Transformer Insulation Materials — Laminates, Tubes, and Coil Forms

Transformer insulation must maintain dielectric integrity across thousands of thermal cycles, resist oil and moisture exposure, and hold dimensional tolerances that keep winding geometry consistent — thermoset laminates in phenolic, glass-epoxy, and glass-silicone grades fulfill these requirements from distribution transformers to high-power utility equipment.

TL;DR — Key Takeaways

• The dominant solid insulation material in oil-filled power transformers is kraft paper (not rigid laminate) — rigid thermoset is used for mechanical spacers, coil forms, clamping structures, and terminals
• G10 (glass-epoxy) is the most widely used rigid thermoset for HV dry-type transformers — best electrical properties, low moisture absorption, good machinability
• For insulation class ratings above 130°C, G11 (glass-epoxy, Tg ~170°C) or G7 (glass-silicone, 220°C) replace standard G10
• Cotton-phenolic and linen-phenolic are used in lower-voltage distribution transformers as spacers and support rings where cost drives selection
• GPO-3 is used for arc barriers and end caps in some dry-type transformer designs

Insulation Classes and Material Selection

IEEE C57.12 and IEC 60076 define transformer insulation classes by maximum hot-spot temperature. Selecting thermoset laminate for transformer service begins with the insulation class:

Key Transformer Insulation Components

Coil Forms and Winding Tubes

Coil forms provide the structural core around which transformer windings are wound. Requirements:

• Precise ID tolerance (winding mandrel must fit consistently)
• High dielectric strength (the coil form is between primary and secondary winding potential)
• Low moisture absorption (prevents winding insulation degradation in humid storage)
• Mechanical strength to resist compressive load from wound coils

Best material: Filament-wound G10 tube (for Class B and below) or G11 tube (for Class F). Tube ID tolerances of ± 0.010″ are standard; ± 0.005″ achievable with precision bore.

For high-temperature applications (Class H), G7 (glass-silicone) coil forms are used — available as filament-wound tube in standard sizes.

See the coil form materials guide for detailed coil form design considerations.

Winding Spacers and Interleaf Barriers

Between winding sections (layer-to-layer, disc-to-disc, or HV-to-LV barriers), rigid spacers maintain clearance for oil or air cooling paths and provide supplementary electrical insulation. Requirements:

• Precision thickness (± 0.005″ or tighter to ensure consistent cooling channels)
• High DS through-thickness (the spacer is stressed from one winding section to the next)
• Oil compatibility (for oil-filled transformers: kraft paper or phenolic; for dry-type: G10)

Best material: G10 sheet cut to bar or plate form for dry-type transformers. In oil-filled units, pressboard (oil-impregnated kraft) is standard for interleaf, with G10 used for mechanical clamping plates.

End Rings and Clamping Plates

The transformer core is clamped between end frames using plates that simultaneously compress the core and provide electrical insulation. Requirements:

• High compressive strength flatwise (to apply uniform clamping pressure without creep)
• Electrical isolation from core-to-ground
• Bolt pattern holes machined precisely

Best material: G10 or G11 sheet (0.250″–1.000″ thick) machined to final end-ring form. For budget transformers, cotton-phenolic (CE) is used for Class A applications.

Terminal Barriers and Bushings

HV terminals exit through the transformer tank or enclosure through bushing insulation. The bushing itself may be porcelain, glass, or epoxy-resin cast — but the flanging, barrier board, and internal bushing tube are often thermoset laminate.

Best material: G10 rod (machined bushing bodies) and G10 sheet (barrier boards around HV terminals). For UL 94 V-0 requirements in dry-type transformers: FR4 sheet.

Oil-Filled vs. Dry-Type Transformer Insulation

Moisture and Chemical Compatibility for Transformer Service

Oil-Filled Transformers

Mineral transformer oil and natural ester fluids (FR3, BIOTEMP) are the two main transformer oil types. Thermoset compatibility:

Phenolic laminates can absorb trace moisture from transformer oil over time — oil quality monitoring (moisture content per IEC 60422) is important when phenolic spacers are used in oil-filled units.

Dry-Type Transformers

Dry-type transformers operate in air, subject to ambient humidity. G10 and G11 laminates are preferred for their low moisture absorption (< 0.15% vs. 0.5–1.3% for phenolic). Phenolic grades are acceptable in ventilated, climate-controlled indoor environments (Class A service).

Thickness and Size Selection for Transformer Components

More related guides

Materials

• G10 Material Hub
• Phenolic-Glass-Silicone (G7) Material Hub

Applications

• Bus Bar Insulation Materials
• Motor Insulation Materials
• Switchgear Insulation Materials