Silicone gel

Compared with the classic dielectrics used in high- and medium-voltage technology, such as cast resin, polyethylene or oil, silicone gels are relatively new insulating materials. They belong to the group of cold-vulcanizing two-component silicone elastomers and differ in their material structure from the silicone rubbers increasingly used in particular in the field of composite insulators and cable fittings by a sometimes considerably lower crosslinking density and a typically high proportion of non-crosslinked components.
From the point of view of high and medium voltage technology, the important properties of silicone gels are, in addition to high electrical strength, their good thermal stability, a high degree of elasticity and their pronounced inherent tackiness, which promises considerable advantages over other insulating materials, especially in the area of electrically highly loaded interfaces.
At present, silicone gels are used primarily as potting compounds for electronic components, e.g. in automotive and power electronics. They are mostly used because of their good mechanical and thermal properties and to protect the components used from corrosion. One application in which the gels are exposed to much higher field strengths is currently used, for example, as a potting compound in high-performance semiconductor modules (IGBT).
In power engineering, silicone gel has so far been used in the field of low-voltage connection technology. In particular, the good compatibility of silicone elastomers with all common insulation materials, their wide operating temperature range (-40°C to 90°C) and their corrosion-protective effect are exploited. The positive experience gained in low-voltage technology also makes silicone gels interesting for use in high- and medium-voltage technology. However, an application in this area requires very precise knowledge of the relevant material properties and extensive knowledge of their dependence on the influencing factors to be expected in the respective area of application.

Against this background, the dielectric properties - in particular the electrical strength - of various silicone gels are to be investigated as a function of the parameters important for high- and medium-voltage insulation and interconnection technology within the scope of this research project. In addition to the behavior under short-term electrical load, the electrical and thermal aging behavior is an essential part of the investigations.
Based on the results obtained, the material silicone gel will be characterized, evaluated and optimized for use in individual applications. The material-related limits will be identified and guidelines for the design and construction of high- and medium-voltage insulations will be developed. In addition to the issue of long-term behavior and thus reliability as an insulating material, which is particularly important for use in electrical power engineering equipment, one focus is on the investigation and optimization of its interfacial properties.
To carry out the research work, our department has a materials laboratory as well as various laboratories for high-voltage testing. In addition to well-known companies in the field of electrical power engineering, Wacker Chemie AG, Europe's largest manufacturer of silicones, is supporting the department in this project as a partner and sponsor in close cooperation.

Building on the silicone gel work, the Department of Systems and High-Voltage Engineering is researching a further step in the modification of insulating materials. Here, field-controlling gels are to be developed and investigated. This type of insulating material has a strongly nonlinear property with respect to electrical conductivity. In the conventional field strength range, the material behaves like a high-quality insulator. Above a certain field strength, however, this behavior tips over into a very conductive range.

This property reduces areas of high field strength in the insulating material, which can lead to partial discharges and ultimately to degradation and destruction of the insulation. The insulating material adjusts itself with regard to its conductivity in such a way that voltage control is achieved, which until now could only be achieved by complex additional components and a very careful design. In the project described below, this material is to be further developed and qualified for use in an industrial product.