Insulating Kraft Paper for Oil-Immersed Transformers

Functions and Selection of Insulating Kraft Paper in Oil-Immersed Transformers In oil-immersed transformers, insulat...

Functions and Selection of Insulating Kraft Paper in Oil-Immersed Transformers

In oil-immersed transformers, insulating kraft paper is one of the most fundamental and critical solid insulation materials.Together with transformer oil, it forms the oil–paper composite insulation system,which plays a decisive role in transformer safety and service life.

1. Functions of Insulating Kraft Paper in Oil-Immersed Transformers

Electrical Insulation (Primary Function)

Insulating kraft paper features high dielectric strength and is widely used for:

Turn-to-turn insulation
Layer-to-layer insulation
Main insulation between windings and the core or clamping structures

When impregnated with transformer oil, its dielectric strength is significantly enhanced,forming a stable and reliable oil–paper insulation system.

Mechanical Support and Structural Stability

During winding, assembly, and operation, insulating kraft paper:
Withstands electromagnetic forces and short-circuit stresses
Prevents winding deformation and displacement
High mechanical strength paper improves the transformer’s short-circuit withstand capability.

Thermal Stability and Life-Determining Factor

The aging rate of insulating paper directly determines transformer service life.
Typical long-term operating temperature in oil: 90–105°C
The decrease in degree of polymerization (DP) is the key indicator of insulation aging
Transformer lifetime is essentially the lifetime of the insulating paper.

Oil Absorption and Degassing Performance

Insulating kraft paper has excellent:
Oil absorption capability
Gas release performance
This facilitates vacuum drying, eliminates air bubbles, and reduces the risk of partial discharge.

Compatibility with Transformer Oil

High-quality insulating paper:
Does not release impurities
Does not accelerate oil aging

This ensures long-term stability of the oil–paper insulation system.

2. Selection Criteria for Insulating Kraft Paper in Oil-Immersed Transformers

Selection by Voltage Class

Transformer Voltage Level	Recommended Selection
≤10 kV	Standard electrical insulating kraft paper
35 kV	High-purity, high-DP insulating kraft paper
≥110 kV	High-density insulating kraft paper or composite insulation systems
UHV	Combined insulating paper and pressboard insulation design

Selection by Thickness (Typical Grades)

Thickness (mm)	Application
0.08 – 0.13	Turn-to-turn insulation
0.15 – 0.25	Layer insulation
0.30 – 0.50	Main insulation, lead wrapping
≥1.0	Insulating pressboard (structural components)

Key Technical Parameters (Critical Requirements)

Parameter	Typical Requirement
Degree of Polymerization (DP)	≥1000 (≥1200 for high-end applications)
Dielectric strength	≥8 kV/mm (oil-impregnated)
Moisture content	≤6% (before drying)
Ash content	≤0.5%
Tensile strength	Above national/international standards
pH value	Neutral or slightly acidic

Use of Modified or Advanced Insulating Papers

Type	Characteristics	Typical Applications
Standard kraft paper	Cost-effective	Distribution transformers
High-density insulating paper	Superior dielectric performance	High-voltage transformers
Thermally Upgraded Paper (TUP)	Service life extended by ~2×	Large power transformers
Aramid composite paper	High thermal class (130–180°C)	Special-purpose transformers

Compatibility with Transformer Oil Types

Mineral oil / naphthenic oil: standard insulating kraft paper
Natural ester oil / synthetic ester oil: thermally upgraded or modified insulating paper is recommended

3. Engineering-Based Selection Summary

Distribution transformers：Standard insulating kraft paper + mineral oil
Power transformers (110 kV and above)：High-DP, high-density insulating paper with strict moisture control
High-temperature or long-life designs：Thermally upgraded insulating paper (TUP) or composite insulation systems

4. Conclusion

Insulating kraft paper is the core material of the oil–paper insulation system in oil-immersed transformers.Its electrical performance, thermal aging behavior, and mechanical strength directly determine transformer reliability and service life. Proper selection of high-quality insulating kraft paper is essential for improving transformer safety and extending operational lifetime.