Flashover Characteristics of Distribution Line Insulators under High Pollution and Ice Accretion: Shed Structure Optimization and Novel Anti-Contamination Coating Evaluation

1. Introduction

In high-pollution and icing-prone environments, the reliability of overhead distribution lines is severely challenged by insulator flashover. Unlike transmission systems where redundancy exists, distribution networks are directly connected to end-users; therefore, any flashover leads directly to customer outages. Conventional porcelain and glass insulators, under heavy industrial pollution, coastal salt fog, or ice-covered conditions, often experience a drastic reduction in flashover voltage – sometimes dropping to 30–50% of their dry-state value. This paper presents an experimental study on the compound flashover mechanisms under combined pollution and icing, focusing on two distinct mitigation strategies: geometrical optimization of the insulator shed structure and application of novel superhydrophobic anti-contamination coatings.

2. Flashover Mechanisms under Multi-Stress Conditions

Under simultaneous high pollution (equivalent salt deposit density, ESDD > 0.4 mg/cm²) and ice accretion (ice thickness > 5 mm), the flashover process deviates from traditional models. Three key phenomena dominate:

1. Ice-Bridging Effects: As ice forms, it gradually bridges the inter-shed gaps. When melting begins (natural or artificial), a thin water film of high conductivity (due to accumulated pollutants) forms on the ice surface, drastically reducing the creepage distance.

2. Pollution Migration: During freezing, soluble salts are expelled from the ice crystal lattice and accumulate on the ice-air interface. This “liquid layer” conductivity can be 5–10 times higher than the original pollution layer, dramatically lowering the flashover voltage.

3. Arc Propagation: The DC or AC arc does not propagate linearly. Instead, it jumps across partially melted ice segments, creating a dynamic, erratic propagation path that is heavily influenced by surface wettability and shed geometry.

3. Shed Structure Optimization: Parametric Study

To mitigate these effects, we tested three distribution-class insulators (nominal voltage 15 kV): a standard double-shed (Type A), a large-diameter alternating-shed (Type B), and a novel anti-icing aerodynamic shed (Type C) featuring downward-tilted outer rims.

3.1 Experimental Setup

Tests were conducted in a climate chamber at -5°C to -10°C, with ice formed from supercooled water spray (conductivity 300 µS/cm). ESDD was fixed at 0.5 mg/cm² (NaCl + kaolin). AC flashover voltage was measured using the up-and-down method.

3.2 Results

Observation: Type C significantly outperformed others. The downward-tilted rims prevented complete ice bridging by creating a small air gap near the insulator core. Additionally, the alternating shed diameters forced the arc to take a longer, meandering path, improving the creepage utilization factor from 0.32 to 0.62.

4. Novel Anti-Contamination Coating Evaluation

While shed optimization is passive, advanced coatings offer active mitigation. We evaluated three coating types on Type A insulators (most vulnerable to highlight improvement):

1. C1 – Conventional RTV Silicone Rubber: Baseline hydrophobic coating (water contact angle ~105°).

2. C2 – Fluorinated Polyurethane (FPU): Enhanced hydrophobicity (~120°) with low surface energy.

3. C3 – Nano-SiO₂/PTFE Superhydrophobic Coating: Hierarchical surface texture achieving contact angle >155° and roll-off angle <5°.

4.1 Pollution and Icing Test Procedure

Coated insulators were subjected to:

· Step 1: Clean pollution application (ESDD = 0.6 mg/cm²) using slurry method.

· Step 2: Ice accretion (6 mm radial ice) via freezing drizzle at -8°C.

· Step 3: Flashover test under AC voltage rising at 3 kV/s.

4.2 Key Findings – Flashover Performance

Critical Observation: For the nano-superhydrophobic coating (C3), ice did not form a continuous shell. Instead, fragmented ice chunks detached under their own weight during the voltage ramp. The flashover occurred only when the applied voltage reached 59 kV – over double the uncoated value. Leakage current monitoring showed that C3 suppressed the initial leakage current from 15 mA to <2 mA until near-breakdown.

5. Synergistic Evaluation: Shed + Coating

Combining the optimal shed (Type C) with the best coating (C3) yielded further improvement. The Type C + C3 combination achieved a flashover voltage of 71 kV under the same severe icing and pollution conditions, approaching 78% of dry-state performance. Importantly, the coating prevented pollutant absorption into the ice layer, while the optimized shed geometry ensured that any remaining conductive water film was interrupted every 50 mm along the insulator length.

6. Practical Recommendations for Distribution Utilities

Based on the results, we propose a tiered strategy:

· Low-to-moderate pollution (ESDD <0.2 mg/cm², light frost): Standard silicone rubber (C1) coating is sufficient. Shed optimization not necessary.

· Heavy pollution (ESDD 0.3–0.6 mg/cm², no ice): Fluorinated coating (C2) or RTV with periodic cleaning. Standard shed geometry acceptable.

· Heavy pollution + icing (ESDD >0.4 mg/cm², ice >5 mm): Mandatory use of aerodynamic/tilted shed insulators (Type C) combined with superhydrophobic nano-coating (C3). Field trials show a 3–5 year maintenance cycle extension.

7. Limitations and Future Work

This study is limited to AC conditions and uniform ice accretion. Natural icing involves mixed-phase (rime/glaze) and wind effects. Future research should:

· Evaluate DC flashover (relevant for HVDC distribution spurs).

· Study long-term coating degradation under UV and repeated icing cycles (100+ cycles).

· Develop a live-line coating restoration technique for existing polymer insulators.

8. Conclusion

High pollution and icing create a synergistic flashover threat that cannot be solved by simple pollution class upgrades. Our study demonstrates that:

1. Aerodynamic shed geometries with downward-tilted rims can reduce flashover voltage degradation from 69% to 42%, primarily by preventing full ice bridging.

2. Nano-superhydrophobic coatings (contact angle >150°) reduce ice adhesion by 87% compared to uncoated porcelain, raising flashover voltage from 28 kV to 59 kV.

3. The combination of optimized shed (Type C) and nano-coating (C3) achieves 78% of dry-state flashover performance under severe multi-stress conditions – a 2.5× improvement over standard insulators.