Buchholz relay for internal faults

A Buchholz relay is a gas-actuated relay fitted in the pipe between the main tank and the conservator of oil-immersed transformers (above about 500 kVA). It detects internal faults by the gas and oil surge they produce.

How it protects

Any fault inside the tank (winding or core) produces heat; the heat decomposes the oil and insulation into gas (H₂, CO, C₂H₂, etc.). The gas rises towards the conservator and passes through the relay.

  Main tank ──pipe──► [ Buchholz relay ] ──► Conservator
                       ┌──────────────┐
                       │ float1 → alarm│ (upper)
                       │ flap/float2 → │
                       │   trip        │ (lower)
                       └──────────────┘

1. Incipient (minor) faults such as insulation breakdown between core laminations, core-bolt insulation failure, poor joints, inter-turn faults or loss of oil: gas forms slowly and collects at the top of the relay. The oil level in the relay falls, the upper float tilts and closes the alarm circuit.
2. Severe faults such as phase-to-earth or phase-to-phase winding faults: a large volume of gas is produced suddenly and forces a surge of oil towards the conservator. This surge deflects the lower float/flap, which closes the trip circuit and opens the transformer circuit breakers.
3. Oil leakage: if oil level falls, first the alarm and then the trip float operate.
4. The gas collected can be drawn off through a cock and analysed: its colour and composition indicate the type of fault (e.g. white: paper insulation; yellow: wood; dark grey: oil).

The relay therefore gives early warning of slowly developing faults that differential protection cannot see, and trips quickly on major internal faults.