TL;DR
A bushing flashover is a high-voltage fault. De-energize immediately, do not approach the transformer until it is confirmed dead, and replace the damaged bushing before returning to service.
What you might see
- transformer tripped by protection relay
- visible arc marks or carbon tracking on bushing porcelain
- oil around the bushing base
- burn marks on the connection compartment interior
Likely causes
Surface contamination (dust, salt, or chemical deposit) tracking across the bushing creepage surface
Cracked or chipped bushing porcelain from mechanical impact or thermal shock
Moisture on a contaminated bushing surface completing a conductive path to ground
Over-voltage transient from a lightning strike or switching surge
Required tools
- Megohmmeter (appropriate kV rating for the bushing voltage class)
- Replacement bushing (ABB-approved, correct voltage class)
- RTV silicone coating for contamination protection
- Full medium-voltage PPE, rubber gloves, face shield, arc flash suit
- Switching permit and grounding equipment
Safety first
- This is a medium-voltage task. All work must be done under a switching permit by personnel qualified for the primary voltage class of the transformer.
- Never touch a bushing or any transformer terminal without confirming de-energization and ground application by authorized switching personnel.
Procedure
- 1
Do not approach the transformer until it is confirmed de-energized and grounded by authorized switching personnel.
Warning: A transformer after a bushing flashover may have residual charge. Approach only after qualified personnel have confirmed de-energization and grounded both primary and secondary terminals. - 2
Coordinate with the utility to open the primary switch and ground the transformer. Follow your facility switching procedure and permit-to-work system.[1]
- 3
After confirmed de-energization, open the connection compartment and inspect the bushings visually. Look for carbon tracking, oil seepage at the base, or visible cracks in the porcelain.
- 4
Photograph all damage before touching anything.
- 5
Test the bushing insulation resistance with a megohmmeter at the appropriate voltage. A cracked or heavily contaminated bushing will show greatly reduced insulation resistance.[1]
- 6
Replace any bushing that shows cracks, significant tracking, or low insulation resistance. Use an ABB-approved replacement bushing of the correct voltage rating.
- 7
Clean undamaged bushings with a dry, clean rag or silicone-treated cloth to remove surface contamination. For severe contamination in coastal or industrial environments, apply RTV silicone coating to increase the creepage distance.[1]
- 8
After replacement, have a qualified engineer review the event and check whether a surge arrester is required on the primary terminals.
Sources
ABB Distribution Transformer Operation and Maintenance Manual, ABB
ABB Pad-Mount Distribution Transformer installation and maintenance instructions, bushing inspection and replacement procedures (general)
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