Aging Mechanisms And Prevention of Composite Insulators: A Detailed Analysis of Hydrophobicity Loss And Fracture in Silicone Rubber Sheds

1. The Critical Role and Loss of Hydrophobicity

Hydrophobicity is the property of a material to repel water. On silicone rubber sheds, water beads up into discrete droplets instead of forming a continuous conductive film. This drastically reduces leakage current and prevents flashover under polluted and wet conditions.

The loss of this property is a primary aging indicator. The main mechanisms are:

· Chemical Degradation (Photo-oxidation): Ultraviolet (UV) radiation from sunlight, combined with ozone and oxygen, breaks the molecular chains of the silicone rubber polymer (polydimethylsiloxane, PDMS). This process, accelerated by heat, destroys the methyl groups on the surface that are responsible for water repellency. The surface becomes hydrophilic, allowing water to form a continuous layer.

· Pollution Layer Interaction: Airborne contaminants (cement dust, industrial emissions, salt fog) settle on the insulator surface. These pollutants can catalyze the photo-oxidation process. More critically, they can absorb and hold water, masking the hydrophobic properties of the underlying SIR. While the bulk material may still be hydrophobic, the surface appears hydrophilic due to the wet pollution layer.

· Aging by Corona and Arcing: Continuous corona discharge or partial arcing, often caused by imperfect hardware or water droplet corona, generates ozone and nitrogen oxides. These compounds are highly reactive and aggressively attack the SIR matrix, leading to surface erosion and permanent loss of hydrophobicity.

2. Mechanisms Leading to Fracture and Mechanical Failure

While hydrophobicity loss affects electrical performance, mechanical failure is catastrophic. The most critical failure is brittle fracture of the fiberglass reinforced plastic (FRP) core.

· Core Fracture Mechanism: Acid-Induced Brittle Fracture: This is the most common and dangerous failure mode. It originates from moisture ingress through seals damaged by weathering, corona cutting, or bird pecking. Once inside, the moisture reacts with air pollutants (e.g., NOₓ, SOₓ) absorbed by the core or with byproducts from corona discharge to form nitric acid (HNO₃) or sulfuric acid (H₂SO₄).

 · The acid attacks the epoxy resin binding the fiberglass rods.

 · It then etches and notches the individual glass fibers, causing them to crack under mechanical tension.

 · This process occurs with no visible plastic deformation, hence the term "brittle fracture." The rod can snap cleanly, often at a fraction of its rated mechanical load.

3. Integrated Prevention and Mitigation Strategies

A proactive approach combining material science, design, and maintenance is key to longevity.

A. Prevention at the Design and Material Level:

· Advanced SIR Formulations: Use of SIR with added ATH (Alumina Trihydrate) improves tracking and erosion resistance. Additives like silica and vinyl groups enhance UV stability and cross-linking density.

· Robust Sealing Systems: Critical to preventing moisture ingress. Multi-layer sealing systems using high-grade polymers and precise engineering at the end-fitting/housing interface are non-negotiable.

· Corona-Free Hardware: Designing hardware (e.g., corona rings) to minimize electric field intensity at the ends prevents corona discharge, protecting the seals and housing from the initial damage that starts the aging chain.

B. Operational and Maintenance Strategies:

· Regular Visual and Thermal Inspection: Schedule regular inspections to identify:

 · Surface Erosion: Tracking, chalking, or cracking of sheds.

 · Damage: Cuts, punctures, or animal damage to the housing.

 · Corona Activity: Use ultraviolet (UV) cameras to detect corona discharges, a precursor to severe damage.

 · Hotspots: Use infrared (IR) cameras to identify abnormal heating caused by leakage currents or internal faults.

· Hydrophobicity Assessment: Perform Hydrophobicity Classification Tests (as per STRI Guide 1/92 or IEC 62073) to quantify the state of the silicone rubber. This is a powerful predictive maintenance tool.

· Cleaning: In heavily polluted areas, periodic washing with deionized water can remove conductive layers that accelerate aging. However, proper technique is crucial to avoid flashover during the process.

Conclusion

The aging of composite insulators is not random but follows predictable chemical and physical pathways. The twin challenges of hydrophobicity loss and acid-induced brittle fracture are manageable through a deep understanding of their root causes. By investing in high-quality materials with robust design features and implementing a diligent inspection and maintenance regime focused on early detection, utilities can fully leverage the benefits of composite insulators while ensuring decades of safe and reliable service for the power grid.