How to Select Kraft Paper for Oil-Immersed Transformers

The selection of Kraft paper for oil-immersed transformers is a core engineering decision based on the transformer's ...

The selection of Kraft paper for oil-immersed transformers is a core engineering decision based on the transformer’s voltage class, operating temperature, and mechanical stress, and must comply with relevant IEC or GB/DL/T standards.

The selection process is not arbitrary but follows engineering principles. The key steps and considerations are outlined below.

1. Establish Core Selection Principles

The fundamental goal is to ensure that the properties of the insulating paper match the actual requirements of the transformer. This is based on three key factors:

Voltage Class: This is the decisive factor. The higher the transformer’s voltage, the higher the requirements for the electrical strength (breakdown voltage) and thickness of the insulating paper. For example, a 500 kV class transformer requires specialized turn-to-turn insulation paper.

Operating Temperature: This determines the required thermal class (temperature index) of the insulating paper. The hot-spot temperature of the transformer directly affects the thermal aging rate of the paper, thus determining the transformer’s service life.

Mechanical Stress: For large, high-capacity transformers, the tensile strength and tear resistance of the insulating paper are crucial to withstand the immense electromagnetic forces and mechanical vibrations generated during operation.

2. Select Based on Standards and Material Classification

Adhering to standards is fundamental to the selection process. Currently, the following authoritative standards are available:

IEC 60554-3-3: Specification for cellulosic papers for electrical purposes.

DL/T 1806-2018: Chinese industry standard for Guidelines for selection of pressboard and insulating components for oil-immersed power transformers, applicable to voltage classes from 110(66)kV to 750kV.

T/CEC 202-2019: Chinese association standard for Selection guidelines for creped insulating paper for oil-immersed power transformers, specifically for standardizing the selection of creped insulating paper.

Based on material characteristics, the two main types of Kraft paper are:

3. Verify Performance Specifications and Testing

After the initial selection, specific performance parameters must be checked, and the product must pass relevant tests. According to standards, insulating paper must meet a series of technical requirements.

Electrical Properties

Dielectric Strength: A key indicator. For example, IEC 60641 requires a dielectric strength of at least 20 kV/mm for insulating paper.

Volume Resistivity: Typically required to be above 10¹³ ohms to ensure high insulation performance.

Physical and Mechanical Properties

Tensile Strength: An important parameter for evaluating mechanical robustness.

Thickness and Thickness Tolerance: Directly affects withstand voltage capability and winding geometry; must be strictly controlled.

Elongation (for creped paper): Due to its elasticity, creped paper is suitable for wrapping irregular surfaces like leads; its elongation is a key parameter.

Chemical and Thermal Properties

Oil Compatibility: Must remain chemically stable in transformer oil.

Moisture Content: Kraft paper is highly hygroscopic; its electrical properties degrade sharply when moist. Therefore, moisture content must be strictly controlled according to standards such as IEC 60422.

Summary

In summary, the selection process is driven by operating conditions, which determine the material grade, and then validated by standards. For over 80% of standard applications, standard A-class Kraft paper meeting IEC or GB standards is the most cost-effective choice. For transformers requiring high reliability, long life, or compact design, thermally upgraded paper should be prioritized. The final selection can be guided by consulting the equipment manual or professional insulation material suppliers.