Christian Cornelissen

Resolution evaluation of ultrasonic diagnosis tools for electrical insulation devices and the detection of electrical trees

Ultrasound is a well-known diagnosis tool that can be used for the condition assessment of insulating systems. For this special purpose, various investigations on the detectability of the included inhomogeneities are performed: The resolution of ultrasonic measurements on high voltage insulations has been evaluated by using standardized Landolt rings in silicone rubber and epoxy resin. Although a dependency exists on the ring position, the diagnostic system that is used is able to image the gap with a sub-millimeter size. In other words, the system has recognition ability for two different objects close to such a distance. Regarding detectability, it has been found that in addition to ball-shaped inhomogeneities, the system can also detect thin fibrous objects with a diameter of a few micrometers. Finally, the results are used to detect and characterize electrical trees in silicone rubber

Non-destructive diagnostic facilities for cable systems

Polymeric materials like polyethylene and silicone rubber used in cables, joints and terminations show an electrical aging behavior. Considering the consequences of dielectric aging, effective diagnostic facilities are currently required, which have to be primarily nondestructive and which should detect as many faults as possible. Based on previous experiences it can be expected, that ultrasonic measurements and nuclear magnetic resonance (NMR) investigations fulfill these requirements. In addition the possible use of these facilities for quality management is presently being investigated. The increasing miniaturization, combined with the possible application of mobile diagnostic devices, is taken into account. First results show the feasible use of these facilities for detecting inhomogeneities like impurities and electrical trees in the insulation material and for identifying variations of the cross-link density.

Ultrasonic and Nuclear Magnetic Resonance for non-destructive diagnostics of cable systems

Due to mechanical, thermal and electrical stress non-reversible damages of the morphological texture can occur in polymeric materials used in cable systems (mainly polyethylene and silicone rubber). Reasons for the preceding damaging are for example included gas-filled microvoids and local inhomogeneities of the cross-linking-density. Considering the consequences of these processes, effective diagnostic methods are currently required, which have to fulfill certain demands (for example non-destructivity). Investigations carried out so far have shown that Ultrasonic and Nuclear Magnetic Resonance (NMR) meet these requirements. Ultrasonic diagnosis rest on the observation of the sound-propagation in materials. NMR is based on the recording of spin-dependent relaxation processes and its characteristics. Both diagnostic methods are well-known in materials science, whereas their use for monitoring equipment of cable systems has to be further developed. Therefore the current investigations focus on the detection, localization and characterization of defects. The analysis regarding the influence of mechanical stress (expansion of joints) shows a non-reversibility of this impact. Furthermore investigations relating to ageing phenomena are depicted. In addition first examinations using an automated ultrasonic measuring device, which has been developed with regard to industrial appliance, confirm the assumption of the ability to detect even small failures in insulation materials.