Common Transformer Bushing Failure Causes and How to Prevent Them

Although a bushing is only one component on a transformer, its failure often leads to severe equipment damage or even fire. Statistics show bushing problems account for a significant share of unplanned transformer outages. Knowing the common failure modes is the first step toward prevention.

1. Moisture ingress (seal failure)

Aged or improperly installed seals allow moisture into the insulation core, sharply increasing tan delta and degrading insulation performance.

• Prevention: perform regular tan delta and capacitance tests and watch for trends; inspect seals.

2. Oil leak / low oil level

Leaks in OIP bushings lower the oil level, exposing insulating oil to air and causing moisture ingress and partial discharge.

• Prevention: inspect the oil level gauge regularly and fix leaks promptly.

3. Partial discharge

Voids, impurities or delamination inside the insulation cause partial discharge that progressively deteriorates the insulation to breakdown.

• Prevention: detect early with partial discharge testing and dissolved gas analysis (DGA).

4. Overheating (poor contact)

Loose connections and high contact resistance cause local overheating and accelerate insulation aging.

• Prevention: use periodic infrared thermography to find hot spots; verify connection torque.

5. External insulation flashover

Contamination on the porcelain surface combined with moisture can flash over under overvoltage or severe weather.

• Prevention: select adequate creepage distance for the pollution level; clean regularly or apply anti-pollution coatings.

6. Mechanical damage

Porcelain cracks or flange deformation caused by transport, lifting or seismic events.

• Prevention: standardise transport and lifting; inspect appearance and fasteners before installation.

We recommend maintaining a full life-cycle record for each bushing and trending tan delta, capacitance, DGA and thermography data — shifting from reactive repair to condition-based maintenance.