Transformer Pressboard Strips (also known as insulation strips or oil duct strips) are core insulation and structural support components used inside the windings of oil-immersed power transformers

1. Introduction to Transformer Pressboard Strips

Pressboard strips are primarily made from 100% high-purity, unbleached insulating sulfate wood pulp. They undergo a high-pressure pre-compression process to achieve exceptional density and stability.
High Mechanical Strength: Exceptional tensile and compressive strength; they will not disintegrate or degrade under high-temperature oil-immersed environments.
Excellent Electrical Properties: Extremely high chemical purity ensures excellent dielectric breakdown resistance and minimal partial discharge (PD) levels.
Stable Oil Absorption: Consistent oil absorption allows seamless integration with transformer oil, optimizing the overall electric field distribution.

2. Core Applications

Pressboard strips function as the “skeleton” within the transformer windings, primarily used for:
Axial Oil Duct Support: Placed axially between winding disks or layers to rigidly maintain uniform oil gaps, ensuring proper convection and heat dissipation of the transformer oil.
Short-Circuit Impact Resistance: Act as rigid supports to prevent coil bending or bulging deformation when a sudden short circuit generates massive electromagnetic forces.
Main Insulation Isolation: Serve as part of the combined solid-liquid insulation structure to block electrical discharge between windings and between the windings and the core.

3. Selection Guide by Voltage Rating

Higher transformer voltage ratings demand stricter requirements for the chemical purity and mechanical compressive strength of the insulation pressboard strips.

Medium & Low Voltage Distribution Transformers (35kV and below)

Selection: Standard rigid insulation pressboard strips cut directly from calendered pressboard.
Features: Typically 2.0mm – 4.0mm thick; the selection focus is mainly on cost-effectiveness and basic oil absorption.

High & Ultra-High Voltage Power Transformers (110kV to 500kV)

Selection: High-density pre-compressed insulation pressboard strips (complying with IEC 60641-3, Grade B.3.1A, such as Weidmann T4 pressboard).
Features: Density must reach 1.15 – 1.30 g/cm³. The edges must undergo precision chamfering and deburring to completely eliminate the risk of corona or point discharge.

Ultra-High Voltage (UHV) & Converter Transformers (750kV, 1000kV and above)

Selection: Ultra-high purity pre-compressed pressboard strips, or electrical laminated pressboard strips made by hot-pressing multiple layers of thin pressboards with insulation resin.
Features: Extreme focus on dielectric constant matching and partial discharge levels. Moisture content must be strictly controlled under 0.5% during factory delivery and assembly.