A transformer can develop a slow internal fault — a winding hot spot, a loose connection, insulation breaking down — long before it becomes a short circuit. No overcurrent relay can see such a fault, because the current is still normal. The Buchholz relay can: it watches the gas and oil movement inside the transformer, and it is the standard protection for oil-immersed transformers. This post explains how it works and where it fails.

Figure 7.1: Buchholz relay — inclined pipe between tank and conservator. (Schematic, not to scale.)

Construction and placement

The relay sits in the inclined pipe (tilt 9°–10°) connecting the main transformer tank to the conservator. The tilt matters: it lets gas bubbles rise along the pipe into the relay chamber instead of collecting elsewhere. Inside the chamber are two elements — an upper float and a lower baffle vane.

Two stages: alarm, then trip

• Incipient faults — alarm: a minor internal fault decomposes the oil into gas — over 70% hydrogen (H₂) — which rises into the chamber, displaces oil and tilts the upper float, closing the alarm contact. The transformer keeps running, but the operator is warned to investigate.
• Severe faults — trip: a serious internal fault produces a violent oil surge toward the conservator. The surge deflects the lower baffle vane, closing the trip contact and taking the transformer out of service.

Limitations

• Oil-immersed transformers with a conservator only — it cannot be fitted to dry-type transformers or sealed-tank designs with no conservator pipe.
• Blind to external faults — a bushing flashover or a fault on the external connections produces no gas inside the tank, so the relay does not see it.
• Slow — operating time is roughly 0.1–0.2 s, far slower than a differential relay, so it is a backup and early-warning device rather than the main fault clearer.
• False trips — vibration, seismic shocks or a sudden oil movement (for example during oil filling) can deflect the baffle and trip the transformer when there is no fault.

Key points to remember

• Fitted in the inclined pipe (9°–10° tilt) between the main tank and the conservator.
• Incipient fault: gas (over 70% H₂) rises, displaces oil, upper float tilts → alarm.
• Severe fault: violent oil surge deflects the lower baffle vane → trip.
• Limitations: oil-immersed with conservator only, blind to external bushing faults, slow (~0.1–0.2 s), vibration or seismic shocks can cause false trips.

Based on Power System Protection & Switchgear, Complete Examination Solution, Exam 07 May 2026, Q7(b).