Insulation Winding Materials for Transformer Industries
At ACC Insulations, we manufacture the complete range of transformer winding insulation materials — from turn-to-turn insulation and layer insulation to phase barriers and end-insulation components.
Every winding in a transformer — HT or LT, oil-immersed or dry-type — relies on insulation materials to contain electrical stress between adjacent conductors, manage thermal loads, and prevent partial discharge. Our materials are engineered precisely for these demands using premium cellulose, aramid fibres and high-grade composite agents, ensuring your transformer maintains optimal energy transmission with zero discharge risk.
How Winding Insulation Works Inside a Transformer
To select the right winding insulation material, it helps to understand where insulation is placed and why each location has different requirements. A transformer winding has three distinct insulation zones:
Transformer Winding Cross-Section — Insulation Zones
Each zone demands a different material. Turn-to-turn insulation needs thin, highly flexible material that wraps tightly without air voids. Layer insulation needs higher dielectric strength per unit thickness. Phase barriers and end-cylinders need structural rigidity to withstand short-circuit electromagnetic forces. Our range covers all three.
Material Classifications — Full Product Range
Our transformer winding insulation materials span four primary product families, each engineered for a specific role within the winding insulation system:
Thermally Upgraded Kraft Paper (TUK)
A chemically modified cellulose paper — cyanoethylated or nitrogen-treated — engineered for turn-to-turn and layer insulation in oil-immersed power transformers. Rated Class A (105°C). Dielectric strength 10–12 kV/mm (in oil). Complies with IEC 60641-3-1. Superior thermal ageing resistance extends transformer service life by up to 7× vs standard kraft paper.
View Kraft Paper RangeElectrical Grade Crepe Paper Tubes & Tapes
Mechanically crinkled cellulose paper with high elongation (up to 120%) for insulating transformer lead wires, crossover points, and terminal connections. The crepe structure conforms to irregular shapes without cracking, creating a void-free insulation layer. Used in both oil-type and dry-type transformers. Available as flat tape, tubes, and split-tube sleeves.
View Crepe Paper RangeLaminated Pressboard Spacers & Cylinders
High-density cellulose pressboard (density 1.1–1.25 g/cm³) used for transformer insulation cylinders, winding spacers, end-rings and clamping rings. Provides the structural rigidity needed to withstand electromagnetic short-circuit forces. Complies with IS 1576 and IEC 60763. Custom CNC-machined shapes available.
View Pressboard RangeDMD & NMN Composite Laminates
DMD (Dacron-Mylar-Dacron) is the standard flexible composite for Class F (155°C) dry-type transformer winding insulation — motor slot insulation, layer insulation, phase barriers. NMN (Nomex-Mylar-Nomex) is the premium composite for Class H (180°C) applications including traction transformers and locomotive applications. Dielectric strength 15–20 kV/mm.
View DMD / NMN RangeWhich Winding Insulation Material Do You Need?
Use this decision guide to quickly identify the right material for your transformer design:
Material Selection Guide
Comparative Performance Matrix
This table gives you the actual performance numbers for each material family so your insulation design is based on real data, not approximations:
| Property | TUK Kraft Paper | Crepe Paper | Pressboard | DMD / NMN |
|---|---|---|---|---|
| Dielectric Strength | 10–12 kV/mm (oil) | 8–10 kV/mm (oil) | 8–10 kV/mm (oil) | 15–20 kV/mm |
| Thermal Class (IEC 60085) | Class A (105°C) | Class A (105°C) | Class E (120°C) | Class F/H (155–180°C) |
| Tensile Strength | Moderate | Low (flexible) | Superior | Excellent |
| Oil Compatibility | Excellent | Excellent | Excellent | Not applicable |
| Primary Winding Use | Turn & layer insulation | Lead wire wrapping | Cylinders, spacers | Layer & phase insulation |
| Transformer Type | Oil-immersed | Oil or dry-type | Oil-immersed | Dry-type |
| Standard Reference | IEC 60641-3-1 | IEC 60554 | IS 1576 / IEC 60763 | IEC 60626 / UL |
Precision Slitting & Custom Fabrication
Every transformer design is unique. ACC Insulations operates an in-house precision slitting and fabrication facility so you receive ready-to-use winding insulation components, not raw material rolls that require additional processing.
Parent Roll Inspection
Incoming kraft paper, crepe paper and pressboard rolls are inspected for thickness, moisture content, dielectric strength and surface defects before entering production.
Precision Slitting to Exact Width
Kraft and crepe paper are slit on CNC slitting machines to your exact winding width (±0.1 mm tolerance) with zero burrs. Multiple slit widths are produced from a single parent roll in one setup to reduce material waste.
Die-Cut Pressboard Components
Pressboard spacers, rings, snouts and cylinders are CNC-machined to your CAD drawings. Custom shapes including stepped cylinders, angled spacers and keyway profiles are available.
Quality Inspection Before Dispatch
All slit and fabricated materials undergo dielectric strength testing, dimensional verification and surface quality checks. Test reports available on request.
Why Precision Winding Insulation Matters
The winding is the heart of the transformer. A single void, contamination point or under-thickness location in the insulation creates a site for partial discharge — the first step toward complete insulation failure. ACC Insulations addresses this at three levels:
Partial Discharge Resistance
- Materials processed to eliminate air voids
- Low ionic contaminant content
- Controlled moisture level before supply
- Verified against IEC 60270 PD limits
Optimised Heat Dissipation
- Open-structure papers allow oil circulation
- Controlled density for oil-wicking performance
- Dimensional stability under thermal cycling
- Low thermal resistance per unit thickness
Voltage Stress Management
- Uniform dielectric constant across the sheet
- No pinhole defects or surface contamination
- Consistent breakdown voltage batch to batch
- Adequate creepage distance via pressboard
Short-Circuit Strength
- Pressboard spacers resist compressive collapse
- High tensile strength paper resists radial expansion
- Pre-compressed grades available for tighter builds
- IEC 60076-5 short-circuit withstand compliant
Applications by Transformer Type
Our transformer winding insulation materials are used by transformer manufacturers across India and exported internationally. Here is how each transformer type uses our product range:
HV Power Transformers (33 kV and above)
Require maximum dielectric integrity. Thermally upgraded kraft paper for turn and layer insulation, thick pressboard cylinders between HV and LV windings, and densified laminated wood (DLW) for clamping structures. Materials must pass rigorous oil-impregnation testing per IEC 60076.
Distribution Transformers (11 kV / 33 kV)
High-volume applications where material consistency is critical for automated winding. TUK kraft paper in slit rolls for automated layer winding; crepe tape for lead wrapping; pressboard spacers for cooling duct formation between winding layers.
Dry-Type Transformers (Cast Resin & VPI)
No mineral oil — synthetic composite insulation required. DMD (Class F) for standard industrial applications; NMN (Class H) for elevated duty cycles. Crepe tape used for lead insulation pre-impregnation in VPI (Vacuum Pressure Impregnation) systems.
Traction & Locomotive Transformers
Operate at 180°C under vibration and shock loads. NMN composites are the material of choice — Class H thermal rating plus superior mechanical strength. Aramid-based materials complying with railway-specific OEM standards.
Special & EV Charging Transformers
Compact, high-frequency designs require thin-gauge insulation with superior dielectric strength per micron. Thin kraft papers and DMD laminates in narrow slit widths for high-frequency winding applications in inverter and EV charging infrastructure.
Technical Specifications
Our transformer winding insulation materials meet strict electrical, thermal and mechanical standards across all product families:
Electrical Properties
- Dielectric strength: 8–20 kV/mm (grade-dependent)
- High insulation resistance (>1000 MΩ)
- Low dielectric loss (tanδ <0.005)
- Partial discharge free (void-free structure)
- Uniform field distribution across surface
Thermal Properties
- Class A (105°C) to Class H (180°C)
- Thermal ageing per Arrhenius Law (IEC 60085)
- Stable properties under continuous thermal load
- Low thermal resistance — efficient heat transfer
- Oil-compatible grades for natural / forced convection
Mechanical Properties
- Tensile strength: 40–180 MPa (grade-dependent)
- High compression resistance (pressboard spacers)
- Crepe elongation up to 120% (crepe paper)
- Dimensional stability under thermal cycling
- Short-circuit force resistance (IEC 60076-5)
Material Composition
- Thermally upgraded kraft paper (TUK)
- Electrical grade crepe paper tapes & tubes
- Laminated pressboard (oil-compatible)
- DMD (Dacron-Mylar-Dacron) composite
- NMN (Nomex-Mylar-Nomex) composite
Standards & Compliance
Our transformer winding insulation materials are manufactured and tested to meet the following international and Indian standards. Compliance certificates are available with each order:
Why Transformer Manufacturers Trust ACC Insulations
With more than three decades of experience supplying winding insulation materials to India's transformer industry, here is what sets our supply apart:
30+ Years of Transformer Insulation Expertise
Deep domain knowledge of insulation system design — our team assists with material selection, thickness calculation and IEC compliance for both oil-type and dry-type windings.
In-House Precision Slitting — ±0.1 mm Tolerance
Slit rolls in exact widths for your automated winding equipment. No secondary processing required at your end — saves time and eliminates edge defects from inaccurate slitting.
Dielectric Strength Tested — Every Batch
Every incoming roll and every slit batch is tested for dielectric strength and dimensional accuracy before dispatch. Test certificates issued on request for traceability.
OEM Transformer Manufacturer Approved
Trusted supplier to leading HV power transformer and distribution transformer OEMs across India. Vendor-qualified for customer-specific material specifications.
Fast Turnaround on Custom Orders
Stock of parent rolls across all material grades enables rapid slitting and dispatch of custom-width orders. Pan-India delivery. Export shipments available.
Engineering Tools Suite
Calculate exact dielectric safety margins, oil impregnation rates, and winding clearances for your transformer design using our interactive calculators.
Frequently Asked Questions
Winding insulation prevents short-circuits between adjacent copper turns (turn-to-turn insulation) and between winding layers (layer insulation), manages electrical stress distribution, and ensures energy is transmitted efficiently without internal partial discharge or failure. Any failure leads to internal arcing, localised heating, oil degradation and catastrophic transformer failure. The insulation system is the single most critical determinant of transformer reliability and service life.
Class F insulation is rated for continuous operation at 155°C (IEC 60085) and uses DMD composites — standard for industrial dry-type transformers with moderate thermal loads. Class H is rated at 180°C and uses NMN (Nomex-Mylar-Nomex) composites, required for traction, locomotive, and high-duty-cycle industrial transformers. Choosing the wrong thermal class results in accelerated insulation ageing and premature transformer failure.
Yes. Our thermally upgraded kraft papers, pressboards and crepe papers are specifically engineered for excellent chemical compatibility and oil absorption in mineral transformer oil. When oil-impregnated, these cellulose materials achieve dielectric strengths of 10–12 kV/mm (kraft paper) and 8–10 kV/mm (pressboard), significantly higher than their dry values. They comply with IEC 60641-3-1 and IS 1576 respectively.
Our winding insulation materials comply with IEC 60641-3-1 (insulation paper for oil-immersed transformers), IEC 60085 (thermal classification Class A to Class H), IS 1576 (pressboard for transformers — BIS), IEC 60554 (cellulose paper for electrical use), and IEC 60626 (DMD composite laminates). Compliance certificates are available with each order.
Yes. We operate an in-house precision slitting facility that produces insulation papers and crepe tapes in exact widths (±0.1 mm tolerance) required by your automated winding equipment. Multiple slit widths from a single parent roll, custom roll diameters, and specific core sizes are all available. Burr-free edges and consistent tension ensure no machine jams during high-speed winding.
Standard kraft paper (untreated cellulose) has a continuous thermal rating of approximately 90–98°C. Thermally upgraded kraft paper (TUK paper) is chemically modified — typically by cyanoethylation or nitrogen-compound treatment — which suppresses the thermal degradation mechanism and raises the continuous rating to 105°C (Class A, IEC 60085). This extends transformer life by up to 5–7× compared to standard kraft paper under the same operating temperature. TUK paper also has lower moisture uptake and superior ageing resistance in hot transformer oil.
