*Article* **Enhancing the Protective Performance of Surge Arresters against Indirect Lightning Strikes via an Inductor-Based Filter**

#### **Mahdi Pourakbari-Kasmaei \* and Matti Lehtonen**

Department of Electrical Engineering and Automation, Aalto University, Maarintie 8, 02150 Espoo, Finland; Matti.Lehtonen@aalto.fi

**\*** Correspondence: Mahdi.Pourakbari@aalto.fi; Tel.: +358-46-538-48-72

Received: 11 August 2020; Accepted: 8 September 2020; Published: 11 September 2020

**Abstract:** Preventing the medium voltage (MV) transformer fault by protecting transformers against indirect lightning strikes plays a crucial role in enhancing the continuous service to electricity consumers. Surge arresters, if selected properly, are e fficient devices in providing adequate protection for MV transformers against transient overvoltage impulses while preventing unwanted service interruptions. However, compared to other protective devices such as the spark gap, their prices are relatively high. The higher the surge arrester rating and energy absorption capacity are, the higher the prices go. This paper proposes an inductor-based filter to limit the energy pushed into the surge arrester, and consequently to prevent any unwanted failure. An energy-controlled switch is proposed to simulate the fault of the surge arrester. Surge arresters with di fferent ratings, e.g., 12 kV, 18 kV, 24 kV, 30 kV, 36 kV, and 42 kV with two di fferent classes of energy, namely, type a and type b, are tested under di fferent indirect lightning impulses such as 100 kV, 125 kV, 150 kV, 175 kV, 200 kV, 250 kV, 300 kV, and 500 kV. Furthermore, these surge arresters are equipped with di fferent filter sizes of 100 μH, 250 μH, 500 μH, and 1 mH. Results prove that equipping a surge arrester with a proper filter size enhances the performance of the surge arrester significantly such that a high rating and somewhat expensive surge arrester can be replaced by a low rating and cheap surge arrester while providing similar or even better protective performance for MV transformers. Therefore, such configurations not only enhance the protective capability of surge arrester, but also reduce the planning and operating costs of MV networks.

**Keywords:** double exponential function; indirect lightning; medium voltage transformer; spark gap; filtered surge arrester; energy-controlled switch
