How to Select Pressboard Strips for Transformer Winding Support & Cooling

Pressboard Strips in Oil-Immersed Transformers In oil-immersed transformers, Pressboard Strips serve as the core com...

Pressboard Strips in Oil-Immersed Transformers

In oil-immersed transformers, Pressboard Strips serve as the core components of the winding structure, often referred to as the “skeleton” of the coils. They provide critical mechanical support and directly determine the cooling efficiency of the transformer.

1. Key Application Scenarios

Strips are typically arranged longitudinally (axially) along the circumference of the winding, fulfilling two primary functions:
Formation of Cooling Oil Ducts: By creating physical gaps between winding layers or between the winding and the insulation cylinder, strips facilitate vertical oil flow. This allows the transformer oil to dissipate heat through natural convection, effectively suppressing “hot spot” temperatures.
Radial Mechanical Support (Short-circuit Resistance): During a sudden external short-circuit, windings are subjected to massive radial electromagnetic forces (inward compression or outward expansion). As rigid supports, the strips must ensure the conductors remain stable, unyielding, and free from displacement.

2. Key Selection Criteria

The industry standard globally is high-density Pre-compressed Pressboard (Grade T4 / Type B.3.1) according to IEC 60641-3-1.

3. Common Physical Profiles

Rectangular Strips: Standard flat bars, easy to process, used for general gap spacing and support.
Dovetail Strips: Feature a dovetail-shaped groove on one side.
Advantage: They can be precisely locked into the longitudinal slots of the insulation cylinder. This ensures absolute positional stability during the winding process and long-term vibration, maintaining uniform oil duct width—a preferred choice for medium-to-high voltage transformers.

4. Technical Recommendations

Edge Beveling: High-quality strips should have rounded or beveled edges to reduce oil flow resistance and prevent electric field concentration at sharp corners.
Vacuum Drying: Since pressboard is highly hygroscopic, the final integrity of the insulation system depends on a rigorous vacuum-drying process after assembly.