Buchholz Relay: Gas-Accumulation Fault Protection for Oil Transformers

What It Is and Where It Lives

A Buchholz relay is a gas-actuated protective relay installed on oil-filled power transformers, shunt reactors, and on-load tap-changers that use a conservator (an elevated expansion tank that lets the oil breathe as it heats and cools). The relay body is a cast housing fitted into the interconnecting pipe, so that every gas bubble or oil movement between the main tank and the conservator must pass through it.

• Location: the pipe run is deliberately inclined 3°–7° (a rise toward the conservator) so buoyant gas migrates naturally into the relay chamber rather than pooling in the tank.
• Coverage: it protects the whole oil volume — windings, core, leads, and bushings inside the tank.
• Two grades: the main-tank relay (double-float) and a smaller single-element protective relay for the diverter switch of an on-load tap-changer (OLTC).

Because it senses the physical consequences of a fault — gas and oil movement — rather than terminal electrical quantities, it catches faults that current- or voltage-based relays cannot see until much later.

How the Mechanism Works

The relay contains an oil-filled chamber with two pivoted floats (or a float plus a baffle vane), each carrying a magnet that operates a sealed reed switch (older units used mercury tilt switches).

• Alarm / slow-gas stage: incipient faults — a loose connection heating up, a partial discharge, a minor turn-to-turn arc — decompose oil and cellulose insulation into gases (H₂, CH₄, C₂H₂, CO). Bubbles rise, collect in the top of the chamber, and displace oil. The upper float sinks as the oil level drops, closing the alarm contact.
• Trip / oil-surge stage: a severe fault (flashover, major short) flash-vaporizes oil, creating a pressure wave that pushes a fast surge of oil toward the conservator. The lower float/vane swings on this flow, closing the trip contact and de-energizing the transformer.

The two stages are independent: gas accumulation drives the alarm; oil velocity drives the trip. A test cock on top lets an operator draw off collected gas for sniff/flammability checks and dissolved-gas analysis.

Key Quantities and a Worked Example

The alarm responds to accumulated gas volume, typically set between 100 and 300 cm³ (commonly ~200–250 cc). The trip responds to oil flow velocity in the connecting pipe, with standard calibration points of 0.6, 1.0, or 1.5 m/s.

• Distribution and smaller units (≈1–10 MVA) use the sensitive 0.6–1.0 m/s setting.
• Large power transformers (>10 MVA) use 1.0–1.5 m/s to avoid nuisance trips from thermal oil circulation.

Worked example. Convert a velocity setpoint to a flow rate. For a DN50 connecting pipe (internal diameter d ≈ 0.05 m), the cross-section is A = π·d²/4 = π·(0.05)²/4 ≈ 1.96×10⁻³ m². At the 1.0 m/s trip setpoint the surge flow is Q = v·A ≈ 1.0 × 1.96×10⁻³ ≈ 1.96×10⁻³ m³/s ≈ 118 L/min. Normal thermal circulation stays well below this, which is why the transformer does not trip on ordinary heating and cooling cycles.

Governing relations: gas alarm on V_gas ≥ V_set; surge trip on v_oil ≥ v_trip, with Q = v·A.

Selection, Installation, and Operation

Correct application is mostly about geometry and calibration:

• Pipe sizing: the relay bore must match the transformer's tank-to-conservator pipe — common sizes are DN25/DN50/DN80 — so it neither throttles normal flow nor desensitizes the surge trip.
• Slope and orientation: mount with the marked arrow pointing toward the conservator, on a pipe rising 3°–7°. A back-to-front install disables gas collection and can invert the trip logic.
• Velocity setpoint: chosen from the transformer rating and cooling type (ONAN/ONAF/OFAF); pumped-oil designs run faster and need a higher setpoint.
• Commissioning: bleed all trapped air first — newly filled or topped-up units generate false gas alarms for days as dissolved air comes out of solution.
• Operation on alarm: do not simply reset. Sample the gas: colorless, odorless, non-flammable gas is usually air; flammable gas indicates a real fault and warrants DGA before re-energizing.

The alarm contact typically routes to SCADA/annunciation; the surge contact hard-wires to the trip circuit alongside differential and sudden-pressure protection.

How It Compares to Other Protection

The Buchholz relay occupies a niche no purely electrical relay can fill: it is the only common device that detects slowly evolving incipient faults from inside the oil, giving days-to-weeks of warning.

• Versus differential (87T): the differential relay is fast (~1–2 cycles) but blind until a fault draws a large through-current imbalance. Buchholz sees the gas first, then trips only on a genuine oil surge.
• Versus sudden-pressure relay (SPR / 63): both react to violent faults; the SPR senses a pressure transient in gas or oil, while Buchholz senses the resulting oil flow. They are complementary and often fitted together.
• Versus DGA: DGA quantifies gas dissolved in the oil (ppm of H₂, C₂H₂, etc.) for diagnosis and trending, but it is periodic/offline; Buchholz continuously captures free gas and acts on it automatically.
• Versus temperature (WTI, 49): temperature relays catch sustained overload; Buchholz catches localized internal breakdown that may never raise bulk oil temperature.

In practice a large transformer carries all of these, layered.

Failure Modes, Trade-offs, and Significance

The relay's strength — sensitivity to any gas or oil motion — is also the source of its nuisance-trip and false-alarm problems:

• Trapped/dissolved air: after oil filling, topping-up, or a cooler-pump start, air liberated from the oil accumulates and triggers spurious alarms.
• Vibration and seismic events: earthquakes or heavy mechanical shock can slosh oil and cause false surge trips; some designs add anti-shock damping.
• Low oil level / leaks: a slow leak drops the oil surface and sinks the float, giving an alarm that is real but not a fault gas.
• Mechanical/contact aging: stuck floats, cracked reed switches, or (in legacy units) spilled mercury switches can fail silent.

Limits: it works only on conservator-type units — sealed/gas-cushion and hermetically sealed transformers can't use it and rely on pressure-relief and SPR devices instead. It also does nothing for external (through) faults.

Significance: a century after Max Buchholz's 1921 patent, this simple float-and-vane device still saves multi-million-dollar transformers by turning the earliest whisper of an internal fault into an actionable signal.

Buchholz relay versus other transformer protection schemes

Protection	Detects	Response	Best for
Buchholz relay	Fault gas + oil surge inside the tank	Alarm (slow) + trip (surge), seconds–weeks	Incipient internal faults, arcing, overheating
Differential (87T)	Current imbalance between windings	Trip in <1–2 cycles (~20–40 ms)	High-energy internal short circuits
Sudden-pressure (SPR)	Rapid tank pressure rise	Trip in ~30–50 ms	Fast, high-current internal arcs
Winding temperature (WTI/49)	Hot-spot temperature (top-oil + gradient)	Alarm/trip over minutes	Sustained overload, cooling loss
Dissolved gas analysis (DGA)	Gas dissolved in oil (ppm)	Offline lab / online monitor	Trending, root-cause diagnosis