**4. Conclusions**

This work has proposed an innovative technical approach in enhancing the performance of surge arresters to protect the MV transformers against indirect lightning overvoltages. More often than not, surge arresters fail after experiencing a thermal runaway as a result of receiving higher energy than their thermal energy absorption limit. An easy way to prevent such failures is to utilize a higher rating surge arrester with a desired energy class, however, it imposes extra costs to the system operator. In this paper, an inductor has been used as a filtering device to limit the energy pushed into the surge arrester. By controlling the energy of a surge arrester, the failure is prevented, and a lower rating surge arrester can be used instead of the high rating expensive surge arresters. In order to provide in-depth analyses, the performance of eleven surge arresters with ratings from 12 kV to 42 kV with two energy classes are investigated under di fferent lightning impulses such as 100 kV, 125 kV, 150 kV, 175 kV, 200 kV, 250 V, 30 kV, and 500 kV. The filters that have been used to control the energy levels are 100 μH, 250 μH, 500 μH, and 1 mH inductors installed before the surge arrester. Besides considering the performance of the proposed filtered surge arrester configuration, the impacts of the spark gap on the performance of this configuration have also been studied. An energy-controlled switch has been proposed to monitor the thermal energy of the surge arrester and to simulate the failure. Results show the e ffectiveness of equipping surge arresters with an inductor-based filter. For instance, equipping a 1 mH filter resulted in a considerable enhancement in the protective performance of a 12 kV rating surge arrester (i.e., SA-12b). A non-filtered surge arrester SA-12b could only protect the MV transformer against 100 kV lightning impulses, while a 1 mH filtered surge arrester SA-12b provides proper protection against 200 kV lightning impulses. The only non-filtered surge arrester, among the considered surge arresters in this paper, that provides proper protection against 200 kV lightning overvoltages is a 42 kV surge arrester (i.e., SA-42b), though, unlike a 1 mH filtered surge arrester SA-12b, it shows inappropriate protection at the beginning of 200 kV lightning overvoltages where a voltage sag is reached to the transformer terminal. Moreover, the surge arrester SA-42b can only decrease the overvoltage tension to 98.7 kV, while a 1 mH filtered surge arrester SA-12b keeps the overvoltage tension below 28.2 kV. It has also been shown that the spark gap installed in series with the surge arrester may help in decreasing the absorbed energy by the surge arrester only slightly. Furthermore, results confirm that by using the filtered surge arrester, the di/dt decreases significantly such that, in this work, without installing a filter, the range of di/dt used was between 5.7 kA/μs and 11.0 kA/μs, and after considering the filtering device the range was between 0.04 kA/μs and 0.11 kA/μs, which is ye<sup>t</sup> another resounding outcome.

All in all, the results (a) show the importance of considering the thermal absorption limit of surge arrester for transient overvoltage studies, and (b) reveal that instead of using an expensive high rating surge arrester to provide proper protection for the MV transformers, a low rating surge arrester, if equipped with a proper filter, can be used. A proper filtered low rating surge arrester not only provides a similar or even better protective performance against the lightning overvoltages, but also proposes a lower residual overvoltage than the proper high rating surge arresters. Therefore, the proposed filter, by boosting the performance of surge arresters, prolongs its lifetime by limiting the energy pushed into the surge arrester and preventing any unwanted failure. Above all, installing low rating surge arresters instead of high rating surge arresters results in considerable savings for the system operator. It is noteworthy to mention that the filtering device that has been applied

to modify the performance of the surge arrester might be its longitudinal withstand voltage level, limited applicability against direct lightning strikes, as well as the size. These limitations need to be studied deeply.

In future works we aim to investigate (1) the impact of the proposed filter on mitigating the effects of lead length [51] and (2) the impact of the earthing system (soil ionization) as well as the conductive coupling of the surge arresters on the performance of the proposed protective device while taking into account the aforementioned limitations.

**Author Contributions:** M.P.-K. proposed the main idea, developed the model, performed the simulations, and prepared the original research draft. M.L. proposed the primary modifications, validated the approach, and supervised the work. All authors have read and agreed to the published version of the manuscript.

**Funding:** The APC was funded by Aalto University.

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