Lignostone® Transformerwood, pressboard, presspaper and insulating tapes suit different transformer winding and lead insulation requirements. The best material depends on the electrical stress, mechanical forces, operating temperature, insulating liquid, geometry and manufacturing process. Transformer manufacturers should therefore select materials as part of the complete insulation system, rather than choosing a material based on dielectric strength alone.
Transformer winding insulation commonly uses cellulose-based materials such as presspaper, pressboard, crepe paper and laminated insulation, with tapes and other insulating materials used for specific winding and lead applications. The material choice depends on the required dielectric strength, mechanical support, thermal performance, oil compatibility and manufacturing process.
For oil-filled transformers, cellulose-based solid insulation remains widely used because it can provide electrical insulation while also contributing to the mechanical structure of the active part.
Common materials include:
IEC 60641-1 defines general requirements for pressboard and presspaper used for electrical purposes, while IEC 60641-3-1 and IEC 60641-3-2 specify requirements for particular pressboard and presspaper types.
Presspaper, crepe paper and electrical insulation tapes are commonly considered for transformer leads because they can conform to irregular shapes and provide layered insulation. The appropriate material depends on the lead geometry, voltage stress, mechanical restraint, oil environment, temperature and required manufacturing process.
Lead insulation presents a different challenge from flat winding insulation.
A lead may need to pass around corners, connect winding conductors to terminals, or occupy a constrained space. The insulation therefore needs to conform closely to the conductor while maintaining the required dielectric separation.
Insulect supplies several materials relevant to these applications, including crepe paper, presspaper, flexible laminates and electrical insulation tapes. Its tape range includes polyester film, glass cloth, reinforced and other electrical insulation constructions.
For a manufacturer, the practical selection question is not simply "Which material has the highest dielectric strength?" It is whether the material can be applied consistently while maintaining the required insulation thickness, clearances and mechanical integrity.
Pressboard is suitable for many transformer insulation components because it provides electrical insulation together with mechanical support. IEC 60641-3-1 specifies requirements for several electrical-purpose pressboard types made from sulphate wood pulp or combinations of sulphate wood pulp and cotton.
Pressboard is commonly used where insulation also needs to maintain a defined physical geometry.
Typical transformer applications include:
IEC 60641-1 establishes general requirements for pressboard and presspaper, while IEC 60641-3-1 specifies individual pressboard materials.
The material grade matters. Different pressboards can have different mechanical, electrical, moisture and processing characteristics. Manufacturers should therefore specify the relevant grade rather than simply stating "electrical pressboard."
Transformer manufacturers should use densified wood when an insulating component also needs significant mechanical strength. Lignostone® Transformerwood is a densified laminated wood material used for transformer components such as pressure rings, platforms, pressing beams, pressure segments and connection elements.
Röchling's Lignostone® L II/2-E3-HQ datasheet identifies the material as laminated densified wood for oil-filled transformers and electrical insulating components. The published data include a density of 0.95 g/cm³, flexural strength of 110 MPa and compressive strength of 200 MPa perpendicular to the material.
The material also has electrical properties relevant to transformer construction. Röchling's datasheet gives a typical electric strength at 90°C under oil of 17 kV/mm in the perpendicular direction for the specified grade.
These properties explain why densified wood can be useful where the insulation must also withstand mechanical loading.
For example, a pressure ring needs to maintain its geometry while transferring mechanical forces through the transformer clamping structure. A purely flexible insulation material would not provide the same structural function.
Pressboard and densified wood both provide electrical insulation, but they serve different structural roles. Pressboard is commonly selected for barriers, spacers and other insulation structures, while densified wood can provide higher mechanical load-bearing capability in components such as pressure rings, beams and supports.
The distinction becomes important when designing the transformer active part.
Pressboard is generally associated with cellulose-based electrical insulation components. IEC 60641 provides the framework for electrical-purpose pressboard and presspaper.
Densified wood, such as Lignostone®, uses laminated wood veneers impregnated with thermosetting resin and compressed under heat and pressure. Insulect describes Lignostone® as a material for transformer, switchboard and other electrical applications.
The choice therefore depends on the component.
A thin barrier separating winding sections may require different characteristics from a structural pressure ring that transfers mechanical force during short-circuit conditions.
Transformer lead wrapping can use crepe paper, presspaper, polyester film, glass cloth and other electrical insulation tapes depending on the application. Flexible materials are useful because lead insulation often needs to follow bends and irregular conductor geometry without creating excessive gaps or damage during assembly.
Insulect's electrical insulation range includes:
Its insulation tape range specifically identifies applications including transformer coils, stress rings and winding work.
For lead insulation, the application method is as important as the material itself. Wrapping tension, overlap, edge treatment, cleanliness and the number of layers can all influence the finished insulation system.
A material that performs well in laboratory testing can still produce a poor result if it is incorrectly applied or contaminated during manufacturing.
Temperature affects insulation selection because transformer materials must maintain their required electrical and mechanical properties throughout the intended operating range. Manufacturers should consider continuous operating temperature, short-term temperature exposure, drying conditions and compatibility with the transformer's insulating liquid.
Temperature is particularly important because transformer insulation is not a single material.
The solid insulation, insulating liquid, conductor, tapes, adhesives and structural components form a combined system.
For example, Röchling's Lignostone® L II/2-E3-HQ datasheet specifies a continuous operating temperature of 105°C and a drying temperature limit of 140°C for that particular grade.
These figures are material-specific, not universal limits for all Lignostone® grades or transformer insulation materials.
This is why manufacturers should use the relevant technical datasheet for the exact material grade selected and confirm that its thermal characteristics suit the transformer design.
Transformer insulation materials must be compatible with the insulating liquid and the transformer's processing conditions. For oil-filled transformers, manufacturers should consider oil absorption, moisture behaviour, chemical compatibility, drying behaviour and dimensional stability when selecting solid insulation materials.
This is particularly relevant for cellulose-based and densified materials.
Röchling's Lignostone® L II/2-E3-HQ datasheet, for example, reports oil absorption of 25% and moisture content of 5% for the specified material.
These properties can affect manufacturing and service behaviour.
Moisture is especially important in transformer insulation because water can affect dielectric performance and ageing behaviour. Transformer manufacturers therefore need controlled material storage, preparation and drying processes in addition to selecting the correct material.
IEC 60076-1 provides general requirements for power transformers, while IEC 60076-3 addresses insulation requirements and dielectric testing for specific transformer windings and terminals.
IEC 60641 applies specifically to electrical-purpose pressboard and presspaper, while IEC 60763 covers laminated pressboard. IEC 60076-3 addresses transformer insulation requirements and dielectric tests. Together, these standards help manufacturers define material requirements within the broader transformer insulation system.
The key references include:
IEC 60641-1:2007
Defines classifications and general requirements for pressboard and presspaper for electrical purposes.
IEC 60641-3-1:2008
Specifies requirements for individual types of electrical-purpose pressboard.
IEC 60641-3-2:2007
Specifies requirements for individual types of electrical-purpose presspaper.
IEC 60763-1:2010
Defines classifications and general requirements for laminated pressboard.
IEC 60076-3:2013+A1:2018
Specifies transformer insulation requirements and corresponding dielectric tests for specific windings and terminals.
These standards do not provide a single table saying which material is "best" for every transformer component. Material selection remains dependent on the design, application and specified material grade.
Transformer manufacturers should choose insulation by matching each component's electrical, mechanical, thermal and processing requirements. The selection process should consider voltage stress, creepage and clearance, short-circuit forces, operating temperature, insulating liquid, moisture, dimensional requirements and manufacturing tolerances.
A practical material-selection process can start with five questions:
The answers can quickly narrow the material options.
For example, a winding barrier may prioritise dielectric and dimensional characteristics, while a pressure ring may require substantially greater mechanical strength. A lead wrap may instead prioritise flexibility, conformability and consistent application.
This component-by-component approach is more reliable than selecting one insulation material for the entire transformer.
Yes. Transformer insulation materials can be supplied as sheets, tubes, rods, cut components, machined parts and complete insulation kits. Custom fabrication allows manufacturers to specify the material and component geometry together, which can simplify assembly and improve repeatability during transformer production or repair.
Insulect supplies insulation materials in multiple formats and provides CNC machining, VMC machining, cutting, tube rolling, bending, slitting and other fabrication services.
This can be useful when a transformer manufacturer needs components made to engineering drawings rather than standard sheet material.
Insulect's transformer capability also includes custom-manufactured transformer insulation kits, with its project portfolio including the Victorian Big Battery, where Insulect supplied custom transformer insulation kits and measurement and protection devices.
For OEMs, the benefit is practical: material selection, machining and component supply can be considered together rather than managed as completely separate processes.
There is no single best insulation material for every transformer component. Presspaper and pressboard suit many cellulose-based electrical insulation applications, densified wood suits mechanically loaded insulating structures, and flexible tapes and papers suit winding and lead insulation. The correct choice depends on the component's electrical, thermal, mechanical and manufacturing requirements.
A useful starting point is:
| Transformer application | Materials to consider | Key selection factors |
|---|---|---|
| Winding insulation | Presspaper, pressboard, flexible laminates | Dielectric strength, thickness, moisture, thermal performance |
| Layer insulation | Presspaper, pressboard | Electrical separation, dimensional stability, processing |
| Barriers and spacers | Pressboard, laminated pressboard | Dielectric and mechanical requirements |
| Lead insulation | Crepe paper, presspaper, electrical tapes | Flexibility, conformability, dielectric performance |
| Mechanical supports | Lignostone® densified wood, composites | Compressive strength, dimensional stability, insulation |
| Pressure rings | Lignostone® Transformerwood, laminated materials | Mechanical loading, electrical insulation, oil compatibility |
| Custom insulation kits | Combination of materials | Design geometry, assembly and repeatability |
The final specification should be based on the complete transformer design and the technical data for the exact material grade selected.
Choosing the right material is only the first step. Insulect can supply transformer insulation materials, custom-machined components and complete insulation kits to suit transformer manufacturing, repair and refurbishment requirements.
Talk to Insulect about your transformer insulation requirements.