How to Select Crepe Paper for Oil-Immersed Transformers

Time: 11-10 2025 Auther: Xujue Electrical

I. Definition and Characteristics of Crepe Paper Crepe paper is a flexible insulating material made from high-purity...

I. Definition and Characteristics of Crepe Paper

Crepe paper is a flexible insulating material made from high-purity electrical insulation paper that has been mechanically creped.
Its main characteristics include:

High flexibility and elasticity — closely adheres to conductor surfaces;

Excellent oil absorption and high dielectric strength when impregnated with oil;

Good oil and heat resistance with stable insulation performance;

Ability to relieve mechanical stress caused by thermal expansion and contraction.

Therefore, crepe paper is one of the most widely used wrapping insulation materials in oil-immersed transformers.

insulation creper paper

II. Main Functions of Crepe Paper in Oil-Immersed Transformers

Function Category	Specific Role	Description
1. Conductor Wrapping Insulation	Provides the primary insulation layer around conductors	Replaces smooth paper; more flexible and less prone to cracking
2. Turn-to-Turn / Layer-to-Layer Insulation	Increases dielectric distance between wires	Improves insulation and voltage withstand performance
3. Lead and End Insulation	Provides mechanical protection and oil resistance	Prevents partial discharge and aging at winding ends
4. Electric Field Smoothing	Balances the electric field at winding ends	Enhances local insulation safety margin
5. Oil Flow Path Formation	Crepe structure promotes oil circulation and heat dissipation	Improves cooling and oil impregnation efficiency

In summary:Crepe paper = flexible insulation + oil channel + electric-field buffer.

III. Key Parameters for Crepe Paper Selection

Parameter	Technical Requirement	Description
Base Paper	100% high-grade unbleached kraft electrical paper	Ensures purity, oil resistance, and dielectric stability
Basis Weight (g/m²)	60–120 (common range)	Thicker paper offers greater mechanical strength
Crepe Stretch Ratio	10–35%	Determines flexibility and wrapping conformity
Oil Absorption	≥300%	Ensures uniform oil impregnation
Dielectric Strength (in oil)	≥35–40 kV/mm	For high-voltage applications, ≥40 kV/mm is recommended
Thermal Class	Class A (105 °C) standard; up to Class H (180 °C) for special grades	Choose according to operating temperature
Appearance	Free from spots, holes, or cracks	Guarantees uniform insulation quality

IV. Selection Recommendations by Transformer Section

Application Area	Recommended Type	Basis Weight (g/m²)	Stretch Ratio	Key Features
Low-Voltage Winding Wrapping	Fine Crepe Paper	60–70	10–20%	Soft, flexible, prevents sharp bends
High-Voltage Layer / Turn Insulation	Medium Crepe Paper	80–90	15–25%	Balanced dielectric and mechanical strength
Leads & End Reinforcement	Heavy Crepe Paper or Aromatic Modified Crepe Paper	100–130	25–35%	Excellent mechanical protection and heat resistance
EHV Transformer End Sections	Polyimide Composite Crepe Paper	≥100	20–25%	Thermal endurance 120–155 °C for critical zones

V. Selection Reference by Voltage Level

Voltage Level	Recommended Paper Type	Thermal Class	Typical Application
≤35 kV	Standard Class A Crepe Paper	105 °C	Conventional oil-immersed distribution transformers
66–110 kV	Modified Class A Crepe Paper	105–120 °C	High-voltage windings
220 kV	Aromatic Modified Crepe Paper	120 °C	Lead wrapping and end insulation
≥500 kV	PI/PTFE Composite Crepe Paper	≥155 °C	Extra-high-voltage and extreme-temperature zones

VI. Quality and Testing Standards

Applicable Standards:

GB/T 5643 – Electrical Crepe Paper

IEC 60554 – Cellulosic Papers for Electrical Purposes

Test Item	Standard Requirement
Moisture Content	≤7%
Tensile Strength	≥40 N/15 mm
Oil Absorption	≥300%
Dielectric Strength (in oil)	≥35 kV/mm
Crepe Uniformity	No visible variation in thickness
Powdering (surface dust)	≤0.05 g/m²
Thermal Aging (120 °C × 96 h)	Property degradation ≤10%

VII. Storage and Handling Guidelines

Storage: Keep in a dry, ventilated environment, humidity ≤60%, avoid direct sunlight.

Before Use: Balance moisture for at least 24 h in the transformer workshop, then vacuum-dry.

Long-Term Storage: Seal against moisture; if stored >6 months, re-dry before use.

During Wrapping: Maintain even tension, avoid tearing or powdering.

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