Buchholz Relay Guide: Gas Accumulation (Alarm) vs. Oil Surge (Trip), Installation Slope 1-1.5%, DIN 42566 & Fault Gas Interpretation

Abstract

The Buchholz relay — named after its inventor Max Buchholz (1921) — is the simplest, most reliable, and most widely used internal-fault detector for oil-immersed conservator-type transformers. Installed in the pipe connecting the main tank to the conservator, it detects two distinct fault conditions: slow accumulation of gas from incipient faults (alarm stage — light gas) and a sudden surge of oil from a severe internal fault (trip stage — heavy gas/oil surge). Despite its name recognition — virtually every power transformer electrician knows the term "Buchholz" — its correct installation, including the critical 1-1.5% slope toward the conservator, and its correct alarm/trip configuration, is frequently botched. This article covers the internal construction and two-stage operation, the installation slope requirement per DIN 42566 (IEC 60076-22-1), gas sampling and visual flame testing for rapid field diagnosis, and the relay's critical complementary role to electrical protection (differential, REF, overcurrent).

1. Internal Construction and Operating Principle

1.1 Physical Arrangement

The Buchholz relay is a cast-iron or aluminum housing containing two float-operated switches, installed in the horizontal pipe section between the transformer tank top cover and the conservator. The housing has a transparent (glass) inspection window on top, a gas-sampling petcock (valve), and a drain plug at the bottom.

Upper float (Alarm element): A hollow metal or plastic float, normally submerged in oil, connected to a mercury switch or a magnetic reed switch. When gas accumulates in the relay housing, the oil level drops, and the upper float descends. When the float angle reaches approximately 15-20° from horizontal, the switch closes, activating the alarm circuit.

Lower baffle/flapper (Trip element): A hinged baffle plate oriented perpendicular to the oil flow direction, held in the closed position by a calibrated spring or magnetic latch. When a sudden oil surge flows from the tank toward the conservator (driven by the rapid generation of gas from an internal arc fault), the oil flow velocity impinges on the baffle, overcoming the spring tension and closing the trip contacts.

1.2 Two-Stage Operation

Stage	Trigger Condition	Response	Time to Actuation	Indication
Alarm (Light Gas)	Slow accumulation of gas in the relay housing (typically 100-300 cm³ of gas displaces the oil level)	Annunciation (alarm) — control room alarm, SCADA notification	Minutes to hours (gas accumulates slowly from an incipient fault)	"Buchholz Gas Alarm"
Trip (Heavy Gas / Oil Surge)	Oil flow velocity exceeds the trip setting (typically 0.6-1.5 m/s, corresponding to a gas generation rate >50-100 L/s from a severe internal arc)	Instantaneous trip of all transformer circuit breakers (HV + LV + tertiary)	Milliseconds to seconds after fault initiation	"Buchholz Trip"

1.3 Typical Relay Ratings

Per DIN 42566 / IEC 60076-22-1:

Transformer Rating	Relay Nominal Diameter (Pipe Size)	Alarm Gas Volume (cm³)	Trip Oil Flow Velocity (m/s)
Up to 5 MVA	25 mm (1 inch)	100-150	0.6-1.0
5-50 MVA	50 mm (2 inch)	150-250	0.8-1.2
50-200 MVA	80 mm (3 inch)	200-350	1.0-1.5
>200 MVA	80-100 mm (3-4 inch)	250-400	1.2-1.8

2. Installation: The 1-1.5% Slope

2.1 Why the Slope Matters

The Buchholz relay must be the highest point in the oil flow path from the transformer tank to the conservator. Any gas generated in the tank must rise and be trapped in the relay housing — not in a pipe bend, not in a dead leg of the piping, and not in the conservator without first passing through the relay.

The slope requirement: The horizontal pipe from the tank top cover to the conservator must slope upward toward the conservator at 1-1.5% (1-1.5 cm per meter of pipe length). This ensures:

Gas generated in the tank rises into the pipe and travels upward along the sloping pipe ceiling to the relay housing — where it is trapped
Oil can still flow freely from the conservator to the tank through the lower portion of the pipe cross-section (the pipe is not completely filled by gas)

If the slope is too shallow (<1%): Gas may become trapped in the horizontal pipe before reaching the relay. The relay will not detect the gas until a very large volume accumulates — delaying the alarm.

If the slope is too steep (>3%): The oil flow velocity required to activate the trip baffle is affected — the relay's calibration assumes near-horizontal installation.

If the slope is reversed (downward toward the conservator): Gas generated in the tank cannot reach the relay — it travels directly to the conservator. The Buchholz relay becomes blind to internal gas generation — a critical protection failure.

2.2 Construction Verification

During transformer assembly:

The tank top cover is slightly domed (crowned) to direct gas toward the Buchholz pipe connection
The conservator support structure is positioned to create the required slope in the connecting pipe
A spirit level or laser level is used to verify the slope before the pipe flanges are tightened
The slope is re-verified after the transformer is filled with oil and the tank and conservator have settled under the oil weight

3. Gas Sampling and Field Diagnosis

3.1 Sampling Procedure

When a Buchholz gas alarm is received:

Do not reset the alarm without sampling the gas. This is a cardinal rule — a gas alarm is always a genuine event (incipient fault, air ingress) and must be investigated.

Wear PPE (arc-flash rated clothing, face shield, insulated gloves). The transformer is energized and the relay contains potentially explosive gas (acetylene + air).

Connect a length of PTFE tubing to the relay's gas-sampling petcock. The other end of the tubing is connected to a gas-sampling syringe (glass, 50-100 mL) or a gas-sampling bag (Tedlar or aluminized Mylar — must be gas-tight and inert).

Open the petcock slowly. Gas (being lighter than oil) will flow out first. Collect the gas sample until the gas volume is exhausted (oil begins to flow). Close the petcock.

Record the volume of gas collected and the total time since the transformer was last inspected (to estimate the gas generation rate).

3.2 The Flame Test (Classic Field Diagnostic)

A flame test on the collected gas provides immediate, albeit crude, diagnostic information:

Observation	Gas Composition	Indication
Gas burns with a yellow, luminous flame	Predominantly hydrogen and hydrocarbon gases (H₂, CH₄, C₂H₂, etc.)	Internal fault — oil or cellulose decomposition
Gas does NOT burn (extinguishes flame)	Predominantly air (O₂ + N₂)	Air ingress into the transformer (leaking gasket, oil pump drawing in air, low conservator oil level)
Gas burns with a blue, non-luminous flame	Predominantly hydrogen (H₂)	Partial discharge (corona) in the oil — low-energy fault
Gas burns and produces black smoke	High hydrocarbon content with unsaturated gases (C₂H₂, C₂H₄)	Arcing fault — high-energy discharge

Safety: The flame test involves introducing a flame to a gas sample that may contain acetylene at concentrations within the explosive range (2.5-80% in air). Perform the test only after lab analysis (gas chromatography) has confirmed the gas composition, or use a spark-type gas tester (which measures the dielectric strength of the gas or ignites a small sample under controlled conditions) rather than an open flame. The classic "match test" is increasingly discouraged due to safety concerns — the gas should be sent to a laboratory for full DGA by gas chromatography (IEC 60567).

3.3 Laboratory Gas Analysis

The gas collected from the Buchholz relay is analyzed by gas chromatography. The results are interpreted using the same diagnostic ratios as dissolved gas analysis (DGA):

Key Ratio	Range	Fault Type
C₂H₂/C₂H₄	>1	Arcing (discharge of high energy)
CH₄/H₂	>1	Thermal fault in oil (>300 °C)
C₂H₄/C₂H₆	>1	Thermal fault (>700 °C)
CO₂/CO	<3	Paper carbonization

Note: Buchholz gas analysis is complementary to oil DGA. The Buchholz gas is the gas that has been released from solution (bubble phase), while DGA measures the gases dissolved in the oil. The two analyses together provide a more complete picture: the Buchholz gas indicates the current gas generation (recent event), while the oil DGA indicates the cumulative gas generation over the transformer's history.

4. The Buchholz Relay's Role in the Protection Scheme

4.1 What the Buchholz Detects That Electrical Protection Misses

The Buchholz relay is sensitive to faults that produce gas but do not produce a significant change in electrical quantities (current, voltage):

Inter-turn short circuit (few turns): A short between 2-3 turns in a winding of 200+ turns changes the winding currents by <2% — below the differential protection's sensitivity threshold. However, the localized arc at the shorted turns generates gas that accumulates in the Buchholz, triggering the alarm. This is the most critical protective function of the Buchholz relay — inter-turn faults are a common failure mode that differential protection may not detect until the fault propagates to involve the full winding.
Core bolt insulation failure: Circulating currents through core clamping bolts generate localized heating and gas without affecting the winding currents.
Localized overheating at core joints or yoke clamps: Generates gas (CO, CO₂ from paper, hydrocarbon gases from oil) with no change in terminal currents.
Slow oil leak with air ingress: Air accumulates in the Buchholz (non-flammable gas), alerting to a sealing problem before moisture ingress degrades the insulation.

4.2 Trip Circuit Wiring

The Buchholz trip contact is wired directly to the transformer's lockout relay (86T), which trips all circuit breakers (HV, LV, tertiary) and blocks manual or automatic reclose. The Buchholz trip must never be wired through a time delay. Unlike differential protection (which may have a short time delay for CT saturation immunity), the Buchholz trip is an absolute indication of an internal fault — any delay is a delay in isolating the fault, and every additional half-cycle of arc duration increases the damage and the fire risk.

For OLTC diverter compartments: A separate surge relay (fast-acting Buchholz-type relay, calibrated for the OLTC compartment's smaller oil volume and higher expected gas generation rate) is installed in the pipe between the OLTC compartment and its separate conservator. This surge relay trips the transformer on detection of a severe OLTC fault — a separate protection function from the main tank Buchholz relay.

FAQ

Q: What is the minimum oil flow velocity required to trip a Buchholz relay?

The trip velocity is calibrated per the transformer rating: typically 0.6-0.8 m/s for small transformers (<5 MVA), 1.0-1.5 m/s for medium and large power transformers. The velocity is set by the baffle spring tension and the baffle surface area. The calibration should be verified during factory testing by injecting oil at a controlled flow rate through the relay. Too low a trip velocity → nuisance trips from normal oil circulation (especially in OFAF/ODAF transformers with oil pumps — pump start can produce a transient oil surge). Too high a trip velocity → failure to trip on a genuine internal fault.

Q: Can the Buchholz relay trip on an external fault?

No — the Buchholz relay is physically located in the pipe between the main tank and the conservator. Only gas or oil flowing from the tank through this pipe can act on the relay. An external fault (outside the transformer tank) does not generate gas inside the tank. However, a severe external through-fault can cause winding movement that displaces oil into the conservator, creating a temporary oil flow that might — in theory — approach the trip velocity. In practice, this is extremely rare because: (1) the oil displacement from winding movement is much slower (oil is incompressible and moves at the mechanical vibration velocity of the winding — not at the gas-expansion velocity of an internal arc), and (2) the trip baffle requires a sustained oil flow (not a momentary pulse) to overcome the magnet latch on modern relays.

Q: Why does the Buchholz relay need a slope toward the conservator?

Gas is lighter than oil — it rises. If the pipe from the tank to the conservator is perfectly horizontal, gas bubbles rising into the pipe will accumulate along the entire length of the pipe ceiling. The Buchholz relay, being a local high point in a horizontal pipe, will only collect gas that happens to rise directly into it. With a 1-1.5% slope toward the conservator, all gas in the pipe naturally migrates upward (along the sloping ceiling) to the relay housing — which is the highest point in the pipe. The relay then functions as designed, collecting and trapping all gas generated in the tank.

Q: My Buchholz relay gas alarm activated. What should I do?

Standard operating procedure: (1) Acknowledge the alarm. (2) Do NOT reset it — leave it active so the gas volume continues to accumulate in the relay for sampling. (3) Send a technician to the substation to sample the gas from the relay. (4) Perform a flame test or, better, send the gas sample to a laboratory for gas chromatography. (5) If the gas is flammable (burns), the transformer has an internal fault — initiate an outage for internal inspection. If the gas is non-flammable (air), there is an air-ingress issue — check the oil level in the conservator, the breather function, the oil pump seals (for forced-oil transformers), and all gasket joints. (6) If the gas generation rate is high (>100 cm³ per day), take the transformer out of service — a rapidly developing fault is in progress. If the gas generation rate is very low (<100 cm³ per month), the transformer may continue in service with intensified monitoring (weekly DGA, monthly Buchholz gas accumulation check).

Q: What is the difference between the Buchholz relay and the differential relay?

The Buchholz relay is a mechanical, gas-actuated device that detects faults generating gas inside the transformer tank. It operates on the physical principle of gas accumulation and oil displacement. The differential relay is an electrical, current-comparison device that detects faults by measuring the difference between primary and secondary winding currents. They are complementary: the differential relay detects turn-to-turn and phase-to-phase faults that produce significant current imbalance (but may not initially generate enough gas to activate the Buchholz), while the Buchholz relay detects incipient faults that generate gas but do not significantly unbalance the terminal currents (inter-turn shorts of few turns). A complete transformer protection scheme includes both.

Q: Does the Buchholz relay need maintenance?

The Buchholz relay itself requires minimal maintenance: (1) annual functional test — press the "test" button (if equipped) to verify the alarm and trip contacts close and the alarm/trip circuits are intact, or inject air into the relay through the petcock to simulate gas accumulation and verify the alarm operates, (2) drain accumulated gas through the petcock after every alarm investigation (the relay housing must be re-filled with oil to reset the floats), (3) clean the inspection window if it becomes opaque due to oil varnish (this prevents visual confirmation of the oil/gas interface), and (4) during major transformer overhauls (every 10-15 years), remove the relay cover, inspect the float mechanism for freedom of movement, check the mercury or reed switches for continuity, and verify the baffle trip setting with a controlled oil flow test. Mercury-switch Buchholz relays (still in service on older transformers) are being phased out under environmental regulations — replacement with reed-switch-based relays is recommended at the next overhaul.

References / Standards

Reference	Title
DIN 42566	Buchholz relays — Specification
IEC 60076-22-1:2019	Power transformer and reactor fittings — Part 1: Buchholz relays
IEC 60567:2011	Oil-filled electrical equipment — Sampling of gases and analysis of free and dissolved gases — Guidance
IEC 60599:2022	Mineral oil-filled electrical equipment in service — Guidance on the interpretation of dissolved and free gases analysis
IEEE C37.91-2021	IEEE Guide for Protecting Power Transformers

*Authored by Du Fu, Production Engineer at ZY POWER. The Buchholz relay is the transformer's "nose" — it smells the first whiff of internal decomposition long before the electrical protection detects a problem. Every Buchholz gas alarm must be investigated; every trip must be respected.*