Paper Covered Wire Insulation Behavior Under Heat

Introduction

Paper-insulated magnet wire is a winding conductor formed by helically wrapping electrical-grade cellulose paper—such as kraft paper, crepe paper, thermally upgraded kraft paper (TUK), thermally upgraded crepe paper (TUCP), or diamond-dotted paper (DDP)—with thicknesses ranging from 0.05 to 0.13 mm around copper or aluminum conductors. It is widely employed in oil-immersed power transformers, distribution transformers, traction transformers, HVDC converter transformers, furnace transformers, and rectifier transformers. Its insulation behavior is highly temperature-dependent: the degradation rate of cellulose polymers increases exponentially with temperature; the viscosity, convective heat transfer coefficient, and dielectric response of mineral oil or ester-based liquids also vary significantly with temperature; furthermore, the conductor’s resistivity, copper catalytic activity, and coefficient of thermal expansion (CTE) introduce additional multi-physics coupling with the insulation system. Thus, “heat” serves not only as a necessary condition for normal operation—enabling oil circulation to remove Joule heating—but also as the primary driver of insulation aging, dielectric degradation, mechanical failure, and ultimate end-of-life.

Throughout the transformer’s service life, the temperature experienced by paper-insulated magnet wire is never truly steady-state. Load fluctuations, diurnal cycles, seasonal ambient variations, short-circuit transient overcurrents, and post-fault cooling processes all subject the insulation to continuous or thermal-shock-type temperature transients. These thermal histories interact synergistically with electric field stress, mechanical strain, moisture, and oxygen dissolved in the insulating oil to ultimately govern the degradation trajectories of critical parameters—including cellulose degree of polymerization (DP), dielectric breakdown strength, dielectric loss factor (tanδ), space charge distribution, and mechanical integrity.

This systematic review focuses specifically on **the insulation behavior of paper-insulated magnet wire under thermal stress**, covering:

• Thermal aging kinetics (Arrhenius model, activation energy, DP degradation, furfural generation);
• Temperature classes and end-of-life criteria (Class A: 105 °C / Class E: 120 °C; Montsinger rule; thermal life assessment);
• Thermo-electrical coupling (thermally induced reduction in partial discharge inception voltage [PDIV], thermally induced increase in tanδ, thermally induced decay of dielectric breakdown strength);
• Thermo-mechanical coupling (CTE mismatch, interfacial shear, paper layer debonding, bubble formation);
• Thermo-chemical coupling (hydrolysis / oxidation / pyrolysis / acidic by-product formation / copper-catalyzed degradation);
• Comparative thermal response across paper types (kraft vs. TUK vs. TUCP vs. DDP vs. crepe);
• Temperature monitoring and lifetime prediction methodologies (IEEE C57.91, IEC 60076-7, IEC 60554, IEC 61198, Duval triangle, dissolved gas analysis [DGA], furfural analysis).

Chemical Basis of Thermal Aging of Paper-Insulated Magnet Wire

Cellulose Molecular Structure and Degradation Mechanisms

The primary constituent of paper insulation is cellulose—a linear polymer composed of D-glucopyranose units linked by β-1,4-glycosidic bonds. Its initial degree of polymerization (DP₀) typically ranges from 1000 to 1200. Under thermal stress, cellulose chains degrade via three parallel pathways: ① **Hydrolytic degradation**—water molecules cleave glycosidic bonds under acid catalysis; ② **Oxidative degradation**—oxygen, in the presence of metal impurities such as copper and iron, oxidizes hydroxyl groups at C2, C3, and C6 positions to form carbonyl and carboxyl groups, subsequently triggering chain scission; ③ **Thermal pyrolysis**—direct homolytic cleavage of glycosidic bonds at elevated temperatures. These mechanisms collectively drive DP reduction from its initial value of 1000–1200 down to the end-of-life threshold of 200–250, accompanied by mass loss, deterioration of mechanical strength, and degradation of dielectric properties.

Aging Byproducts and Oil-Soluble Species

Small-molecule degradation products—including water, furfural (2-furaldehyde), 5-hydroxymethylfurfural (HMF), acetic acid, formic acid, CO, and CO₂—diffuse from the paper insulation into the transformer oil. Among these, **furfural concentration** serves as the most specific indicator of paper aging:

• In healthy paper insulation, oil furfural content remains below 0.1 mg/L;
• Moderate aging corresponds to 0.5–2.0 mg/L;
• When DP approaches 400, cumulative furfural concentration reaches 2–4 mg/L;
• Furfural levels exceeding 4 mg/L indicate that paper insulation is nearing its end-of-life threshold.

As another critical marker, **5-hydroxymethylfurfural (HMF)** helps distinguish the relative contributions of hydrolytic versus oxidative degradation pathways, making it especially valuable for diagnosing mixed aging conditions.

Role of Moisture: A Synergistic Aging Accelerant

Moisture acts both as a direct reactant in cellulose hydrolysis and as a trigger for dielectric breakdown at the oil-paper interface. In a well-dried new transformer, paper moisture content is typically below 0.3% (w/w); however, during service life, moisture accumulates to 1–3% due to aging of breather gaskets, seal failure, and slow interfacial diffusion between oil and paper. Each 1% increase in paper moisture content accelerates hydrolytic degradation rates by several-fold and significantly elevates dielectric loss factor (tanδ), while reducing partial discharge inception voltage (PDIV) and dielectric breakdown strength. Within the temperature range of 0–100 °C, moisture in paper exhibits Curie-von Schweidler relaxation behavior in its dielectric response, with a distinct tanδ peak observed near 60–80 °C.

Temperature Classes and End-of-Life Criteria

Temperature Class Classification

According to IEC 60085 and IEEE C57.12.00, paper-insulated windings primarily fall into the following temperature classes:

• **Class A (105 °C)**: Conventional kraft paper + mineral oil; the most common insulation system for oil-immersed power transformers
• **Class E (120 °C)**: Thermally upgraded kraft paper (TUK); widely adopted in distribution and traction transformers
• **Class B (130 °C)**: High-purity mineral oil + partial synthetic esters; rarely used with paper-wrapped conductors
• **Class F (155 °C)**: Synthetic ester (e.g., Midel 7131) or silicone oil + TUK; applied in specialized traction and locomotive transformers

Classes A and E dominate paper-wrapped conductor applications; higher classes are typically achieved through synergistic effects of insulating liquids or varnishes. Class H (180 °C) and above generally rely on high-temperature synthetic papers—such as Nomex 410 or Kapton PI—rather than conventional cellulose-based papers.

Montsinger Rule and Equivalent Lifetime

The classical Montsinger empirical rule states that the service life of cellulose insulation is approximately halved for every 8–10 °C increase in temperature. This rule applies within the 80–150 °C range and aligns closely with the guidance in IEC 60076-7, which specifies a halving of lifetime for every 6 K rise in hotspot temperature. Based on this rule:

• Continuous operation at 95 °C: kraft paper lifetime = 20–25 years
• Continuous operation at 105 °C: lifetime = 12–18 years
• Continuous operation at 115 °C: lifetime = 5–8 years
• Continuous operation at 130 °C: lifetime = 1–2 years

In contrast, TUK and thermally upgraded crepe paper (TUCP), enhanced with chemical additives such as dicyandiamide and melamine to neutralize acidic degradation by-products, shift the Arrhenius lifetime curve to the right—enabling continuous operation at 110 °C for over 30–40 years.

Thermal End-of-Life Criteria for Paper Insulation

The industry commonly adopts any one of the following criteria as the thermal end-of-life threshold for paper insulation: ① Degree of polymerization (DP) declines to 200–250; ② Tensile strength drops below 50% of its initial value; ③ Breakdown electric field strength falls below 50% of its initial value; ④ Cumulative furfural concentration in oil exceeds 4 mg/L; ⑤ Rate of increase of CO + CO₂ concentration in oil surpasses a critical threshold. Employing multiple criteria in combination enhances the reliability of life assessment.

Thermal–Electrical Coupling: Temperature Effects on Dielectric Properties

Temperature Dependence of PDIV and Breakdown Field Strength

The dielectric breakdown strength of paper-insulated winding wire exhibits pronounced temperature dependence. For oil-impregnated cellulose paper, the AC breakdown field strength gradually decreases from approximately 50–70 kV/mm at 25–120 °C to about 35–50 kV/mm; beyond 130 °C, the decline accelerates sharply, dropping below 25 kV/mm at 150 °C. This behavior arises from three primary mechanisms: ① Redistribution of moisture within cellulose with increasing temperature—water migrates toward hotter regions, forming localized high-moisture zones; ② Reduced viscosity of mineral oil, lowering the energy barrier for bubble detachment and promoting growth of larger bubbles; ③ Accelerated diffusion of copper ions, which catalyze oxidative degradation of cellulose, generating additional oxygen-containing polar functional groups and thereby increasing dielectric loss. PDIV (Partial Discharge Inception Voltage) exhibits comparable thermal sensitivity: over the 20–100 °C range, PDIV typically declines at a rate of 5–8% per 10 °C.

Temperature Response of tanδ

For oil-impregnated paper, the dielectric loss factor (tanδ) is approximately 0.002–0.004 at 25 °C and 50 Hz. With rising temperature, tanδ displays two distinct relaxation peaks: the first occurs in the 60–80 °C range and corresponds to dipolar relaxation of moisture and polar by-products; the second appears at 130–150 °C and reflects segmental motion of cellulose chains and interfacial polarization. When paper moisture content increases to 1–2%, the tanδ peak in the 60–80 °C range rises from ~0.005 to >0.015, exhibiting the characteristic “high-temperature tanδ peak of wet paper.” IEC 60076-1 and IEC 60270 explicitly require that, prior to transformer commissioning, the tanδ versus temperature curve be measured under 1.0× rated voltage across the 20–90 °C range, verifying monotonic increase and compliance with the limit of tanδ ≤ 0.005 at 90 °C.

Space Charge and Polarization Effects

Under elevated temperatures, Maxwell–Wagner interfacial polarization at the oil–paper interface intensifies significantly. The permittivity mismatch between mineral oil (εr ≈ 2.2) and cellulose paper (εr ≈ 3.5–4.5) drives strong interfacial polarization, particularly prominent in the 80–120 °C range, resulting in distortion of the internal electric field distribution under AC voltage. In HVDC converter transformers, space charge accumulates within paper layers under DC bias, further enhancing local electric field stress. The space charge decay time constant is ~10³–10⁴ s at 100 °C but extends to 10⁵–10⁶ s at 60 °C—this fundamental thermal dependence underpins the critical operational and testing principle for HVDC insulation: “low-temperature operation and low-temperature testing.”

Thermal–Mechanical Coupling: CTE Mismatch and Interface Failure

CTE Mismatch and Cyclic Thermal Stress

The coefficient of thermal expansion (CTE) of copper conductors is approximately 17 × 10⁻⁶/K, while that of aluminum conductors is about 23 × 10⁻⁶/K. In contrast, cellulose paper exhibits anisotropic CTE: ~30–40 × 10⁻⁶/K along the fiber direction and ~60–80 × 10⁻⁶/K perpendicular to the fiber direction. Repeated thermal cycling between 80–120 °C induces significant interfacial shear stress at the conductor–paper interface: a single temperature change of 30 K can generate 5–10 MPa shear stress at the paper–copper interface. After thousands of thermal cycles in service, microcracks or localized delamination may develop between the paper insulation and the conductor, providing pathways for oil penetration and bubble nucleation.

Bubble Formation and Bubble-Induced Failure

In oil-immersed transformers, rapid load transitions from full-load to no-load can cause winding temperatures to drop by 30–50 K within tens of minutes. During such cooling, moisture in the paper and dissolved gases in the oil become locally supersaturated due to the abrupt decrease in solubility, leading to bubble formation. Under electric field stress, these bubbles trigger partial discharges (PD), substantially lowering the partial discharge inception voltage (PDIV). Studies show that even a 0.5-mm-diameter bubble can produce detectable PD at only 1 kV. IEC 60076-7 provides a “bubble safety curve” for load switching, specifying maximum permissible load-decline rates to prevent rapid localized cooling at hotspots and subsequent bubble formation.

Winding Loosening and Mechanical Strength Degradation

With increasing service age, the elastic modulus of paper insulation declines from an initial 4–6 GPa to below 2 GPa, while its elongation-at-break drops from 3–5% to less than 1%. As the paper embrittles, the winding’s resistance to electromagnetic forces during short-circuit events deteriorates markedly; both axial and radial stiffness decrease significantly. If paper layers fracture prematurely during short-circuit mechanical stress, irreversible geometric deformation accumulates in the winding, potentially culminating in winding collapse. This is why IEC 60076-5 explicitly mandates short-circuit withstand capability assessment for aging transformers and recommends appropriate derating during operation.

Thermal–Chemical Coupling: Moisture, Oxygen, and Copper Catalysis

Temperature Amplification of Hydrolytic Aging

The activation energy for cellulose hydrolysis is approximately 110 kJ/mol for conventional kraft paper or 130 kJ/mol for thermally upgraded kraft (TUK) paper. According to the Arrhenius model, the hydrolysis rate constant *k* follows:

*k* = *A* · exp(−*E*_a / *RT*)

where *A* is the pre-exponential factor (frequency factor), *R* is the universal gas constant, and *T* is the absolute temperature. A temperature increase of 10 °C accelerates the hydrolysis rate by a factor of ~1.8–2.2. This pronounced temperature amplification effect renders “hot-spot temperature” the dominant controlling parameter for paper insulation lifetime—underpinning the “6 K rule” (halving of lifetime per 6 K rise in temperature) specified in IEEE C57.91.

Oxidation and Copper Catalysis

Copper conductors in oil–paper systems release Cu⁺/Cu²⁺ ions, which strongly catalyze cellulose oxidation reactions. Under conditions of temperature >100 °C and presence of dissolved oxygen in the oil, copper catalysis can accelerate oxidative degradation by a factor of 5–10. This explains why IEC 60296 mandates that copper content in new transformer oil be limited to <0.2 mg/kg and emphasizes deaeration and dehydration during oil processing.

Oil Degradation and Thermal Synergy

Mineral oil begins significant oxidation above 120 °C, generating acidic compounds, peroxides, and sludge. These polar by-products, in turn, accelerate both hydrolytic and oxidative degradation of paper insulation. Synthetic esters (e.g., Midel 7131) and natural esters (e.g., FR3, Envirotemp FR3) exhibit markedly superior oxidation stability compared to mineral oil, with antioxidant service temperatures reaching 150–170 °C. The combination of TUK paper with natural ester fluid represents a preferred solution for high-temperature, long-life oil-immersed transformers.

Comparative Thermal Response of Different Paper Types

Conventional Kraft Paper

Kraft paper (sulfate process kraft paper) is the mainstream base material for transformer paper insulation, manufactured from unbleached softwood sulfate pulp, with standard thicknesses of 0.05/0.08/0.10/0.13 mm. Its dielectric strength is ≥30 kV/mm (after oil impregnation), tensile strength along the fiber direction is ≥4.5 kN/m, and perpendicular to the fiber direction is ≥2.0 kN/m. However, kraft paper is rated only for Class A thermal endurance, with a long-term hotspot temperature range of 95–105 °C and an expected service life of 20–25 years.

TUK — Thermally Upgraded Kraft Paper

TUK (thermally upgraded kraft paper) incorporates chemical additives—such as dicyandiamide, melamine, and polyacrylamide—into the kraft pulp during manufacturing, ensuring uniform distribution of these additives throughout the cellulose matrix. During aging, the additives are gradually released and neutralize acidic by-products (e.g., acetic acid, formic acid), thereby retarding acid-catalyzed hydrolysis. TUK elevates the thermal class to Class E (120 °C), enabling a continuous operating life of 30–40 years at 110 °C.

TUCP — Thermally Upgraded Crepe Paper

TUCP (thermally upgraded crepe paper) combines the mechanical conformability of crepe paper with the chemically enhanced aging resistance of TUK. Its creped structure provides elongation of approximately 30–50%, making it especially suitable for irregular conductor surfaces, lead-out regions, and locations subject to high bending stress. TUCP is widely used in traction transformer lead-outs and high-voltage winding end-region insulation of power transformers.

DDP — Diamond-Dotted Paper

DDP (diamond-dotted paper) features discrete epoxy resin dots applied onto the paper substrate at intervals of 10–30 mm. In the dry state, these resin dots remain solid and non-conductive. During transformer winding vacuum drying and hot-press curing (at ~100–130 °C), the resin dots melt and subsequently cure, forming mechanical bonding between winding layers. DDP significantly enhances short-circuit withstand capability and vibration resistance; however, due to the increased dielectric interfaces introduced by the resin dots, interfacial compatibility between the resin and paper must be carefully evaluated under thermal aging conditions.

Crepe Paper

Crepe paper is mechanically crimped to achieve elongation of 50–200%, specifically designed for irregular conductors (e.g., rectangular conductors, transposed conductor CTC lead-outs) and regions subjected to concentrated bending stress. Its more open, less dense fiber structure enables oil impregnation rates 2–3 times faster than those of kraft paper, though its dielectric strength is slightly lower (25–40 kV/mm after oil impregnation). Crepe paper is commonly used in combination with kraft paper: kraft serves as the inner layer providing dielectric strength, while crepe paper forms the outer layer delivering mechanical cushioning and stress relief.

Temperature Monitoring and Thermal State Assessment

Optical Fiber-Based Temperature Measurement and Hotspot Localization

Conventional resistance temperature detectors (RTDs) and thermocouples can only measure either the average winding temperature or localized representative points, failing to capture true hotspots. In contrast, Fiber Bragg Grating (FBG) sensors and distributed Raman optical fiber temperature sensing systems enable real-time, full-length temperature profiling of windings with a spatial resolution of 0.5 m—accurately identifying hotspot locations and temperature rise gradients. This technology has become the mainstream solution for thermal state monitoring in HVDC converter transformers and large-capacity power transformers.

IEEE C57.91 Loading Guide

IEEE C57.91 provides a loading assessment methodology based on “equivalent aging,” with the key metric being the “relative aging rate” *V* (*V* = 1 corresponds to the design-life aging rate). When *V* remains persistently above 1, insulation enters accelerated aging; a *V* > 4 triggers an early warning; and *V* > 8 mandates immediate load reduction. This model uses hotspot temperature as its primary input and outputs a statistical estimate of remaining paper insulation life.

IEC 60076-7 Hotspot Temperature Rise Limits

IEC 60076-7 specifies hotspot temperature rise limits for oil-immersed power transformers: under ONAN cooling, average winding temperature rise ≤ 65 K and hotspot temperature rise ≤ 78 K; under ONAF cooling, average rise ≤ 70 K and hotspot ≤ 83 K; under OFAF cooling, hotspot ≤ 93 K. Operation exceeding these limits constitutes “overnameplate operation,” which significantly shortens insulation service life.

Dissolved Gas Analysis (DGA)

DGA diagnoses internal faults by quantifying concentrations of dissolved gases in insulating oil—including H₂, CH₄, C₂H₆, C₂H₄, C₂H₂, CO, and CO₂. Among these, CO and CO₂ are characteristic gases generated during cellulose degradation. IEC 60559 and IEEE C57.104 provide diagnostic criteria based on “absolute gas generation rates” and the “Duval triangle.” The Duval triangle classifies faults into three categories: discharge (D), thermal fault (T), and mixed (DT). Within the T region, thermal faults are further subdivided into T1 (< 300 °C), T2 (300–700 °C), and T3 (> 700 °C)—all closely correlated with thermal aging of paper-insulated magnet wire.

Furfural Analysis and Degree of Polymerization (DP) Estimation

ASTM D5837 and IEC 61198 specify high-performance liquid chromatography (HPLC) methods for determining furfural concentration in transformer oil. An empirical relationship links furfural concentration (*F*, in mg/L) to the degree of polymerization (DP) of cellulose insulation:

DP ≈ 1850 / (log₁₀(*F* + 0.8)) − 100

This empirical formula enables estimation of the average DP of paper insulation without requiring core lifting or physical sampling. When combined with 6–8 years of trend data, it yields a reliable prediction of remaining insulation life.

Accelerated Thermal Aging Tests and Modeling

Laboratory Accelerated Aging

The laboratory accelerated aging protocols recommended by IEC 60076-7 and IEEE C57.91 involve continuous loading of either full-scale transformer models or paper insulation specimens at three temperature levels—110 °C, 130 °C, and 150 °C—while monitoring degradation curves of key parameters: degree of polymerization (DP), dielectric breakdown strength, dielectric loss factor (tanδ), and furfural concentration. Lifetime extrapolation to service temperature is performed using the Arrhenius model. This approach enables acquisition of equivalent lifetime data corresponding to 20–40 years of service within 6–12 months.

Arrhenius Parameter Identification

The Arrhenius lifetime model is expressed as: t = B · exp(Eₐ / RT) where *t* is lifetime (hours), *B* is a pre-exponential constant, *Eₐ* is activation energy (kJ/mol), *R* is the universal gas constant (8.314 J/(mol·K)), and *T* is absolute temperature (K). After measuring lifetimes at the three specified temperatures, linear regression of ln(*t*) versus 1/*T* yields both *Eₐ* and *B*. Typical activation energies are: ~105–115 kJ/mol for conventional kraft paper; ~125–140 kJ/mol for thermally upgraded kraft paper (TUK); and ~80–95 kJ/mol for mineral oil.

Multi-Stress Coupling Model

In actual transformers, thermal stress does not act in isolation: electric field (~2–3 kV/mm), mechanical vibration (10–100 Hz), oil-flow-induced shear, and moisture collectively degrade paper insulation. Multi-stress lifetime modeling employs either Miner’s linear cumulative damage law or the Eyring-corrected model to synthesize contributions from individual stresses via weighted summation: Lₜₒₜₐₗ = 1 / Σ(1 / Lᵢ) where *Lᵢ* denotes the lifetime under the *i*-th stress acting alone. This methodology is especially critical for lifetime assessment of transformers operating under extreme conditions—such as HVDC converter transformers and electric arc furnace transformers.

Thermal Shock and Transient Overload Response

Short-Term Overload Capability

Transformers are permitted to operate under short-term overload conditions during emergencies. Typical guidelines include: ① 1.1 times rated load for 30 minutes; ② 1.3 times rated load for 30 minutes; ③ 1.5 times rated load for 15 minutes. During overload, the hotspot temperature may temporarily exceed 120 °C (Class E), and instantaneous degradation of the degree of polymerization (DP) is acceptable—provided that the load is promptly reduced afterward and the hotspot temperature is maintained at ≤95 °C for several hours to “heal” the thermal history. Both IEC 60076-7 and IEEE C57.91 explicitly permit such short-term overloads.

Cold-Start Thermal Shock

Upon re-energization after shutdown, winding temperature may rise abruptly from as low as –20 °C (in cold outdoor environments) to above 80 °C, inducing significant thermal stress between paper insulation layers and conductors. Research indicates that mobile transformers (e.g., wind-power pad-mounted units) subjected to repeated energization/de-energization cycles within the –10 °C to +30 °C ambient range exhibit a 3–5 times higher probability of paper-wrapped conductor delamination compared to units operating continuously at ambient temperature. Preheating is recommended prior to full-load energization when ambient temperature falls below –5 °C.

Cooling After Fault Clearance

Following clearance of a short-circuit fault, winding temperature may drop rapidly—from above 200 °C to approximately 80 °C—within seconds, resulting in severe thermal shock. Rapid oil flow-induced cooling near hotspots can cause bubble formation within the paper insulation, leading to a 30–50% reduction in partial discharge inception voltage (PDIV) within one hour after cooling. Consequently, transformers must not be immediately re-energized following fault clearance; adequate cooling and degassing of dissolved gases in the insulating oil are required prior to restart.

Impact of Manufacturing Processes on Thermal Behavior

Vacuum Drying and Moisture Content Control

Paper-insulated magnet wire windings must undergo vacuum drying prior to oil impregnation, with the target paper moisture content reduced to below 0.3%. A typical process comprises: ① Preheating at 80–110 °C for 12–24 hours; ② Vacuum application at <100 Pa for 24–48 hours; ③ Further vacuum reduction to <50 Pa for an additional 12–24 hours. Excess moisture by just 1% can reduce cellulose insulation lifetime by 5–10 years.

Oil Impregnation and Elimination of Air Gaps

Vacuum impregnation is another critical process: degassed insulating oil (moisture content <10 ppm, dissolved gas content <1%) is introduced under vacuum, enabling capillary action and pressure differential to drive oil penetration into every inter-fiber gap within the paper layers. Residual air gaps constitute the primary source of partial discharge (PD), potentially reducing the partial discharge inception voltage (PDIV) by 30–50%.

Compaction Force and Interlayer Bonding

When Diamond Dotted Paper (DDP) is used in combination with kraft or thermally upgraded kraft (TUK) paper, thermal compression molding of the winding is a key manufacturing step. Insufficient compaction force (<0.5 MPa) increases the risk of interlayer air gaps and delamination; excessive compaction (>5 MPa) may damage cellulose fibers, thereby shortening insulation life. The optimal compaction force range is typically 1–3 MPa.

Thermal Considerations in Design and Selection

Engineering Control of Hot-Spot Temperature Rise

The following measures can be implemented during design to control hot-spot temperature rise: ① Reducing current density (trading cost for extended service life); ② Increasing the number and width of oil ducts (enhancing ONAN/ONAF cooling efficiency); ③ Employing low-loss core steel (minimizing core loss conversion to heat); ④ Optimizing winding arrangement (reducing stray-flux-induced additional losses); ⑤ Using synthetic ester fluid (e.g., Midel 7131) to raise allowable hot-spot temperature.

Paper-Type Selection Decision Tree

• **Class A oil-immersed transformer + conventional mineral oil** → Select kraft paper
• **Class E oil-immersed transformer + mineral oil + long-service-life requirement** → Select thermally upgraded kraft (TUK) paper
• **Traction and locomotive transformers (high vibration + Class E)** → Select thermally upgraded crepe paper (TUCP) or TUK combined with crepe paper
• **HVDC converter transformers (DC bias superimposed + elevated operating temperature)** → Select TUK combined with mineral oil or natural ester (e.g., FR3, Envirotemp FR3)
• **Wind-power pad-mounted transformers (large diurnal temperature swings + frequent load fluctuations)** → Select TUK combined with natural ester
• **High-impedance electric furnace transformers (enhanced short-circuit withstand capability)** → Select diamond-dotted paper (DDP) combined with TUK

Trade-off Between Service Life and Reliability

A typical design target is a **30-year service life**, with a hot-spot temperature safety margin of **5–10 K** reserved to avoid operation at failure boundaries. For transformers operating under extreme conditions—such as HVDC, electric furnace, and wind-power applications—the service life target is often increased to **40 years**, necessitating integrated solutions comprising **TUK + natural ester + optimized cooling system**.

Conclusions and Outlook

The insulation behavior of paper-wrapped magnet wire under thermal stress exhibits a multi-physics coupling characteristic—governed collectively by Arrhenius degradation of cellulose polymers, temperature-dependent dielectric response of oil-paper systems, mechanical failure induced by coefficient of thermal expansion (CTE) mismatch, and synergistic chemical catalysis between copper and moisture. These interrelated mechanisms jointly determine the service life of oil-immersed power transformers. Temperature classes (e.g., Class A at 105 °C and Class E at 120 °C) establish a life-prediction framework via the Montsinger rule and the Arrhenius model, enabling engineers to quantitatively assess the impact of thermal operating conditions on transformer lifetime during the design phase.

Distinct thermal responses among different paper types—kraft paper, thermally upgraded kraft (TUK), thermally upgraded crepe paper (TUCP), diamond-dotted paper (DDP), and crepe paper—offer optimization opportunities for diverse application scenarios. TUK combined with natural ester fluid is emerging as the mainstream solution for high-temperature, long-life transformers; DDP demonstrates superior performance in withstanding short-circuit electromagnetic forces; and crepe paper remains irreplaceable for irregularly shaped conductors and regions subject to stress concentration.

Future development directions include: ① Online lifetime assessment systems integrating fiber-optic temperature monitoring and furfural trend analysis; ② Exploration of nanocellulose and high-performance synthetic papers for higher temperature classes (Class F at 155 °C and Class H at 180 °C); ③ Multi-stress coupled lifetime modeling based on digital twin technology; ④ Penetration of environmentally friendly natural ester fluids (e.g., FR3, Envirotemp FR3) into HVDC and renewable-energy transformers; ⑤ Rapid online detection techniques for cellulose degradation by-products (e.g., mid-infrared spectroscopy).

As power systems evolve toward higher capacity, compactness, and enhanced reliability, research into the thermal behavior of paper-wrapped conductors will continue to provide a scientific foundation for transformer design, operation, and maintenance.