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Ansorge S.,PFISTERER SEFAG AG | Ansorge S.,ETH Zurich | Schmuck F.,PFISTERER SEFAG AG | Papailiou K.,Hellbuhlstrasse 37
IEEE Transactions on Dielectrics and Electrical Insulation | Year: 2015

The paper presents the influence of various fillers having different sizes (0.3 μm to 18 μm) and surface modifications (unmodified, modified by the material supplier and in-situ modified during compounding) to the erosion resistance of HTV Silicone Rubber (SR) composites. The particles used were Aluminium-Trihydrate (ATH), Alumina (Al2O3) and Silica (SiO2), while the main research focus was on ATH fillers as it has the ability to release water at elevated temperature. First, by a simple water storage test under defined conditions the water uptake for different composites was analyzed to prove the effectiveness of an in-situ modification compared with the modification by the particle supplier. The Inclined Plane Test (according IEC 60587, further named IPT) and the high voltage, low current dry arc test (according IEC 61621, further named arc test) were used to determine the erosion resistance of the different samples. For the IPT, an adapted evaluation model was applied: This test is known to have a wide scatter in the case of material formulations, which are on the borderline to pass or to fail the test and therefore show substantial erosions on certain samples. The scatter could be reduced by evaluating the eroded volume by using samples only, which showed a limited erosion length. It was found for ATH, that larger particles show slightly better results than smaller particles. This can be explained by the formation of boehmite [AlO(OH)] for the larger particles, which causes a release of the bound water over a wider temperature range. This effect could be confirmed by thermo gravimetric analysis (TGA). The surface modification of the particles with Vinyltrimethoxysilane (VTMS) and Methyltrimethoxsilane (MTMS) did not improve the erosion resistance significantly, but reduces the water-uptake to a large extent, which is advantageous for the retention of the hydrophobicity. In order to achieve a low erosion rate, high filler loadings are essential. © 1994-2012 IEEE.

Ansorge S.,ETH Zurich | Ansorge S.,Pfisterer Sefag AG | Papailiou K.,Hellbuhlstrasse 37
Journal of Elastomers and Plastics | Year: 2015

It is known that Young's modulus of elastomers increase strongly due to high filler loadings, which is critical for practical applications and processing. An experimental study was performed in order to investigate the mechanical properties of silicone rubber, which was highly filled with alumina trihydrate (ATH) under uniaxial tensile tests. Different sizes of ATH particles having different surface conditions were used. The composites were prepared in a Z-blade mixer, and the particles were partly in situ modified. In preliminary investigations, the correct amount of silane for the in situ modification was determined and the effect of the mixing time on the mechanical properties of the cured rubber was studied. Long mixing times show generally a decrease in the modulus. This is explained by the increasing formation of bound rubber. A substantial dependence of the particle size and particle surface modification on the Young's modulus was found. This effect can be simulated using a model based on a stiff particle surrounded by a soft shell, which has a higher modulus than the polymer. Smaller particles show a stronger increase of the modulus than larger ones, which is due to (1) higher volume of interphase area and (2) shorter distance between the particles increasing the modulus of the interphase. © The Author(s) 2015.

Ansorge S.,Pfisterer Sefag AG | Schmuck F.,Pfisterer Sefag AG | Papailiou K.O.,Pfisterer Holding
IEEE Transactions on Dielectrics and Electrical Insulation | Year: 2012

The paper gives a comparison of the tracking and erosion properties and the hydrophobicity behaviour between different kind of silicone rubbers: Unfilled gum, HCR silicone rubber filled with silica, silicone rubber containing silica and ATH and liquid silicone rubber containing silica filler only. The Inclined Plane Test was used to determine the tracking and erosion performance. It is shown on an example that the principle set-up of the Inclined Plane Test can be used to evaluate the hydrophobicity as well. It is found that the performance of gum in the Inclined Plane Test is very poor. The addition of silica with filler loadings around 20 - 25 wt% improves this behavior, but failures still occur at 6 kV in the Inclined Plane Test. Commercially available liquid silicone rubber shows a better performance than high consistency rubber, in the case that both systems contain only silica fillers of similar loadings. By the additional addition of high volumes of surface modified ATH, the Inclined Plane Test can be passed reliably at 6 kV. Special focus is put in the proper statistical evaluation of the results from Inclined Plane testing. In order to obtain statistically significant data for a precise comparison of the results, the sample size for the Inclined Plane Test should be at least 20 specimens. It was further found that the type and size of ATH particles have a substantial influence on the results. It was shown that a voltage of 6 kV in the Inclined Plane Test is better suitable for the identification of weak compositions of silicone rubber than a voltage of 4.5 kV. In order for high filler loadings not to be detrimental to the hydrophobicity, the use of surface modified particles is critically important. © 2012 IEEE.

43rd International Conference on Large High Voltage Electric Systems 2010, CIGRE 2010 | Year: 2010

The ageing and pollution performance of a composite insulator is closely linked to the properties of the housing material. Of particular importance are its tracking and erosion performance as well as its hydrophobicity. Good hydrophobicity will result in the leakage current staying in the A-range and prevent the formation of dry band activity. However under severe conditions causing corona discharges or pollution layer wetting, the hydrophobicity can be temporary lost. Leakage current in the mA-range as well as dry band arcing can then occur and erode inappropriate material. Under special circumstances, nitric acid can accumulate on the insulator. Nowadays in high voltage applications, the use of silicone rubber for composite insulators is a standard as it offers excellent properties with respect to the above mentioned criteria. Nevertheless, large variations between different silicone rubber formulations can be observed, which can affect the lifetime of composite insulator housings considerably. It is therefore of paramount importance to understand how different additives and fillers affect the performance of silicone rubber. Here the focus is on aluminia trihydrate (ATH), as nowadays this is the most comon filler used in silicone rubber for this application. It was found that for the silicone rubber base formulation utilised, a high filler loading of 57 wt% is needed in order to achieve a superb tracking and erosion behaviour, and a consistent pass in the inclined plane test at 6 kV according to IEC 60587. The materials were also subjected to a dynamic drop test and an acid imersion test, where the importance of surface modified particles can be clearly seen. A problem of the tracking and erosion test is its inherent scatter. In order for the erosion performance of different compounds to be evaluated in greater detail, like the eroded mass, an increase in the sample size of up to 30 specimens is recomended. Nanoparticles have the potential to improve the tracking and erosion behaviour of silicone rubber. It is shown that the optimal filler loading is around 5 wt% for the specific particle used here.

Papailiou K.O.,Pfisterer Holding | Schmuck F.,Pfisterer Sefag AG
CIGRE 2011 Bologna Symposium - The Electric Power System of the Future: Integrating Supergrids and Microgrids | Year: 2011

Compact lines using the concept of insulated crossarms are a well accepted solution to solve right-ofway constraints and to provide a technical solution to reduce electromagnetic field on ground. They have been introduced for voltage levels up to 420 kV so far. The paper reflects more than ten years of experience in designing and manufacturing of insulators for insulated crossarms in 420kV-networks, mainly with a pivoted attachment to the tower. Furthermore, general technical aspects of compact lines such as alternative designs, advantages and disadvantages are presented and specific issues in the design stage and for the practical testing are discussed. With the availability of high strength cores with relatively large diameters, solutions with a single post became the preferred option, because of the better aesthetic appearance. Typically, the electrical design of the insulated crossarm is less challenging in comparison to the mechanical design, because under certain load scenarios, the composite insulators will be subjected to buckling loads. It has also been shown that the mechanical performance of the high strength cores have to be balanced with the strength of the hardware, which provides the attachment of the conductors and towards the pole. For this reason, a full scale mechanical testing of the critical load scenarios is recommended for the complete crossarm set. With the introduction of higher voltage classes, routes of transmission lines with 800 kV have sometimes also right-of-way constraints and for this reason the development of a corresponding insulated crossarms is continued as a technically and commercially acceptable option.

Papailiou K.O.,PFISTERER HOLDING | Thaddey S.,PFISTERER SEFAG AG | Schmuck F.,PFISTERER SEFAG AG | Armschat C.,Siemens AG | Stankewitz J.C.,Siemens AG
43rd International Conference on Large High Voltage Electric Systems 2010, CIGRE 2010 | Year: 2010

Transmission line projects with voltage levels of 800 kV HVDC or 1000 kV AC have been recent milestones in the further development of electrical equipment, insulators and apparatuses. In converter or sub-stations of such projects, the use of long post insulators is required. Station post insulators for lower voltage levels are currently seen as commodity products. However, no optimized products were available for an 800 kV HVDC project and the "up- scaling" of an existing product range was technically not feasible. The paper describes the development process of different composite post insulator types, with the initial main objective being to use well-developed technology, proven over many years in the transmission network. After an analysis of the existing technologies, the development was focused on a tripod post solution. In this solution, composite solid core insulators were used with a housing made of high-temperature vulcanizing silicone rubber and flexible shed geometry options. Initial calculations of the mechanical performance using analytical methods and finite element analysis, appeared to be unsatisfactory because of the missing correlation between each other and also probably to the performance of the product in reality with the combination of different materials. For this reason, specimens with a height of seven (7) meters were erected for systematic practical testing in the design stage. The results of the practical tests were fed back into the finite element program for optimization. The findings at the design stage were used for the manufacture of the ten (10) meters long prototype, which was successfully type-tested to prove the values of the specified parameters. The results of the type test were again used to confirm or to optimize the model of the finite element simulation. Due to the good correlation found between test and simulation results, the expected failure values of the tripod post were simulated without further testing. With this development, an innovative solution was introduced, which can be adopted for various applications in stations in many ways.

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