Bushing Failure Analysis
A transformer bushing provides insulation between the energized
conductor and the grounded transformer tank.
Bushing failure is particularly dangerous because it can result in:
. External flashover
. Internal dielectric breakdown
.Oilfire
. Transformer tank damage
Main causes:
· Moisture ingress
. Insulation ageing
· Partial discharge
. Surface contamination
· Cracked porcelain
. Loss of oil in oil-filled bushings
· High dielectric stress
· Poor sealing
· Internal capacitance-grading deterioration
Important diagnostic tests:
. Capacitance measurement
. Tan 8/power factor
. Visual inspection
· Infrared thermography
· Oil testing where applicable
. Variable-frequency dielectric response in appropriate cases
The CEA manual notes that variable-frequency tan-delta testing canhelp assess insulation condition, including moisture and paper ageing,while a conventional 50 Hz capacitance/tan-delta measurement may notidentify every problem such as certain partial-discharge or grading-layer
issues.