Jaroslav Hornak

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.

Comparison of the electrical properties of canola oil with commercially available mineral oil

Insulating liquids are used as electrical insulation and cooling medium for power transformers. The most commonly used insulation liquid is mineral oil. Mineral oil is characterized by its high flammability and poor biodegradability. On the other hand, this oil has very good electrical properties. The future development of biodegradable oil is most focused on vegetable oils. The disadvantage of this oil is faster aging process compared to mineral or synthetic oils. This paper discusses the experiment, which is divided into two parts. In both parts of the experiment are investigated mineral oil and canola oil. The first part of the experiment follows the dissipation factor, permittivity and resistivity. The second part follows the dielectric strength of these oils.

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.

Statistical approach to endurance models - data processing using regression models

The paper presents a methodology for processing experimental data related to the issue of the electrical insulation system endurance, using the method of accelerated laboratory aging. Several possible approaches are proposed. Traditionally used is the phenomenological approach combined with mathematical regression of measured data. This method is complemented by confidence intervals to ensure statistical certainty of the results of the experiment statements. The paper also describes a statistical approach to endurance modeling, however similarly supplemented with statistical certainty by using confidence intervals. Statistical certainty based on confidence intervals brings very valuable information for the practical use of electrical insulation materials and systems, especially considering design and dimensioning.

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.

Aspects of PD measurement in hybrid gaseous insulation at DC voltage

The increased application of HVDC components in electrical energy systems has led to new challenges in diagnostic testing and measurement of electrical insulation. In addition to other diagnostic methods Partial Discharge (PD) diagnosis at DC voltage is currently subject to much discussion worldwide. This is because the established and approved procedures, methods and evaluation techniques at AC voltages cannot be so easily applied under DC conditions. This paper deals with some problems associated with PD measurement, which can be carried out in hybrid gaseous insulation using DC testing. In principle, such insulation types are mostly characterized by the presence of insulating (solid) materials. These interfaces can become electrically charged, should an electric field be present within the insulation, e.g. by the application of an external electric field during PD testing. For the study of such phenomena, a simple needle-plane electrode set with an insulating sheet was investigated at DC voltage with different polarities and electrical stress durations. It could be shown, that the charging process on the insulating sheet already takes place at test voltages below the measured PD inception voltage level. The charge intensity on interface, and as a consequence, its impact on the electrical field conditions e.g. the inception field strength, depends both on polarity and applied voltage duration. It is assumed, that any charging process below the (self-sustaining) ionisation level, characterized by the electrical inception field strength, should be caused by natural ionisation processes which take place in the test object. This assumption was supported by different results obtained if the test set was under open- and closed- volume condition. In the same manner, the results obtained were confirmed by electrical field calculations for different charging conditions on the insulating sheet and applied voltage polarity. The results described in this paper lead to the main conclusion that the charging ability of interfaces in hybrid gaseous insulation must be considered if PD measurement at DC voltage should be performed. For a deeper understanding of these processes further investigation, e.g. in a more compact insulation type appears to be necessary.