A Comparative Study on Flashover Characteristics of Porcelain, Glass, and Composite Insulators under Coastal Contamination

1. Introduction

Insulators are critical components in overhead transmission lines, tasked with both electrical insulation and mechanical support. In coastal regions, airborne salt particles accumulate on insulator surfaces, forming a conductive layer when combined with high humidity or fog. This significantly reduces flashover voltage, leading to outages, equipment damage, and safety hazards.

Among the widely used insulator types – porcelain, glass, and composite – each exhibits different surface properties, pollution accumulation mechanisms, and flashover behaviors. Understanding their comparative performance under identical coastal contamination levels is essential for utility engineers and asset managers.

This article presents a controlled laboratory study comparing the AC flashover characteristics of porcelain, glass, and composite suspension insulators under simulated coastal pollution. Key parameters include equivalent salt deposit density (ESDD), non-soluble deposit density (NSDD), and flashover voltage gradients.

2. Experimental Setup and Methods

2.1. Test Specimens

· Porcelain insulator: Standard disc type (XP-70), 146 mm diameter, 140 mm spacing.

· Glass insulator: Toughened glass disc (LXY-70), identical dimensional profile to porcelain.

· Composite insulator: Silicone rubber insulator (FXBW-110/70), with fiberglass-reinforced plastic core and silicone rubber sheds.

All specimens were new, with no prior surface aging, and cleaned according to IEC 60507 standards before contamination.

2.2. Coastal Contamination Simulation

Coastal pollution was simulated using a mixture of NaCl (representing sea salt) and kaolin (representing inert dust). The ESDD was set to three levels: 0.05 mg/cm² (light), 0.10 mg/cm² (medium), and 0.20 mg/cm² (heavy). NSDD was fixed at 0.5 mg/cm². Contamination was applied using the solid-layer method, followed by steam fog humidification at 100% relative humidity.

2.3. Flashover Test Procedure

Tests were conducted in a high-voltage laboratory (up to 200 kV AC). The clean-dry flashover voltage was first recorded for each insulator type. Contaminated specimens were then subjected to a step-up voltage method (IEC 60507) until flashover occurred. For each contamination level and insulator type, five measurements were taken, and the average flashover voltage was calculated.

3. Results and Discussion

3.1. Clean-Dry Flashover Voltage

All three types showed similar clean-dry flashover voltages (approximately 75–80 kV per unit), confirming that under uncontaminated conditions, their dielectric strength is comparable. However, this metric is not indicative of real-world coastal performance.

3.2. Flashover Voltage under Coastal Contamination

The table below summarizes the average flashover voltage reduction across contamination levels:

Insulator Type ESDD = 0.05 mg/cm² ESDD = 0.10 mg/cm² ESDD = 0.20 mg/cm²

Porcelain 58 kV (↓26%) 42 kV (↓47%) 28 kV (↓65%)

Glass 60 kV (↓23%) 44 kV (↓45%) 30 kV (↓62%)

Composite 68 kV (↓13%) 56 kV (↓28%) 48 kV (↓38%)

Key Observations:

· Composite insulators consistently maintained higher flashover voltages across all contamination levels.

· Porcelain and glass performed similarly, with glass showing a slight advantage (approximately 2–3 kV higher) due to its smoother surface and slightly lower pollution adherence.

· The performance gap widened as ESDD increased, indicating that composite materials are particularly advantageous in heavy coastal pollution.

3.3. Hydrophobicity and Leakage Current

Water contact angle measurements revealed:

· Porcelain and glass: hydrophilic (contact angle ~20–40°), forming continuous water film.

· Composite (silicone rubber): hydrophobic (contact angle ~100–120°), water droplets remain spherical.

Leakage current monitoring during fog application showed that composite insulators experienced negligible leakage current until just before flashover, while porcelain and glass exhibited continuous creeping currents exceeding 50 mA at heavy contamination, leading to localized dry-band arcing.

3.4. Surface Contamination Distribution

Post-test inspection indicated:

· Porcelain: salt deposits concentrated on lower surfaces of sheds.

· Glass: similar distribution but with less adhesion due to smoother glazing.

· Composite: the lotus-like effect and low surface energy caused many salt particles to be washed off during fog condensation, effectively self-cleaning.

4. Discussion of Practical Implications

4.1. Design and Maintenance

For coastal lines, using composite insulators allows either:

· Shorter creepage distances compared to porcelain/glass for the same voltage level, or

· Higher contamination withstand capability without increasing string length.

Maintenance frequency: Porcelain and glass require periodic washing (e.g., every 6–12 months in severe coastal zones), while composite insulators may extend intervals to 3–5 years, significantly reducing lifecycle costs.

4.2. Limitations of This Study

This research used new, unaged insulators. In real coastal environments, UV radiation, salt fog cycling, and mechanical stress degrade hydrophobic properties over time. Long-term field studies are needed to confirm whether composite insulators retain their advantage after 10–15 years of service.

4.3. Recommendations

· Light to moderate coastal pollution: All three types are acceptable, but composite reduces maintenance.

· Heavy coastal pollution (e.g., within 1 km of shoreline): Composite insulators are strongly recommended. If porcelain or glass must be used, increase creepage distance by at least 30–40% above standard recommendations (e.g., from 25 mm/kV to 35–40 mm/kV).

· For existing porcelain/glass lines: Consider applying silicone grease coatings or installing booster sheds to improve flashover performance.

5. Conclusion

This comparative study demonstrates that under simulated coastal contamination, composite (silicone rubber) insulators significantly outperform porcelain and glass in flashover voltage retention. The superior hydrophobicity and lower pollution adhesion of composite materials explain the performance gap, which becomes more pronounced as ESDD increases.

Porcelain and glass insulators, while reliable in clean or mildly polluted environments, require heavier design margins or frequent cleaning in coastal areas. For new transmission lines in coastal zones, composite insulators offer a compelling technical and economic advantage, provided that long-term aging effects are properly managed through material selection and periodic inspection.