**8. Conclusions**

Current measurements are a promising technique for the investigation of the partial discharge behavior in gas-insulated systems under DC voltage stress. The evaluation of the measured electron and ion current increased the understanding of discharge processes at one common PD source in gas-insulated systems, a protrusion. The analysis of the measurement data allowed classifying four PD types depending on the polarity of the protrusion, the electric field stress, and the pressure of the insulating gas used. The results improved the interpretation of the measurement data gained during tests and provided therefore a basis for a meaningful and robust PD analysis.

Challenges during partial discharge measurement in gas-insulated DC systems may arise as a result of varying time differences between subsequent impulses, PD current amplitudes changing over orders of magnitude, and pulseless PD currents. The time differences could be in the range from several tens of nanoseconds to several minutes and require therefore PD measurement over a sufficiently long time and with high bandwidth of the measurement system in order to obtain a detailed analysis. The precise measurement of partial discharge current impulses with a low amplitude could be beneficial for the interpretation (at the conductor or enclosure) of a defect, due to the different behavior of positive and negative protrusion. The measurement of the pulseless PD direct current in addition to the well-established measurements according to IEC 60270 and in the UHF range could be used advantageously during laboratory tests to increase the knowledge about the defect.

To obtain a meaningful PD analysis, the applied voltage during PD measurements should not be fixed to a certain value or polarity, due to the fact that glow discharges could occur at negative protrusions, which are not detectable with the state-of-the-art measurement techniques.

The studies complement the knowledge necessary for safe and reliable operation of gas-insulated DC systems, since they contributed to a meaningful PD measurement and analysis for one common PD source, a protrusion. Future research is necessary to facilitate the changeover from the currently used insulating gas SF6 to more environmentally friendly gases like synthetic air or others.

**Author Contributions:** Conceptualization, T.G. and K.B.; methodology, T.G.; validation, T.G. and K.B.; formal analysis, T.G.; investigation, H.K. and T.G.; data curation, T.G. and H.K.; writing, original draft preparation, T.G.; writing, review and editing, T.G. and K.B.; visualization, T.G.; supervision, K.B.; funding acquisition, T.G. All authors read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Project Number 379542208. Open Access Funding by the Publication Fund of the TU Dresden.

**Acknowledgments:** The authors gratefully thank Joachim Speck, former leader of the high voltage group of the IEEH , for the valuable discussions during the funding acquisition, the construction of the experimental setup, and the interpretation of the results. Further, the authors would like to thank Maria Kosse for her continuous support and the discussions during the funding acquisition. The authors thank ABB Power Grids Switzerland Ltd. (Uwe Riechert) for providing the test vessel necessary for the execution of the experiments.

**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; nor in the decision to publish the results.
