What is pressboard?
Pressboard is a technical cellulose-based insulating material that, in the power industry, performs both an insulating and a structural function. It is not ordinary paper or cardboard, but a high-density material designed for long-term operation in an electric field, under mechanical load, and in the presence of transformer oil.
In industrial practice, pressboard is treated as a load-bearing element of the insulation system. It separates live parts, stabilizes windings, and helps control the distribution of the electric field. After oil impregnation, it becomes an integral part of the paper-oil insulation system, rather than merely a structural add-on.
This dual role is what distinguishes pressboard from many other insulating materials used in industry. Pressboard differs fundamentally from technical paper. Paper is thin, flexible, and primarily designed for surface insulation. Pressboard has significantly greater thickness, density, and rigidity. Its structure is optimized for mechanical strength and dimensional stability, not only for electrical properties.
What is pressboard made of?
The basis of pressboard is high-purity wood cellulose. It is most commonly derived from softwood species, as long fibers provide better mechanical strength and greater resistance to degradation.
The cellulose used in pressboard production is not chemically bleached. Preserving the natural fiber structure is crucial for dielectric performance and for the material’s ability to interact with transformer oil.
Fiber length is also important. Long, uniform fibers form a compact and homogeneous structure that carries loads more effectively and ages more slowly. Contaminants, ash, or chemical residues can significantly reduce insulation durability. For this reason, pressboard production requires strict control of raw materials already at the stage of pulp preparation.
Why pressboard is used in the power industry?
The power industry places exceptionally high demands on insulating materials. Pressboard meets these requirements thanks to a combination of good electrical, mechanical, and thermal properties.
As an electrical insulator, pressboard effectively limits current flow and helps shape the electric field in power and distribution transformers. Its structure enables uniform distribution of electrical stresses, reducing the risk of local discharges.
A key feature of pressboard is its ability to be impregnated with transformer oil. Once oil-impregnated, the material is no longer just an insulating barrier. It becomes part of a coherent paper-oil system responsible for both cooling and insulation of the entire transformer.
Pressboard ages in a predictable manner. Degradation occurs gradually and can be indirectly monitored through oil analysis. This allows grid operators to assess insulation condition and plan refurbishments in advance, rather than reacting to failures.
The role of pressboard in transformers
In a transformer, pressboard performs far more functions than insulation alone. It actively contributes to the operating conditions of the entire system. Its presence affects electric field distribution, mechanical stability of windings, and the unit’s resistance to fault conditions.
One of the key roles of pressboard is shaping the electric field. Pressboard components such as spacers, rings, and flanges smooth local voltage concentrations. This reduces the risk of partial discharges, which are one of the main causes of insulation degradation.
Pressboard also stabilizes transformer windings. During normal operation and especially during short circuits, significant electrodynamic forces arise in the windings. The material must withstand these loads for many years without creep or permanent deformation.
Under short-circuit conditions, pressboard acts as a structural element. Its mechanical strength limits winding movement that could otherwise lead to insulation damage or core deformation. This is why pressboard quality has a direct impact on a transformer’s resistance to fault events.
How pressboard is formed in industrial practice
Pressboard forming in the power industry is based on wet processes that allow the material to be shaped precisely before full densification of its structure. Cellulose pulp, containing a large amount of water, is placed on molds and pre-pressed.
In practice, this process is carried out on machines using high frequency (HF), such as PRESA. In this type of equipment, energy is not supplied by surface heating, but through the interaction of a high-frequency electric field with the moist pressboard structure. This results in volumetric heating, meaning a uniform temperature increase throughout the entire cross-section of the formed component.
As a result, high-frequency machines make it possible to combine wet forming with simultaneous pressing and drying in a single technological cycle. The component is compressed and dried at the same time, and moisture is removed from within the material, not only from its surface. This approach significantly reduces internal stresses and improves the dimensional stability of finished parts.
The use of high frequency also enables precise control of process parameters. Heating time, temperature, and final moisture content of the pressboard can be accurately controlled. This is critical in the power industry, where even small deviations in moisture or geometry can affect the dielectric strength of insulation.
Moisture control remains one of the most important aspects of pressboard forming in high-frequency processes. Excessive water content reduces electrical performance and accelerates material aging. Overly aggressive drying, on the other hand, can lead to deformation or micro-cracking. Therefore, the process requires carefully selected settings and continuous technological supervision.
PRESA – a machine for manufacturing insulating flanges for transformers
Pressboard and power system safety
The safety of the power system largely depends on the condition of transformer insulation. In practice, degradation of cellulose insulation is one of the most common causes of high-voltage equipment failures.
Pressboard ages gradually. This process is related to temperature, moisture, and electrical load. Over time, the material loses mechanical strength and its structure weakens.
Importantly, the condition of pressboard often reveals more about a transformer’s health than its calendar age. A unit operated under favorable conditions can work safely for several decades. A transformer that is overloaded or exposed to moisture may degrade much faster.
Pressboard and modern insulating materials
With technological progress, new insulating materials such as aramids and polymer composites have emerged. They are used where higher thermal classes or specific operating conditions are required.
Nevertheless, pressboard still dominates in conventional grid power engineering. It is a proven, predictable material whose aging behavior is well understood. Its long-term performance can be effectively monitored.
Modern materials do not always offer the same level of predictability. They are often more expensive, more difficult to diagnose, and less compatible with existing paper-oil insulation systems.
How pressboard production and processing are evolving
Pressboard production is also evolving. Increasing emphasis is placed on shortening process times, improving repeatability, and reducing energy losses. High-frequency drying enables faster and more uniform moisture removal, allowing the material to reach stable parameters in a shorter time. This is particularly important when producing thicker components.
At the same time, process automation is advancing. Parameter control is increasingly continuous, and quality repeatability is becoming the norm rather than the exception.
Why pressboard will remain a foundation of the power industry
The power industry is a sector where reliability and predictability are essential. Pressboard meets both requirements, as its properties are well known and confirmed by decades of operation. Although new materials and technologies continue to emerge, pressboard remains the reference point. It is a material that combines long-established experience with the potential for further technological development. For these reasons, everything indicates that pressboard will remain a foundation of the power industry for many decades to come.
