Early Warning Signs of Transformer Bushing Failure: Causes and Prevention

Transformer bushings play a pivotal role in power systems: they function as current conductors, linking the transformer’s internal high- and low-voltage leads to its external casing. Beyond conducting electricity, they offer mechanical stability to the leads and maintain insulation between the leads and the grounded tank. In operation, these components endure constant load currents and sudden short-circuit currents during external faults, making them essential for steady power delivery and grid integrity. Failing to address bushing issues promptly can escalate into major problems, threatening the entire power network.

Root Causes of Bushing Failures

       Production Imperfections

• Weak terminal seals let moisture seep in, sparking internal partial discharges that may lead to flashovers or explosions.

• Defects in capacitive cores—such as misaligned shielding, ill-fitting aluminum foils, folded insulation layers, or trapped air bubbles—distort electric fields, causing partial discharges or insulation breakdown.

• Issues like improper end-screen grounding, loose lead welds, or grounding failures from electromagnetic vibrations.

• Excessive electrical stress on capacitive screens speeds up insulation aging, increasing breakdown risks.

Operational & Maintenance Mistakes

• Inadequate vacuum oil filling, oil leaks, or moisture intrusion degrade insulation performance over time.

• Bent lower voltage grading spheres touching conductive tubes generate parasitic currents, leading to localized overheating.

• Rough handling during testing (e.g., improper disassembly) causes contact overheating or damaged end-screen leads.

• Errors in oil sampling—such as insufficient refilling or loose plugs—result in oil leaks.

• Overvoltage from lightning strikes, switching surges, or surface contamination triggers flashovers.

Design & Assembly Shortcomings

• Mismatched thread sizes between conductive tubes and winding leads raise contact resistance, leading to overheating.

• Using low-melting-point solder (e.g., tin) for lead connections, which can melt at 200°C under high loads.

Typical Bushing Issues

• Overheating: Stemming from poor internal or external electrical contact, or excessive operational load.

• Oil Leakage: Caused by worn seals or uneven flange connections at the bushing base.

• Elevated Acetylene in Insulation Oil & End-Screen Arcing: Often due to design flaws or ungrounded end screens.

Fixing Bushing Problems

       Addressing Overheating

• Welding: Clean and polish surfaces thoroughly before welding. Use phosphor copper solder with even heat application to ensure a large, secure contact area.

• Crimping: Match lead and connector diameters precisely. Use standardized crimping techniques with adequate crimp length for optimal conductivity.

Stopping Oil Leaks

• Use compatible nuts, clamps, and gaskets. For larger cables, install crimped terminals to ensure a tight fit.

• Secure connections with dual nuts to boost tightness and improve heat dissipation.

• Prevent prolonged overloading by balancing load distribution. Conduct regular inspections and fix leaks as soon as they’re detected.

Resolving Acetylene Buildup & End-Screen Discharge

• Replace faulty bushings with certified, rigorously tested units to restore safe operation.