What Is Paper Covered Wire Used for in Transformers

I. Introduction

Paper-covered wire, as one of the most classic and widely used winding conductor materials in the transformer manufacturing field, has been used in oil-immersed power transformers, distribution transformers, and special transformers for over a century. Despite the emergence of new materials such as polyimide film insulation and NOMEX paper insulation in recent years, paper-covered wire, with its excellent electrical insulation performance, mature manufacturing process, and significant cost advantages, still holds an irreplaceable dominant position in the high-voltage and ultra-high-voltage transformer field.

This article aims to systematically describe the main application scenarios, core functional characteristics, key points of technology selection, and common application pitfalls of paper-covered wire in transformers, providing professional reference for transformer design engineers and procurement technicians.

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II. Definition and Basic Structure of Paper-Insulated Conductors

2.1 Definition

Paper-insulated conductors are winding cables made by wrapping cable paper (Kraft Paper) or modified cable paper around the surface of a copper conductor or aluminum conductor. Cable paper is an electrical insulation paper made from sulfate wood pulp, possessing excellent dielectric properties and mechanical strength, and is a core component of the transformer oil-immersed insulation system.

2.2 Basic Structure

The basic structure of paper-insulated conductors is a concentric layered distribution, from the inside out:

Conductor matrix: Typically, oxygen-free copper (OFC) or electrolytic copper is used as the conductor material, with a conductivity of over 101% IACS; the aluminum conductor is made from high-purity electrical aluminum ingots, with a conductivity of approximately 61% IACS. The conductor cross-sectional area is determined according to the transformer design current, ranging from a few square millimeters to several thousand square millimeters.

Paper Insulation Layer: The number of paper winding layers in cables is typically 1 to 6, with single-layer paper thicknesses including 0.45mm, 0.60mm, 0.75mm, 0.95mm, 1.25mm, 1.35mm, 1.60mm, 1.95mm, and 2.95mm, among others. Transformer oil is used as the impregnation medium between the paper layers, forming a composite insulation structure.

Outer Sheath (Optional): Some special paper-insulated wires have an additional polyester film tape or semiconductor paper layer on the outer layer to enhance end moisture resistance or improve electric field distribution.

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III. Core Application Functions of Paper-Insulated Wires in Transformers

3.1 Electrical Insulation Function

The primary function of paper-insulated wires is to provide reliable electrical insulation. During oil-immersed transformer operation, high voltage differences exist between winding conductors, between windings and the iron core, and between windings and the oil tank. Paper-insulated conductors effectively block current leakage paths and prevent breakdown discharge through their paper insulation layer.

The dielectric constant of the cable paper is approximately 3.5–4.5 (at 50 Hz), the dielectric loss factor (tanδ) in oil at 90°C is approximately 0.002–0.005, and the breakdown electric field strength can reach over 40 kV/mm. These electrical performance parameters ensure the long-term stable operation of paper-insulated wires under high-voltage environments.

From the perspective of insulation coordination, the paper insulation layer and transformer oil constitute a typical “paper-oil” composite insulation system. The two work synergistically: the transformer oil fills the capillary pores of the paper layer, significantly improving the dielectric strength of the composite insulation; simultaneously, the paper layer acts as a supporting framework for the oil, preventing oil leakage and slowing down oil aging. This synergistic mechanism makes the “paper-oil” insulation system one of the most reliable insulation solutions in the transformer field.

3.2 Heat Conduction and Dissipation Function

During transformer operation, the winding conductors generate a large amount of heat due to copper and iron losses. The paper insulation layer of paper-insulated conductors serves not only as an electrical barrier but also as a heat conduction channel. Heat must pass through the conductor surface, the paper insulation layer, and the transformer oil before finally being transferred to the iron core and the oil tank radiator.

The thermal conductivity of the paper insulation layer is approximately 0.15–0.20 W/(m·K), lower than that of metallic conductors but higher than that of air. In an oil-immersed environment, the heat transfer efficiency of paper-insulated wire windings is relatively high. Combined with forced oil circulation or natural oil circulation cooling systems, the winding temperature rise can be effectively controlled.

It is worth noting that the thermal aging characteristics of the paper insulation layer are a key factor affecting the transformer’s lifespan. When cable paper operates for extended periods at 110°C–130°C, aging phenomena such as cellulose degradation, decreased polymerization degree, and reduced mechanical strength occur. Studies have shown that for every 10°C increase in the operating temperature of oil-immersed paper insulation, the chemical aging rate approximately doubles. Therefore, the hot spot temperature of the paper-insulated wire must be strictly controlled during transformer design.

3.3 Mechanical Support and Short-Circuit Resistance Electromechanical Function

During operation, the transformer may encounter sudden short-circuit faults. The short-circuit current can reach 10 to 25 times the rated current, generating enormous electrodynamic forces on the winding conductors. It is estimated that a 1000MVA giant transformer winding can withstand electrodynamic forces of up to tens of tons during a short circuit.

The paper-insulated wires are not independent within the winding; rather, they are mechanically bonded to adjacent wires, support bars, and spacers through the paper insulation layer. The adhesive effect of the paper layer and the overall curing process of the winding make the entire winding a rigid whole, effectively resisting radial compression and axial vibration caused by short-circuit electrodynamic forces, preventing winding deformation, displacement, or insulation damage.

Furthermore, the winding process of the paper-insulated wire has a significant impact on the mechanical strength of the winding. Strict control is required in aspects such as wire transposition design, winding tension control, and baking curing treatment to ensure that the winding possesses sufficient short-circuit resistance mechanical strength.

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IV. Typical Applications of Paper-Insulated Conductors in Oil-Immersed Transformers

4.1 Power Transformer Windings

Power transformers with winding voltage levels of 66kV and above (also known as main transformers or grid transformers) are the primary application areas for paper-insulated conductors. These transformers are characterized by large capacity (from tens of MVA to thousands of MVA), high voltage, and stringent operational reliability requirements.

In 330kV to 750kV ultra-high voltage transformers, paper-insulated conductors typically employ a multi-layer interlayer insulation structure. The conductor cross-section is mostly rectangular (flat wire), with a width ranging from 30mm to 80mm and a thickness from 8mm to 20mm. The paper insulation thickness of a single conductor is determined based on the interlayer voltage of the winding, generally within the range of 1.35mm to 2.95mm.

In ±800kV and above UHVDC converters, the valve-side windings must withstand complex AC/DC superimposed voltage stresses, placing more stringent requirements on the insulation performance of paper-insulated wires. These transformers typically employ the HPFF (High Pressure Formed Paper) process, where a pre-formed paper insulation layer is shaped under high temperature and pressure and then wrapped around the conductor surface, resulting in significantly better insulation quality and consistency than traditional wrapping processes.

4.2 Distribution Transformer Windings

Distribution transformers (also known as secondary transformers or distribution transformers) with voltage ratings of 6kV to 35kV are another important application area for paper-insulated wires. Compared to power transformers, distribution transformers have smaller single-unit capacities (typically 50kVA to 2500kVA), larger batch sizes, and higher cost sensitivity.

Distribution transformer windings mostly use round wires or small-sized flat wires. Paper insulation thickness is typically 0.45mm to 0.95mm, and single or double-layer paper sheathing structures are sufficient to meet insulation requirements. In power distribution transformer manufacturing, paper-insulated wire is used in the following forms:

• Paper-covered Round Wire (PCRW): Used for winding low-voltage windings or small-capacity high-voltage windings, with a conductor diameter ranging from 0.80mm to 5.00mm.
• Paper-covered Flat Wire (PCFW): Used for winding high-voltage windings, with a width of 5mm to 25mm and a thickness of 1.00mm to 5.00mm.
• Paper-covered Stranded Wire: Multiple round wires or flat wires wound in parallel, used for high-current windings to reduce eddy current losses.

4.3 Special Transformer Windings

Besides power and distribution transformers, paper-insulated conductors are also widely used in the following special transformer fields:

• Electric Furnace Transformers: Used in high-current output applications such as industrial electric arc furnaces and induction furnaces. The windings typically use water-cooled paper-insulated wire, with internal water cooling and external paper insulation to meet the technical requirements of high current, low voltage, and high heat dissipation.
• Rectifier Transformers: Used in DC power supply equipment for electrolysis, electroplating, etc. The current waveform of rectifier transformer windings contains a large number of harmonic components, and the dielectric loss characteristics of the paper-insulated wire require special attention to the impact of harmonic temperature rise.
• Rail Transit Traction Transformers: Installed on electric locomotives or EMUs, subjected to strong vibration and impact loads. The paper-insulated wire used for traction transformer windings must possess excellent vibration fatigue resistance.

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V. Key Technical Selection Points for Paper-Insulated Conductors

5.1 Matching Voltage Rating and Insulation Thickness

The insulation thickness of paper-insulated conductors must be precisely matched to the voltage rating of the transformer winding. The selection criteria mainly include:

• Winding Rated Voltage: Determines the operating voltage intensity that the insulation layer can withstand.
• Test Voltage: Includes type test voltage values such as power frequency withstand voltage and impulse withstand voltage, which determine the electrical strength margin of the insulation layer.
• Partial Discharge Level: High-voltage transformers typically require partial discharge levels below 10 pC; the insulation design must ensure no significant partial discharge under the highest operating voltage.
• Insulation Coordination Factor: Usually taken as 1.3 to 1.5 to ensure that the insulation layer does not break down under overvoltage conditions.

Based on engineering experience, the recommended insulation thickness for paper-insulated wires of different voltage levels is as follows:

Voltage Level	Insulation Thickness Range	Typical Paper Insulation Specifications
6kV～10kV	0.45mm～0.75mm	Single layer 0.45mm or 0.60mm
35kV	0.95mm～1.25mm	Double layer 0.45mm+0.45mm
66kV～110kV	1.35mm～1.95mm	Double layer 0.60mm+0.60mm or triple layer
220kV	2.95mm～3.85mm	Three to four layer combination
500kV and above	≥5.00mm	Multi-layer composite insulation structure

5.2 Conductor Material Selection: Copper vs Aluminum

Paper-insulated wires can be selected as either copper conductors or aluminum conductors. Both materials have their advantages and disadvantages, and a comprehensive consideration is needed when selecting:

Performance Indicators	Copper Conductor	Aluminum Conductor
Conductivity	Excellent (~101% IACS)	Good (~61% IACS)
Density	High (8.9 g/cm³)	Low (2.7 g/cm³)
Coefficient of Thermal Expansion	Moderate (17×10⁻⁶/°C)	Relatively High (24×10⁻⁶/°C)
Tensile Strength	High	Relatively Low
Cost	High	Low (approximately 30%～40%)
Welding Difficulty	Relatively Difficult	Relatively Easy

In high-voltage windings of power transformers, copper conductors are commonly used due to space utilization and mechanical strength requirements. In power distribution transformers or special transformers, paper-insulated wires can be considered to reduce costs, but this requires a corresponding increase in conductor cross-sectional area, which may lead to an increase in winding size.

5.3 Determining the Thermal Class of Paper-Insulated Wires

The thermal class of paper-insulated wires is determined by the heat resistance of both the cable paper and the impregnating varnish. According to IEC 60076-2 and GB/T 1094.2 standards, transformer insulation thermal classes are classified as follows:

• Class A (105°C): Oil-impregnated paper insulation system, with a maximum hot spot temperature of 105°C. The normal operating temperature of paper-insulated wire windings in an oil-immersed environment is typically controlled below 95°C.
• Class B (130°C): Using modified cable paper or high-voltage fully cured paper (HPFF) insulation, capable of withstanding higher operating temperatures.
• Class F (155°C): Using polyester film composite paper insulation, mainly used in dry-type transformers.
• Class H (180°C) and above: Insulation using Nomex paper or polyimide film, for high temperature resistance applications.

Choosing a higher thermal class means higher material costs and manufacturing difficulty. In oil-immersed power transformers, Class A insulation remains the mainstream choice because it has the best compatibility with the transformer oil system, the most mature technology, and the best cost-effectiveness.

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VI. Common Application Misconceptions and Quality Control Points

6.1 Mislabeling of Insulation Thickness

Some suppliers in the market have discrepancies between the nominal and actual values of paper insulation thickness. For example, a paper-insulated wire nominally labeled as 0.95mm may actually only be 0.85mm. Although a deviation of 0.1mm seems small, it can lead to a 10% to 15% drop in breakdown voltage in high-voltage transformers, posing a serious safety hazard.

Quality Control Measures: During incoming inspection, an insulation thickness measuring instrument with an accuracy of not less than 0.01mm should be used to measure the first, middle, and last ends of each roll of paper-insulated wire, record the measurements, and compare them with the nominal values. Simultaneously, suppliers should be required to provide third-party testing reports, including key parameters such as paper insulation thickness, breakdown voltage, and dielectric loss.

6.2 Conductor Resistivity Exceeding Standards

The resistivity of the copper conductor in the transformer winding directly affects the transformer’s load loss and temperature rise. The resistivity of high-quality oxygen-free copper should be ≤0.017241Ω·mm²/m (20°C), equivalent to a conductivity ≥101% IACS. If the conductor impurity content is too high, the increased resistivity will lead to increased transformer operating losses and decreased efficiency.

6.3 Missing Type Testing Issues

Paper-insulated conductors must complete a full set of type tests before being supplied in bulk. Routine testing items include:

• Appearance and dimensional inspection (conductor dimensions, paper insulation thickness, sheathing quality)
• Electrical strength test (voltage resistance, breakdown field strength)
• Dielectric loss factor determination (tanδ)
• Bending test (verifying mechanical flexibility)
• Tensile test (verifying tensile strength)
• Heat aging test (verifying heat resistance life)

Some purchasers, in order to save costs, only perform sampling inspection or even no inspection at all, and directly put the products into winding and use. This practice is extremely risky; once a batch of quality problems occur, it will cause huge losses.

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VII. Conclusion

Paper-insulated wire, as a traditional core material for transformer windings, is widely used and technologically mature in oil-immersed power transformers, power distribution transformers, and special transformer fields. Its comprehensive performance in electrical insulation, heat conduction, and mechanical support makes it irreplaceable in high-voltage and ultra-high-voltage transformer fields.

When selecting paper-insulated wire, the following factors should be emphasized: precise matching of voltage level and insulation thickness, appropriate selection of conductor material, proper determination of thermal class, and the supplier’s quality control level. Avoiding low-price traps, emphasizing type testing, and strictly implementing incoming inspection are essential to ensure the long-term safe operation of transformer products.