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IEC 61628-1: Corrugated Pressboard for Electrical Purposes — Specifications and Applications
IEC 61628-1 is the international standard that specifies the requirements for corrugated pressboard used in electrical equipment, particularly in oil-filled power transformers. Corrugated pressboard is a specialized form of transformerboard (cellulose-based insulation) that has been corrugated to create a fluted structure with alternating ridges and grooves. This geometry provides unique advantages in transformer insulation systems, including improved oil flow guidance, enhanced mechanical stiffness, controlled creepage distance, and optimized heat transfer in windings and lead structures. For transformer design engineers, understanding the specifications of IEC 61628-1 is essential for optimizing the insulation system’s electrical, thermal, and mechanical performance.
Tip: Corrugated pressboard is not a structural substitute for flat pressboard — it serves a different function. The corrugations create oil circulation channels that enhance cooling, whereas flat pressboard provides solid barrier insulation. The two materials are complementary in transformer design.
1. Material Composition and Manufacturing
IEC 61628-1 specifies that corrugated pressboard shall be manufactured from high-grade cellulose fibers derived from unbleached softwood kraft pulp, processed to achieve specific purity and consistency requirements. The cellulose pulp must have a high alpha-cellulose content (minimum 90%) with low hemicellulose, lignin, and ash content to ensure long-term thermal stability and dielectric performance in transformer oil. The fibers are processed using a Fourdrinier or cylinder machine to form a continuous web, which is then pressed and dried under controlled conditions to achieve the required density, thickness, and mechanical properties.
The corrugation process is a critical manufacturing step. The base pressboard, with a typical thickness of 1.0–3.0 mm, is passed through heated corrugating rolls that form the fluted profile while the material is in a controlled plasticized state. The standard specifies that the corrugation pitch (distance between adjacent ridges) shall be 8 mm or 14 mm, with a corrugation height of 3.5 mm or 6.5 mm, depending on the type. The corrugations must be uniform across the full width and length of the board, with no cracks, delamination, or surface defects.
The standard defines three grades of corrugated pressboard based on density and mechanical properties. Grade A is high-density (1.10–1.25 g/cm³) for applications requiring maximum mechanical strength and stiffness. Grade B is medium-density (1.00–1.15 g/cm³) offering a balance of mechanical and oil-flow properties. Grade C is lower-density (0.90–1.05 g/cm³) optimized for maximum oil flow and thermal performance in lightly loaded structures.
| Property | Grade A (High Density) | Grade B (Medium Density) | Grade C (Low Density) |
|---|---|---|---|
| Density (g/cm³) | 1.10 – 1.25 | 1.00 – 1.15 | 0.90 – 1.05 |
| Corrugation pitch (mm) | 8 or 14 | 8 or 14 | 8 or 14 |
| Corrugation height (mm) | 3.5 or 6.5 | 3.5 or 6.5 | 3.5 or 6.5 |
| Tensile strength MD* (MPa) | ≥ 90 | ≥ 75 | ≥ 60 |
| Tensile strength CD* (MPa) | ≥ 50 | ≥ 40 | ≥ 30 |
| Compressive strength (kPa) | ≥ 800 | ≥ 600 | ≥ 400 |
| Moisture content as supplied (%) | 4.0 – 7.0 | 4.0 – 7.0 | 4.0 – 7.0 |
| Ash content (%) | ≤ 1.0 | ≤ 1.0 | ≤ 1.0 |
*MD = Machine Direction, CD = Cross Direction
Warning: Corrugated pressboard is hygroscopic and will absorb moisture from ambient air. The moisture content as supplied (4–7%) is in equilibrium with typical factory humidity. If the material is exposed to high-humidity conditions before transformer assembly, the moisture content can exceed 10%, which will degrade the dielectric strength and increase the drying time required during transformer manufacturing. Store corrugated pressboard in a controlled environment at 40–60% RH.
2. Dielectric and Mechanical Performance
The corrugated geometry serves a dual purpose in transformer insulation. Mechanically, the fluted structure provides exceptional stiffness in the direction perpendicular to the corrugations, allowing the board to be used as a self-supporting spacer and oil-flow director in winding structures. The compressive strength of corrugated pressboard, measured perpendicular to the plane of the board per the standard’s test method, must exceed the minimum values specified for each grade.
Dielectrically, the corrugated profile extends the creepage distance along the surface of the board compared to a flat sheet of the same projected area. This is particularly important in regions of high electrical stress, such as the end insulation of transformer windings and lead exit structures. The standard specifies that the dielectric strength of the pressboard base material must meet the requirements of IEC 60641-3-1 for the corresponding thickness and density, ensuring that the corrugation process does not degrade the intrinsic dielectric properties of the cellulose material.
The oil flow characteristics of corrugated pressboard are a key design parameter. The corrugations form parallel channels that direct oil flow through the winding structure, improving cooling efficiency by ensuring that oil reaches hot spots that would otherwise be stagnant zones. The standard does not directly specify flow characteristics, but the geometry (pitch and height) determines the cross-sectional area available for oil flow, which directly impacts the thermal performance of the transformer.
| Application | Corrugation Pitch | Corrugation Height | Primary Function |
|---|---|---|---|
| Winding axial cooling ducts | 14 mm | 6.5 mm | Oil flow guidance and heat transfer |
| Winding radial spacers | 8 mm | 3.5 mm | Mechanical spacing and partial discharge control |
| Lead support structures | 14 mm | 6.5 mm | Creepage distance extension and mechanical support |
| Shield insulation | 8 mm | 3.5 mm | Combined barrier and oil flow function |
Design Insight: The orientation of corrugations relative to the oil flow direction is critical. Corrugations should be aligned with the primary oil flow direction to minimize flow restriction. When corrugated pressboard is used in winding axial cooling ducts, the corrugations should run vertically (parallel to the winding axis) to allow oil to flow freely through the ducts. Placing corrugations perpendicular to the oil flow can reduce flow rates by 30–50%, severely degrading cooling performance.
3. Quality Assurance and Testing
IEC 61628-1 defines a comprehensive testing program for corrugated pressboard qualification and lot acceptance. Type tests performed on representative samples include: dimensional measurements (corrugation pitch, height, board thickness, width, length), density determination, tensile strength in both machine and cross directions, compressive strength, dielectric strength in oil (per IEC 60243-1), moisture content determination, ash content, and pH and conductivity of aqueous extract.
The standard also specifies routine tests that are performed on each manufacturing lot. These include visual inspection for surface defects, corrugation uniformity verification, dimensional check, and moisture content measurement. The standard provides detailed sampling plans and acceptance criteria based on statistical quality control principles, with lot sizes up to 500 sheets and sample sizes determined by AQL (Acceptable Quality Level) values.
Tip: When receiving corrugated pressboard, immediately inspect the edges for signs of delamination or cracking. The corrugation process creates mechanical stress at the ridge lines, and boards that are too dry (moisture content below 3%) become brittle and prone to cracking at the corrugation peaks. If edge cracks are visible, reject the lot — cracks will propagate during transformer assembly and may lead to partial discharge sites in service.
FAQs
Q1: Can corrugated pressboard be used in dry-type transformers?
A: Corrugated pressboard is designed for oil-immersed transformers where the oil provides both dielectric strength and thermal conductivity. In dry-type transformers (cast resin or vacuum-pressure impregnated), the absence of oil means the corrugations do not serve their primary oil-flow function. Moreover, the cellulose material is not suitable for open-air operation without oil impregnation due to its hygroscopic nature and relatively low surface resistivity in air.
A: Corrugated pressboard is designed for oil-immersed transformers where the oil provides both dielectric strength and thermal conductivity. In dry-type transformers (cast resin or vacuum-pressure impregnated), the absence of oil means the corrugations do not serve their primary oil-flow function. Moreover, the cellulose material is not suitable for open-air operation without oil impregnation due to its hygroscopic nature and relatively low surface resistivity in air.
Q2: What is the maximum operating temperature for corrugated pressboard?
A: Cellulose-based pressboard undergoes thermal degradation above 105°C (220°F) in the presence of oxygen. In a sealed oil-impregnated transformer operating under a nitrogen blanket, the cellulose can withstand continuous operation at 98°C (hot spot temperature) for the rated life of the transformer (typically 20–30 years). Temperature excursions up to 120°C are acceptable for short-term emergency overloads. IEC 61628-1 refers to IEC 60641-1 for thermal classification, which rates cellulose pressboard as Class A (105°C) insulation.
A: Cellulose-based pressboard undergoes thermal degradation above 105°C (220°F) in the presence of oxygen. In a sealed oil-impregnated transformer operating under a nitrogen blanket, the cellulose can withstand continuous operation at 98°C (hot spot temperature) for the rated life of the transformer (typically 20–30 years). Temperature excursions up to 120°C are acceptable for short-term emergency overloads. IEC 61628-1 refers to IEC 60641-1 for thermal classification, which rates cellulose pressboard as Class A (105°C) insulation.
Q3: How does corrugated pressboard compare to creped paper for oil-flow applications?
A: Corrugated pressboard provides more rigid and precisely controlled oil-flow channels compared to creped paper wraps. The corrugated structure maintains its geometry under mechanical clamping pressure, ensuring consistent oil flow paths throughout the transformer’s life. Creped paper, while more flexible and easier to apply to irregular surfaces, can compress and deform under clamping pressure, potentially restricting oil flow. Corrugated pressboard is preferred for main cooling ducts, while creped paper is often used for lead insulation and flexible barrier applications.
A: Corrugated pressboard provides more rigid and precisely controlled oil-flow channels compared to creped paper wraps. The corrugated structure maintains its geometry under mechanical clamping pressure, ensuring consistent oil flow paths throughout the transformer’s life. Creped paper, while more flexible and easier to apply to irregular surfaces, can compress and deform under clamping pressure, potentially restricting oil flow. Corrugated pressboard is preferred for main cooling ducts, while creped paper is often used for lead insulation and flexible barrier applications.
Q4: What is the shelf life of corrugated pressboard?
A: When stored in its original packaging in a controlled environment (temperature 15–30°C, relative humidity 40–60%), corrugated pressboard has an indefinite shelf life. The cellulose fibers do not chemically degrade under normal storage conditions. However, the material will absorb or desorb moisture to reach equilibrium with the storage environment. If stored at high humidity (> 70% RH) for extended periods, the moisture content may exceed 10%, requiring extended drying before use. The drying process can cause slight shrinkage and distortion, which should be accounted for in precision applications.
A: When stored in its original packaging in a controlled environment (temperature 15–30°C, relative humidity 40–60%), corrugated pressboard has an indefinite shelf life. The cellulose fibers do not chemically degrade under normal storage conditions. However, the material will absorb or desorb moisture to reach equilibrium with the storage environment. If stored at high humidity (> 70% RH) for extended periods, the moisture content may exceed 10%, requiring extended drying before use. The drying process can cause slight shrinkage and distortion, which should be accounted for in precision applications.
