**1. Introduction**

Indirect lightning phenomena are more common than direct lightning and are considered as one of the primary sources of failures and damages in the medium voltage (MV) equipment by causing stress on their insulation system [1,2]. The MV transformers, among all the equipment, are more expensive, and thus providing adequate protection for them is of high importance. Proper protection of transformers not only enhances the system reliability as well as social welfare, but also, by controlling the transient overvoltage stress, prevents/defers extra expenses imposed by transformer failure or severe damages [3].

In practice, typically spark gaps are used to protect the transformers in MV networks against lightning impulses [4]. Although spark gaps are rather cheap protective devices, their operation yields a service interruption due to voltage chop and such voltage chopping imposes steep voltage stress across the transformer terminal [5,6]. Besides, transients may also occur due to the energization of the transformer [7] after the follow current interruption due to spark gap operation. On the other hand, surge arresters, by their charming nonlinear behavior as well as surge energy capabilities, can provide adequate protection against lightning impulses while preventing any unwanted service interruption [8]. However, utilizing a surge arrester as the protective device may not always protect the MV transformers against lightning [9,10]. A report by the Cigre Study Committee A2.37 on the failure rate of MV transformers lists the failure rate of surge arrester-protected transformers due to lightning as ~3% [11].

In the literature, there exist several works that either only conventional surge arresters are considered to protect MV transformers against lightning, or some external or internal modifications are made aiming at enhancing the protective performance of surge arresters. In internal modifications, the microstructure and electrical properties of surge arresters are optimized to withstand di fferent overvoltage conditions [12–15]. In external modifications, which is the focus of our work, more often than not, only other protective devices are combined with surge arresters, or di fferent sides of the transformer are chosen for installing the surge arresters aiming at enhancing the protection outcome [6]. When a conventional surge arrester is used for protecting MV transformers, several characteristics should be taken into account to prevent unwanted failures or returning to normal operating condition after absorbing surge energy [16]. Among all, the thermal energy absorption limit plays a crucial role in guaranteeing the healthy operation of the surge arrester while absorbing surge energy. Moreover, the authors of [17] showed that if the surge arrester is installed closer to the transformer, the overvoltage stress transferred to the LV side of transformer tends to increase; although, to keep the overvoltage stress as low as possible on the MV side of the transformer, the surge arrester needs to be installed as close as possible to the transformer. The authors of [18] provided adequate information on selecting the surge arresters for residential areas. In a distribution network, the placement of surge arresters is a crucial challenge [19], where installing too many surge arresters increases the probability of surge arrester failure, and consequently unwanted outages may occur [20]. One approach is to decrease the number of surge arresters in the network [20], while strengthening the surge arrester and enhancing its performance against lightning overvoltages can be another alternative. In [6], the authors concluded that instead of using a surge arrester with a rather high rating, a series connection of a spark gap and a surge arrester might provide a similar protection level, and a lower rating surge arrester can be considered. However, in this work, the thermal energy absorption limit was not thoroughly investigated. Several other combinations of the spark gap and surge arresters have been studied to enhance the protection quality. For example, in [21], the spark gaps were installed at the MV terminals of the transformer and the LV terminals were protected by low rating surge arrester, while in [22] surge arresters and spark gaps were installed at the MV and LV terminals of the transformer, respectively. In [23], experts advised installing surge arresters at both the MV and LV terminals, which will impose additional costs to the system operator. The authors of [24] proposed a novel idea in which the surge arrester wa replaced by an inexpensive LC filter, and it was shown that even by removing all surge arresters in power system, the lightning overvoltages were kept below the BIL.

According to the literature review above, and to the best of our knowledge, (a) there is a gap on modeling the failure of surge arrester due to absorbing excess energy; (b) no work, so far, considered a filter for controlling the energy pushed into the surge arrester; and (c) there is a gap in investigating the impact of series-connected spark gap on the energy absorbed by surge arresters.

Therefore, the primary contributions of this work are fourfold:


The remainder of this paper is organized as follows. Section 2 presents the simulation set-up for different components of the test. Case studies and simulation results are provided in Section 3. Section 4 presents the concluding remarks and prospects of future works.
