Thermal faults in power transformers refer to abnormalities or malfunctions within the transformer system primarily caused by excessive heat or temperature-related issues.

Introduction

Thermal faults arise from a build-up of “energy” levels in a contained area or the transfer of elevated energy across a medium. IEC 60599 defines a thermal fault as an excessive temperature rise in the insulation caused typically by insufficient cooling, excessive circulating currents in adjacent metal parts or through the insulation, and overheating of internal winding or bushing connection lead [IEC 60599]. Figure 1 below provides an overview of the typical causes of thermal faults in power transformers.

External Factors

External factors may not be easily managed especially after the transformer has been designed and installed. External factors must be carefully assessed during the concept phase of purchasing a transformer.

Ambient Temperatures

The ambient conditions must be catered for taking into account both historical data and future trends to include possible changes over 20 years.

Ambient temperature affects the ability of a transformer to dissipate heat with higher ambient temperatures reducing the temperature gradient between the transformer’s internal components and the surrounding environment. This reduced cooling efficiency can lead to elevated internal temperatures, increasing the risk of thermal faults such as overheating and insulation breakdown.

Proximity to Heat Sources

The proximity of heat sources to a power transformer can negatively impact its thermal capacity by increasing ambient temperatures, reducing cooling efficiency and causing localized heating. The uneven distribution of heat within the transformer can lead to temperature gradients and hot spots, potentially accelerating insulation aging and compromising thermal capacity.

Increased Core and Winding Temperatures: External heat sources can elevate the ambient temperature around a transformer leading to a corresponding rise in the core and winding temperatures of the transformer. The higher operating temperatures accelerate the aging of insulation materials and can lead to thermal degradation.

Insulation Deterioration: The insulation system in transformers is designed to withstand certain temperature limits. Proximity to additional heat sources can push these limits, causing faster deterioration of insulation materials such as paper, pressboard, and oil. Degraded insulation can lead to partial discharges, reduced breakdown voltage, and eventually catastrophic failures like short circuits or winding faults.

Oil Degradation: Transformer oil acts as both an insulator and a coolant. Elevated temperatures due to nearby heat sources can accelerate the oxidation and decomposition of the oil. Oil degradation products, such as acids, sludge, and moisture, can further degrade insulation and reduce the cooling efficiency, creating a vicious cycle of increasing temperatures and faults.

Hot Spot Formation: Localized hot spots can form in parts of the transformer exposed to external heat sources. These hot spots cause uneven thermal stresses and can significantly affect the longevity and reliability of the transformer. Hot spots are particularly dangerous as they can go unnoticed until they cause significant damage or trigger thermal runaway conditions.

Thermal Expansion and Mechanical Stress: Heat sources can cause uneven thermal expansion within the transformer components, leading to mechanical stress. This stress can weaken structural integrity and connections, increasing the likelihood of mechanical and electrical faults. Components like bushings, winding connections, and tap changers are particularly susceptible to damage from differential expansion.

Cooling System Overload: Transformers rely on efficient cooling systems to maintain safe operating temperatures. Proximity to additional heat sources can overload these cooling systems, reducing their effectiveness. Overloaded cooling systems can lead to higher than intended operating temperatures, exacerbating the risk of thermal faults.

Mitigation Strategies:

• Proper Siting: Ensure transformers are sited away from external heat sources, such as industrial equipment, direct sunlight, or other transformers.
• Ventilation and Cooling: Enhance ventilation and cooling around the transformer to dissipate additional heat.
• Temperature Monitoring: Implement advanced temperature monitoring systems to detect and respond to abnormal temperature rises.
• Regular Maintenance: Conduct regular maintenance to check and replace degraded insulation, oil, and other components.
• Thermal Barriers: Use thermal barriers or shields to protect transformers from nearby heat sources.

Understanding and mitigating the effects of proximity to heat sources is crucial for maintaining the reliability and longevity of power transformers. By addressing these factors, the risk of thermal faults can be significantly reduced.

Internal Factors

The internal factors affecting the thermal faults of a transformer, such as effective energy dissipation mechanisms, play a critical role in maintaining optimal operating temperatures and preventing overheating.

Overheating

Overheating faults in power transformers due to overloading, circulating cuurents and insufficient cooling can lead to serious consequences, including insulation degradation, winding deformation, and even catastrophic failure.

Overloading

Overloading a power transformer significantly increases the risk of thermal faults, which can lead to serious operational issues and potential failure of the transformer.

Increased Core and Winding Temperatures: Overloading causes the transformer to operate above its rated capacity, leading to excessive current flow through the windings. This elevated current generates more heat due to the resistive losses (I²R losses) in the windings. The core also experiences higher flux densities, contributing to additional heat generation. Both these factors cause a significant rise in the internal temperatures of the transformer.

Accelerated Insulation Degradation: Higher operating temperatures due to overloading accelerate the aging and thermal degradation of insulation materials such as paper, pressboard, and transformer oil. Degraded insulation loses its dielectric strength, becoming more susceptible to electrical faults like short circuits and partial discharges, which can culminate in catastrophic failures.

Oil Degradation and Sludge Formation: Transformer oil serves as both a coolant and an insulator. Overloading increases the temperature of the oil, accelerating its oxidation and the formation of degradation products such as acids and sludge. Sludge can impede oil flow and cooling efficiency, creating hot spots that further exacerbate thermal stresses and the risk of faults.

Hot Spot Formation: Overloading leads to uneven heating, particularly in areas with higher resistance or less effective cooling. These hot spots can cause localized thermal stress, significantly affecting the longevity and reliability of the transformer. Hot spots are particularly problematic as they can lead to thermal runaway conditions, where localized heating causes further insulation degradation and increased resistance, creating a feedback loop of rising temperatures.

Cooling System Overload: The cooling system of a transformer is designed to dissipate a specific amount of heat based on the transformer’s rated capacity. Overloading can exceed the cooling system’s capacity, resulting in inefficient heat dissipation. An overwhelmed cooling system fails to maintain safe operating temperatures, leading to sustained high temperatures and increased risk of thermal faults.

Mechanical Stress and Expansion: Elevated temperatures due to overloading cause differential thermal expansion in transformer components, leading to mechanical stress. This can weaken structural integrity and connections within the transformer. Mechanical stresses can result in the deformation of windings and core, misalignment of components, and increased wear on moving parts like tap changers.

Mitigation Strategies

• Load Management: Ensure transformers operate within their rated capacity. Employ load forecasting and management strategies to avoid overloading conditions.
• Temperature Monitoring: Implement advanced temperature monitoring systems to detect early signs of overheating and initiate preventive actions.
• Regular Maintenance: Conduct routine maintenance to check for signs of insulation degradation, oil quality, and overall transformer health.
• Upgraded Cooling Systems: Enhance the cooling system capacity or employ auxiliary cooling measures to handle occasional overload conditions.
• Real-Time Load Adjustment: Use automated systems to adjust loads in real-time based on transformer temperature and loading conditions to prevent sustained overloading.

Circulating Currents

Circulating currents in power transformers can significantly impact the occurrence and severity of thermal faults. These currents typically arise due to imbalances in the transformer’s windings or connections, leading to localized heating and potentially serious damage.

The main causes of circulating currents are Magnetic Imbalance caused by unequal magnetic flux in different phases due to asymmetries in the core construction or discrepancies in winding inductance. Differences in impedance between parallel windings or between phases can lead to unequal current distribution, causing circulating currents. Phase Displacement caused by slight phase shifts between the secondary windings can create circulating currents, particularly in the delta-connected windings. Non-linear loads can introduce harmonics into the system. These harmonic currents, especially the third harmonic, can cause circulating currents in the transformer windings.

The effects of circulating currents can result from:

Eddy Currents

Eddy currents are circulating currents induced within conductors by a changing magnetic field. In a power transformer, alternating current (AC) in the windings creates a time-varying magnetic field. When this magnetic field intersects conductive materials within the transformer (such as the core, windings, or structural components), it induces eddy currents. These currents flow in loops perpendicular to the magnetic flux.

Heat Generation: Eddy currents cause localized heating due to the I²R losses (Joule heating), where ‘I’ is the current and ‘R’ is the resistance of the material. This can raise the temperature of the affected components.

Core Losses: In the transformer core, eddy currents contribute to core losses, which are part of the overall power losses. These losses manifest as heat, further increasing the thermal load on the transformer.

Localized Overheating: Eddy currents can create hotspots, areas where the temperature is significantly higher than the surrounding regions. These hotspots can degrade the insulation material, leading to insulation failure, which is a common type of thermal fault. Over time, repeated heating and cooling cycles can cause thermal stress and eventual damage to the transformer’s components.

Impact on Insulation: The insulation system in transformers is designed to withstand certain temperature limits. Excessive heat from eddy currents can accelerate insulation aging, reducing its dielectric strength and potentially leading to partial discharges or short circuits.

Mitigation Strategies

To minimize the detrimental effects of eddy currents and their contribution to thermal faults, several design and operational strategies are employed:

• Laminated Core: Transformer cores are made from thin laminated sheets of electrical steel, insulated from each other, which restricts the path of eddy currents and significantly reduces eddy current losses.
• Use of Non-Magnetic Materials: Components that might be exposed to the magnetic field are often made from non-magnetic materials or coated to reduce eddy current formation.
• Design Optimization: Careful design of the transformer windings and core to ensure uniform magnetic flux distribution and minimize areas where eddy currents could form.
• Cooling Systems: Efficient cooling systems (such as oil cooling or forced air cooling) are employed to dissipate the heat generated by eddy currents and other losses, ensuring the transformer operates within safe temperature limits.

Eddy currents are a natural consequence of the electromagnetic operation of power transformers but can lead to significant thermal issues if not properly managed. By inducing localized heating, they contribute to thermal faults, potentially compromising the transformer’s insulation and overall integrity. Effective transformer design, material selection, and cooling strategies are crucial to mitigate these effects and ensure reliable transformer operation.

Stray Flux

Stray flux in power transformers, which refers to magnetic flux that escapes from the core and flows through unintended paths, can have a significant impact on the occurrence of thermal faults.

Stray flux arises due to imperfections in the transformer design, manufacturing defects, or operational conditions that deviate from the ideal. Not all magnetic flux generated by the primary winding is confined to the magnetic core. Some flux leaks out and travels through surrounding structures and components. Stray flux can flow through the transformer tank walls, support structures, and other metallic components not intended to carry magnetic flux.

Thermal Effects of Stray Flux

Localized Heating:  Stray flux can induce eddy currents in metallic parts of the transformer, like those within the core. These eddy currents generate heat due to I²R losses, causing localized heating.  The heat generated by these eddy currents can create hotspots. These areas of elevated temperature can damage insulation materials, leading to thermal degradation.

Insulation Degradation:  Persistent heating from stray flux-induced currents can cause thermal stress on insulation materials, reducing their lifespan and dielectric strength. Insulation breakdown due to overheating can lead to partial discharges, short circuits, and ultimately, thermal faults.

Increased Core Losses:  Stray flux can cause parts of the core to operate closer to saturation, increasing core losses and, consequently, the temperature of the core and surrounding components.

Mitigation Strategies

To minimize the adverse effects of stray flux and associated thermal faults, various design and operational measures are implemented:

• Magnetic Shields: Placing magnetic shields or flux diverters around the core and windings helps to contain the stray flux and direct it away from sensitive areas.
• Shielding Tank Walls: Non-magnetic materials or shields are often used in transformer tank walls to prevent stray flux from inducing eddy currents.
• Laminated Steel: Similar to the core, using laminated steel for structural components can reduce the path for eddy currents, minimizing localized heating.
• Optimal Design: Ensuring that components such as windings and structural supports are placed to minimize the impact of stray flux.
• Core Design: Designing the magnetic core to minimize leakage flux by optimizing the core shape and material properties.
• Cooling Systems: Efficient cooling systems (such as oil cooling or forced air cooling) help dissipate heat from areas affected by stray flux, maintaining safe operating temperatures.

Stray flux in power transformers contributes to thermal faults primarily through the induction of eddy currents and the resultant localized heating. This heating can degrade insulation materials and lead to hotspots, increasing the risk of thermal faults. Implementing strategies like magnetic shielding, laminated structures, optimal design, and effective cooling can mitigate the effects of stray flux, enhancing the reliability and lifespan of power transformers.

Insufficient Cooling

Cooling System Faults

Cooling system faults in power transformers can manifest in various ways, potentially leading to overheating and other related issues. Here are the common cooling system faults:

1. Coolant Flow Obstruction:
   • Blocked Radiators or Pipes: Dirt, debris, or sludge buildup can block the radiators or cooling pipes, restricting the flow of the coolant (oil or water).
   • Valve Malfunctions: Faulty or closed valves can impede the flow of coolant through the system.
2. Pump Failures:
   • Mechanical Failures: Pump breakdowns due to worn-out bearings, impeller damage, or motor failure.
   • Electrical Issues: Problems such as motor burnout, electrical shorts, or connection failures can cause pumps to stop functioning.
3. Cooling Fan Issues:
   • Mechanical Damage: Broken blades, worn-out bearings, or misalignment can reduce the efficiency of cooling fans.
   • Electrical Failures: Issues like motor failures, electrical shorts, or control circuit problems can cause fans to stop working.
4. Thermostat or Control Failures:
   • Faulty Sensors: Temperature sensors may fail to accurately measure the transformer’s temperature, leading to improper cooling system operation.
   • Control Circuit Malfunctions: Problems in the control circuitry can result in incorrect operation of cooling components, such as fans and pumps not turning on or off at appropriate times.
5. Oil Leaks:
   • Seals and Gaskets: Leaking seals or gaskets can lead to a loss of insulating and cooling oil, reducing the overall cooling efficiency.
   • Tank Leaks: Cracks or punctures in the oil tank can cause significant oil loss, compromising cooling.
6. Cooling Medium Contamination:
   • Oil Contamination: Water, dirt, or other contaminants in the oil can degrade its cooling and insulating properties.
   • Degraded Coolant Quality: Over time, the coolant can degrade, losing its effectiveness in heat transfer.
7. Heat Exchanger Malfunctions:
   • Fouling: Accumulation of deposits on heat exchanger surfaces can reduce their efficiency.
   • Leakages: Leaks within the heat exchanger can mix coolant with contaminants, reducing cooling efficiency.
8. Fan and Pump Control Problems:
   • Control System Failures: Malfunctions in the systems that control the operation of fans and pumps can result in improper cooling.
   • Relay and Contact Issues: Faulty relays or contacts can prevent fans and pumps from operating correctly.
9. Ambient Temperature Sensor Failures:
   • Incorrect Readings: Faulty ambient temperature sensors can lead to incorrect cooling adjustments, either overcooling or undercooling the transformer.
10. Inadequate Maintenance:
    • Neglected Servicing: Lack of regular maintenance can lead to the buildup of contaminants, wear and tear on mechanical parts, and undetected minor issues escalating into major faults.
    • Failure to Replace Components: Overdue replacement of components like filters, oil, and worn-out parts can degrade cooling system performance.

Identifying and addressing these cooling system faults promptly is crucial for maintaining the efficient and reliable operation of power transformers. Regular inspections, preventive maintenance, and timely repairs can help mitigate these issues and extend the lifespan of the transformer.

Sludge Buildup

Sludge buildup in a power transformer can significantly impact its thermal performance, leading to various operational and reliability issues.

1. Reduced Heat Transfer Efficiency:
   • Blocking Cooling Pathways: Sludge can accumulate in the cooling ducts, radiators, and heat exchangers, obstructing the flow of the cooling oil. This obstruction reduces the efficiency of heat transfer from the core and windings to the cooling medium, leading to higher internal temperatures.
   • Insulating Effect: Sludge acts as an insulating layer on the surfaces of the transformer components, preventing effective heat dissipation. This causes the temperature of the windings and core to rise.
2. Increased Operating Temperature:
   • Core and Windings Overheating: As the sludge impedes heat dissipation, the core and windings operate at higher temperatures. Prolonged overheating can lead to accelerated aging of the insulation and other materials, reducing the transformer’s lifespan.
   • Hot Spot Formation: Sludge buildup can cause localized hot spots within the transformer. These hot spots can degrade the insulation more rapidly in specific areas, increasing the risk of thermal faults and potential failure.
3. Accelerated Insulation Degradation:
   • Thermal Stress on Insulation: Higher operating temperatures due to sludge buildup increase the thermal stress on the insulation system. This accelerated aging process can reduce the dielectric strength of the insulation, making it more susceptible to breakdowns and partial discharges.
   • Oil Degradation: Elevated temperatures can also accelerate the degradation of the transformer oil, leading to the formation of more sludge and other byproducts. This creates a vicious cycle where degraded oil leads to more sludge, further impairing thermal performance.
4. Impact on Load Capacity:
   • Derated Performance: With reduced cooling efficiency, the transformer may need to operate at a lower load capacity to avoid excessive overheating. This derating can affect the overall efficiency and reliability of the power distribution system, especially during peak demand periods.
5. Increased Risk of Thermal Runaway:
   • Self-Accelerating Heat Generation: The insulating effect of sludge can lead to thermal runaway conditions where the transformer generates more heat than can be dissipated. This self-accelerating process can cause rapid temperature increases, potentially leading to catastrophic failure.
6. Maintenance and Reliability Issues:
   • Frequent Maintenance Needs: Sludge buildup necessitates more frequent maintenance and cleaning to restore the transformer’s thermal performance. This increases operational costs and downtime.
   • Decreased Reliability: Persistent sludge buildup and associated thermal issues reduce the overall reliability of the transformer, increasing the risk of unexpected failures and outages.

Mitigation Strategies

To mitigate the effects of sludge buildup, regular maintenance and monitoring are essential. This includes:

• Oil Filtration and Purification: Regularly filtering and purifying the transformer oil to remove contaminants and prevent sludge formation.
• Periodic Inspections: Conducting routine inspections of the cooling system to detect and address any blockages or sludge deposits.
• Oil Testing: Performing periodic oil testing to monitor the condition of the oil and detect early signs of degradation or contamination.
• Cooling System Maintenance: Ensuring that radiators, heat exchangers, and other cooling components are kept clean and free of obstructions.

By addressing sludge buildup promptly and maintaining a proactive maintenance schedule, the thermal performance and overall reliability of power transformers can be significantly improved.

Diagnosing Thermal Faults

Diagnosing thermal faults in power transformers is critical for ensuring their reliability and efficiency. Various methods are employed to detect and analyze these faults, leveraging advanced technologies and analytical techniques. Here are some key methods:

Dissolved Gas Analysis (DGA) involves extracting and analyzing gases dissolved in the transformer oil. Different gases are produced by different types of faults (e.g., Hydrogen, Methane, Ethane, Ethylene, Acetylene, Carbon Monoxide). By monitoring the concentration of these gases, engineers can detect and diagnose overheating, arcing, and insulation degradation early​ [IEC60599]​. Some of the more common methods of diagnosis include Duvals Triangle 1, Duvals Triangle 4, Duvals Triangle 5, Duvals Pentagon 1, LEDT – Low Energy Degradation Triangle, Doernenberg Method, and Rogers Ratio Method.

Thermal Imaging (Infrared Thermography) cameras are used to capture thermal images of the transformer. This non-invasive technique identifies hotspots and temperature anomalies, indicating potential overheating issues and helping to prevent thermal faults before they cause damage​ [Balakrishnan1]​.

Partial Discharges (PD) are small electrical sparks that occur within the insulation system. PD analysis involves detecting and measuring these discharges. Detecting partial discharges can reveal insulation weaknesses and potential thermal faults. PD analysis helps in early fault detection and prevention​ [Hussain1]​.

Temperature Sensors and Fiber Optic Monitoring by placing Fiber optic sensors within the transformer provide real-time temperature monitoring. Continuous temperature data helps in detecting abnormal thermal behavior, enabling proactive management of thermal faults​ [Badar1]​

Frequency Response Analysis (FRA) involves measuring the transformer’s electrical response to a range of frequencies. Changes in the frequency response can indicate mechanical deformations and insulation issues associated with thermal faults​ [Alsuhaibani1]​.

Regular testing of the transformer oil for moisture, acidity, and dielectric strength to identify degradation in oil quality can lead to reduced cooling efficiency and insulation performance, which can be indicators of thermal faults​ [Gutiérrez1]​.

Load Tap Changer (LTC) Monitoring the operation and condition of load tap changers (LTCs), which are essential for voltage regulation. Faulty LTCs can cause thermal problems. Monitoring their performance helps in diagnosing and preventing thermal faults​ [Kang1]​.

By integrating these diagnostic methods, power transformer operators can achieve a comprehensive understanding of the transformer’s health and detect thermal faults early. This proactive approach helps maintain the reliability and extend the operational life of power transformers.

Conclusion

In conclusion, addressing thermal faults requires a comprehensive and diligent approach to transformer maintenance and monitoring. Through consistent and informed efforts, the risks associated with thermal faults can be minimized, ensuring a stable and reliable power supply. As we continue to advance in technology and techniques, the goal remains clear: to keep our transformers running cool, efficient, and fault-free.

References

IEC 60599	Mineral oil-filled electrical equipment in service – Guide to the interpretation of dissolved and free gases analysis, IEC, ICS 17.220.99; 29.040.10; 29.180
[Balakrishnan1]	Balakrishnan, G.K.; Yaw, C.T.; Koh, S.P.; Abedin, T.; Raj, A.A.; Tiong, S.K.; Chen, C.P. A Review of Infrared Thermography for Condition-Based Monitoring in Electrical Energy: Applications and Recommendations. Energies 2022, 15, 6000. https://doi.org/10.3390/en15166000
[Hussain1]	M. R. Hussain, S. S. Refaat and H. Abu-Rub, “Overview and Partial Discharge Analysis of Power Transformers: A Literature Review,” in IEEE Access, vol. 9, pp. 64587-64605, 2021, doi: 10.1109/ACCESS.2021.3075288.
[Badar1]	M. Badar, P. Lu, Q. Wang, T. Boyer, K. P. Chen and P. R. Ohodnicki, “Real-Time Optical Fiber-Based Distributed Temperature Monitoring of Insulation Oil-Immersed Commercial Distribution Power Transformer,” in IEEE Sensors Journal, vol. 21, no. 3, pp. 3013-3019, 1 Feb.1, 2021, doi: 10.1109/JSEN.2020.3024943.
[Alsuhaibani1]	Alsuhaibani, S.; Khan, Y.; Beroual, A.; Malik, N.H. A Review of Frequency Response Analysis Methods for Power Transformer Diagnostics. Energies 2016, 9, 879. https://doi.org/10.3390/en9110879
[Gutiérrez1]	Gutiérrez, Cristina Méndez, Alfredo Ortiz Fernández, Carlos Javier Renedo Estébanez, Cristian Olmo Salas, and Riccardo Maina. “Understanding the ageing performance of alternative dielectric fluids.” IEEE Access 11 (2023): 9656-9671.
[Kang1]	P. Kang, D. Birtwhistle, J. Daley and D. McCulloch, “Noninvasive on-line condition monitoring of on load tap changers,” 2000 IEEE Power Engineering Society Winter Meeting. Conference Proceedings (Cat. No.00CH37077), Singapore, 2000, pp. 2223-2228 vol.3, doi: 10.1109/PESW.2000.847701.