TL;DR
A Buchholz relay alarm means gas is accumulating inside the transformer, which signals an internal fault. Do not reset and ignore. De-energize the transformer and perform a dissolved gas analysis before returning to service.
What you might see
- Buchholz relay alarm or trip signal active
- visible gas bubbles in the Buchholz relay sight glass
- transformer tripped by relay protection
- unusual noise or humming from the tank
Likely causes
Internal arcing generating combustible gas from oil decomposition
Partial discharge in the winding insulation
Moisture ingress causing steam generation under load heat
Surge transient causing localised breakdown and gas production
Required tools
- Oil sampling syringe or vacuum bottle (clean, dry)
- Gas sample tube for Buchholz relay
- PPE for medium-voltage switching
- Switching log and permit-to-work documentation
Safety first
- All switching on a medium-voltage transformer must be done by authorized personnel under a switching permit. This is not a task for a general maintenance technician.
- Buchholz relay alarms should never be silenced and ignored. An unresolved internal fault can cause a catastrophic failure including tank rupture and fire.
Procedure
- 1
Treat every Buchholz alarm as a serious fault indicator. Do not reset the alarm and return the transformer to service without investigation.
Warning: A Buchholz trip indicates a potentially dangerous internal fault. Do not re-energize until a dissolved gas analysis is complete and authorizes return to service. - 2
If the relay is in alarm (gas accumulation) rather than trip, coordinate with your utility or electrical authority to schedule a controlled de-energization.
- 3
If the relay has tripped the transformer, do not attempt to reclose. Follow your switching procedure to isolate the transformer from both the primary and secondary sides.
- 4
With the transformer de-energized and isolated, collect an oil sample from the bottom sampling valve using a clean, airtight syringe or vacuum bottle per your oil sampling procedure.[1]
- 5
Send the oil sample to a laboratory for dissolved gas analysis (DGA). The gas ratios will identify the fault type: thermal, partial discharge, or arcing.
- 6
Collect the gas from the Buchholz relay through its gas sampling cock into a gas sample tube. This is also sent to the lab.[1]
- 7
Do not return the transformer to service until the lab results are reviewed by a qualified engineer and the fault is either cleared or assessed as safe.
Sources
ABB Distribution Transformer Operation and Maintenance Manual, ABB
ABB Pad-Mount Distribution Transformer installation and maintenance instructions, Buchholz relay operation and oil sampling procedures (general)
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