eliminated and replaced by natural products, which are easily biodegradable. In past, it was shown, that natural esters may reach qualities of mineral oils in some parameters (from electrical parameters it is typically electric strength), while they remain worse in others (typically dissipation factor). Although there are some difficulties, by proper design they should be eliminated. The paper concentrates on specific aspects of designing insulation elements by preserving the same lifetime as in case of mineral oils. Not only endurance curves should be considered, but also other aspects as partial discharge resistance, oxidation and viscosity rise, regeneration and the impact on solid components. Valuable information can be obtained from FEM field calculations, which are recommended. Voltage distribution and especially electrical field intensity may point out high stressed areas where potential problems may occur during material operation as well as temperature rise can be calculated. During material aging in service high stressed areas deteriorates by continuous irreversible changes which lead to outages and downtimes or to human health threat in the worst case.

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.

Possibilities of modification of polymer/SiO 2 composite by various types of carrier

The main aim of this paper is to describe possibilities of modification of polymer insulating materials by various types of carrier. The major problem of currently used insulation system is their inhomogeneous structure. Therefore, there is an efforts to develop material that is macroscopically homogeneous. New composite materials are created using nano-particles silica (~1 wt %) and epoxy type of resin. Polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and fabric with micrometric thicknesses were chosen as carrier for this research. Composites were created by using a mixing method, in which a nano-fillers were applied to the epoxy resin and applied on PET, PEN or glass fabric carrier component. The dissipation factor, volume resistivity, dielectric strength and polarization indexes were measured to determine the electrical insulating properties.

Composites with nanosilica

There are only a few materials that are promising composites. For decades high-voltage electrical insulation technology used ternary insulation systems; usually based on modified epoxy resins and reconstructed mica. Adding additional components into existing systems, it is possible to influence their properties. However, the disadvantage is an inhomogeneous composite structure that emerges from the very quality of these materials. Inhomogeneous structure material causes defects during operation that increase material and this can limit the life of the entire device. The key to solving this problem is to reduce the dimensions of the individual components, in this case the filler. This paper deals with the study of electrical insulation properties of nanocomposites based on epoxy resin filled with hydrophobic and hydrophilic silica. Nanocomposities were created by using a dissolution method, in which a nanofiller was applied to the melted matrix DGEBA. We made a special mould to cure the samples. To determine the electrical insulating properties the following characteristics were measured: polarization index, volume resistivity, dissipation factor and dielectric strength. The aim of this study is to demonstrate that hydrophobic silica combined with water with little or no hydrogen bonds will have better electrical properties than hydrophobic silica on its own. The results show that the best results were achieved with 1% hydrophobic silica.