As the external connection hub of a transformer, the bushing performs the electrical function of leading out high-voltage leads, while undertaking current-carrying capacity, mechanical support and tank sealing. Once a bushing fails, it is highly likely to trigger unplanned transformer outages and even grid accidents. Mastering bushing fault patterns and implementing effective defect control is a core priority for power equipment operation and maintenance.
1. Functions and Main Types of Transformer Bushings
Bushings are mounted on the top of the transformer tank. Their core function is to bring internal high-voltage winding leads out of the tank and establish electrical connection with the external power grid.
Under operating conditions, bushings must withstand rated system voltage and continuously carry load current. Mechanically, they support electrical leads and resist wind loads and tensile forces from busbars. Meanwhile, they seal tank openings to prevent transformer-oil leakage and block ambient moisture from entering the tank interior.
Classified by voltage level and insulation structure: capacitor-type bushings are widely adopted for systems of 110 kV and above. They optimize internal electric-field distribution via capacitive voltage division and reduce local field strength. For medium- and low-voltage applications, plain porcelain bushings and oil-immersed bushings are commonly used. Composite-insulation bushings with silicone-rubber sheds feature excellent anti-pollution-flashover performance and are widely deployed in complex environments such as outdoor sites and heavily-polluted industrial plants.
2. Common On-site Fault Modes and Root-cause Analysis
2.1 Abnormally High Operating Temperature
Temperature rise is a latent defect that is easily overlooked. Heating faults fall into two categories: heating in current-carrying circuits and heating in insulating dielectrics.
Heating in current-carrying circuits originates from excessive contact resistance. Loose terminal bolts, improper lead crimping, galvanic corrosion caused by direct copper-to-aluminum contact without transition connectors, and oxidation-corrosion on mating contact surfaces all increase contact resistance and generate substantial heat under current flow. This creates a vicious cycle: higher temperature accelerates oxidation and corrosion, further raising contact resistance and aggravating overheating.
Dielectric heating arises from degradation of the insulation system. Overheating occurs when capacitive cores absorb moisture, internal insulating oil deteriorates, porcelain housings crack and admit water ingress, or electric-field distortion produces excessive local field strength, leading to significant growth of dielectric loss. In addition, heavy contamination accumulated on external sheds increases leakage current and induces local overheating.
2.2 Seal Failure and Oil Leakage
Oil leakage is the most frequently-reported field defect. Three primary root causes are listed below:
– Sealing rubber components age, harden and lose elasticity after long-term exposure to high temperature and immersion in transformer oil.
– Improper field installation: uneven flange mating surfaces, non-compliant bolt torque or tightening sequence, and displaced or distorted gaskets.
– Manufacturing defects: imperfect flange welding and sand holes on porcelain components.
Oil leakage directly lowers internal oil level of the bushing and degrades insulation capability. Spilled oil contaminates external insulation surfaces, attracts dust and pollutants and further deteriorates external insulation performance.
2.3 Insulation Discharge and Breakdown Failures
Such catastrophic failures mostly evolve from minor latent defects. Contributing factors include:
– Damaged sealing allowing moisture ingress and rapid drop of insulation resistance;
– Inadequate vacuum impregnation during manufacturing leaving internal bubbles and persistent partial discharge;
– Heavy surface contamination on outdoor bushings leading to surface flashover under rainy or foggy conditions;
– Inadequate preventive testing that fails to detect progressive insulation degradation.
3. Practical On-site Defect-handling Procedures
Handling Bushing Overheating Defects
Use infrared thermography to locate hot-spots and distinguish current-induced heating from voltage-induced heating. For current-induced heating: de-energize equipment for maintenance. Disassemble electrical joints, polish off surface oxidation, replace deformed terminals, and fasten bolts with specified torque. Install transition connectors for copper-aluminum joints to eliminate galvanic-corrosion risks. For voltage-induced heating: perform insulation-resistance, dielectric-loss and partial-discharge tests. Clean contaminated external insulation. Replace the bushing directly if test results confirm severe internal insulation degradation.
Handling Oil-leakage Defects
Replace aged sealing rubber parts with new oil-resistant, high-temperature-resistant seals. For misaligned flanges or uneven mating surfaces, re-align interfaces, re-position gaskets and tighten bolts in diagonal cross-tightening sequence. Minor sand holes and tiny cracks may be repaired with dedicated repair compounds. Replace the complete bushing if structural components suffer severe damage.
Mitigation of Insulation-discharge Risks
If flashover burn marks are found on bushing sheds, clean external insulation immediately and trace moisture-ingress paths. Shorten test cycles and continuously monitor insulation trends when test data shows anomalies. Replace the bushing promptly if partial-discharge levels exceed limits or fault-characteristic gases appear in oil-chromatography results to avoid major accidents.
4. Full-life-cycle Control Measures for Bushing-fault Prevention
Bushing-fault prevention relies not only on post-fault remediation, but also on multi-dimensional protection covering type selection, incoming inspection, operation, maintenance and asset-life management.
Reasonable Type Selection: Select bushing models according to system voltage, short-circuit level, site pollution severity and installation conditions. Prefer long-creepage-distance bushings or composite-insulation bushings for heavily-polluted outdoor locations.
Strict Incoming Acceptance: Before commissioning new bushings, re-verify key tests including dielectric loss, capacitance and partial discharge to screen manufacturing defects. Reject non-conforming units for service.
Deploy On-line Condition Monitoring: For main transformers at critical substations, install on-line monitoring devices for bushing dielectric loss and capacitance to realize continuous condition tracking.
Routine Inspection Regimes: Mandate infrared temperature measurement in daily inspections. Visually check porcelain parts for cracks, oil levels and oil-leakage traces.
Standardize Field Maintenance & Installation: Issue standardized work instructions and strengthen technician training to minimize human-induced defects.
Life-assessment for Aged Equipment: For long-serving capacitor-type bushings, analyze historical test datasets to evaluate insulation aging trends. Schedule replacement in advance before performance reaches alarm thresholds to prevent failures.
Conclusion
Safe and reliable bushing performance depends on both manufacturing quality and full-cycle operation-and-maintenance management. Field experience shows that the vast majority of bushing faults can be detected at early stages via routine inspections, infrared thermography and periodic insulation tests.
The core of maintenance work lies in identifying early-stage defects accurately and implementing targeted corrective actions. Shifting from reactive emergency repair to proactive prevention effectively guarantees long-term safe and reliable operation of transformers and power grids. For reliable transformer solutions and professional support, QXG provides dependable products and services for customers across North America and Latin America.