Introduction
As the core insulation material of high-voltage transformer windings, the operating state of paper-sheathed wire directly affects the safety and reliability of the transformer and even the entire power system.
During long-term operation, the paper-sheathed wire insulation system inevitably suffers from the combined effects of multiple stresses, including electrical, thermal, mechanical, and chemical stresses, gradually leading to performance degradation and ultimately insulation failure.
Insulation failure is one of the most common failure modes of transformers.
Once the paper-sheathed wire insulation fails, it can cause anything from a decrease in transformer performance and shortened lifespan to serious accidents such as short circuits, fires, and even explosions, resulting in widespread power outages and huge economic losses.
Therefore, a deep understanding of the mechanism of paper-sheathed wire insulation failure, identification of key risk factors, and mastery of scientific diagnostic and prevention methods are of significant practical engineering importance for the safe operation of power equipment.
This article systematically analyzes the main types of insulation failure in paper-insulated wires and their formation mechanisms, focusing on five aspects of failure causes: thermal aging, electrical aging, mechanical stress, environmental factors, and manufacturing defects.
It also introduces corresponding diagnostic techniques and preventative measures, providing technical reference for transformer design, manufacturing, and maintenance personnel.
Overview of Paper-Insulated Wire Insulation System
Insulation System Structure
The insulation system of paper-insulated wire is a multi-layered composite structure, consisting of a conductor layer, an insulation layer, and an insulating paper layer from the inside out.
The conductor is typically made of copper or aluminum and is responsible for conductivity.
The insulation layer is coated on the conductor surface, providing basic electrical insulation.
The insulating paper layer is wound around the outside of the enameled wire, forming a reinforced insulating protective layer.
In oil-immersed transformers, the paper-insulated wire insulation system also works in conjunction with transformer oil.
The transformer oil fills the voids in the insulating paper, forming a paper-oil composite insulation system, significantly improving overall insulation performance.
The porous structure of insulating paper allows it to absorb and retain transformer oil, which significantly improves its electrical strength and heat dissipation.
Performance Parameter Evaluation of Insulating Materials
The performance of paper-insulated wire systems requires comprehensive consideration of multiple technical parameters.
Dielectric strength is a core indicator of an insulating material’s ability to withstand electric field strength, typically expressed in kV/mm.
Kraft paper has a dielectric strength of approximately 40 to 60 kV/mm, while NOMEX aromatic polyamide paper can reach over 80 to 100 kV/mm.
The dielectric loss factor (tanδ) reflects the degree of energy loss of the insulating material in an alternating electric field and is an important parameter for assessing the degree of insulation aging.
The dielectric loss factor increases significantly after the insulating material ages.
Partial discharge performance characterizes the insulating material’s ability to withstand micro-discharges in high electric field regions, directly affecting the long-term reliability of the insulation system.
Tensile strength and elongation reflect the mechanical properties of the insulating material and are closely related to the winding process and its ability to withstand mechanical stress during operation.
Stages of Insulation Failure
Stages of Insulation Failure Insulation failure is a gradual process, typically divided into three stages.
The first stage is the latency period, during which subtle chemical or physical changes occur within the insulation material, but there are no obvious external abnormalities, and electrical performance indicators show minimal changes.
The second stage is the development stage, where insulation deterioration worsens, partial discharge increases, the dielectric loss factor rises, and detectable changes appear in the insulation performance indicators.
The third stage is the failure stage, where the insulation material breaks down or loses its function, leading to transformer failure.
The duration of the three stages of insulation failure varies and is closely related to factors such as stress intensity, material properties, and operating conditions.
Under severe stress conditions, insulation may directly transition from the latency period to the failure stage, manifesting as a sudden failure.
Therefore, online monitoring and periodic inspection of the insulation condition are crucial, allowing for timely detection and intervention during the failure development stage.
Thermal Aging Failure
Mechanism of Thermal Aging
Mechanism of Thermal Aging Thermal aging is one of the most common causes of paper-insulated wire insulation failure.
Under high-temperature conditions, the insulating paper and enameled layer undergo complex chemical reactions, leading to a gradual deterioration of material properties.
The main component of insulating paper is cellulose.
Cellulose molecules undergo depolymerization, oxidation, and hydrolysis under high temperatures, resulting in molecular chain breakage, a decrease in the degree of polymerization, and a reduction in mechanical properties.
The thermal class of kraft paper insulation is typically 105°C to 120°C.
When the operating temperature exceeds the design limit, the thermal aging rate accelerates significantly.
Studies show that for every 8°C to 10°C increase in temperature, the expected lifespan of the insulating paper is reduced by approximately half.
This rule of thumb, known as the “10°C rule,” is an important basis for assessing the thermal aging of transformers.
In oil-immersed transformers, the insulating paper and transformer oil coexist and interact.
During thermal aging, the transformer oil undergoes oxidation and rancidity reactions, and the generated organic acids and moisture accelerate the hydrolytic aging of the insulating paper.
Conversely, products such as furfural produced during the aging of the insulating paper also affect the performance of the transformer oil.
This synergistic effect makes the thermal aging process of oil-immersed transformers more complex.
Factors Affecting Thermal Aging
Factors Affecting Thermal Aging Many factors influence the thermal aging of paper-insulated wire insulation, primarily including temperature, temperature duration, oxygen content, and moisture content.
Temperature is the dominant factor in thermal aging, directly determining the rate of chemical reactions.
Higher temperatures lead to more vigorous molecular motion, higher frequency of chemical bond breakage and recombination, and faster aging.
Fluctuations in operating temperature also affect the thermal aging process.
Temperature cycling caused by transformer load changes accelerates the thermal fatigue of insulating materials, leading to the generation and propagation of microcracks.
Aging under temperature cycling conditions is often more severe than under constant high temperatures.
Oxygen is a promoting factor for thermal aging.
Dissolved oxygen in oil-immersed transformers participates in the oxidation reaction of the insulating paper.
Vacuum drying can effectively reduce the dissolved oxygen content in the transformer oil, thereby slowing down the thermal aging rate.
Moisture is another important influencing factor.
Insulating paper has strong water absorption; insulating paper containing moisture is more prone to hydrolysis at high temperatures.
The decrease in the degree of polymerization of insulating paper during aging further reduces its ability to remain dry, creating a vicious cycle.
Identification Characteristics of Thermal
Identification Characteristics of Thermal Aging After thermal aging, insulating paper exhibits a series of detectable characteristic changes.
In terms of appearance, aged insulating paper darkens in color, gradually changing from light yellow to brownish-red, and in severe cases, it becomes black and brittle.
In terms of mechanical properties, tensile strength and elongation decrease significantly, and aged insulating paper is prone to breakage and detachment.
Chemical analysis can detect a decrease in the degree of polymerization and an increase in furfural content.
The degree of polymerization (DP) is a parameter characterizing the length of cellulose molecular chains.
The degree of polymerization of new insulating paper is typically above 1000, while it can drop below 200 after severe aging.
Furfural is a characteristic product of cellulose thermal degradation; detecting the furfural content in insulating paper using high-performance liquid chromatography (HPLC) can assess the degree of thermal aging.
Chemical analysis of the oil can also provide indirect information about insulation aging.
During the aging process, products such as moisture, organic acids, and metal particles accumulate in the oil.
Regular testing of the transformer oil’s acid value, moisture content, and dissolved gas analysis (DGA) can help determine the general condition of the insulation system.
Electrical Aging Failure
Impact of Partial Discharge
Impact of Partial Discharge Partial discharge (PD) is the main cause of electrical aging in paper-insulated wire.
Under high voltage, a small discharge phenomenon occurs at locations with defects inside or on the surface of the insulation material; this is called partial discharge.
Although the discharge energy of partial discharge is small, its long-term continuous action gradually erodes the insulation material, eventually leading to breakdown.
The destructive mechanism of partial discharge on insulation materials includes both physical bombardment and chemical corrosion.
High-speed charged particles generated during the discharge process directly bombard the surface of the insulation material, causing molecular chain breakage and micropore formation.
Simultaneously, reactive gases such as ozone and nitrogen oxides generated during the discharge react chemically with the insulation material, accelerating the deterioration process.
Insulation aging caused by partial discharge manifests in various forms.
The fiber structure of the insulating paper gradually becomes loose and fragile, and fine cracks and pits appear on the surface.
The accumulated heat generated by the discharge also leads to a local temperature increase, further accelerating thermal aging.
This combined electro-thermal aging is often more destructive than simple thermal aging.
Discharge Types and Characteristics
Discharge Types and Characteristics Common discharge types in transformer insulation systems include internal discharge, surface discharge, and corona discharge.
Internal discharge occurs at bubbles or defects within the insulating material.
Because the dielectric constant of bubbles is much lower than that of the surrounding solid material, the electric field strength concentrates at the bubbles, easily triggering discharge.
Surface discharge occurs along the interface between solid insulation and gas (such as transformer oil), usually related to improper insulation structure design or surface contamination.
Corona discharge occurs at sharp or protruding points on the conductor surface, where the gas in the high electric field region ionizes, generating discharge.
The discharge pulse characteristics and severity vary among different types of discharge.
Internal discharge is the most damaging to insulation because the active material generated by the discharge is trapped inside the insulation and difficult to dissipate.
Although surface discharge is relatively less harmful, it gradually erodes the insulation surface, reducing insulation strength.
Laws of Insulation Strength Decrease
Under the continuous action of partial discharge, the electrical strength of paper-insulated wire insulation gradually decreases.
Studies have shown that insulation aging caused by partial discharge follows a specific law of damage accumulation.
The greater the discharge quantity and the longer the duration, the more significant the decrease in insulation strength.
When the partial discharge level exceeds a certain threshold, insulation breakdown may occur within a short period of time.
There is an approximately inverse relationship between insulation breakdown voltage and discharge duration.
Under high discharge levels, insulation may break down within hours to days.
Under low discharge levels, insulation can withstand tens or even hundreds of thousands of hours of discharge before finally failing.
This pattern provides an important basis for condition-based maintenance of transformers.
Partial discharge measurement is an important means of assessing insulation condition.
By detecting the apparent discharge quantity, number of discharges, and discharge phase distribution of partial discharges, the degree and trend of insulation aging can be determined.
National standards have clear provisions on the partial discharge level of transformers, and transformers in operation should undergo partial discharge tests periodically.
Mechanical Stress Failure
Mechanical Stress in Operation
During operation, the windings of a transformer are subjected to various mechanical stresses.
The magnetic field generated by the load current interacts with the winding current, generating electromagnetic force.
Under normal operating conditions, the electromagnetic force is relatively small.
However, during a short-circuit fault, the short-circuit current can reach tens of times the rated current, resulting in a sharp increase in electromagnetic force, which may cause winding deformation, displacement, or even collapse.
The mechanical effect of electromagnetic force on the paper-insulated wire insulation is mainly manifested in two aspects.
First, axial force, which may cause the insulation pads at the winding ends to loosen, or the winding to loosen or stand up.
Second, radial force, which may cause the inner winding to be pressed outward and the outer winding to be pressed inward, resulting in winding shape instability.
Repeated mechanical stress can lead to fatigue cracks and delamination in the insulation material.
Vibration is another common source of mechanical stress.
The transformer core will undergo magnetostriction under the action of magnetic force, causing vibration of the core and windings.
Vibration is transmitted to the windings through the core, causing fatigue damage to the insulation material during long-term operation.
Vibration may also cause friction between the winding conductors, accelerating the wear of the insulating paper layer on the surface of the paper-insulated wire.
Impact of Short-Circuit Faults
Impact of Short-Circuit Faults Short-circuit faults are the operating conditions under which the transformer experiences the most severe mechanical stress.
When a transformer experiences an outlet short circuit, the peak short-circuit current can reach 20 to 30 times the rated current, and the resulting enormous electromagnetic force can cause severe deformation of the windings.
Damage to the paper-insulated wire insulation from short-circuit faults includes axial displacement of the windings, radial deformation of the windings, and damage to the conductor insulation.
Axial electromagnetic forces can cause the winding end clamping devices to fail, leading to winding loosening.
Radial electromagnetic forces generate opposing forces between the inner and outer windings, potentially causing the inner winding to be crushed and the outer winding to be stretched or even stretched apart.
Even if the short-circuit current does not cause immediate damage to the insulation, the short-circuit impact will accelerate the aging process of the insulation.
The combined effect of mechanical damage and electrical and thermal stress will significantly shorten the remaining life of the transformer.
Studies have shown that transformers subjected to multiple short-circuit impacts experience a significantly faster insulation aging rate.
Transportation and Installation Damage
Transportation and Installation Damage Paper-insulated wire insulation may suffer mechanical damage during transportation and installation.
Bumps, collisions, and vibrations during transportation can cause cracks, detachment, or deformation of the insulation layer.
Improper handling during installation, such as impacts, excessive bending, or stretching, can also damage the insulation.
The minimum bending radius of paper-insulated wire is an important parameter in the design and installation process.
A bending radius that is too small will cause the insulation paper layer to crack or peel off, affecting insulation performance.
For paper-insulated wire with a rectangular cross-section, stress concentration at corners is more pronounced and requires special protection.
On-site inspection and insulation testing before commissioning are important means of detecting mechanical damage.
Ultrasonic testing can detect internal cracks and delamination in the insulation.
The low-voltage bridge method can measure the capacitance and dielectric loss of the insulation, and detect problems such as moisture absorption and aging.
Environmental Factors Failure
The Effect of Moisture
The Effect of Moisture Moisture is one of the main environmental factors causing insulation failure in paper-insulated wire.
Insulating paper has strong hydrophilicity and can absorb moisture from the air or oil.
Insulating paper containing moisture will have significantly reduced electrical and mechanical strength.
The main sources of moisture in insulating paper include: residual moisture from incomplete insulation drying, moisture generated by the oxidation reaction of transformer oil, moisture absorbed from the outside through respiration, and moisture produced by the aging and decomposition of the insulating paper.
Throughout the entire lifespan of the transformer, the moisture content of the insulating paper gradually increases.
Moisture affects the electrical properties of insulation in several ways.
First, moisture reduces the dielectric strength of the insulating paper; in high electric field regions, moisture easily accumulates to form conductive channels.
Second, moisture increases the dielectric loss of the insulation, accelerating insulation aging.
Furthermore, moisture promotes the oxidation of transformer oil, producing acidic substances that further corrode the insulation.
In oil-immersed transformers, the transformer oil plays a crucial role in absorbing and removing moisture from the insulation system.
Therefore, regularly monitoring the trace moisture content of the transformer oil and performing vacuum dehydration and regeneration are important measures to maintain the dryness of the insulation.
Chemical Corrosion Chemical corrosion
Chemical Corrosion Chemical corrosion is another important environmental failure mechanism.
Transformer oil undergoes oxidation and rancidity during long-term operation, producing organic acids, moisture, and peroxides.
These substances corrode the insulating paper and enameled layer.
Organic acids are one of the main products of transformer oil oxidation.
Fatty acids and other organic acids react with cellulose in the insulating paper, causing cellulose hydrolysis and molecular chain breakage.
Organic acids also react with conductors such as copper or aluminum, forming metallic soaps that affect the conductor’s electrical properties.
Sulfide corrosion also requires attention in certain situations.
Sulfide impurities in the transformer oil may decompose under discharge or high temperatures, producing reactive sulfur that reacts with the copper conductor to form products such as cuprous sulfide.
Cuprous sulfide is conductive and reduces the insulation performance of the enameled layer.
The rate of chemical corrosion is closely related to temperature.
The higher the temperature, the faster the chemical reaction rate and the more obvious the corrosion.
Therefore, controlling the operating temperature of the transformer and maintaining the quality of the oil are effective measures to mitigate chemical corrosion.
Contamination and Foreign Matter
Contamination and Foreign Matter Contamination and foreign matter in the operating environment can also affect the reliability of paper-insulated wire insulation.
Solid contaminants such as dust and metal particles, after entering the transformer oil, will move and accumulate under the influence of an electric field, causing electric field distortion or directly forming conductive channels at weak points in the insulation.
If metal powder or conductive particles adhere to the surface of the paper-insulated wire, tiny discharge channels may be generated under a high electric field, gradually eroding the insulation layer.
Insulation fragments, burrs, and other foreign matter remaining during winding and assembly may also cause localized electric field concentration or mechanical damage.
Dissolved gas analysis (DGA) in the transformer oil is an effective method for detecting internal faults and contamination.
Different fault types produce different gas compositions and contents.
A sharp increase in hydrogen and acetylene content usually indicates a discharge fault, while an increase in methane and ethane may indicate an overheating fault.
Manufacturing Defects
Material Quality Issues
The quality of raw materials is a fundamental factor affecting the reliability of paper-insulated wire insulation.
Parameters such as the uniformity of insulation paper thickness, tensile strength, and thermal class must meet standard requirements.
Inferior insulation paper may have defects such as uneven thickness, uneven fiber distribution, and impurities, severely weakening insulation performance.
The quality of the enameled coating is equally critical.
Insufficient enameled coating thickness, poor adhesion, and excessive pinholes will reduce basic insulation performance.
The compatibility between the enameled wire and the insulation paper is also important; in some cases, incompatible material combinations may lead to interfacial delamination or chemical reactions.
The surface quality of the copper conductor or aluminum conductor also affects the overall performance of the paper-insulated wire.
Burrs, oxide layers, oil stains, etc., on the conductor surface may cause poor enameled wire adhesion or loose insulation paper winding.
Insufficient purity of the conductor material may contain impurities that affect conductivity and mechanical properties.
Process Defects Improper process
Process Defects Improper process control during manufacturing can also introduce defects.
Loose winding of insulating paper can create air bubbles and gaps between paper layers, reducing insulation strength and partial discharge performance.
Overly tight winding may damage the insulation layer or cause cracks in the paper layers.
The drying process has a significant impact on insulation quality.
Insufficient drying temperature and time will result in excessive moisture residue in the insulation, accelerating aging after commissioning.
Excessively high drying temperature or excessively long drying time may damage the insulation material itself.
Vacuum drying requires precise control of temperature, vacuum level, and processing time to achieve optimal drying results.
The impregnation process is particularly important for paper-insulated wires used in oil-impregnated transformers.
Insufficient impregnation will leave air bubbles in the insulating paper, which are prone to causing partial discharge under high electric fields.
Good impregnation should ensure that the insulating paper fibers are completely filled with transformer oil, forming a uniform and dense composite insulation structure.
Improper Design Improper transformer
Improper Design Improper transformer insulation structure design is also a significant cause of failure.
Insufficient insulation distance design can lead to excessively high electric field strength, accelerating insulation aging and even causing breakdown.
Non-uniform insulation structures can cause electric field distortion, creating over-electric fields in localized areas.
Improper conductor selection can lead to insulation stress exceeding the material’s capacity.
For example, using ordinary kraft paper-insulated wire in high-thermal-class applications will accelerate insulation aging under long-term high-temperature conditions.
Using single-layer insulation structures in high-voltage applications without sufficient margin increases the risk of partial discharge and breakdown.
Insulation coordination design needs to consider the combined effects of various stress factors.
Simply meeting one aspect is insufficient; reliability under simultaneous electrical, thermal, and mechanical stresses must also be guaranteed.
Detailed insulation strength calculations and aging assessments should be performed during the design phase, with sufficient insulation margin.
Diagnostic Techniques and Preventive Measures
Insulation Condition Monitoring Insulation
Insulation Condition Monitoring Insulation condition monitoring is a crucial means of timely detection of insulation degradation and prevention of failure.
Offline testing includes insulation resistance measurement, dielectric loss factor measurement, capacitance measurement, and partial discharge testing.
These testing methods are mature and reliable, and are routine items for transformer factory inspection and periodic maintenance.
Online monitoring technology can monitor the insulation condition of the transformer in real time during operation.
Online analysis of dissolved gases in oil can continuously monitor the content and growth rate of characteristic gases in the oil, enabling timely detection of internal faults.
Online partial discharge monitoring can acquire discharge signals in real time and analyze discharge modes and development trends.
Ultrasonic testing can detect internal cracks and discharge locations in the insulation.
Infrared thermal imaging technology can detect localized overheating areas during transformer operation, assisting in assessing the status of the insulation and cooling systems.
Vibration monitoring can detect loosening and deformation of windings and assess mechanical integrity after being subjected to short-circuit current impacts.
Maintenance Strategies Scientific maintenance
Maintenance Strategies Scientific maintenance strategies are crucial for extending the life of paper-insulated wire insulation.
Transformer oil maintenance includes regular sampling and analysis of indicators such as trace water content, acid value, and dielectric loss factor, and timely dehydration, regeneration, or replacement.
Severe oil deterioration accelerates the aging of insulating paper.
Winding temperature monitoring and control can prevent thermal aging of insulation.
For every 10°C reduction in operating temperature, the insulation life can approximately double.
Load management should avoid prolonged overload operation; load operation should be reduced when the ambient temperature is high.
Regular partial discharge testing can promptly detect discharge defects in the insulation.
An increase in partial discharge is usually a signal that insulation degradation has progressed to a certain stage, requiring timely intervention.
Short-circuit impedance measurement can detect winding deformation and displacement, assessing the mechanical condition after being subjected to a short-circuit impact.
Preventive Design Considering insulation
Preventive Design Considering insulation reliability from the design stage can effectively reduce the risk of failure during operation.
Insulation design should have sufficient margins, considering combinations of the most unfavorable operating conditions.
Material selection should match the expected operating conditions; cost savings should not be pursued at the expense of reliability.
Optimized insulation structure design can improve electric field distribution and reduce local electric field concentration.
Appropriate shielding and voltage equalization measures can effectively reduce the electric field strength in high-electric-field areas such as ends and corners.
The configuration of insulating paper layers should be uniform, avoiding abrupt changes in thickness or air bubbles.
Quality control during manufacturing is equally important.
Strict inspection of incoming raw materials is required; unqualified materials must not be used.
Process inspection points should be set up for key processes to promptly identify and correct deviations.
Factory testing should be comprehensive to ensure that every transformer meets technical requirements.
Summary
Paper-insulated wire insulation failure is a key risk factor in high-voltage transformer operation, with complex mechanisms and numerous influencing factors.
Thermal aging, electrical aging, mechanical stress, and environmental factors work together to gradually degrade insulation performance.
A deep understanding of the interactions between these failure mechanisms is crucial for developing effective monitoring strategies and preventative measures.
Prevention of paper-insulated wire insulation failure requires a multi-stage approach, encompassing design, manufacturing, operation, and maintenance.
The design phase should ensure sufficient insulation margin and a reasonable structural design; the manufacturing phase should strictly control material quality and processes; the operation phase should strengthen insulation condition monitoring and load management; and the maintenance phase should involve regular inspections and upkeep.
With the development of online monitoring and intelligent diagnostic technologies, real-time assessment of insulation condition and prediction of remaining life have become possible.
This provides technical support for the shift from periodic maintenance to condition-based maintenance, helping to improve economic efficiency while ensuring reliability.