K. Kannus

Dielectric properties and partial discharge endurance of polypropylene-silica nanocomposite

This paper presents the results of the dielectric properties and partial discharge endurance measurements conducted on polypropylene (PP)-silica nanocomposite. The material compounds were analyzed with micro-Raman spectroscopy, X-ray tomography and transmission electron microscopy (TEM). ac and dc breakdown strength of the materials was measured. Dielectric response, capacitance and loss factor of the film samples were measured as a function of temperature and frequency. Partial discharge (PD) endurance of the reference PP and PP Silica nanocomposite was studied as a function of ac voltage. Material surfaces were analyzed after PD stress with optical microscopy. All dielectric measurements were done for oriented thin films with a thickness of 11-23 μm. The results were analyzed statistically to determine the effects of the additive on the properties of PP. The paper discusses the potential of PP Silica nanocomposite with a view to high voltage applications, especially power capacitors.

Dielectric properties of nanostructured polypropylene-polyhedral oligomeric silsesquioxane compounds

This paper presents the results of the dielectric measurements conducted on polymer nanocompounds consisting of polypropylene (PP) and polyhedral oligomeric silsesquioxane (POSS). The material compounds were analyzed with a scanning electron microscope (SEM) and Raman-atomic force microscope (Raman-AFM). Ac and lightning impulse (LI) breakdown strength of the material compounds were measured. Relative permittivity, loss factor and volume resistivity measurements were also conducted on the material samples. Two types of POSS, octamethyl and isooctyl, were used in different quantities. The thickness of the samples was approximately 600 mum. Statistical analysis was applied to the results to determine the effects of the additive type and amount on the breakdown strength of polypropylene. The paper discusses the possibilities and restrictions in order to achieve advantages in high voltage applications using polyhedral oligomeric silsesquioxanes.

Evaluation of the operational condition and reliability of surge arresters used on medium voltage networks

Laboratory studies were conducted on gapped silicon-carbide arresters and gapless metal-oxide surge arresters (MOAs) gathered from 24 kV networks in Finland. A total of 410 arresters of 21 different types from eight different manufacturers were investigated. A total of 246 silicon-carbide arresters (age from 14 to 38 years) and 164 MOAs (age from 4 to 15 years) were studied at the High Voltage Laboratory of Tampere University of Technology. The results of the measurements on MOAs were compared to the measurements conducted earlier on 16 new, unused MOAs of the same types as those taken from the field. A portion of 16.7% (41 specimens) of the silicon-carbide arresters failed in the ac voltage withstand test and 34.5% (85 specimens) did not pass the lightning impulse sparkover test. The current impulse test of the silicon-carbide arresters resulted in a lot of damage. Due to the damage and failures in the electrical tests the authors recommend the electricity companies in Finland to exchange 7 types of silicon-carbide arresters to new MOA types in order to reduce the risk of arrester failures and to improve the protection levels in MV networks. All the ten MOA types studied were generally in good condition after being used 4-15 years in networks. Only three specimens (out of 164) could be evaluated to be faulty. Comparisons between the MOAs gathered from the networks and the unused MOAs showed that the protection levels of the MOA types studied had remained stable and the ac durability of these MOA types had not remarkably weakened during the use in networks.

Laboratory Investigations of the Electrical Performance of Ice-covered Insulators and a Metal Oxide Surge Arrester

This review paper summarizes the main results of the laboratory investigations conducted at Tampere University of Technology (Finland) on ice-covered insulators and a metal oxide surge arrester (MOA). The most important factors affecting the dielectric strength of high voltage (HV) insulators in freezing rain conditions are the resistivity of the freezing water, the length and the number of icicles between insulator sheds and electrodes and the state of the ice deposit (i.e. how wet it is). The studies verify that most of the commercially available HV insulators may flashover even with a normal phase-to-ground ac voltage in rather light salty (or otherwise slightly contaminated), but long-duration freezing rain conditions. The ac strength of a clean insulator string under rime ice during melting period was approximately 75% of the ac strength in dry conditions. Accordingly, the ac strength of the contaminated (ESDD ap 0.3 mg/cm2 NaCl) insulator strings during the melting period of rime was on average 65% of the AC strength in dry conditions. The icing of an MOA consisting of two or more units in series may have harmful effects on the electrical performance of the MOA. With ac voltage stress an unevenly ice-coated MOA may be thermally stressed due to the leakage current transition from the ice covering of one unit to the interior of another unit not covered with so much ice. With switching impulse current surges the residual voltage across an ice-covered unit rather easily causes an external flashover. This leads to a sudden increase in the surge current: with the MOA of two units studied the current peak reached as much as twice the peak value in normal operation without ice covering.

Behaviour of the DC leakage currents of polymeric metal oxide surge arresters in water penetration tests

The behaviour of seven commercially available polymer-housed metal oxide surge arrester types was studied in +70/spl deg/C and boiling water immersion tests. The DC leakage current of the arresters was used to diagnose the humidity penetration inside an arrester. The recovery features after an immersion test for different arrester types were measured and discussed. Together with the immersion and recovery tests, a humidity penetration test in very humid air was performed. For the first time, the behaviour of different arrester types in immersion tests was compared to their behaviour in a humidity penetration test with realistic ambient conditions. The resistance to moisture penetration varied greatly in the arrester types tested. Also, recovery of the DC leakage current varied between the arrester types depending on their structure. The results from the tests in very humid air indicate similar arrester behaviour than the results from other tests and suggest ideas for diagnosing arrester behaviour.

Diagnostic methods in revealing internal moisture in polymer housed metal oxide surge arresters

Diagnostic measurements performed for surge arresters are normally studied by evaluating their suitability to detect ageing and degradation of metal oxide material. The suitability of different methods for revealing intemal moisture in arresters has not been studied much. This aspect is considered in the paper based on measurement results obtained during a 1.5 year laboratory test series conducted for six different types of polymer housed distribution class arresters. AC power losses, third harmonic content of resistive leakage current, DC leakage current and partial discharge measurement results over the test period are given and discussed. Details of the measurement procedures suitable for measuring polymeric arresters with intemal moisture are also studied. AC power loss and DC leakage current measurements were found to be good methods, while the other two can only be used as additional methods when diagnosing intemal moisture in polymer housed arresters.

Nonlinear DC voltage-current characteristics of new polymeric composite materials based on semiconductive polyaniline emeraldine base filler

The aim of this study is to develop new polymer-based compounds with nonlinear current-voltage properties and good processability. We have formulated polymeric compounds based on semiconductive polyaniline emeraldine base (PANI-EB). The reference PANI-EB specimen was prepared by powder compacting and solvent casting. Silicone rubber was chosen as the matrix polymer for the composites. We studied the effects of the filler particle loading and of the graphite co-filler on the composite j-E-characteristics. We found that nonlinear j-E-curves can be attained by using either conductive graphite or semiconductive (PANI-EB) fillers alone or combined. Their properties depend strongly on the composite processing and composition. The composites show electrical properties that could readily be utilized in stress grading. In all the sample types and reference samples, we observed alpha-values much higher than the requirement for stress grading applications - the highest being about 25 for bulk PANI-EB with graphite electrodes.

Possibilities to reveal internal moisture in polymeric metal oxide surge arresters by means of PD and EMR measurements

This paper describes the results of a test series where six different types of gapless, polymer housed metal oxide surge arresters were subjected to humidity and impulse current stresses for ca. 1.5 years. The results of partial discharge and electromagnetic radiation measurements are given in the paper together with the results of internal leakage current measurements providing an indication of the moisture content inside the arresters tested. PD activity was measured in most of the specimens of arrester types with tight interfaces between the polymeric housing and inner parts of an arrester. Arresters with considerable internal gas spaces showed no PD activity during the test despite internal moisture content. Characteristics of the PD type measured in arresters are presented. In all the cases with PD activity the moisture problem was noticed earlier from the internal leakage current measurements.

A comparison between the DC leakage currents of polymer housed metal oxide surge arresters in very humid ambient conditions and in water immersion tests

The behaviour of leakage currents of seven different types of commercially available polymer housed metal oxide surge arresters was studied in very humid ambient conditions during a 16 month test. During the test series many different combinations of mechanical, electrical and humidity stresses were applied to the test specimens in order to obtain the range of leakage current behaviour possible for the different arrester types in humid ambient conditions. The leakage current behaviours measured for the different arrester types are compared to their behaviour in the boiling water immersion test. This wide comparison gives valuable information of possible leakage current behaviours of several modern arrester types in humid ambient conditions and especially of the suitability of the boiling water immersion test for sealing testing of polymer housed arrester types. In general, both tests revealed the same kinds of DC leakage current behaviour in the different arrester types although some effects of different arrester structures and water penetration phenomena were observed.

Feasibility of density functional methods to predict dielectric properties of polymers.

Feasibility of density functional theory (DFT) to predict dielectric properties such as polarizability of saturated polymers is investigated. Small saturated molecules, methane and propane, which is a monomer of polypropylene chain, are used in testing the methods. Results for polarizabilities based on several density functionals together with different basis sets are compared and contrasted with each other, with results by Hartree-Fock and second-order Moller-Plesset perturbation theory, as well as experimental data. The generalized gradient approximation PW91 method together with the 6-311++G(**) basis set is found to be the most suitable method, in terms of sufficient accuracy and computational efficiency, to calculate polarizabilities for large oligomers of polypropylene. The dielectric constant is then determined using the calculated polarizabilities and the Clausius-Mossotti equation. The molecular DFT methods at the PW916-311++G(**) level together with the Clausius-Mossotti equation give dielectric constants for saturated polymers such as polypropylene in good accordance with the experimental values.