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Date：09-28-2026
Transformer bushings are among the most critical components for high-voltage transmission transformers. Industry field data shows bushing-related failures account for around 9.8% of total transformer failures, making it the second-largest contributor to transformer outages.
This technical brief draws on a field case documented by engineers from State Grid Tianjin Electric Power Company (Zhang Tianxu et al.), describing an overheating incident on a 220 kV transformer unit. It walks readers through fault detection, on-site troubleshooting, corrective maintenance and practical business insights for asset owners and procurement stakeholders.
Infrared Thermography Detects Severe Hot-spot Condition
The anomaly was uncovered during routine monitoring work at a 220 kV substation. The main transformer (Model SFSZ-180000/220) was generally running within operational limits, yet infrared inspection identified an obvious hot spot on the top cap of the B-phase, 110 kV-side bushing.
Measurements were recorded under 28 °C ambient conditions:
- Phase A: 45.9 °C
- Phase B: 78.4 °C (fault location)
- Phase C: 44.0 °C
A substantial relative temperature difference was observed for Phase B. Against widely accepted assessment criteria for current-induced heating defects, this was classified as a severe fault condition. As operational load rises, such concealed defects can progress rapidly and raise the threat of catastrophic equipment failure.
Root-cause Investigation: Minor Assembly Mis-alignment Triggers Progressive Damage
Emergency de-energisation was performed for fault investigation. DC resistance testing revealed significantly elevated resistance values for Phase B, with inter-phase unbalance exceeding the acceptance threshold defined in Q/GDW 168-2008 Condition-Based Maintenance Test Regulations.
Once technicians disassembled the bushing top cap, physical evidence became visible. Both internal threads on the cap and external threads of the lead connector exhibited heavy oxidation. The bakelite insulating gasket, tasked with sustaining consistent clamping force, showed clear signs of ageing, brittleness and cracking.
The root cause traced back to an assembly deviation from the original installation phase. The support shoulder beneath the lead thread has a width of only 2 mm, while the radial dimension difference (outer minus inner diameter) of the insulating gasket stands at 4.5 mm. During assembly, the gasket shifted out of its intended position. Merely 2 mm of the gasket rested fully on the supporting shoulder, leaving a 2.5 mm segment unsupported and hanging free.
Over years of cyclic thermal expansion and contraction in service, uneven mechanical stress built up and eventually fractured the gasket. This reduced clamping compression between the top cap and lead connector. Loosening contact interfaces pushed contact resistance higher, setting off a destructive feedback loop: higher contact resistance generates extra heat; heat accelerates thread oxidation; oxidation further degrades electrical contact performance.
Corrective Maintenance and Post-repair Validation
The maintenance team completed systematic rectification work following standard service practice:
Thread mating surfaces were thoroughly cleaned using alcohol and abrasive pads to remove oxidation layers.
The damaged anti-rotation ring and degraded insulating gasket were replaced with new spare components.
Special care was taken to ensure full seating of the replacement gasket onto the support shoulder before reassembling the top cap.
Technicians added positional markings for flange-cap alignment, implemented new locking features, and fitted spring washers to maintain sustained clamping pre-load over service life.
Post-repair DC resistance testing showed inter-phase unbalance fell well below the regulatory limit. Follow-up infrared thermal inspection confirmed the hot spot had been eliminated, and the transformer was safely returned to commercial operation.
Practical Takeaways for Utility & EPC Procurement
High-voltage transformers are large-scale heavy assets, yet long-term operational reliability heavily depends on small auxiliary parts and quality of field assembly. Even millimetre-level mis-placement of gaskets and similar accessories can build hidden mechanical stress over multiple operating years, which may result in unexpected component failure and expensive unplanned outages.
For power utilities, EPC contractors and project procurement teams, two key practical considerations emerge:
Component tolerance and quality assurance: Enforce strict dimensional tolerance requirements for auxiliary parts such as insulating gaskets and bushing top caps. Include accessory conformance checks alongside main-equipment acceptance procedures.
Standardise on-site installation execution: Installation and service crews should strictly follow official assembly procedures. Verify proper seating of internal components (anti-rotation rings, insulating gaskets and others) to avoid uneven pressure distribution across contact surfaces.
Reference: Zhang Tianxu, Ren Zhiyong, Ning Xin, Wang Pengfei, Lu Xuan. Analysis and Treatment of Abnormal Heating in 220kV Transformer Bushing Joints, State Grid Tianjin Electric Power Company High-Voltage Branch.
Transformer bushings are among the most critical components for high voltage transmission transformers. Industry field data shows bushing related failures account for around 9.8% of total transformer failures, making it the second largest contributor to transformer outages.
This technical brief draws on a field case documented by engineers from State Grid Tianjin Electric Power Company (Zhang Tianxu et al.), describing an overheating incident on a 220 kV transformer unit. It walks readers through fault detection, on site troubleshooting, corrective maintenance and practical business insights for asset owners and procurement stakeholders.
Infrared Thermography Detects Severe Hot spot Condition
The anomaly was uncovered during routine monitoring work at a 220 kV substation. The main transformer (Model SFSZ 180000/220) was generally running within operational limits, yet infrared inspection identified an obvious hot spot on the top cap of the B phase, 110 kV side bushing.
Measurements were recorded under 28 °C ambient conditions:
Phase A: 45.9 °C
Phase B: 78.4 °C (fault location)
Phase C: 44.0 °C
A substantial relative temperature difference was observed for Phase B. Against widely accepted assessment criteria for current induced heating defects, this was classified as a severe fault condition. As operational load rises, such concealed defects can progress rapidly and raise the threat of catastrophic equipment failure.
Root cause Investigation: Minor Assembly Mis alignment Triggers Progressive Damage
Emergency de energisation was performed for fault investigation. DC resistance testing revealed significantly elevated resistance values for Phase B, with inter phase unbalance exceeding the acceptance threshold defined in Q/GDW 168 2008 Condition Based Maintenance Test Regulations.
Once technicians disassembled the bushing top cap, physical evidence became visible. Both internal threads on the cap and external threads of the lead connector exhibited heavy oxidation. The bakelite insulating gasket, tasked with sustaining consistent clamping force, showed clear signs of ageing, brittleness and cracking.
The root cause traced back to an assembly deviation from the original installation phase. The support shoulder beneath the lead thread has a width of only 2 mm, while the radial dimension difference (outer minus inner diameter) of the insulating gasket stands at 4.5 mm. During assembly, the gasket shifted out of its intended position. Merely 2 mm of the gasket rested fully on the supporting shoulder, leaving a 2.5 mm segment unsupported and hanging free.
Over years of cyclic thermal expansion and contraction in service, uneven mechanical stress built up and eventually fractured the gasket. This reduced clamping compression between the top cap and lead connector. Loosening contact interfaces pushed contact resistance higher, setting off a destructive feedback loop: higher contact resistance generates extra heat; heat accelerates thread oxidation; oxidation further degrades electrical contact performance.
Corrective Maintenance and Post repair Validation
The maintenance team completed systematic rectification work following standard service practice:
Thread mating surfaces were thoroughly cleaned using alcohol and abrasive pads to remove oxidation layers.
The damaged anti rotation ring and degraded insulating gasket were replaced with new spare components.
Special care was taken to ensure full seating of the replacement gasket onto the support shoulder before reassembling the top cap.
Technicians added positional markings for flange cap alignment, implemented new locking features, and fitted spring washers to maintain sustained clamping pre load over service life.
Post repair DC resistance testing showed inter phase unbalance fell well below the regulatory limit. Follow up infrared thermal inspection confirmed the hot spot had been eliminated, and the transformer was safely returned to commercial operation.
Practical Takeaways for Utility & EPC Procurement
High voltage transformers are large scale heavy assets, yet long term operational reliability heavily depends on small auxiliary parts and quality of field assembly. Even millimetre level mis placement of gaskets and similar accessories can build hidden mechanical stress over multiple operating years, which may result in unexpected component failure and expensive unplanned outages.
For power utilities, EPC contractors and project procurement teams, two key practical considerations emerge:
Component tolerance and quality assurance: Enforce strict dimensional tolerance requirements for auxiliary parts such as insulating gaskets and bushing top caps. Include accessory conformance checks alongside main equipment acceptance procedures.
Standardise on site installation execution: Installation and service crews should strictly follow official assembly procedures. Verify proper seating of internal components (anti rotation rings, insulating gaskets and others) to avoid uneven pressure distribution across contact surfaces.
Reference: Zhang Tianxu, Ren Zhiyong, Ning Xin, Wang Pengfei, Lu Xuan. Analysis and Treatment of Abnormal Heating in 220kV Transformer Bushing Joints, State Grid Tianjin Electric Power Company High Voltage Branch.
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