Views: 0 Author: Site Editor Publish Time: 2026-06-12 Origin: Site
Lightning strike is one of the leading causes of transmission line trip-outs worldwide. In China, for instance, lightning accounts for over 50% of power system failures, and operational data from five provinces within the China Southern Power Grid over a five-year period indicated that approximately 62% of all trip-out faults were lightning-related. Externally Gapped Line Arresters (EGLAs) have emerged as highly effective countermeasures for transmission line lightning protection. By connecting an external series gap in parallel with the line insulator, the EGLA provides reliable overvoltage limitation while isolating the metal-oxide varistor (MOV) from the power frequency operating voltage under normal conditions, thereby eliminating continuous leakage current and significantly enhancing device longevity.
The discharge characteristics of EGLAs are governed by a complex interaction among three essential components: the external series gap, the MOV valve blocks, and the overall insulation coordination with the protected insulator string. This article reviews the key principles of discharge behavior for gapped line arresters, examines the design requirements for external series gaps, and presents an integrated optimization methodology that combines experimental validation with multi-physics simulation for engineering applications.
The EGLA is installed in parallel with the line insulator string. Under normal power frequency operation, the external gap isolates the MOV unit from the system voltage. When a lightning overvoltage occurs, the gap is designed to spark over before the insulator string flashes over. Once the gap conducts, the lightning surge current is diverted through the MOV unit to ground, and the residual voltage across the arrester is clamped to a level substantially below the flashover voltage of the insulator string, thereby protecting the line insulation.
Experimental studies have shown that the relationship between the overall discharge voltage of the EGLA, the breakdown voltage of the series gap, and the V–A characteristics of the MOV unit is non-trivial. Under power frequency voltage, the gap and the MOV unit primarily share voltage according to their equivalent capacitances. Under lightning impulse voltage, however, the overall discharge voltage depends on both the gap discharge voltage and the MOV V–A characteristics. The influence of the MOV V–A characteristics increases with the steepness of the lightning wavefront, and the disparity between the gap discharge voltage and the overall arrester discharge voltage becomes more pronounced as the wave steepness rises.
Quantitatively, studies on 220 kV transmission lines have demonstrated EGLA protection effectiveness exceeding 99.993%. The positive and negative lightning impulse discharge voltage–time curves of the arrester were found to be at least 16% and 15% lower than those of the insulator string, respectively, ensuring a sufficient coordination margin. Furthermore, the lightning impulse coordination factor for the same system reached 1.39 for positive polarity and 1.34 for negative polarity, with a synthetic coordination factor above 1.18, guaranteeing reliable protective action across a wide range of operating conditions.
The design of the external series gap must reconcile two inherently conflicting requirements: the gap must spark over reliably under lightning overvoltage conditions, yet it must remain stable and not spark over under switching impulses or power frequency temporary overvoltages. This fundamental tension has driven the development of systematic design criteria.
3.1 Gap Distance Determination
The gap distance is determined based on two complementary principles: (1) it must be sufficiently large to withstand the maximum overvoltage that the line can experience under normal operation, and (2) it must be sufficiently small to ensure that the arrester sparks over before the insulator string flashes over when a lightning strike occurs. The practical approach involves first testing the 50% lightning impulse sparkover voltage of the air gap and comparing it with that of the insulator string. The distance is selected such that a specified impulse coordination factor is satisfied. Subsequently, the volt–time characteristics of the selected gap configuration are experimentally validated against those of the insulator string to confirm adequate coordination across the entire wavefront duration. Finally, power frequency withstand tests are conducted to ensure that the gap does not spark over under the maximum temporary overvoltage expected on the system.
For ultra-high-voltage applications, the challenges associated with simultaneously satisfying lightning sparkover and switching impulse immunity become particularly severe. Advanced designs have incorporated secondary sparkover gap units to relax these constraints, enabling the primary gap to be optimized primarily for lightning performance while a secondary gap addresses switching impulse coordination.
3.2 Gap Structure Selection
Two primary gap structures have been widely adopted: the separated air gap and the fixed (or integrated) gap. The separated gap design, pioneered in Japan, isolates the two discharge electrodes solely by air, necessitating complex arc-shaped electrodes to maintain a constant gap distance under varying mechanical loads such as wind and conductor swing. The fixed gap design employs a composite insulator to rigidly mount the two ring-shaped electrodes, ensuring that the gap distance remains unchanged regardless of external forces. This structure has become the predominant choice for transmission line arresters in China because it provides stable discharge voltages with minimal dispersion.
However, the fixed gap design introduces its own considerations: the composite insulator used to mount the gap is subjected to almost the entire continuous operating voltage across the EGLA, because the MOV unit is isolated by the gap. For 110 kV fixed gap arresters, the capacitance of the gap (approximately 0.4 pF for a 500 mm gap length) is substantially lower than that of the MOV stack (approximately 37 pF for 28 disks), meaning that the composite insulator must withstand nearly the full system voltage. This imposes stringent requirements on insulator quality and long-term aging performance.
The optimization of EGLA protection gaps follows an integrated approach combining theoretical modeling, simulation analysis, and experimental validation.
4.1 Electrode Geometry Optimization
The geometry of the discharge electrodes directly influences the electric field distribution within the gap, which in turn determines the consistency of the sparkover voltage. Simulation studies using finite element analysis (e.g., COMSOL Multiphysics) have revealed that for a given electrode type, the electric field decreases in a quadratic fashion as the electrode radius increases. Conversely, increasing the electrode spacing produces a linear reduction in the electric field magnitude. Both ring-type and rod-type electrodes can satisfy practical protection requirements, but ring electrodes offer greater manufacturing convenience and more uniform field distributions. For 10 kV externally gapped arresters, a ring electrode with a spacing of 52 mm was determined to be optimal based on 50% lightning impulse discharge voltage testing. In addition, field simulation using COMSOL can guide the placement of the gap relative to the arrester body to minimize electrical stress on the unit: positioning the gap between the high-voltage terminal and the arrester body significantly reduces the electric field acting directly on the MOV unit, promoting long-term operational stability.
4.2 Multi-Physics Discharge Simulation
Traditional gap design relies heavily on empirical testing, which does not fully capture the multi-physics coupling effects that govern the transient discharge process. Recent advances have introduced magnetohydrodynamic (MHD) modeling of the arc discharge, enabling prediction of key parameters across all stages of the discharge event. Simulations that consider the coupling between the electrical field, thermal field, and fluid dynamics reveal that the amplitude of the lightning current dominates the early-stage arc temperature rise, while the duration of the current controls the later stages. Furthermore, when the initial phase of the system voltage aligns with the polarity of the lightning impulse, arc energy is significantly enhanced, increasing the risk of failed follow-current interruption. The simulation results also demonstrate that increasing the conductivity of the MOV valve blocks enhances arc discharge intensity and improves current diversion efficiency, thereby more effectively suppressing overvoltage stress on the line. This modeling capability provides a powerful tool for optimizing gap parameters before physical prototyping.
4.3 Verification of Arc Interruption Capability
After the lightning surge has been diverted, the power frequency follow current must be extinguished reliably. For EGLA applications, the follow-current interruption capability is a critical performance metric that must be verified through dedicated testing. One promising design direction is the use of multi-short-gap structures connected in series with a large discharge gap. In this configuration, the multiple short gaps break down sequentially, establishing arc channels that are then expelled under the action of high pressure generated within the short gaps. The arc extinguishes at the natural zero crossing of the power frequency follow current, and because the maximum voltage distributed across any individual short gap is insufficient to reignite the arc, sustained conduction is avoided. This multi-short-gap approach offers both reliable sparkover under lightning impulses and robust arc extinction after the event, without requiring complex auxiliary interruption mechanisms.
While EGLAs provide effective lightning protection, their long-term reliability must be systematically addressed. Field failure of EGLAs, particularly under multiple lightning strokes, can cause both ablation damage from short-circuit current arcs and impact damage from ejected components, potentially leading to conductor breakage accidents. These failure modes highlight the importance of robust mechanical design in addition to electrical performance optimization. Moreover, degradation of the composite insulator in fixed-gap designs due to continuous application of power frequency voltage represents another potential weakness that requires careful material selection and accelerated aging tests to validate long-term performance.
International standards such as IEC 60099-8:2017 provide comprehensive guidance for EGLA design, including verification tests for insulator coordination, follow-current interruption capability, and mechanical load withstand. Adherence to these standards is essential for ensuring that optimized gap designs translate into reliable field performance.
The discharge characteristics of line arresters with series gap are the result of a delicate balance among gap geometry, MOV V–A characteristics, and insulation coordination with the line insulator string. Systematic optimization of the external protection gap requires combined application of theoretical coordination principles, finite element electric field simulation, multi-physics modeling of the transient discharge process, and experimental validation of sparkover and arc interruption performance. With careful consideration of both electrical and mechanical reliability factors, properly designed EGLAs offer an exceptionally high level of lightning protection while minimizing the risk of long-term degradation or failure. As transmission voltages continue to increase and lightning-related outages remain a persistent challenge, ongoing advances in simulation-driven gap design and robust engineering will further enhance the effectiveness of series-gapped line arresters as the preferred solution for overhead line lightning protection.
