Thermal conductivity analysis of transformer insulating pressboard is essential for evaluating heat dissipation performance, predicting winding hot-spot temperatures, and extending the service life of oil-immersed power transformers. This technical article explains the thermal properties of cellulose-based pressboard, the applicable international standards, laboratory testing methods, and practical applications in transformer design and quality control.
Technical Overview
Transformer insulating pressboard is a dense, lignin-free cellulose material manufactured from unbleached sulfate (kraft) pulp. It is widely used in oil-immersed power and distribution transformers as winding spacers, axial and radial sticks, oil duct barriers, angle rings, end-ring insulation, and core insulation components. Because these components are located directly in the heat-flow path between the windings and the cooling oil, the thermal conductivity of the pressboard has a direct influence on winding temperature gradients and the hot-spot temperature of the transformer.
Cellulose pressboard is a relatively poor thermal conductor. Typical through-thickness thermal conductivity values range from approximately 0.10 to 0.16 W/(m·K) for dry board and from approximately 0.17 to 0.25 W/(m·K) for oil-impregnated board. Several material and environmental factors significantly affect the measured values, including:
- Density (apparent density typically 0.9 to 1.3 g/cm³, with higher density generally producing higher thermal conductivity)
- Moisture content (absorbed water increases thermal conductivity but degrades dielectric performance)
- Oil impregnation state (oil-filled pores conduct heat far better than air-filled pores)
- Fiber orientation and lamination structure, which create anisotropy between the in-plane and through-thickness directions
- Operating temperature and thermal aging, including the degree of polymerization (DP) of the cellulose
Accurate thermal conductivity data is therefore a critical input for thermal-hydraulic winding models, numerical heat-transfer simulations, and insulation aging-life assessments. Thermal analysis is typically performed by accredited third-party laboratories in accordance with recognized international standards such as IEC 60641 for pressboard and presspaper, IEC 60076-7 for transformer loading and hot-spot calculation, and ISO or ASTM methods for the thermal measurements themselves.
Testing Samples
Standard Pressboard Sheets and Plates
Samples are cut from production sheets or plates with thicknesses commonly ranging from 0.5 mm to 8 mm. Depending on the selected test method, specimens may be prepared as circular discs (typically 12.7 mm or 25.4 mm in diameter for laser flash analysis), square plates (50 mm to 150 mm per side for heat flow meter testing), or rectangular bars for in-plane measurements. Specimens must be flat, free of delamination, and representative of the machine and cross-machine directions of the board.
Thick Laminated Pressboard Components
Thick laminated blocks, molded components, and glued laminated board (for example, components bonded with polyester or casein adhesives) are tested as supplied or as sectioned specimens. The adhesive layers introduce additional thermal resistance, so through-thickness measurements on laminated assemblies reflect the effective thermal conductivity of the complete stack rather than the base cellulose material alone.
Oil-Impregnated Specimens
To simulate the actual in-service condition, specimens may be dried under vacuum and then impregnated with transformer mineral oil or ester fluid in accordance with the impregnation procedures referenced in IEC 60641 and IEC 60642. Oil-impregnated specimens must be handled and transported in sealed containers to prevent moisture re-absorption, and testing should be completed within a defined time window after impregnation.
Conditioned Reference Specimens
For comparative material qualification, specimens are conditioned in a standard atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity per ISO 291 or ASTM D618 prior to testing. Parallel measurements on dry, conditioned, and impregnated states allow laboratories to quantify the moisture and oil sensitivity of the material.
Testing Items
A comprehensive thermal characterization program for transformer insulating pressboard typically includes the following parameters:
- Thermal conductivity, λ, expressed in W/(m·K), measured in both the through-thickness and in-plane directions
- Thermal diffusivity, α, expressed in mm²/s, which characterizes transient heat propagation
- Specific heat capacity, cp, expressed in J/(g·K), determined as a function of temperature
- Apparent density, ρ, in g/cm³, required to verify the internal consistency of λ = α · ρ · cp
- Thermal anisotropy ratio between in-plane and through-thickness conductivity
- Temperature-dependent thermal property curves across the service range, typically from ambient temperature up to 120 °C to 180 °C
- Comparative evaluation of dry, moisture-conditioned, and oil-impregnated states
Optional supporting tests, such as moisture content determination, oil content, degree of polymerization, and tensile or compressive properties, are frequently combined with the thermal measurements to provide a complete material profile.
Testing Methods
Guarded Hot Plate Method (ASTM C177 / ISO 8302)
The guarded hot plate method is an absolute steady-state technique and the primary reference method for low-conductivity materials. A heated main plate surrounded by guard rings establishes one-dimensional heat flow through two symmetrically placed specimens toward cooled plates. It offers the highest accuracy for insulating board but requires large, flat specimens and long stabilization times.
Heat Flow Meter Method (ASTM C518 / ISO 8301)
The heat flow meter method is a comparative steady-state technique using calibrated heat flux transducers. It is faster than the guarded hot plate method and well suited for routine quality control of pressboard sheets. Calibration is performed against certified reference materials traceable to national metrology institutes, and the method is applicable over the typical conductivity range of dry and impregnated pressboard.
Transient Plane Source Method (ISO 22007-2)
The transient plane source technique uses a spiral nickel sensor sandwiched between two pieces of the specimen. A controlled electrical pulse heats the sensor while its resistance response records the transient temperature rise, from which thermal conductivity and diffusivity are calculated simultaneously. It requires only modest specimen sizes and can measure both directions of anisotropic board, making it a practical choice for engineering data on transformer insulation materials.
Laser Flash Analysis (ASTM E1461 / ISO 22007-4)
Laser flash analysis measures thermal diffusivity by applying a short laser pulse to the front face of a disc specimen and recording the temperature response on the rear face with an infrared detector. It requires small specimens, covers a wide temperature range, and is ideal for generating temperature-dependent diffusivity curves. Thermal conductivity is then calculated from diffusivity, density, and specific heat.
Specific Heat Capacity by Differential Scanning Calorimetry (ASTM E1269 / ISO 11357-4)
Specific heat capacity is measured by DSC using the sapphire reference method. The results feed into the calculation of thermal conductivity from laser flash data and into transient thermal models of transformer windings.
Measurement Quality and Traceability
All methods are executed with calibrated instrumentation, defined uncertainty budgets, and, where applicable, verification against certified reference materials. Test reports issued by accredited third-party laboratories state the specimen state (dry, conditioned, or oil-impregnated), test direction, temperature, method, and measurement uncertainty so that the data can be used directly in transformer thermal design.
Applications
Thermal conductivity data for insulating pressboard supports a broad range of engineering and quality activities throughout the transformer life cycle:
- Calculation of winding hot-spot temperature and temperature-rise distribution in accordance with IEC 60076-7 loading guides
- Input material data for computational fluid dynamics (CFD) and finite-element thermal-hydraulic models of oil-immersed windings
- Dimensional optimization of oil duct spacers, barriers, and winding supports to balance dielectric clearances with cooling requirements
- Incoming material qualification and supplier comparison for pressboard manufacturers and transformer OEMs
- Thermal aging and insulation life studies, where accurate thermal data supports kinetic models referenced in IEC 60216
- Development of advanced materials such as aramid-pressboard laminates, nanocellulose composites, and thermally enhanced insulation systems
- Failure analysis and forensic investigation of decommissioned transformers, including assessment of thermal aging and moisture effects
In high-power transformers, even a modest improvement in the effective thermal conductivity of the solid insulation can reduce the hot-spot temperature and significantly extend insulation life, since the aging rate of cellulose roughly doubles for every 6 K increase in temperature.
FAQ
What is the typical thermal conductivity of transformer insulating pressboard?
Dry pressboard generally measures about 0.10 to 0.16 W/(m·K) in the through-thickness direction, while oil-impregnated pressboard typically ranges from 0.17 to 0.25 W/(m·K). In-plane values are usually 10% to 30% higher due to fiber orientation.
Why does oil impregnation change the thermal conductivity?
Pressboard contains a porous network of inter-fiber voids. When these voids are filled with transformer oil, which conducts heat considerably better than air, the effective thermal conductivity of the composite material increases substantially.
How does moisture affect the results?
Absorbed moisture raises the apparent thermal conductivity but simultaneously degrades dielectric and mechanical performance. For this reason, laboratories report the conditioning state and often perform tests on dry, conditioned, and impregnated specimens to bracket the real service behavior.
What specimen sizes and quantities are required?
Requirements depend on the method: laser flash analysis uses small discs of roughly 12.7 mm diameter, transient plane source testing requires two matching pieces approximately 40 mm square, and heat flow meter testing requires plates of 100 mm to 150 mm square. Duplicate or triplicate specimens are recommended to assess material homogeneity.
Over what temperature range can testing be performed?
Most engineering data is generated between ambient temperature and 180 °C, which covers normal and overload operating conditions of oil-immersed transformers. Sub-ambient measurements can also be arranged for special applications.
How are the results used in transformer design?
The measured conductivity, diffusivity, and specific heat values are entered into thermal network or CFD models to predict winding gradients and hot-spot temperatures, verify compliance with temperature-rise limits, and support loading-guide calculations and insulation aging estimations.