Experimental Investigations of the Multiple Impulse Energy Handling Capability of Metal-Oxide Varistors for Applications in Electrical Power Engineering Translation of: Experimentelle Untersuchungen zur Mehrfachimpulsbelastbarkeit von Metalloxidvaristoren für Anwendungen in der elektrischen Energietechnik

In this thesis, the results of the research on multiple impulse energy handling capability of metal-oxide varistors (MOVs) are described. The focus here is on repetitive impulse stresses with a high energy density where the specimens cool down to ambient temperature after each energy injection. The repetitive energy injection handling capability of MOVs is compared to their single impulse energy handling capability. It is widely known that the single impulse energy handling capability of MOVs increases with the current density of the impulse stress. This is due to the homogeneous current distribution within MOVs when applying higher current densities. The findings described in this thesis suggest that the temperature dependence of electrical resistance causes a further homogenization of the current distribution within the material of the MOVs at higher current densities. However, the impact of this seems to be small as its influence on the single impulse energy handling capability of the MOVs is not measurable. Furthermore, the results of experiments conducted for this thesis show that short interruptions of longduration current impulse stresses up to the range of seconds do not increase the impulse energy handling capability of MOVs. Additionally, the research results illustrate that the repetitive impulse energy handling capability of MOVs does not correlate with their single impulse energy handling capability. Concerning repetitive energy injections at lower current densities, the stress handling capability is very close to the single impulse energy handling capability. However, repetitive impulse stresses with higher current densities cause a reduced stress handling capability compared to the single impulse energy handling capability. Nonetheless, the energy handling capability when applying repetitive energy injections does not drop below the characteristic volume-related nominal energy handling capability of standard surge arresters. With regard to these findings, it is also shown that a change in reference voltage due to prior repetitive stressing of the MOV is not a sure sign for a pre-damage that leads to a decrease in single impulse energy handling capability. The results gained from this research are discussed with regard to routine tests of MOVs, the usage of surge arresters within electric power systems and the international standardization of surge arresters.