Iranian Journal of Electrical and Electronic Engineering

---

Gas-insulated substations (GIS) have different specifications in proportion to air-insulated substations. Transformer failures related to lightning and switching are often reported in the gas insulated substation (GIS). This problem is the voltage magnifications due to reflections of switching and lightning surges at various junctions within the GIS. thereby overvoltages in GIS are more important than air-insulated substation. There are methods to suppress the stresses created by lightning and switching. However, these methods are suitable before installing the substation and during the substation design period. This paper presents feasible methods for mitigation of the overvoltage magnitude. The advantages of the proposed methods are their simplicity and low cost for implantation along with producing minimal changes in the installed GIS.

---

In this paper, the performance of the EGLA (Externally Gaped Line Arresters) and its impact on the back flashover rate of a 400 kV transmission line have been investigated. The frequency behavior of the grounding system and soil resistivity has been modeled. To analyze the EGLA performance in relation to the grounding system's frequency behavior, a rod-shaped grounding system model has been implemented. By placing the EGLA at different phases of the transmission line, the best scenario has been identified to minimize back-flashover occurrences. Furthermore, the performance of the frequency grounding system to that of the nonlinear grounding system has been compared. The results clearly indicate that using a nonlinear grounding system leads to higher back flashover rates compared to the frequency grounding system. Additionally, the EGLA absorbs less energy when connected to a nonlinear resistor compared to the frequency grounding system. It can be concluded that modeling the grounding system's frequency behavior using the frequency grounding model provides more accurate results, especially in investigations related to power grid insulation coordination.

---

Ensuring the protection of all components within power systems from lightning-induced overvoltage is crucial. The issue of power interruptions caused by both direct and indirect lightning strikes (LS) presents significant challenges in the electrical sector. In medium voltage distribution feeders, the relatively low dielectric strength makes them susceptible to insulation degradation, which can ultimately lead to failures in the distribution system. Therefore, implementing effective protective measures against LS is vital for maintaining an acceptable level of reliability in distribution systems. This paper presents an analytical assessment of LS-induced system overvoltage through high-frequency modeling of components within a 20kV distribution system. The study utilizes EMTP-RV software for precise component modeling, including the grounding system, surge arresters, and distribution feeders. Additionally, the operational impacts of protective devices, such as ZnO surge arresters, shield wires, and lightning rods, are evaluated to mitigate LS-induced overvoltage. A frequency grounding system is implemented using the method of moments (MOM) to analyze the grounding system's influence on LS-induced overvoltage. Furthermore, eight different scenarios are explored to assess the anti-LS capabilities of the 20kV distribution system. Each scenario involves evaluating dielectric breakdown and overvoltage across the insulator chain while proposing suitable protective solutions. The results indicate that the absence of shielding wires and surge arresters leads to higher breakdown voltages, with the lowest breakdown voltage occurring when surge arresters are installed during LS events. Additionally, the use of a frequency grounding system, due to its accurate modeling, yields more precise results compared to a static resistor approach. The MOM simulation reveals a 50% reduction in breakdown voltage under the worst-case scenario, and overall overvoltage experiences a 2% decrease.

---

Overvoltage at the insulator terminal caused by a lightning strike can occur in two ways, i.e., a direct lightning strike on the phase line and ground wire. The insulator can be exposed to the phenomenon of back flashover (BFO) if the terminal voltage of the insulator is higher than its insulator critical voltage The lightning current characteristics are distinguished by the maximum current and the steepness. Differences in the characteristics in this study are identified as International Electrical Commission (IEC) and Conseil International des Grands Reseaux Electriques (CIGRE) impulse waveform standards. The footing-tower grounding system comes in different configurations, such as horizontal, vertical, and grid. Alternative transient program (ATP) software was used for simulating lightning strikes on ground wire and phase lines. The results exhibit that the highest critical voltage of the insulators on the footing tower through grid grounding when the surge current strikes ground wire (3308kV – 3395 kV), with the magnitude of the lightning current ranging from (48 kA – 3395 kA). For lightning direct stroke on the phase line, the critical voltage on vertical grounding is highest on (2938 kV -3021 kV).  The surge current flow footing-tower is highest on the grid. The currents magnitude flow in footing tower were influenced by impedance of grounding.