Partial discharges may cause damage to electrical insulation of high voltage equipment. They initiate elastic waves in the insulating material, e.g. in the stress cone of an outdoor termination. Localisation of the origin of such elastic waves can help to predict serious damaging processes in the electrical insulation. In order to measure and evaluate the wave propagation effects in typical multilayered elastomeric structures, knowledge of the material properties is required. The propagating velocity and the attenuation of longitudinal waves are important parameters. Values for these quantities found in the literature were not appropriate. Therefore, for cross-linked polyethylene (XLPE) and cured liquid silicone rubber (LSR), the longitudinal wave velocity and the attenuation were evaluated in the temperature interval from -20°C to 50°C and in the frequency range from 200 kHz to 600 kHz using a two-sample ultrasound technique. The loss factor was determined from these measured quantities. Additionally, low frequency Dynamic Mechanical Thermal Analysis (DMTA) was applied to investigate LSR and XLPE in a temperature interval between -100 and 50°C and to check qualitatively the ultrasound data.
Fibre-optic sensors for early damage detection in plastic insulations of high-voltage facilities
(2011)
Fibre-optic sensors (FOS) have great potential as online damage detectors when integrated in HV accessories. Among their well-known use as temperature and strain sensors, there are some more opportunities of use, e. g. they can intimately be embedded in polymeric insulations of HV cable terminations and joints to detect and monitor partial discharges right at the location of their origin. Two FOS types for early PD detection were investigated: an embeddable fibre-optic acoustic sensor to measure acoustlc waves in polymeric insulations generated by PDs, and a fluorescent optical fibre to detect first optical effects during ionization processes in the insulation material. The paper descrlbes these methods, related monitoring Problems and shows first test results.
For prediction of wave propagation in multilayered structures it is necessary to know the material properties. Important parameters are sound velocity (c) and attenuation ( ). Values found in the literature were not satisfactory mainly due to insufficient accuracy of measured attenuation. Experiments were carried out in Division VI.3 - Durability of Polymers. The ultrasonic test equipment was provided by Divisions VIII.1 - Measurement and Testing Technology; Sensors and VIII.4 - Acoustical and Electromagnetical Methods.
Acoustic emission (AE) sources caused by partial discharges in solid dielectric devices like transition joints or outdoor terminations are investigated. Together with a prepared high voltage cable end this device forms a multilayer structure. The hypothesis is that due to the high damping of polymeric insulation materials no AE signal can be detected on the surface of the prescribed structures. An analytical technique is used to determine the transmission for a typical multilayer insulation. For quantitative computations it is necessary to know the material properties such as complex Young´s modulus, density and the Poisson´s ratio. Since the properties found in the literature were not fully satisfactory, measurements had to be done. The experimental setup and the results are shortly presented. Finally it will be shown that the transmission path plays not the significant role as was assumed in the beginning.