**4. Conclusions**

Physical modeling (using an artificial thunderstorm cell) of the possible influence of upward streamer discharges from the rod or elongated model elements of a power transmission line monitoring systems on their operation in the electric field of a thunderstorm cloud and/or lightning produced many results. Firstly, upward streamer discharges that formed on the monitoring systems model elements (i.e., sensors, and receiving and transmitting devices) can affect the functioning of these systems due to the fact that in the spectrum of their currents, there are frequencies of up to tens of megahertz, which is close to the operating frequencies of power transmission line monitoring systems. Secondly, upward streamer discharges on the model elements of power transmission lines can also induce signals in the neighboring rod or elongated elements of the monitoring systems. These signals are dangerous for the monitoring systems' operation, since the discharge spectrum contains frequencies of tens to hundreds of megahertz (up to units of gigahertz).

It was found that the parameters of the current pulse of upward streamer discharges, the signals that they induce in the neighboring elements, and their spectral characteristics depend on the electric field distribution near the model rod or elongated element (electric field amplification factor). The highest frequencies in the spectrum of the current pulse of the streamer corona flash are typical for the model elements with a high electric field amplification coefficient of >25. The highest frequencies in the spectrum of the signal induced in the neighboring element by the upward streamer discharges are typical for model elements with an amplification coefficient of 10–25. Thus, both the frequency ranges in the spectrum of the current pulse of upward streamer discharges formed from the model elements of sensors and receiving-transmitting devices of the power transmission line monitoring system and the frequency ranges in the spectrum of signals induced by close upward streamer discharges on these elements may appear close to the operating frequency ranges of analog-to-digital converters of sensors and/or devices for transmitting data in monitoring systems. This can lead to failures in monitoring systems operation, false alarms, incorrect transmission of information, and, as a result, create significant risks for the functionality of these systems during thunderstorms.

**Author Contributions:** Project administration and methodology, A.T.; software, L.C. and O.B.; visualization, T.K.; investigation, A.O. and N.L.; writing—original draft preparation, V.V.; funding acquisition, D.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study conducted by Moscow Power Engineering Institute was financially supported by the Ministry of Science and Higher Education of the Russian Federation (project No. FSWF-2020-0019).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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