Introduction: From Cotton Cloth to Kraft Paper—A Century of Transformation
At the dawn of the electric power industry in the late 19th century, engineers faced a challenge that is difficult to imagine today: how to provide reliable electrical insulation for high-voltage windings? In an era without polymer films or synthetic resins, the earliest transformer insulation materials were extraordinarily diverse—cotton cloth, silk, rubber, mica, asbestos, wood, oil-impregnated paper… Various materials took turns dominating the field for a decade or two each, and most were eventually eliminated by history.
However, one seemingly ordinary material has remained steadfast throughout this 120+ year “insulation material war”—it is Kraft Paper.
Beginning in the 1930s, Kraft Paper gradually replaced cotton cloth, silk, rubber, and other early insulation materials, becoming the most mainstream, reliable, and widely used insulation medium in oil-immersed transformers. To this day, more than 90% of oil-immersed power transformers still use Kraft Paper as the primary winding insulation. This proportion has barely changed over the past 50 years.
Why did Kraft Paper win? Why has it not been eliminated in an era when synthetic materials flourish? How did it transform from an ordinary “packaging paper” into the “gold standard” for transformer insulation? Behind this success lies both the inevitability of technological evolution and the consolidation of standards systems, as well as the engineering culture of persistent trust built by generations of transformer engineers.
This article systematically tells the “century-long counterattack history” of Kraft Paper: from the “hundred schools of thought” contest of early insulation materials, to the breakthrough of the Kraft pulping process in the 1930s; from the establishment of international standards systems in the 1950s–1970s, to the “Kraft Paper is irreplaceable” status in large power transformers; to the challenges of new insulation materials in the 21st century, and Kraft Paper’s self-evolution through process improvements.
We will answer the following 5 core questions:
- What exactly is Kraft Paper? What unique physical and chemical properties does it possess?
- Before Kraft Paper appeared, what insulation materials did transformers use? Why were they all eliminated?
- What technological breakthrough allowed Kraft Paper to “become famous overnight”?
- How did the international standards system consolidate its industry position?
- Under the challenge of modern materials such as Nomex, polyimide, and polyester film, why does Kraft Paper remain the “gold standard” to this day?
Whether you are a transformer engineer, power equipment procurement specialist, industry researcher, or a technology enthusiast interested in engineering material history, this article will take you through a century of power industry history, understanding how a seemingly ordinary material became an irreplaceable cornerstone of the industry.
1. Kraft Paper Basics: An “Underestimated” Industrial Material
In the public perception, Kraft Paper is merely the raw material for common shopping bags, envelopes, and file folders. However, in the eyes of transformer engineers, Kraft Paper is an extremely precise, demanding industrial-grade material. To understand why it became the industry standard, we first need to understand its essence.
1.1 What Is Kraft Paper
Kraft Paper is unbleached or semi-bleached industrial paper manufactured using the Kraft Process (sulfate pulping method). The term “Kraft” derives from the German word “Kraft” (meaning “strength”), specifically referring to the high-strength cellulose characteristics brought by this process.
Core composition of Kraft Paper:
- Cellulose: Accounts for 80%–90%, serving as the main skeletal material of the paper.
- Hemicellulose: Accounts for 5%–15%, providing flexibility and hygroscopicity.
- Lignin: Accounts for 1%–5% (in semi-bleached Kraft Paper), providing rigidity.
- Moisture: Typically controlled at 5%–8% (transformer grade requires even lower).
1.2 Key Physical and Chemical Properties of Kraft Paper
Transformer-grade Kraft Paper has 5 key properties, each corresponding to specific transformer insulation requirements:
| Property | Value Range | Transformer Application Significance |
|---|---|---|
| Thickness | 0.05mm–0.25mm | Determines insulation thickness and outer diameter |
| Tensile Strength | ≥80 N/cm (longitudinal) | Withstands paper wrapping tension and winding stress |
| Moisture Content | ≤6% (transformer grade) | Affects breakdown voltage and aging rate |
| Breakdown Voltage | ≥10 kV/mm in air | Basic electrical insulation capability |
| Ash Content | ≤1% | Reduces conductive particle contamination |
1.3 The Essential Difference Between Kraft Paper and Ordinary Paper
Ordinary paper (such as A4 printing paper, tissue paper) typically uses the mechanical pulping method, retaining large amounts of lignin, causing the paper to easily yellow and become brittle. Kraft Paper adopts the Kraft pulping process, removing most lignin through strong alkali treatment while retaining high-purity cellulose, thus providing:
- Higher strength
- Longer lifespan
- Better electrical performance
- Lower moisture content
- More stable chemical properties
It is precisely this characteristic of “seemingly ordinary but actually precise” that makes Kraft Paper one of the optimal choices for transformer insulation.
2. Historical Phase One (1900s–1930s): The “Hundred Schools of Thought” Contest of Insulation Materials
At the dawn of transformer invention (1880s), engineers experimented with various insulation materials. This phase can be called “The Hundred Schools of Thought Contest of Insulation Materials”—various materials took turns, each with its own strengths and weaknesses.
2.1 List of Early Transformer Insulation Materials
| Insulation Material | Era of Appearance | Main Advantages | Main Disadvantages | Application Outcome |
|---|---|---|---|---|
| Cotton Cloth | 1880s | Flexible, easy to process | Hygroscopic, perishable | Eliminated in 1920s |
| Silk | 1890s | Thin, good insulation | High cost, low strength | Eliminated in 1920s |
| Rubber | 1890s | Waterproof, flexible | Ages easily, poor heat resistance | Eliminated in 1920s |
| Shellac | 1890s | Good adhesion | Brittle, cracks easily | Eliminated in 1930s |
| Mica | 1900s | High temperature resistance, excellent insulation | Extremely high cost, difficult to process | Retained only in high-end applications |
| Asbestos | 1900s | High temperature resistance, flame retardant | Health hazards (carcinogenic) | Eliminated in 1980s |
| Wood | 1900s | Easy to obtain, low cost | Flammable, hygroscopic, unstable | Eliminated in 1910s |
| Oiled Paper | 1890s | Improved breakdown voltage | Easily degraded, poor temperature resistance | Replaced by Kraft Paper |
2.2 Why Were Cotton Cloth and Silk Eliminated First?
Cotton cloth and silk, as the earliest textile insulation materials, were widely used in the late 19th century due to their flexibility and ease of manual wrapping. However, they had fatal shortcomings:
- High hygroscopicity: Cotton cloth’s insulation performance drops sharply after absorbing moisture.
- Perishability: Organic fibers undergo hydrolysis and decay during long-term immersion in transformer oil.
- Limited strength: Difficult to withstand the winding stress of modern transformers.
By the 1920s, as power system voltage levels increased (10kV → 35kV), cotton cloth and silk could no longer meet high-voltage insulation requirements.
2.3 Why Were Rubber and Shellac Also Eliminated?
- Rubber: Easily ages and becomes brittle at high temperatures, and hardens and cracks at low temperatures, unable to adapt to transformer operating environments.
- Shellac: High brittleness, cracks easily, and is sensitive to oil, showing delamination during long-term use.
2.4 Mica: The Only “Survivor” Among Early Materials
Among all early insulation materials, Mica is the only one still used in high-end transformers. Mica possesses:
- Extremely high thermal class (Class C, 220°C+)
- Excellent electrical performance (breakdown voltage ≥20 kV/mm)
- Good chemical stability
However, mica’s cost is extremely high (50–100 times that of Kraft Paper), and it is difficult to process. Therefore, it is only used as local insulation in EHV, UHV, and high-temperature special transformers, and cannot become a mainstream material.
2.5 Early Oiled Paper: The “Predecessor” of Kraft Paper
It is worth noting that as early as the 1890s, engineers began experimenting with oiled paper as insulation medium. However, the paper used at that time was mainly Chemical Wood Pulp Paper, which contained more lignin and impurities, resulting in:
- Unstable electrical performance after oil impregnation
- Tendency to produce acidic substances that corrode copper wire during long-term use
- Short lifespan (10–15 years)
This phase laid the groundwork for Kraft Paper’s later debut—engineers realized that paper-based insulation had great potential, but better pulping processes were needed to improve performance.
3. Historical Phase Two (1930s–1950s): The Kraft Process Brings Electrical Performance Leap
The 1930s was the watershed in the history of transformer insulation materials. A key pulping process—the maturity and popularization of the Kraft Process—propelled Kraft Paper to become the “protagonist” of transformer insulation overnight.
3.1 What Is the Kraft Pulping Process
The Kraft pulping process was invented by German chemist Carl F. Dahl in 1884 and began large-scale industrial application in the 1930s. Its core principles are:
- Under high temperature and high pressure, treating wood chips with “white liquor” containing NaOH (sodium hydroxide) and Na₂S (sodium sulfide).
- Strong alkali dissolves lignin and hemicellulose, retaining high-purity cellulose.
- The resulting pulp has high strength, high purity, and good stability.
Key Chemical Equation (simplified):
Wood chips + NaOH + Na₂S → Cellulose (retained) + Lignin (dissolved) + Hemicellulose (partially dissolved)
3.2 The 4 Major Performance Leaps Brought by the Kraft Process
Compared to early chemical wood pulp paper, Kraft Paper brought by the sulfate process has:
- Tensile strength improved by 50%–100% — Paper does not easily break during wrapping, able to withstand higher tension.
- Moisture content reduced by 30%–50% — Reduces the impact of moisture on electrical performance.
- Lignin residue reduced to 1%–5% — Reduces acidification tendency during long-term use.
- Post-oil-impregnation breakdown voltage improved by 100%–200% — This is the most critical leap!
Breakdown Voltage Comparison (after oil impregnation):
| Paper Type | Breakdown Voltage (kV/mm) | Application Era |
|---|---|---|
| Chemical Wood Pulp Paper (early) | 20–30 | 1890s–1920s |
| Cotton Cloth | 15–20 | 1880s–1910s |
| Kraft Paper (1930s) | 40–60 | 1930s to present |
| Modern High-Purity Kraft Paper | 60–80 | 1970s to present |
3.3 The Significance of Doubling Breakdown Voltage
The increase in breakdown voltage from 20–30 kV/mm to 40–60 kV/mm means:
- At the same insulation thickness, the withstood voltage doubles — Directly supports transformer voltage levels upgrading from 35kV to 110kV and 220kV.
- At the same voltage level, insulation thickness can be halved — Reduces transformer volume and lowers cost.
- Insulation redundancy greatly improved — Reduces failure rate and extends lifespan.
This is the key technical foundation for the transformer industry to move from 35kV to 110kV and 220kV. Without the Kraft Paper breakthrough, there would be no modern large power transformers.
3.4 Representative Applications in the 1930s–1950s
In the 1930s–1950s, Kraft Paper began to replace cotton cloth, silk, and chemical wood pulp paper on a large scale, with application scenarios including:
- Distribution Transformers (10kV): Single-layer Kraft Paper wrapping.
- Power Transformers (35kV): Double-layer Kraft Paper wrapping.
- High-Voltage Reactors: Double-layer Kraft Paper + oil duct design.
- Instrument Transformers: Kraft Paper as main insulation.
- Oil-Immersed Capacitors: Kraft Paper as dielectric layer.
By the 1950s, approximately 80% of oil-immersed transformers globally had adopted Kraft Paper as main insulation.
4. Historical Phase Three (1950s–1970s): Establishment of International Standards System
The process breakthrough in the 1930s allowed Kraft Paper to “become famous overnight”, but what truly made it the “industry standard” was the establishment of the international standards system in the 1950s–1970s.
4.1 Key Standards Organizations and Standards Documents
| Standards Organization | Standards Documents | Applicable Scope | Key Parameters |
|---|---|---|---|
| IEEE | IEEE C57.12.00, IEEE 4 | North America | Thickness, breakdown voltage, moisture content |
| IEC | IEC 60317, IEC 60641 | Europe, Global | Thickness, tensile strength, ash content |
| ASTM | ASTM D202, ASTM D580 | North America | Moisture content, ash content, pH |
| GB (China) | GB/T 7673, GB/T 19264 | China | Aligned with IEC |
| JIS (Japan) | JIS C3202, JIS P8111 | Japan | Moisture content, tensile strength |
| NEMA | NEMA MW 31, MW 1000 | North America | Aligned with IEEE/UL |
4.2 Core Significance of Standardization
The contribution of standardization to Kraft Paper becoming the “industry standard” is mainly reflected in 3 aspects:
Significance One: Unified Quality Benchmarks
Before standards emerged, Kraft Paper produced by different manufacturers varied in quality, leading to unstable transformer insulation performance. Standards (such as IEC 60641, IEEE C57) unified:
- Thickness tolerance (e.g., 0.075mm ± 5%)
- Tensile strength (e.g., ≥80 N/cm)
- Moisture content (e.g., ≤6%)
- Breakdown voltage (e.g., ≥10 kV/mm)
- Ash content (e.g., ≤1%)
This allows transformer manufacturers to confidently purchase any Kraft Paper meeting standards without worrying about quality issues.
Significance Two: Establishing International Common Language
IEC standards enable global transformer manufacturers, paper mills, and testing institutions to communicate using a unified technical language. This greatly reduces international trade barriers and accelerates the global popularization of Kraft Paper.
Significance Three: Solidifying Application Rules
Standards not only specify paper performance but also regulate how paper is applied in transformers:
- Wrapping layers (single vs double layer)
- Overlap ratio (30%–60%)
- Oil impregnation temperature (80–120°C)
- Oil impregnation time (≥8 hours)
- Moisture content requirements (≤6% before shipment, ≤0.5% before use)
These rules transformed Kraft Paper application from “empirical craft” to “standardized craft”, dramatically improving transformer quality consistency.
4.3 Milestone Significance of IEC 60641 and IEEE C57
- IEC 60641 (Pressboard and hardboard for electrical purposes): Regulates thickness, density, tensile strength, and breakdown voltage of Kraft Paperboard.
- IEEE C57.12.00 (General requirements for liquid-immersed transformers): Regulates application requirements for transformer insulation paper.
The promulgation of these two standards officially established Kraft Paper’s “standard status” in the field of oil-immersed transformer insulation.
5. Historical Phase Four (1970s–2000s): Transformer Scaling Consolidates Kraft Paper’s Position
During the 1970s–2000s, the global power system underwent a dual transformation of “voltage level escalation + transformer capacity scaling”. During this process, Kraft Paper’s position was not weakened but further consolidated.
5.1 Technical Challenges of Transformer Scaling
As power system voltage levels rose from 110kV to 220kV, 500kV, and 1000kV (UHV), the electric field strength and thermal stress that transformer windings must withstand grew increasingly severe:
- Electric Field Strength: 220kV transformers may experience internal electric fields of 5–10 kV/mm.
- Temperature Rise: Large transformers under full load may have winding temperature rises of 60–80°C.
- Short-Circuit Electrodynamic Force: 500kV transformer short-circuit electrodynamic force can exceed 100 tons.
- Long-Term Operation: Transformer design lifespan ≥30 years.
5.2 Why Did Kraft Paper Win in the Scaling Era?
Facing these challenges, Kraft Paper demonstrated 4 comprehensive advantages unmatched by other materials:
Advantage One: Mature and Stable Oil Impregnation Process
Large transformers require insulation materials to be impregnated in transformer oil. The compatibility between Kraft Paper and transformer oil has been optimized over 100+ years, and the impregnation process is highly mature:
- Impregnation temperature, time, and pressure all have standard specifications
- Oil permeability, viscosity, and acid value are optimized for Kraft Paper
- Oil-paper synergistic aging models have been established
Advantage Two: Processability
Large transformer windings require insulation paper to be manually or machine wrapped onto conductors, forming complex inter-turn, inter-layer, and inter-segment insulation structures. Kraft Paper offers:
- Good flexibility (can be bent into various shapes)
- High tensile strength (withstands wrapping tension)
- Easy to cut, fold, and laminate
Advantage Three: Affordable Cost
Large power transformers require massive amounts of insulation paper. Taking a 500kV transformer as an example, it may require several tons of insulation paper. At this scale:
- Kraft Paper cost: $1–3/kg ✅ Affordable
- Mica cost: $50–200/kg ❌ Cost explosion
- Nomex cost: $30–100/kg ❌ Too expensive
- Polyimide film cost: $50–150/kg ❌ Too expensive
Advantage Four: Recyclability and Sustainability
When transformers reach end-of-life and are retired, the Kraft Paper + transformer oil system can be relatively easily separated and recycled. Kraft Paper itself is natural cellulose, biodegradable, and complies with modern sustainable development requirements.
5.3 Representative Applications in the 1970s–2000s
- 500kV EHV Transformers: Double-layer + triple-layer Kraft Paper wrapping, combined with Vacuum Pressure Impregnation (VPI).
- ±500kV Converter Transformers: Kraft Paper + oil duct design, addressing DC bias current.
- 1000kV UHV Transformers: Multi-layer Kraft Paper + composite insulation, representing the world’s top-tier transformers.
- Large Hydropower Station Main Transformers: 300MVA, 500MVA, 1000MVA capacities, Kraft Paper as main insulation.
- Nuclear Power Plant Main Transformers: High reliability requirements, Kraft Paper as the preferred choice.
By the 2000s, approximately 95% of oil-immersed power transformers globally used Kraft Paper as main insulation, and Kraft Paper’s “gold standard” status had become unshakeable.
6. Historical Phase Five (2000s–Present): Modern Competition and Coexistence
Since the 21st century, various new materials have emerged in the field of transformer insulation. Has Kraft Paper been challenged?
6.1 List of New Insulation Materials
| New Material | Era of Appearance | Main Advantages | Main Disadvantages | Application Field |
|---|---|---|---|---|
| Nomex (meta-aramid paper) | Invented 1960s, popularized 2000s | High temperature resistance (220°C), flame retardant | High cost, poor transformer oil compatibility | Dry-type transformers, Class H and above |
| Polyimide Film (Kapton) | Invented 1960s, popularized 2000s | High temperature resistance (250°C+), thin | High cost, poor oil compatibility | Special motors, aerospace |
| Polyester Film (Mylar/PET) | Invented 1950s, popularized 2000s | Thin, high mechanical strength | Low temperature resistance, poor oil compatibility | Small transformers, capacitors |
| Mica Tape | 1960s | High temperature resistance, excellent insulation | Extremely high cost | High-temperature motors, UHV |
| DMD (Polyester film + non-woven + polyester film) | 1970s | High mechanical strength | General temperature resistance | Dry-type transformers |
| NMN (Nomex + Mylar + Nomex) | 1980s | High temperature resistance, good mechanics | High cost | Class H dry-type transformers |
| Epoxy Resin Cast Insulation | 1990s | Integrated, moisture-proof | Not repairable | Cast resin dry-type transformers |
6.2 Kraft Paper vs. Nomex: Essential Differences
Nomex is the modern material most commonly compared with Kraft Paper. The core differences between the two are:
| Dimension | Kraft Paper | Nomex |
|---|---|---|
| Thermal Class | 105°C (Class A, up to 120°C after oil impregnation) | 220°C (Class C) |
| Transformer Oil Compatibility | ⭐⭐⭐⭐⭐ Excellent | ⭐⭐ Poor (absorbs water, easily expands) |
| Breakdown Voltage | 60–80 kV/mm (oil impregnated) | 20–30 kV/mm (in air) |
| Cost | $1–3/kg | $30–100/kg |
| Recyclability | ⭐⭐⭐⭐⭐ Biodegradable | ⭐ Difficult to degrade |
| Main Application | Oil-immersed transformers | Dry-type transformers, Class H and above |
Conclusion: In the field of oil-immersed transformers, Nomex cannot replace Kraft Paper due to poor oil compatibility and high cost. However, in dry-type transformers and Class H and above high-temperature scenarios, Nomex is the preferred choice.
6.3 Kraft Paper’s “Coexistence” Strategy in Modern Transformers
Modern large power transformers typically adopt a “Kraft Paper + other materials” composite insulation strategy:
- Main insulation (between winding and core): Kraft Paper (low cost, good performance)
- Inter-turn insulation (between different turns of the same winding): Kraft Paper + enameled wire enamel coating (dual insulation)
- Inter-segment insulation (between different voltage segments): Kraft Paper + oil ducts
- Local high-temperature points: Nomex, mica (targeted reinforcement)
- Lead insulation: Nomex sleeves (temperature resistance)
This “Kraft Paper as primary + other materials as auxiliary” strategy allows Kraft Paper to maintain its core position in modern transformers.
7. Five Major Reasons Why Kraft Paper Remains the “Gold Standard” Today
Drawing from 100+ years of engineering practice, Kraft Paper’s ability to become the industry standard and maintain its advantages is mainly attributed to the following 5 core reasons:
7.1 Reason One: Optimal Balance of Cost and Performance
Kraft Paper, at an extremely low price of $1–3/kg, provides a high breakdown voltage of 60–80 kV/mm (after oil impregnation). At equivalent cost, no other material can provide such high electrical performance. This cost-performance ratio advantage is the fundamental reason why Kraft Paper remains invincible.
7.2 Reason Two: Perfect Synergy with Transformer Oil
The cellulose structure of Kraft Paper is perfectly compatible with various transformer oils including mineral oil, synthetic ester, and silicone oil:
- Oil has good permeability and can fill the microscopic gaps between paper layers
- Mature oil-paper synergistic aging model with predictable lifespan
- Oil acid value and moisture monitoring can indirectly reflect paper aging
- Oil replacement and replenishment can extend paper lifespan
This oil-paper synergy is a core advantage difficult for other materials (such as Nomex and polyimide) to replicate.
7.3 Reason Three: Mature and Stable Manufacturing Process
After 90+ years of optimization, the Kraft Paper manufacturing process has reached a near-perfect level:
- Thickness tolerance can be controlled within ±2%
- Moisture content can be controlled at ≤6% (≤0.5% before use)
- Tensile strength can reach ≥80 N/cm
- Ash content can be controlled at ≤1%
- Automated production with stable quality
This process maturity allows transformer manufacturers to confidently purchase and stably use Kraft Paper.
7.4 Reason Four: Complete Industry Chain and Recycling System
Kraft Paper has a complete global industry chain:
- Wood pulp raw materials (North America, Europe, Southeast Asia, Brazil)
- Paper mills (hundreds of specialized manufacturers globally)
- Transformer insulation paper deep processing (slitting, calendering, lamination)
- Application manufacturers (transformer factories, motor factories, reactor factories)
- Recycling system (paper-oil separation and recycling after transformer retirement)
This industry chain scale and maturity cannot be established by other materials in a short time.
7.5 Reason Five: Environmental Friendliness and Sustainability
Kraft Paper’s main component is natural cellulose, with the following environmental advantages:
- Biodegradable (naturally degrades within months)
- Renewable raw materials (wood from sustainable forestry)
- Relatively low carbon emissions during manufacturing (vs. synthetic fibers)
- Simple processing after transformer retirement (incineration or composting)
As ESG (Environmental, Social, Governance) receives increasing attention, Kraft Paper’s environmental friendliness has become an important factor for its continued selection.
8. Application Map of Kraft Paper in Different Transformers
Kraft Paper’s application scope covers almost all types of oil-immersed transformers:
8.1 Classification by Voltage Level
| Voltage Level | Kraft Paper Application | Typical Scenario |
|---|---|---|
| ≤1kV | Single-layer 0.075mm Kraft Paper | Control transformers, instrument transformers |
| 1–10kV | Single or double layer 0.075–0.125mm Kraft Paper | Distribution transformers |
| 10–35kV | Double layer 0.125mm Kraft Paper | Medium-voltage power transformers |
| 35–110kV | Double + triple layer 0.125–0.20mm Kraft Paper | High-voltage power transformers |
| 110–500kV | Multi-layer Kraft Paper + oil ducts | EHV power transformers |
| ≥500kV | Multi-layer Kraft Paper + composite insulation + VPI | UHV power transformers |
8.2 Classification by Transformer Type
| Transformer Type | Kraft Paper Application Characteristics |
|---|---|
| Oil-Immersed Distribution Transformer | Single or double layer Kraft Paper, optimal cost-performance |
| Oil-Immersed Power Transformer | Double or multi-layer Kraft Paper, VPI impregnation |
| Dry-Type Transformer | Usually does not directly use Kraft Paper (unless Class H oil-immersed) |
| Furnace Transformer | Double layer Kraft Paper, heat and impact resistant |
| Rectifier Transformer | Double layer Kraft Paper, handles harmonics |
| Converter Transformer | Multi-layer Kraft Paper, addresses DC bias |
| Traction Transformer | Double layer Kraft Paper, vibration resistant |
| Nuclear Power Transformer | High-reliability multi-layer Kraft Paper |
8.3 Classification by Operating Environment
| Operating Environment | Kraft Paper Application Considerations |
|---|---|
| Conventional Indoor | Standard Kraft Paper is sufficient |
| Outdoor | Requires waterproof, UV-resistant design |
| High Altitude | Need to consider the impact of low air pressure on breakdown voltage |
| High Humidity Heat (Tropical) | Need to strictly control moisture content |
| High Cold (Polar) | Need to consider low-temperature fluidity of oil |
| Offshore (Wind Power) | Need salt spray protection design |
| Chemical Corrosive Environment | Need acid and alkali resistant design |
9. Modern Manufacturing Process of Kraft Paper
To understand why Kraft Paper maintains high quality, let us briefly review its manufacturing process.
9.1 Kraft Pulping Process Flow
Wood (softwood such as pine, fir) → Debarking → Chipping → Cooking (NaOH + Na₂S) → Washing → Screening → Bleaching (partial) → Beating → Paper Making → Calendering → Reeling → Rewinding
9.2 Key Process Parameters
| Process Step | Key Parameters | Control Requirements |
|---|---|---|
| Cooking Temperature | 165–175°C | Affects cellulose strength |
| Cooking Time | 2–4 hours | Affects lignin removal rate |
| NaOH Concentration | 10%–15% | Affects pulping efficiency |
| Bleaching Process | ECF (Elemental Chlorine Free) | Environmental requirements |
| Beating Degree | 30–50 °SR | Affects paper density |
| Paper Machine Speed | 500–1500 m/min | Affects thickness uniformity |
| Calendering Pressure | 50–200 kN/m | Affects density and thickness |
9.3 Key Differences Between Transformer-Grade and Regular-Grade Kraft Paper
| Indicator | Regular-Grade Kraft Paper | Transformer-Grade Kraft Paper |
|---|---|---|
| Moisture Content | 6%–10% | ≤6% |
| Ash Content | ≤2% | ≤1% |
| pH Value | 5–9 | 6–8 (neutral) |
| Conductive Particles | Not required | ≤0.05% (strictly controlled) |
| Thickness Tolerance | ±10% | ±5% |
| Tensile Strength | ≥50 N/cm | ≥80 N/cm |
Transformer-grade Kraft Paper has far higher requirements for purity, thickness tolerance, and strength than regular grade, which is key to ensuring transformer insulation reliability.
10. Key Indicators for Kraft Paper Quality Control
When transformer manufacturers purchase and use Kraft Paper, they need to strictly control the following 5 core indicators:
10.1 Thickness
- Measurement Method: Thickness gauge, 10-point average.
- Tolerance Requirement: ±5% (transformer grade).
- Impact: Directly determines insulation thickness and outer diameter.
10.2 Moisture Content
- Measurement Method: Oven drying method or infrared moisture meter.
- Requirement: ≤6% before shipment, ≤0.5% before use.
- Impact: Moisture content is a key factor affecting breakdown voltage and aging. For every 1% increase in moisture content, breakdown voltage decreases by approximately 5%–10%.
10.3 Tensile Strength
- Measurement Method: Tensile testing machine.
- Requirement: Longitudinal ≥80 N/cm, transverse ≥40 N/cm.
- Impact: Determines whether the paper can withstand tension without breaking during wrapping.
10.4 Ash Content
- Measurement Method: High-temperature burning method.
- Requirement: ≤1%.
- Impact: Conductive particles in ash may cause partial discharge.
10.5 Breakdown Voltage
- Measurement Method: High-voltage breakdown test.
- Requirement: ≥10 kV/mm in air, ≥60 kV/mm after oil impregnation.
- Impact: Directly determines insulation reliability.
10.6 Other Key Indicators
| Indicator | Requirement | Measurement Method |
|---|---|---|
| pH Value | 6–8 (neutral) | pH meter |
| Conductive Particles | ≤0.05% | Particle counter |
| Tearing Resistance | ≥600 mN | Tearing tester |
| Air Permeability | 5–50 μm/Pa·s | Air permeability tester |
| Oil Absorption | ≥20 mm/10min | Oil absorption test |
11. Kraft Paper vs. Modern Insulation Materials: Detailed Comparison
To more comprehensively understand Kraft Paper’s advantages, let us compare it in detail with modern mainstream insulation materials:
11.1 Comprehensive Performance Comparison
| Dimension | Kraft Paper | Nomex | Polyimide Film | Mica Tape | DMD |
|---|---|---|---|---|---|
| Thermal Class | 105°C (A) | 220°C (C) | 250°C+ | 500°C+ | 130°C (B) |
| Breakdown Voltage (kV/mm) | 60–80 (oil impregnated) | 20–30 (air) | 100–200 | 20–30 | 40–60 |
| Oil Compatibility | ⭐⭐⭐⭐⭐ | ⭐⭐ | ⭐⭐ | ⭐⭐⭐⭐ | ⭐⭐ |
| Cost | $1–3/kg | $30–100/kg | $50–150/kg | $50–200/kg | $5–15/kg |
| Recyclability | ⭐⭐⭐⭐⭐ | ⭐ | ⭐ | ⭐⭐⭐ | ⭐⭐ |
| Processability | ⭐⭐⭐⭐⭐ | ⭐⭐⭐ | ⭐⭐ | ⭐⭐ | ⭐⭐⭐ |
| Main Application | Oil-immersed transformer | Dry-type transformer | Special motor | High-temperature motor | Dry-type transformer |
11.2 Kraft Paper Remains the First Choice for Oil-Immersed Transformer Insulation
In the main battlefield of oil-immersed transformers, Kraft Paper’s advantages are overwhelming:
- Cost is only 1/30 of Nomex.
- After oil impregnation, breakdown voltage reaches 60–80 kV/mm, exceeding Nomex’s 20–30 kV/mm in air.
- Perfect synergy with transformer oil, mature aging model.
- Complete industry chain and recycling system.
- Environmentally friendly.
These advantages make Kraft Paper virtually irreplaceable in the field of oil-immersed transformers.
11.3 Dry-Type Transformers Use Other Materials
In the field of dry-type transformers (oil-free), Kraft Paper cannot be used (because without oil protection, the paper will absorb moisture, degrade, and burn dry), so the following are used:
- Nomex (DuPont): First choice for Class H, Class C dry-type transformers.
- DMD, NMN: Commonly used for Class F, Class H dry-type transformers.
- Epoxy Resin: Cast resin dry-type transformers.
However, it should be noted that more than 80% of power transformers globally are still oil-immersed, so the application scale of Kraft Paper remains huge.
12. Future Trends: Sustainable Evolution of Kraft Paper
12.1 Process Improvement Directions
Although the Kraft Paper process is already very mature, the transformer industry’s demand for higher performance continues to drive Kraft Paper process improvements:
| Improvement Direction | Target | Current Progress |
|---|---|---|
| Purity Improvement | Ash content ≤0.5%, conductive particles ≤0.02% | Partially achieved |
| Moisture Content Reduction | Shipment ≤4% | Partially achieved |
| Temperature Resistance Improvement | After oil impregnation 130°C | Experimental stage |
| Tensile Strength Improvement | ≥120 N/cm | Partially achieved |
| Dielectric Loss Reduction | tan δ ≤0.002 | Partially achieved |
12.2 Sustainable Pulping
Traditional Kraft pulping consumes large amounts of chemicals and energy. Modern processes are transitioning to green pulping:
- Biomass Refining: Converting pulping by-products (hemicellulose, lignin) into biofuels or chemicals.
- Closed-Loop Water Recycling: Reducing water consumption and wastewater discharge.
- ECF/TCF Bleaching: Elemental chlorine free or totally chlorine free bleaching, reducing pollution.
- FSC Certified Wood: Raw materials from sustainably managed forests.
12.3 Composite with New Materials
In the future, Kraft Paper may form “composite insulation” with new materials:
- Kraft Paper + Nanocellulose: Improves strength and temperature resistance.
- Kraft Paper + Ceramic Coating: Improves temperature resistance and flame retardancy.
- Kraft Paper + Flame Retardants: Improves flame retardant performance.
- Kraft Paper + Smart Sensors: Enables online insulation condition monitoring.
12.4 Expansion in New Energy Sector
The rapid development of new energy (wind power, photovoltaics, energy storage, electric vehicles) has brought new application growth points for Kraft Paper:
- Wind Power Transformers: High reliability requirements, Kraft Paper as main insulation.
- Photovoltaic Inverters: Small transformer volume, Kraft Paper wrapping for compact windings.
- Energy Storage Transformers: High-frequency operation, Kraft Paper oil impregnation performance is stable.
- Electric Vehicle Charging Pile Transformers: High power density, Kraft Paper as insulation.