IEC TR 62039: Selection of Polymeric Materials for Outdoor Use Under HV Stress

Introduction

IEC TR 62039:2007 is a Technical Report providing comprehensive guidance on the selection and qualification of polymeric materials for outdoor high-voltage insulation applications. Unlike porcelain or glass, polymeric insulating materials are susceptible to surface degradation mechanisms including tracking, erosion, UV embrittlement, and loss of hydrophobicity. This document synthesizes decades of field experience and laboratory research into a practical framework for material selection, covering silicone rubbers (HTV, LSR, RTV), ethylene-propylene rubbers (EPDM/EPR), cycloaliphatic epoxies, and their filled compounds.

The report addresses four critical performance axes: tracking and erosion resistance, weathering and UV durability, hydrophobicity and its recovery, and compatibility with filling materials (particularly alumina trihydrate, or ATH). This article examines each axis in detail, with practical recommendations for engineers designing composite insulators, cable terminations, surge arresters, and polymer-housed HV equipment.

1. Tracking and Erosion Resistance

1.1 The Tracking Mechanism

Tracking is the formation of conductive carbonized paths on the surface of an organic insulating material. It occurs when leakage current across a contaminated wet surface causes localized drying, forming a dry band across which arcing occurs. The heat of the arc carbonizes the polymer, creating a permanent conductive path that grows incrementally with each arcing event, ultimately leading to flashover.

1.2 Test Methods for Tracking and Erosion

IEC TR 62039 references several test methods for evaluating tracking and erosion resistance:

• Inclined Plane Test (IEC 60587): A contaminant solution (typically 0.1% NH₄Cl with wetting agent) flows down an inclined test specimen while HV is applied. The time to tracking failure or the erosion depth after a fixed test duration is recorded. Materials are classified into 1A (highest tracking resistance) through 1C.
• Rotating Wheel Dip Test (IEC 62217 / ANSI C29.13): Full-size insulator sheds are alternately dipped in saline solution and exposed to HV in a rotating fixture, simulating accelerated aging. The test duration is typically 1000-5000 hours.
• 1000 h Salt Fog Test (IEC 62217): The insulator is exposed to a salt fog environment under continuous HV for 1000 hours, with leakage current monitored throughout.

2. Hydrophobicity and Hydrophobicity Transfer

2.1 The Hydrophobicity Advantage

The defining advantage of silicone rubber over other polymeric materials is its ability to maintain and recover hydrophobicity. When a silicone surface is contaminated by salt, dust, or industrial pollutants, low-molecular-weight (LMW) silicone oil chains diffuse from the bulk material to the surface, encapsulating the contaminant particles and restoring the water-repellent property. This “hydrophobicity transfer” mechanism effectively prevents the formation of continuous water films and suppresses leakage current.

2.2 Classification and Testing

IEC TR 62039 references the hydrophobicity classification (HC) system, ranging from HC1 (completely hydrophobic, water contact angle > 110°) to HC7 (completely hydrophilic, continuous water film). The standard recommends that materials for outdoor HV use should maintain HC1-HC3 under clean conditions and recover to HC3-HC5 within 24-72 hours after contamination and wetting.

3. Weathering and UV Durability

UV radiation from sunlight, particularly in the 290-400 nm band, causes chain scission and cross-linking in polymer materials, leading to surface cracking, chalking, and loss of mechanical strength. IEC TR 62039 recommends the following approach for weathering qualification:

• Accelerated UV exposure: 2000-5000 hours per ISO 4892-2 (Xenon-arc) or ISO 4892-3 (Fluorescent UV). The test cycle includes UV exposure, water spray, and condensation phases.
• Performance criteria: After exposure, the material must retain ≥ 70% of its initial tensile strength and ≥ 50% of its initial elongation. Surface crazing deeper than 0.1 mm constitutes a failure.
• Carbon black stabilization: For black-colored compounds, 2-3% well-dispersed carbon black provides the most effective UV stabilization. For colored housings (grey, brown), use hindered amine light stabilizers (HALS) at 0.5-2.0% by mass combined with UV absorbers (benzotriazoles or benzophenones).

4. ATH Filler Technology

Alumina trihydrate (Al(OH)₃, or ATH) is the most important filler for tracking-resistant polymeric insulation. It functions through endothermic decomposition at approximately 220 °C: ATH releases water of crystallization, absorbing heat and cooling the surface, while the water vapor dilutes flammable gases and suppresses carbonization. Key design parameters include:

• Particle size: Median particle diameter (D50) of 1-5 µm provides optimal dispersion. Particles larger than 10 µm act as stress concentrators.
• Filler loading: 35-50% by mass (15-25% by volume) is typical. Above 50%, the compound becomes difficult to process and mechanical properties degrade.
• Surface treatment: Silane coupling agents (e.g., vinyl silane or amino silane at 0.5-1.0% by mass of filler) improve filler-polymer bonding, enhancing both tracking resistance and mechanical strength.

5. Engineering Design Insights

• Housing vs. shed material: For composite insulators, the housing (sheath) and sheds may use the same or different formulations. A typical strategy uses a higher-ATH-content formulation (45-50%) for sheds that face the highest tracking stress, and a more flexible, lower-ATH formulation (35-40%) for the housing to improve interface sealing with the fiberglass core.
• Corona resistance: Silicone rubber has inherently better corona resistance than EPDM due to its inorganic backbone (Si-O-Si). In applications with known corona activity (e.g., compact transmission lines), specify HTV silicone regardless of cost.
• Brittle fracture prevention: The housing material must be sufficiently elastic to accommodate the difference in thermal expansion between the polymer housing and the fiberglass-reinforced plastic (FRP) core. A minimum elongation at break of 200% is recommended for housing compounds.
• Interface quality: The bond between the polymer housing and the FRP core is the most common failure site in composite insulators. Material selection must consider adhesion characteristics — use a primer system validated for the specific polymer-core combination.

6. Frequently Asked Questions