Lukas Harvanek

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.

Influence of thermal aging on electrical properties of inhomogeneous dielectric material

This paper deals with diagnostic of composite insulation material which are currently used in high voltage appliances e.g. in large rotating machines - turbo or hydro generators. This composite is consist of calcinated mica paper, polyethylene terephthalate (PET) foil and epoxy-novolac resin. The insulation composite material with inhomogeneous structure was subjected to accelerated thermal aging and dielectric parameters such as dissipation factor, permittivity, capacity, absorption characteristics with its related parameters and breakdown voltage were measured in predetermined time intervals. Two temperature levels (180 and 195 °C) and various times of exposure were investigated. The temperature and time of exposure dependences of individual parameters are presented in this paper.

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.

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.

Influence of nano and microparticles on the development of electrical trees

This paper deals with the effect of inorganic micro/nanofillers on electrical tree growth in high voltage insulation during application of AC voltage of 20 kV, 50 Hz. Pure epoxy samples were subject of experiment as well as epoxy micro/nanocomposites with hydrophobic nanosilica or hydrophilic microsilica. The times to breakdown were evaluated and compared. A transparent resin was used allowing observation of electrical tree grow with the help of an optical microscopes. Also the X-ray computed tomography was used for the 3D visualization of electrical trees in evaluated samples. The results show that the time required for the tree development and breakdown increases when a nanofiller or a microfiller is used. The tree growth optical patterns are different for pure epoxy, epoxy microcomposites and epoxy nanocomposites. The possible reasons why and how nanoparticles prolongs the time to breakdowns is discussed.

Behavior of alternative electro insulating liquid at solidification process

The most commonly used insulating liquid is mineral oil. Mineral oil is characterized by high flammability and poor biodegradability. On the other hand, this oil has very good electrical properties. For reasons of environmental protection is an effort to replace the mineral oil by alternative insulating liquids. The vegetable or synthetic oils may be used as an alternative insulating liquid. The rapeseed oil is a suitable replacement for mineral oil in terms of dielectric properties, environmental friendliness, and financial costs. The main objective of this paper is to analyze the influence of solidification of rapeseed oil on its dielectrics parameters by dielectric spectroscopy measurement. The main emphasis is placed on the area near the pour point. The comparison of the dielectric strength of rapeseed oil in the liquid state and in the solid state is also part of this research.

Effects of individual chemical cleaning detergents on conditions of winding surface

The aim of this paper is to describe cleaning processes of rotating machines in detail and to analyze the effect of chemical cleaning detergents on condition of the insulation system of the electrical rotating machines. The study emphasizes the importance of cleaning procedure as a part of the maintenance of electrical rotating machines. Three chemical solutions which contain different types of isoalkanes and acetone cleaner were investigated. The real stator windings were used for this investigation. The changes of surface resistivity were measured in initial conditions and also after long-term exposure of chemical solutions at predetermined intervals. A nonlinear regression model was defined in order to describe changes of surface resistivity during this investigation.

Modification of epoxy resin used in high-voltage technology

This paper focused on modified resin that contain epoxy matrix on base improving the thermal conductive, a new composite system was designed by applying new phase represented by the nano or microparticles of silica. In this paper, low quantities bisphenol A (DGEBA) with diamine crosslinker (Laromin C260). In order to assess the challenge of improving the electrical properties while not changing or even of nanosilica were used (1 wt%), which were derived from previous measurements. The sample achieved the best electrical properties with this quantity of nanoparticles. We used also the same quantities of microparticles (1 wt%). The impact of the additional filler on the electrical and thermal properties of the nano and microcomposites were investigated. Using thermal conductivity measurements, it was found that low quantities of microfiller added to the epoxy resin can improve the thermal conductivity. Moreover, using dielectric characterization techniques (volume resistivity, dissipation factor and dielectric breakdown), it was found that the electrical properties of the material with microfiller remain changed or are slightly improved. By contrast, nanoparticles improve electrical properties significantly but slightly increase thermal properties compared to the pure epoxy resin.

Effect of electrical/thermal aging on dielectric properties of polymer/SiO 2 nanocomposites

This paper deals with experimental results of the combined electrical and thermal aging of two epoxy resins with nanosilica subjected to AC voltages. Overall 20 samples were prepared. Ten samples contain laboratory epoxy resin DGEBA, hardener Laromin C260, and nanosilica. Other 10 samples contain industrially used epoxy resin Epoxylite also with nanosilica. These ensembles had been measured prior to ageing and then exposed for long-time. Samples were measured in the course of ageing in a time series after 100, 500 and 1000 hours. To determine the electrical insulating properties the following characteristics were measured: polarization index, volume resistivity, dissipation factor vs temperature, permittivity vs temperature and dielectric strength. The measurements and subsequent evaluation show that Epoxy resin ages slowlier than Epoxylite. Our benefit is two-factor and long-term aging of the nanocomposite and determining their dielectric properties.