Pavel Totzauer

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 insulating fluids — Some other features

Natural esters are candidate fluids for transformer insulation system. Recently the most common insulation system in power transformers is paper-oil system. Oil generally mean petroleum based insulating fluid. 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.

Antioxidant variations in the nature ester oil

The insulating liquids for power transformers are still evolving area. The current trend is to move away from currently used mineral oil because it poses a threat to the environment and use the renewable, biodegradable nature ester oils instead. These oils are now well known but what keeps them from widespread usage are unbalanced properties. In some areas these natural oils excel (fire point, sulfur content, etc.) but in others they still need improvement (dissipation factor, oxidative stability). This paper deals primarily with dielectric properties of the nature ester oil (rapeseed oil) with a focus on improvement of an oxidation stability. The emphasis is placed on a conservation of good dielectric properties for a long time thanks to the addition of an antioxidant. The experiment is based on preparation and measurement of the rapeseed oil with the different amount of several antioxidants. Evaluation of the impact of antioxidants on the dielectric properties of oils in the supplied state is performed via measurement of a breakdown voltage and a dissipation factor. The effect of antioxidant addition on the properties of oil, which is exposed to elevated temperature, is determined via a test of oxidation stability. The evaluation of this test is based on the comparison of values of the dissipation factor at the beginning and at the end of the test. The additional method for assessing of changes in the chemical structure (associated with oxidation processes) is Fourier transform infrared spectroscopy.

Dielectric properties of modified natural ester — Perspective electrical insulating liquid

Natural oils are divided into vegetable and mineral oils. Compared with vegetable oils, the mineral oils show better dielectric properties, however, they are rather dangerous for the living environment and their flammability is higher. The vegetable oils are composed of natural ester molecules with a triacylglycerol structure. The mineral oils are a mixture of hydrocarbons produced from crude oil. Recently, additives in the form of dispersed nanoparticles, for example SiO2, TiO2, ZnO, Al2O3, are used for improvement of certain dielectric properties of mineral oils. Another way is how the dispersed nanoparticles will improve the dielectric properties of natural esters. This paper deals with modified rapeseed oil with dispersed adapted nanoparticles TiO2. The results showed that the polarization index and resistivity were both improved. Modified rapeseed oil with dispersed adapted nanoparticles TiO2 was evaluated as perspective electrical insulating liquid.

Dielectric spectroscopy and absorption/resorption analysis of composite dielectric with MgO filler

Composite insulation systems are inseparable part of most electrical installations. Strongly inhomogeneous structure of these systems is the main problem which may lead to defect during operation life. Main aim of this paper is to introduce novel composite material designed for insulating system of high voltage equipment. This material is composed of epoxy resin matrix with dispersed magnesium oxide (MgO) nanoparticles and polyethylene naphthalate (PEN) film. The main emphasize is placed on behaviour of this material in AC electric field depending on frequency of testing voltage and ambient temperature and also on the behaviour in DC field. The characterization in AC field is based on broadband dielectric spectroscopy, on the other hand the behaviour in DC field is characterized by dielectric absorption. Parameters like real permittivity, loss factor and dielectric absorption and resorption are mentioned in this paper.

The effect of space charge accumulation in high voltage insulation systems

Because of the financial savings and with respect for the environment, several efforts are made to replace AC voltage long distance transmission lines for DC voltage transmission lines. This may lead to increases of charging of insulation materials. The main aim of this paper is to describe the space charge accumulation in insulation material and its effect on local overloading of insulation material. The space charge measurement principles are mentioned in this paper. The finite element method (FEM) models are used for demonstrations of different space charge behavior and its influence on intensity of electric field. The dependence of space charge level or applied voltage level on deformation of electric field distribution is also determined.

The benefits of using online diagnostic tools for portable diagnostic device: With focus on accelerated ageing and partial discharge

The world of diagnostics has to keep up with the needs and possibilities todays world requires and offers. Everyone want to have the information they desire at the moment and, of course, at minimal cost. The world of diagnostics is no different. One tool that offers that is the online diagnostics. It has many benefits that allow, with proper usage of tools like CBM (condition based maintenance), to save money on maintenance. It also gives a better understanding of the reasons behind failures because we can track down the problem which caused the failure. The problem with these tools is the high price. One solution can be developing the small portable diagnostic tool that will be able to give you the much-needed information for a fraction of a price.

Effect of inhibitors on thermal degradation of vegetable oils

Electrical insulation system is an important part of electrical equipment with high requirements - accuracy, quality, reliability and environmental protection. In power and distribution transformers is the most common electrical insulating system oil-paper system. The liquid component of the insulation system is used as a cooling and insulating medium as well. The most commonly used liquid insulating material is a mineral oil, which has excellent electrical properties - low dissipation factor, high dielectric strength, sufficient oxidation stability etc. The current research is focused on alternative to mineral oil because mineral oils are a threat for environment in case of a leakage from the transformer. Another reason why we look for a replacement is that a mineral oil comes from a non-renewable source. The research has focused on selecting appropriate mixtures of synthetic esters and on modification of natural esters. The electrical and physical properties of these newly created oils were compared using the mineral oil. In this paper, two different of vegetable oils (rapeseed oil and sunflower oils) are tested. The aim of the paper is to show the electrical properties of sunflower oil and his lifetime curve depending on temperature and on different time of exposure. At the end of the paper, the results of the experiment are discussed and life time curves for different temperatures are calculated.

Diagnostics of composite insulation materials for simple online diagnostics tools

The world of diagnostics has to keep up with the needs and possibilities todays world requires and offers. Everybody want the desired information in a cheap way and with minimum delay. The world of diagnostics is no different. Online diagnostic tools can cover these demands. Proper usage of tools like CBM (Condition Based Maintenance) can save us lots of money otherwise spend on unnecessary maintenance. Another benefit of online monitoring is the ability to track down the true reason behind failures. This will help with the recognition of future problems and their elimination before they can develop into severe failure. The major problem so far is the price of such diagnostics. Our research deals with the possibility of developing a small portable or online diagnostic device that will allow such as diagnostics with a fraction of price. For the reason of the remaining life calculation a lifetime curves for modeling the aging of observed insulation systems has to be laboratory measured. The demonstration of such as measurement is presented in the paper on composite insulation material as for rotary machines and for oil transformers.