Views: 0 Author: Site Editor Publish Time: 2026-07-23 Origin: Site
Surge arresters are critical assets in power transmission and distribution networks, providing protection against overvoltages caused by lightning strikes and switching operations. Their external insulation system—whether porcelain or polymeric—must maintain adequate dielectric strength under all environmental conditions. However, in regions characterized by high airborne pollution (industrial emissions, coastal salt fog, desert dust, or agricultural contaminants) combined with frequent and heavy rainfall, the external insulation of surge arresters faces a particularly severe threat: rain flashover.
Rain flashovers occur when rain water bridges the insulating surface, reducing the creepage distance and creating a conductive path for leakage currents. In polluted environments, this problem is exacerbated because contaminants deposited on the arrester surface dissolve in rainwater, forming an ionic conductive layer that dramatically lowers the flashover voltage. Field statistics indicate that rain flashovers account for a significant proportion of DC external insulation failures.
Understanding the physics of rain flashover is essential for designing effective countermeasures. Under dry conditions, the external insulation of a surge arrester provides sufficient creepage distance to withstand the system voltage. However, during heavy rainfall, several phenomena converge to create flashover conditions:
First, rainwater forms a continuous film or water bridges across the insulation surface, particularly on vertically oriented arresters or those with inadequate shed geometry. Second, in high-pollution areas, accumulated contaminants—salt, cement dust, carbon particles, and industrial fallout—dissolve in the rainwater, creating an electrolyte that supports leakage currents. Third, these leakage currents cause localized heating and drying of the surface, leading to the formation of dry bands. When the voltage stress across a dry band exceeds the dielectric strength of air, dry-band arcing occurs. If these arcs persist and elongate, they can bridge the entire insulation length, resulting in a complete flashover.
Computational simulations have demonstrated that under heavy rain conditions, the electric field strength between arrester sheds and grounded components can increase by approximately 2.5 times compared to dry conditions, making air-gap breakdown highly probable. The situation is particularly critical for arresters with conical or tapered housings, where rainwater flows along the surface and bridges adjacent sheds.
Creepage extension skirts—also known as shed boosters or auxiliary sheds—are auxiliary insulating components installed on existing surge arrester housings to increase the effective creepage distance. Typically manufactured from high-grade silicone rubber through high-temperature vulcanization, these skirts are designed to be retrofitted onto porcelain or composite arrester housings.
3.1 Working Principles
The primary function of a creepage extension skirt is to lengthen the surface leakage path, thereby increasing the creepage distance per unit of system voltage. For a given pollution level, international standards specify minimum creepage distances (typically 25–35 mm/kV depending on pollution class). When existing arresters fall short of these requirements, creepage extension skirts offer a practical retrofit solution.
Beyond simply increasing creepage distance, silicone rubber creepage skirts provide several additional protective mechanisms:
· Hydrophobicity and Hydrophobic Migration: The silicone rubber material is inherently water-repellent. Moreover, it exhibits hydrophobic migration—the ability to transfer its hydrophobic properties to contaminant layers deposited on its surface. This means that even when the skirt becomes covered with pollution, the outer contamination layer acquires water-repellent characteristics, preventing the formation of continuous water films.
· Self-Cleaning Effect: The hydrophobic surface causes water to form discrete droplets that roll off the surface, carrying loose contaminants with them. This natural cleaning action reduces the accumulation of pollution over time.
· Arc-Path Zigzag Effect: The geometry of the extension skirt forces any developing arc to follow a longer, more tortuous path, increasing the voltage required to sustain the arc.
3.2 Installation Considerations
Creepage extension skirts are available for voltages ranging from 10 kV to 500 kV and can be applied to various substation equipment including post insulators, circuit breakers, transformers, and surge arresters. Installation typically involves cleaning the existing insulator surface, applying a specialized silicone adhesive, and securing the skirt around the porcelain or glass shed. The skirt extends beyond the original shed by approximately 40–50 mm—sufficient to provide meaningful creepage extension while avoiding excessive overhang that could cause deformation.
Research has shown that for 35 kV class insulators, adding a single creepage extension skirt provides the most significant improvement in pollution flashover voltage. Adding two skirts yields comparable or sometimes slightly reduced benefits, depending on installation position. Optimal placement is typically at the mid-section of the insulator for single-skirt installations, or at the top and middle for two-skirt configurations.
While creepage extension skirts address the issue of insufficient creepage distance, hydrophobic coatings tackle the problem at the surface level by fundamentally altering the wetting behavior of the arrester housing.
4.1 RTV Silicone Rubber Coatings
Room Temperature Vulcanizing (RTV) silicone rubber coatings are the most widely adopted hydrophobic coating technology for high-voltage outdoor insulation. Applied as a thin layer (typically 150–300 μm) over the arrester housing, RTV coatings create a hydrophobic surface that prevents water from forming a continuous film.
The key performance attributes of RTV coatings include:
· Excellent Hydrophobicity: Water contact angles exceeding 90° ensure that water forms discrete droplets rather than continuous films.
· Hydrophobicity Transfer: Like silicone rubber skirts, RTV coatings can transfer their hydrophobic properties to surface contaminants, maintaining protection even when the coating becomes covered with pollution.
· Arc and Tracking Resistance: The coating material is formulated to resist degradation from dry-band arcing, with the ability to recover hydrophobicity after arcing events.
· UV and Weathering Resistance: Modern RTV formulations incorporate UV stabilizers and inorganic fillers that provide long-term durability in outdoor exposure.
· Self-Cleaning Capability: The hydrophobic surface promotes natural cleaning by rain, reducing the frequency of manual washing.
4.2 Advanced Superhydrophobic Coatings
Recent advances in materials science have led to the development of superhydrophobic coatings with water contact angles exceeding 150° and sliding angles below 10°. These coatings achieve their extreme water repellency through a combination of low-surface-energy chemistry and micro/nano-scale surface roughness, mimicking the lotus leaf effect.
Superhydrophobic coatings offer several advantages over conventional RTV coatings:
· Enhanced self-cleaning: Water droplets roll off inclined surfaces, removing >95% of surface particulates within seconds.
· Reduced ice adhesion: The ultralow ice adhesion strength (<20 kPa) prevents the formation of ice bridges that can trap contaminants.
· Photocatalytic decomposition: Some advanced formulations incorporate TiO₂ nanoparticles that break down organic pollutants under UV exposure.
While each approach provides substantial protection individually, the combination of creepage extension skirts and hydrophobic coatings offers synergistic benefits that address the rain flashover problem comprehensively.
The creepage extension skirt increases the fundamental creepage distance, providing a longer surface path that must be bridged by leakage currents. The hydrophobic coating, applied to both the original housing and the extension skirt, ensures that the extended surface remains dry and clean, preventing the formation of conductive water films. Together, these measures provide:
1. Redundancy: If the coating degrades over time, the increased creepage distance still provides a margin of safety.
2. Extended Service Life: The coating protects the silicone rubber skirt from UV degradation and contamination, while the skirt shields portions of the coating from direct rain impact.
3. Optimized Performance: Testing has demonstrated that the combined approach provides flashover voltages significantly higher than either measure alone.
For surge arresters operating in high-pollution, heavy-rainfall regions, the following implementation strategy is recommended:
1. Assessment: Evaluate the existing creepage distance against the requirements for the site-specific pollution level. If the creepage distance is below the recommended value (typically >31 mm/kV for heavily polluted areas), creepage extension skirts should be added.
2. Surface Preparation: Thoroughly clean the arrester housing to remove all contaminants, oils, and loose materials before applying any coating or installing skirts.
3. Skirt Installation: Install silicone rubber creepage extension skirts using manufacturer-recommended adhesives and procedures. Ensure proper alignment and secure bonding to prevent gaps that could allow moisture ingress.
4. Coating Application: Apply RTV or superhydrophobic coating to the entire arrester housing and the installed skirts. Spray application is typically preferred for uniform coverage, though brushing or dipping may be suitable for specific geometries.
5. Curing and Inspection: Allow the coating to cure at ambient temperature. Inspect for uniform coverage, proper adhesion, and absence of defects.
6. Periodic Maintenance: Conduct regular inspections to assess coating condition and skirt integrity. Reapply coating or replace damaged skirts as needed.
Rain flashover of surge arrester external insulation in high-pollution and heavy-rainfall regions presents a significant reliability challenge for power systems. The combined application of silicone rubber creepage extension skirts and hydrophobic protective coatings provides an effective, cost-efficient solution. The skirts increase the fundamental creepage distance while the coatings ensure that the insulation surface remains hydrophobic and self-cleaning. Together, these technologies offer a robust defense against the complex failure mechanisms that lead to rain flashovers, ensuring the long-term reliability of surge arresters in the world's most demanding environments.
As power systems continue to expand into regions with challenging climates and increasing pollution levels, the adoption of comprehensive external insulation protection strategies will become not just beneficial but essential for maintaining system reliability and safety.
