**6. Conclusions**

The purpose of this manuscript is to discuss the factors affecting the PD and their detection in EMs and especially epoxy resin, one of the most significant components of EMs' insulation system. Epoxy resin was chosen, as it is used not only on the insulation system of the SG described above but also in many other similar systems. It was therefore essential for this research effort to simulate the insulation of this SG in order to investigate how the above (and many other) factors affect its "health" condition and prognose possible faults. PD tests, Offline or Online, are the most suitable tests for evaluating the insulation condition. Offline tests are mainly used for determining the point where a potential fault starts, while Online PD tests are able to monitor the insulation condition continuously during operation.

The simulation results show that, when increasing the applied voltage, the number and the amplitude of PDs increase. Furthermore, if the volume of the void decreases, the PDs decrease. Other factors that play a significant role in PDs are humidity and contamination. Experiments with epoxy resin samples and different droplet arrangements were carried out in order to investigate which factors can affect the PDs and provided interesting results. Water droplet conductivity, the distance of the water droplets from the electrodes, and the volume and number of the water droplets on the surface of the epoxy resin affect the flashover performance and the PDs.

As for future work, experiments with different water conductivities and water droplet arrangements would be useful in confirming the results and providing statistical significance. As for the simulations, more iterations with different geometries and dimensions of voids would be useful. The simulations and the experiments could be combined with the onsite measurements of PDs across a wider range of existing SGs in power plants in order to determine the situation of their insulation system for planning maintenance actions more efficiently.

**Author Contributions:** Conceptualization, D.V., A.K., M.G.D. and J.A.A.-D.; methodology, D.V., A.K., M.G.D. and J.A.A.-D.; software, D.V.; validation, D.V., A.K., M.G.D. and J.A.A.-D.; formal analysis, D.V., A.K., M.G.D. and J.A.A.-D.; investigation, D.V., A.K., M.G.D. and J.A.A.-D.; resources, D.V., A.K. and M.G.D.; data curation, D.V.; writing—original draft preparation, D.V., A.K., M.G.D. and J.A.A.-D.; writing—review and editing, D.V., A.K., M.G.D. and J.A.A.-D.; visualization, D.V., A.K., M.G.D. and J.A.A.-D.; supervision, A.K., M.G.D. and J.A.A.-D.; project administration, D.V., A.K., M.G.D. and J.A.A.-D.; funding acquisition, A.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors of this paper would like to thank Pericles Stratigopoulos, Director of Komotini Power Plant, Public Power Corporation S.A.—Hellas, for providing the pictures and the results of the PD measurements for this SG.

**Conflicts of Interest:** The authors declare no conflict of interest.
