In-Service Testing and Evaluation of Composite Insulators
The primary characteristics of composite insulators during operation are hydrophobicity and hydrophobicity migration, which determine the insulator’s anti-pollution flashover performance.
Critical Risk Points in Operation:
- Mechanical strength of the connection between the end-fitting and the core rod.
- Quality of the epoxy FRP (Fiber Reinforced Plastic) rod.
- Quality of the silicone rubber material.
- Integrity of the sealing system.
- Correct installation of grading (corona) rings.
1. Sampling Principles for In-Service Testing
Based on performance characteristics and potential risks, the following criteria are recommended for selecting test samples from the grid:
- Insulators located on towers within a 5km radius downwind of industrial pollution sources.
- Insulators in heavy multi-pollution zones within a 3km radius.
- Insulators on towers at river or lake crossings.
- Insulators on towers within a 1km radius of wetlands.
- Insulators on towers within a 1km radius of villages.
- Insulators on towers with large vertical spans.
- Insulators located in wind gaps or outlets.
- Insulators in heavy icing sections.
- Insulators on towers in lightning-prone areas.
- Insulators in sections with frequent bird activity.
- Sampling Cycle: Generally every two years; annually if necessary.
2. Hydrophobicity Classification (HC) Standards
The following table outlines the Hydrophobicity Classification (HC 1-7) based on the state of water droplets on the surface:
| Level | Wetting State and Contact Angle Analysis |
|---|---|
| HC 1 | Only discrete droplets. Most receding angles θ ≥ 80°. |
| HC 2 | Only discrete droplets. Most receding angles 50° < θ < 80°. |
| HC 3 | Discrete droplets (non-circular). Most receding angles 20° < θ < 50°. |
| HC 4 | Both discrete droplets and wetted streaks. Wetted area < 2 cm²; total area < 90% of the tested region. |
| HC 5 | Wetted areas > 2 cm². Total wetted area < 90% of the tested region. |
| HC 6 | Total wetted area > 90%. Few dry spots or streaks remain. |
| HC 7 | Continuous water film forms over the entire tested area. |
3. Criteria for Shed and Sheath Materials
- Hydrophobicity Angle: Average angle θav ≥ 100°, minimum angle θmin ≥ 90°.
- Classification: Should generally be HC 1–HC 2; no more than one sample at HC 3.
4. Aging Determination
- HC 1 – HC 2: Good hydrophobicity; no significant aging.
- HC 3: General surface aging.
- HC 4 – HC 5: Moderate to severe aging.
- HC 6 – HC 7: Complete material surface aging.
5. Temporary Loss and Recovery of Hydrophobicity
Under extreme humidity, silicone rubber may temporarily lose its hydrophobicity. Recovery time depends on the silicone variety, filler, aging status, and pollution levels.
- Recovery Standards: For new insulators (HC 1) after 24h water immersion, the average recovery time is 37.57s. If recovery takes > 38s, the hydrophobicity is considered unstable.
- Laboratory Test: Samples are soaked in distilled water (conductivity < 10 μS/cm) for 96h at 20±5°C. The recovery time after testing should not exceed 85s. If the HC level is > 3 or recovery takes > 85s, the insulator is flagged for aging-related hydrophobicity instability.
