R. Polanský

Dielectric analysis of vegetable and mineral oils

This paper deals with analysis and comparison of the specific dielectric properties and chemical structure of sunflower, rapeseed and commonly used transformer mineral oil. The measured sunflower and rapeseed oils are natural ester molecules with a triglyceride structure and have mainly excellent fire resistance and biodegradability. The measured common mineral oil is produced from mixture of hydrocarbons from crude oil and belongs to the group of naphthenic oils consisting mainly of cycloalkanes and is relatively flammable and dangerous for living environment. The measurement was performed by means of the dielectric spectroscopy method. The real and imaginary parts of complex permittivity of measured samples were analyzed in a frequency range from 50 mHz to 100 kHz and in temperature range from -50 °C to +90 °C. The measured spectrums displayed relaxation processes (α and β) as well as the conductive component (σ). The results showed that the measured dielectric properties of the investigated vegetable oils are very similar to each other in the evaluated frequency range, but differs from the common mineral oil properties. The common mineral oil had these properties much better than the sunflower and rapeseed oils. Adapted mixtures of vegetable and mineral oils with suitable inhibitors can be considered to using as an alternative insulation liquid into power transformers instead of mineral oils.

MECHANICAL BEHAVIOR AND THERMAL STABILITY OF EVA ENCAPSULANT MATERIAL USED IN PHOTOVOLTAIC MODULES

Abstract The mechanical behavior and the thermal stability of an encapsulant based on ethylene-vinyl acetate (EVA) were studied. The EVA properties were verified at temperatures ranging from −70 °C to 500 °C. Thermogravimetry, differential scanning calorimetry and dynamic mechanical analysis were used in this study. It has been shown that the encapsulant has a good weight stability; however, the encapsulant passes through both a glass transition and a melting phase in the range of operating temperatures. The kinetic parameters of crosslinking were also analyzed. It is possible to achieve 65% crosslinking at a temperature of 150 °C and a time of 5 minutes. The activation energy of crosslinking is 95.6 kJ/mol.

Structural and mechanical behaviour of LLDPE/HNT nanocomposite films

The paper briefly describes structural and mechanical influences of Halloysite nanotubes (HNT) in different level of fulfilment (0, 1, 3, 7 wt%) in the LLDPE commonly used in the cable industry. The influence of HNT on the polymer has been observed and evaluated through the average crystallite size and the micro- deformation by X-Ray diffractometry and the imaging of SEM. Despite the certain inter-phase tension between the polymer and HNT, the influence on the mechanical and combustion behaviour was observed. Measurement showed a higher content of agglomerates in the sample with 7 wt% HNT fulfilment.

Dielectric analysis of halloysite nanotubes LLDPE nanocomposite compounds

The aim of this paper is to investigate the influence of HNT on the dielectric properties of LLDPE (linear low density polyethylene) nanocomposite compounds in the different level of fulfillment (0, 1, 3, 7 wt%). The utilization of these HNT/LLDPE nanocomposite compounds can be seen in the cable industry, where the dielectric properties are essential. Used experimental compounds have been prepared in the laboratory blades mixer Brabender W50 EHT and the plasticorder Brabender PLE 651. Immediately after mixing, the molten compounds were removed from the mixer, inserted into the spacer, and pressed into films of 0.25 mm thickness. Dielectric properties in the DC field (polarization current, reading of polarization index, volume resistivity) of the individual samples were measured and the temperature and frequency range by Broadband dielectric spectroscopy Alpha-A (Novocontrol) were investigated. Samples have been subjected to the dielectric strength test according to standards. As results showed, experiment...

Determination of the thermal endurance of transformer oil by structural analyses

Determination of the thermal endurance of transformer oil Nynas Lyra X was studied. The oil was exposed to thermal treatment at temperatures of 110, 120 and 130 °C for times ranging from 100 to 2400 hours. New methods for determination of materials thermal endurance were searched. Attention was drawn to the group of structural analyses from which two methods were selected: Fourier transform infrared spectroscopy (FT-IR) and differential scanning calorimetry (DSC). The dissipation factor was also measured according to standard EN 60247 as a comparison test and for a determination of end-point criteria of all newly monitored observables. Infrared spectral band of low temperature inhibitor and the temperature of the first thermo-oxidative DSC peak were used for further evaluation. It has been shown that the both analysed observables are continuously changing along applied thermal treatment and hence meet essential requirements of IEC 60216 standards. End-point criteria of analysed observables were determined and Arrhenius diagrams were subsequently plotted. The proposed observables, as determined via structural analyses, provide promising alternatives to dissipation factor measurement.

A new method of lifetime estimation for high-voltage insulating systems in rotating machines

The stator wedge supporting material is used for diagnosing the state of the main insulation in rotating machines. The stator wedge is placed in a stator slot, and the material is aged using the same degradation conditions experienced by the bar winding insulating system itself. The possibility of easy replacement of the supporting material without causing any damage to the whole insulating system during device shutoff is a major advantage of this approach. The samples were tested with differential scanning calorimetry (DSC) and were prepared from isolated bars of a real hydroelectric generator (after 57,984 operation hours). The bar insulating system material (glass-mica-based) and the stator wedge material (laminated glass-fabric and laminated cellulose paper (paper-phenolic laminates) were tested. Additional laboratory thermal aging of the samples from both the bar winding insulating system and the supporting material was conducted to verify the suitability of the DSC method. Based on previous experience and on the thermal class of the insulating system (F - 155 °C), the aging temperature was set to 180 °C. Enthalpies were measured after 52, 70, 100 and 200 hours of aging. The samples from three bars (phase, zero and changed bar) were analyzed. Powdered samples from compacted bar insulation and stator wedge material were measured using DSC in a pure air atmosphere. The area of the first thermal oxidative peak (enthalpy) was evaluated. The experiment showed good agreement with the enthalpy results for the insulating system and the supporting material.

Analysis of influence of thermal and voltage treatments on silicone/mica electrical insulation by FT-IR ATR

An influence of thermal and voltage treatments on properties of high-temperature electrical insulating material based on mica and silicone binder was analyzed. Tested material is intended for an operation at extremely high temperatures along with electric field stress. The main aim was to describe the high temperature and electric field influence on the material inner structure. Thermal treatment was simulated by unrepeated thermal stress of 320°C for the time of 500 hours. The material was also exposed to electric field intensities ranging from 9.6 to 15.9 kV/mm for times ranging from 7 to 280 hours. The samples were analyzed via Fourier Transform Infrared Spectroscopy, using the Attenuated Total Reflectance (FT-IR ATR) technique, and also via Microscopic Analysis. According to the measurement, accelerated thermal treatment has no significant effect on the inner structure. On the contrary, voltage treatment causes expressive degradation of the material.