**4. Conclusions**

In this paper, electrical detection of creeping discharge under AC voltage application was presented. The complete procedure, starting from the numerical simulation of the electrode configuration until the final test implementation was described in detail. Emphasis was given to the selection of current sensing and recording devices so that their cut-off frequency is kept as high as possible.

Flashover tests results showed that, for the examined electric field distribution and for all the examined insulating gaseous media (dry air, CO2 and N2), lower flashover voltages were measured for electrode configurations with insulating materials having higher relative permittivity. Furthermore, high-frequency recordings of the current pulses associated with creeping discharges clearly indicated that there is more significant electrical discharge activity when the relative permittivity of the insulating material is higher. For lower permittivity insulator sample materials, fewer current pulses were detected however, the activity was spread greatly within a range corresponding to relatively higher amplitudes. It could be said that, the higher the difference between the relative permittivities of the insulator sample material and the insulating gaseous medium, the stronger the detected creeping discharge events are, especially for the cases of dry air and N2. For the case of CO2, the higher the relative permittivity of the insulator material the narrower the margin between creeping discharge and flashover is. Back-discharges were detected for all the cases tested in this investigation.

Future research will extend the current work and further explore the presented scenario using different physical perspectives and detection techniques.

**Author Contributions:** Conceptualization, M.M., P.W., A.B. and A.H.; methodology, M.M.; software, M.M.; validation, P.W., A.B. and A.H.; formal analysis, M.M.; investigation, M.M.; data curation, M.M.; writing—original draft preparation, M.M.; writing—review and editing, M.M., P.W., A.B. and A.H.; visualization, M.M.; supervision, P.W., A.B. and A.H.

**Funding:** The initial research leading to this work received funding from the Royal Academy of Engineering through a DVF to Professor A. Beroual to visit Professor Manu Haddad at Cardiff University, "Environmentally-friendly insulating gases for high voltage equipment", Grant number (DVF1617\5 \28): https://www.raeng.org.uk/grants-and-prizes/grants/international-research-and-collaborations/ distinguished-visiting-fellowships. In support to the DVF award, the PhD studentship awarded to Michail Michelarakis was funded by the School of Engineering at Cardiff University.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
