Overview of Silicone Elastomers, Gels & Auxiliary Materials for Cable Accessory Applications

Transcription

1 Overview of Silicone Elastomers, Gels & Auxiliary Materials for Cable Accessory Applications Authors: Dr. Jens Lambrecht, Dr. Konrad Hindelang Wacker Chemie AG Munich, Germany 1. INTRODUCTION The first silicone cable accessories were developed in the late 1960ies. They appeared together with the first extruded XLPE-cables. Focus of the engineers at this time was a design with field-grading and insulating parts in a prefabricated body that allowed easy assembling and avoiding mounting errors. The first parts of both, terminations and joints were still rather massive as the outstanding electrical behavior of silicone was not yet fully understood but test results and first service experiences in comparison to the - at this time state-of-the-art - polyurethane and other alternative solutions were found encouraging. Thus, the main idea of a prefabricated, silicone-elastomeric body remained and was further refined since. Today - having gained more and long-term experiences of the rather unique combination of properties of silicone elastomers customers enjoy availability of a wide portfolio of all types of silicone elastomers and auxiliary silicone materials for medium- and highvoltage cable accessories. The paper deals with some important aspects of the subject in an overview. 2. TYPES AND TASKS OF CABLE ACCESSORIES The majority of all power cables are extruded XLPEinsulated cables. These cables consist of a conductor, an inner conductive layer, the main insulation layer, an outer conductive layer, an outer conductor and an outer insulation/jacket (Figure 1). At places where two cable parts are to be connected theses layers have to be re-built in the same way by applying a so called cable joint or cable splice (Figure 2) and at the terminating end of a cable the layers have to be treated in such a way that they can withstand the resulting electrical and environmental stresses. This is done by the mounting of a cable termination (Figure 3). Figure 1 Extruded XLPE-cable, side view Figure 2 Typical design of a silicone elastomer cable joint (principle) Figure 3 Typical design of a silicone elastomer cable termination (principle)

2 Besides these two main types of cable accessories there are several combinations and modification, such like connectors and pluggable systems. The resulting major tasks are: 1. Electrical connection, 2. Field grading, 3. Sealing, 4. To withstand weathering. Medium-voltage plug-in systems are a good example for the first task (Error! Reference source not found.). A rather soft silicone elastomer cone (shown here) becomes pressed by a metallic spring into the male resin part (not shown) and thus provides a perfect electrically insulated interface. Besides the low hardness of the silicone elastomer the high gas permeability of the silicone elastomer is of advantage as any entrapped gas in the interface is going to diffuse within a very short time after mounting As mentioned above, it was a main goal during the early development of the silicone accessories to be able to pre-fabricate a ready-to-use part consisting of all insulating and field grading parts. Figure 5 shows a perfect example of an older design of a joint exhibiting this. All grading parts here were made from conductive silicone elastomers. Other solutions applying silicone elastomers a high permittivity are possible, too. Silicone elastomers, especially liquid silicone elastomers were found to be an ideal material for the necessary stepmolding processes as they are quite low in viscosity and allow a good flow into the mold and towards the triplepoints between the mold and the pre-inserted conductive parts. Figure 4 Silicone insulated medium voltage cable plug Figure 5 Field grading parts in a silicone elastomer joint (left: at the cable side, right: covering the connector, top: outer conductive layer) Figure 6 Cold shrink silicone joint widened and sitting at a support spiral Figure 7 Silicone termination showing a perfect hydrophobicity Proper sealing is definitely an important task of a silicone cable accessory. Outstanding mechanical properties of soft silicone elastomers allow a quite easy slip-on mounting. The perfect and lasting ability to shrink to the original dimensions even after a rather long expansion time allows the so called cold-shrink technology (Figure 6). Both mounting technologies benefit from the high gas permeability of silicones as well. Proper mounting and sealing is supported by special silicone pastes that are available for the T&D-industry. Pastes can be modified as remaining type that allows easy dismantling even after long time or a migrating type that disappears from the interface after rather short time. Most important for outdoor applications is the general ability of silicone elastomers to withstand weathering. Nevertheless, our silicones for these applications are especially modified to show a long lasting stability against the weathering stresses and possibly resulting electrical and erosive stresses in service. An example is shown in Figure 7. This termination provides a perfect hydrophobicity. Experience and know-how allow to provide silicones that can withstand both, the rather short term stresses during types testing (e.g. salt-fog-test) and the long term stresses in service. In that context it is important to take the pollution of outdoor cable accessories into consideration and the ability of suitable silicone elastomers to not only remain hydrophobic themselves but to show a good hydrophobicity transfer towards pollution layers. Decades of experience is required to be able to tune silicone materials to provide the fine balance of all mentioned properties.

3 TYPES OF SILICONE MATERIALS Main materials for silicone cable accessories are the silicone elastomers. The early years of production were characterized by the use of 2-component, low viscosity silicone elastomers that were designed to cure at rather low temperatures. Parts were mainly casted or vacuum casted. The output in production was rather low in comparison to the more sophisticated technologies that are available today. Nevertheless, these materials are still available, e.g. for prototyping or for manufacturing of small series of goods. State-of-the-art materials for cable accessories are either low hardness, high-consistency silicone rubber (HCR) (Figure 8) that are available as peroxide curing types or as 1- or 2-component addition curing materials for fast and very fast injection molding applications or extrusion respectively. Press molding is not recommended as the state-of the art accessories require a quite high accuracy of molds. The other are liquid silicone rubbers (LSR), (Figure 9) that usually come as 1:1 2-component systems. These materials are today widely used to manufacture all types of cable accessories. (30 50) Shore A liquid silicone rubbers are typically applied for the high-throughput injection molding production. Modified versions of both HCR and LSR with a low electrical volume resistivity or a high permittivity are available. Figure 8 High consistency silicone rubber (HCR) being packed in an automated packaging machine Figure 10 Different types of silicone pastes are widely used as auxiliary materials for mounting of silicone cable accessories Figure 9 Liquid silicone rubber (LSR) waiting for shipment at the Wacker Chemie plant in Burghausen/Germany Figure 11 Silicone gels, especially modified types, start to become applied for medium voltage cable accessories Our portfolio of materials for cable accessories is supplemented by silicone pastes (Figure 10). Several modifications are available (compare sub-clause 2). Extremely soft silicone elastomers (Figure 11), so called silicone gels, started their application in the energysector in the low voltage area below 1000 V. First products were so called Gel-joints that allow proper electrical installation and sealing by just closing shell-like parts pre-filled with soft silicone gel. Meanwhile - thanks to the use of special fillers - extensive application possibilities for silicone gels are gaining momentum.

4 3. SELECTED PROPERTIES OF SILICONES FOR CABLE ACCESSORIES There is no standard for the definition of general requirements for materials for cable accessories. IEC 62039TR:2007, Selection guide for polymeric materials for outdoor use under HV stress, covers the material for outdoor cable accessories and defines the following properties: Resistance to Tracking and Erosion, Resistance to Corona and Ozone, Resistance to Chemical and Physical Degradation by Water, Tear Strength, Volume Resistivity, Breakdown Field Strength, Resistance to Chemical Attack, Resistance to Weathering and UV, Resistance to Flammability, Arc Resistance, Glass Transition Temperature, Hydrophobicity. Resistance to Tracking and Erosion is generally understood to be a very important property of insulating materials for outdoor applications. Resistance to Tracking and Erosion is tested acc. to IEC (Figure 12). The general minimum requirement acc. to IEC 62039TR:2007 is a classification 1A 3,5. Quality siliconeelastomers can usually withstand the higher classification of 1A 4,5 (Figure 13) and therefore provide an improved security for the network when in service. Figure 12 Silicone elastomer specimen for outdoor use under test in an inclines-plane-test acc. to IEC Figure 13 Typical appearance of a well-designed silicone insulating grade for outdoor after test Breakdown field strength is understood to be another important property for insulating materials for outdoor application. IEC 62039TR:2007 sets a minimum value of 10 kv/mm when measured acc. to IEC It is important to take into consideration that this minimum value does not at all reflect the properties of silicone elastomers. Moreover, the old version of IEC has meanwhile been replaced by a new edition, the IEC :2013, Electric strength of insulating materials - Test methods - Part 1: Tests at power frequencies. The new edition of the standard contains a new sub-clause Elastomers with the following recommendations: Use test specimens of (1,0 ± 0,1) mm thickness, Unequal electrodes shall be used (Figure 14), In the case of elastomers of low hardness, e.g. silicone rubbers, a suitable casting material shall be used as embedding material or surrounding medium, respectively (Figure 15).

5 Figure 14 Proposed electrode arrangement in the new edition of IEC Figure 15 Electrodes, ball shaped at their bottom, to measure the specific break-down voltage embedded in a soft silicone elastomer Application of the conditions of the new edition of IEC leads to measuring results that are higher in general in comparison to values that were collected using electrodes in air/fluids (Figure 16). It may be claimed that only by applying the proposed electrode arrangement in the combination with embedding, differences of the specific breakdown voltage of silicone elastomers can be sorted out. Unfortunately the pure number is of limited interest, as it usually does neither reflect the real electrical field conditions in molded parts, nor gives information about the long-term behavior of material under stress. Nevertheless, the variation of the values shows, that there are obviously difference between different silicone elastomers that we were not even aware about. Figure 16 Specific breakdown voltage of 18 different silicone elastomers measured with ball-platearrangement (orange bars) and embedded ball-ball electrodes (green bars) Naturally, only a few aspects of the use of silicone elastomers for cable glands and their properties could be considered here. Please do not hesitate to contact the authors if you have any remaining questions. Dr.-Ing. J. Lambrecht Studied Radio Engineering and Power Engineering. Received his doctor s degree from Dresden University of Technology. Employed at Wacker Chemie AG with the Application Engineering of silicones for power applications. Member of several working groups of DKE, CIGRE AND IEC. jens.lambrecht@wacker.com Dr. rer. nat. K. Hindelang Studied Chemistry with a focus on Macromolecular Chemistry and received his PhD from Technical University of Munich. Joined Corporate R&D of Wacker Chemie AG in Responsible for technical support and material developments for power applications at WACKER SILI- CONES since konrad.hindelang@wacker.com