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 inception field strength may be reached/exceeded. The partial discharges that then occur and the resulting degradation can lead to the failure of the entire insulation system. According to current design guidelines, insulation systems are designed in such a way that the causes of increased electric field strength described above are avoided under all circumstances. These solutions are usually highly complex and correspondingly cost-intensive. Consequently, the development of fault-resistant products is a general 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 has specialized in materials that automatically reduce excessive field strengths in local areas of the insulation system by reducing the volume resistance. These so-called non-linear-conductive materials (NLCM) homogenize the electrical field stress in the insulating material so that the resulting insulation systems are significantly more resistant to various defects. The advantages of using these innovative insulating materials are insulating systems with less complexity, fewer production steps and simpler installation and therefore lower overall costs.

Practical example of a cable joint

In the following, the operating principle of an NLCM will be illustrated using 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 tests were first carried out with COMSOL Multiphysics. The final design for the NLCM used is shown in the following figure. The maximum value of the field strength scale (in kV/mm) was set to the operating point (above this field strength, the material reduces its volume resistance). This means that the material actively contributes to field strength reduction in the areas shown in dark red.

Electrical field strength simulation of a cable joint with an NLCM

In a second step, the sleeve design was transferred to a real application. To do this, the sleeve housing was first produced using a 3D printer. The housing was then placed around a simple cable connection and encapsulated with the NLCM.

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

The medium-voltage cable joint was then tested in accordance with 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, in which, for example, the tear-off screw was left on the connector, also showed promising results. In the next step, we are working with our partners to develop a robust and thermally stable housing in order to carry out the necessary ageing tests.

Our 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 testing and characterization.