Buchholz relay (gas relay) — two-stage gas protection for oil-filled power transformers

The guide on transformer vector groups covers the winding connection and phase shift of a power transformer. This article covers a completely different aspect of the same machine: a mechanical protection that is fully independent of electrical differential or overcurrent relays, and that often gives the first indication of an incipient internal fault — the Buchholz relay.

Where the relay sits and which transformers it applies to

The Buchholz relay is mounted in the pipe between the main tank of an oil-filled power transformer and the overhead conservator vessel (the expansion vessel that accommodates the oil's volume changes with temperature). This construction — main tank plus conservator — is a precondition for a Buchholz relay: a hermetically sealed transformer (without a conservator, for example with a nitrogen cushion or a fully filled, pressure-rated tank) or a dry-type transformer has no Buchholz relay and relies on alternative protections, such as a sudden-pressure relay or online gas-analysis monitoring.

Two separate stages

The Buchholz relay works via two independent mechanisms in the same housing:

Alarm stage (slow gas accumulation): an incipient, low-energy fault — for example partial discharge, a hot spot in the winding, or slowly progressing insulation ageing — gradually decomposes the oil and forms gas. This gas rises to the relay housing, displaces oil there, and lowers a float that closes a contact. This activates an alarm only, not a trip.
Trip stage (sudden oil surge): a severe, rapidly developing fault — for example an internal short circuit between windings or a breakdown to the core — causes a sudden, forceful oil flow from the main tank to the conservator. This surge strikes a flap or vane in the relay housing, which closes a second, independent contact. This trips the transformer's circuit breaker immediately.

The two stages are therefore not two settings of the same signal, but two physically separate detection principles — one reacts to gradual gas formation, the other to a sudden mechanical oil surge.

Why this complements electrical protection

Differential and overcurrent protections detect faults through the electrical quantities at the transformer terminals. An incipient, low-energy fault — such as slowly progressing partial discharge in the insulation — often does not yet produce an electrical signal large enough to operate an electrical relay, while the fault is already producing gas. The Buchholz relay catches this type of "silent" fault well before it develops into an electrically detectable short circuit.

What to do on an alarm signal

Note: a Buchholz alarm must never be reset or ignored without investigation. The usual follow-up step is to take an oil/gas sample from the relay for dissolved gas analysis (DGA) per the sampling method of IEC 60567 and the interpretation guideline of IEC 60599; the composition of the collected gases (for example hydrogen, methane, ethylene, acetylene) indicates the type and severity of the underlying fault. The relay itself only identifies that gas has formed, not which fault caused it — that identification requires a separate laboratory analysis of the sample.

Practical relevance

During periodic maintenance of an oil-filled power transformer with a conservator vessel, it is worth checking whether the Buchholz relay is present, correctly connected, and periodically function-tested (the float and flap mechanisms can seize after years of standstill without this being visible from the outside). On an alarm signal, a gas sample must always be taken and assessed before the transformer is kept in service without reservation.

Common mistakes

Resetting or ignoring an alarm signal without taking a gas sample for DGA — a persistent low-energy alarm can indicate a fault that is gradually worsening.
Assuming every oil-filled transformer has a Buchholz relay — hermetically sealed and dry-type transformers do not have this protection and require alternative safeguards.
Confusing the alarm stage with the trip stage, or thinking that any gas accumulation automatically leads to a trip — these are two physically separate mechanisms with a different trigger.
Not periodically function-testing the float and flap mechanism as part of the maintenance programme, so that a relay that has seized after years of standstill goes unnoticed until it is needed.