Kari Lahti

Large-area dielectric breakdown performance of polymer films - part i: measurement method evaluation and statistical considerations on area-dependence

A multi-breakdown measurement method for large-area dielectric breakdown characterization of polymer films is presented and evaluated. Based on the self-healing breakdown capability of metalized film, large amount of breakdown data can be obtained from a relatively large total film area, thus enabling the execution of detailed breakdown performance analysis. The studied films include non-metalized laboratory-scale, pilot-scale and commercial capacitor-grade bi-axially oriented polypropylene films in the thickness range of 14-25 μm. With the active measurement area of 81 cm2 per sample, breakdown distributions covering total film areas of 486-972 cm2 are presented. Various aspects encompassing the sample film preparation, measurement procedure, breakdown progression, discharge event characterization, breakdown field determination, data validation and statistical analysis are discussed. Comparative small-area breakdown measurements were performed in order to study the relationship between the large-area multi-breakdown measurement method and a conventional small-area (1 cm2) manual breakdown measurement method. Implications of the area-dependence and the applicability of the Weibull area-scaling are also discussed.

Transmission line corona losses under hoar frost conditions

Power transmission line corona losses under hoar frost conditions were studied in the climate room of the high voltage laboratory of Tampere University of Technology, Finland. The measurements were performed using a coaxial measurement arrangement with different bundle and conductor types. The effects of conductor and bundle type, temperature, applied voltage and hoar frost thickness on corona losses were investigated. A two-conductor bundle had corona losses about 2.5...5 times higher than a three-conductor bundle. Relatively thin hoar frosts were used in the tests. Even the thinnest hoar frost resulted in remarkable corona losses and the losses were very sensitive to changes in the hoar frost thickness. The ambient temperature had a strong influence on the measured losses.

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.

New approach to evaluate area-dependent breakdown characteristics of dielectric polymer films

A new dielectric breakdown measurement method for determining breakdown characteristics of polymer films has been developed and evaluated. The method is based on measurement of multiple breakdowns per sample area beyond the weakest point of the film by utilizing low-energy self-healing breakdown of metallized polymer film. A data selection process based on the discharge energy characteristics of each measured self-healing breakdown is utilized prior to the statistical analysis in order to validate the mutual independence of the results. Even with a relatively small sample area, the method yields a large amount of breakdown data from a wide voltage spectrum, thus enabling the formation of detailed material-specific breakdown fingerprints. The measurement system and the area dependence of the breakdown results have been evaluated with capacitor-grade metallized film. For the statistical analysis, additively mixed Weibull distributions are utilized as multiple breakdown mechanisms are found to be operative in the dielectric. Defects in the dielectric volume are found to have a profound effect on the structure of the mixed distribution with increasing area.

Large-area dielectric breakdown performance of polymer films – Part II: Interdependence of filler content, processing and breakdown performance in polypropylene-silica nanocomposites

In this study, large-area dielectric breakdown performances of various bi-axially oriented polypropylene (BOPP)-silica nanocomposite films are studied by utilizing the self-healing multi-breakdown method presented in the Part I of this publication. In particular, the effects of silica filler content, pre-mixing method, co-stabilizer content and film processing on the large-area breakdown performance are analyzed. Nanostructural and film cross-sectional analyses are correlated to the breakdown responses. The optimum silica filler content is found to reside at the low fill fraction level (~1 wt-%) and automatic pre-mixing of the raw materials and the optimization of the orientation temperature are found to be preferable. The co-stabilizer Irgafos 168 is found to have a significant effect on the breakdown distribution homogeneity of the reference BOPP films. The breakdown response of the silica nanocomposites is found to be not only dependent on the active measurement area but also on the voltage ramp rate, indicating that the silica nanocomposites exhibit altered internal charge behavior under DC electric field. The area- and ramp-rate-dependence results exemplify the importance of careful breakdown strength evaluation of dielectric polymer nanocomposites. Above all, the results emphasize the fact that a thorough understanding and the optimization of the film processing parameters are crucial for achieving improved breakdown response in dielectric polymer nanocomposite films.

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.

Effect of inter-layer pressure on dielectric breakdown characteristics of metallized polymer films for capacitor applications

Relatively high level of inter-layer pressure is often promoted for wound metallized film capacitors as it has been found to reduce the capacitance loss during self-healing breakdown. However, the effect of inter-layer pressure on the dielectric breakdown strength of metallized polymer films is not well documented and further analysis is needed. In this study, the effect of inter-layer pressure on the dielectric breakdown characteristics of various metallized polymer films is studied. By utilizing low-energy self-healing breakdown, multiple breakdowns can be measured per sample film. The results suggest that high inter-layer pressure has a negative impact on the breakdown characteristics of the metallized films studied and, in the worst case, may lead to insulation resistance failure.

Dielectric properties of HVOF sprayed ceramic coatings

Thermally sprayed ceramic coatings can be used as electrical insulators for example in high temperature applications (e.g. fuel cells) or in other demanding conditions. In electrical insulation applications the mostly used coating materials are aluminum oxide, magnesium oxide and magnesium aluminate. In general, only few reports of dielectric properties of thermally sprayed ceramic coatings can be found in literature and further analysis is thus needed. In addition, the measurement methods and conditions in previous research are often not fully documented, complicating the evaluation and comparison of the properties of different coatings. The aim of this paper was to characterize dielectric properties of thermally sprayed ceramic spinel coating sprayed with high-velocity oxygen fuel (HVOF) technique. The studied dielectric properties are DC resistivity, DC dielectric breakdown strength, as well as permittivity and dielectric losses at different frequencies. All measurements were made at temperature of 20 °C and at relative humidity of 20 %. Dielectric properties and the composition of coating material are presented and analyzed.