Smart Materials

In conventional insulation systems, inhomogeneities in the insulation material (e.g. air inclusions, conductive particles) or the shape or surface quality of adjacent electrodes can lead to increased electric field strengths. As a result, the partial discharge insertion field strength may be reached / exceeded. The partial discharges that then occur and the resulting degradation can lead to failure of the entire insulation system. According to current design guidelines, insulation systems are designed to avoid the causes of increased electric field strength described above under all circumstances. These solutions are usually highly complex and correspondingly cost-intensive. Consequently, the development of fault-resistant products is a commonly pursued goal.

Smart Materials

A new approach to solving the problem described above is innovative materials that reduce high electric field strengths by adapting their material properties to the load that occurs. In this area, CRW Engineering specializes in materials that automatically reduce excessive field strength in localized sections of the insulation system by reducing volume resistance. These so-called non-linear conductive materials (NLCM) homogenize the electric field stress in the insulating material, so that the resulting insulating systems are significantly more resistant to various faults. The advantages of using these innovative insulating materials are insulating systems with less complexity, fewer manufacturing steps and simpler installation, and thus lower overall costs.

Practical example of a cable sleeve

In the following, the operating principle of an NLCM will be illustrated based on a research project carried out by CRW Engineering on the subject of cable joints for medium voltage (12/24 kV).

In order to optimize the design of the joint, various simulation investigations were first carried out with COMSOL Multiphysics. The final design for the NLCM used is shown in the figure below. The maximum value of the field strength scale (in kV/mm) was set to the working point (above this field strength the material reduces its volume resistance). Thus, the material actively contributes to the field strength reduction in the areas shown in dark red.

Electric field strength simulation of a cable joint with an NLCM

In a second step, the socket design was transferred to a real application. For this purpose, the socket housing was first produced using a 3D printer. Subsequently, the housing was placed around a simple cable connection and cast with the NLCM.

3D model and 3D print of the cable sleeve housing (top) and the finished demonstrator (bottom).

Subsequently, the medium-voltage cable sleeve was tested according to certain sections of the DIN VDE 0278-629-1 standard. Both the AC voltage tests (including 52 kV - 5 min) and the lightning impulse voltage tests (125 kV - 10 times positive and negative) were passed. Initial tests with provoked fault sources, where for example the breakaway screw was left on the connector, also show promising results. In the next step, we are striving with our partners to develop a robust and thermally stable housing in order to carry out the necessary aging tests.

The cooperation partner in this project is CRW Engineering(www.crw-engineering.de). A spin-off from the University of Kassel with a focus on electrical material investigations and characterizations.