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Arabic (عربي) Why Gas Is the Signal
Every internal fault in an oil-immersed transformer produces gas. An electric arc breaks mineral oil into hydrogen, methane, ethylene and acetylene; overheated paper insulation releases carbon monoxide and carbon dioxide. The gas is lighter than oil, so it rises. In a conservator-type transformer there is only one path upward — the pipe connecting the tank cover to the conservator. The Buchholz relay sits in that pipe.
This is what makes the relay different from electrical protection. A differential or overcurrent relay responds to current: it needs a fault large enough to disturb the electrical balance of the circuit. A Buchholz relay responds to the physical by-product of the fault, which appears far earlier. A loose core clamping bolt heating a few square centimetres of steel, or partial discharge eroding a spacer, draws no measurable fault current at all. It produces gas from the first minute.
The consequence is practical: a Buchholz relay usually gives warning of a developing fault days or weeks before any other protection notices it. That warning is the reason the device has remained standard equipment on conservator transformers for more than a century.
The relay is installed in the rising pipe between the tank cover and the conservator, positioned so that all gas leaving the tank must pass through it. Two independent elements inside the housing respond to two different physical events.
Two-Stage Operation
The upper element responds to gas that accumulates slowly; the lower one responds to a sudden flow of oil. They are wired to separate contacts and mean entirely different things.
Stage 1 — Gas Accumulation (Alarm)
Gas produced by a slowly developing fault collects in the upper part of the relay housing and displaces oil. As the oil level inside the relay falls, a float drops with it and closes the alarm contact. The transformer stays in service. The operator is warned, can draw off the collected gas through the sampling cock and have it analysed before deciding on any action.
Stage 2 — Oil Surge (Trip)
A severe internal fault — a winding short circuit or a flashover to earth — vaporises oil violently and drives a surge of oil and gas towards the conservator. The flow strikes a flap in the lower part of the relay, deflecting it against a spring and closing the trip contact. The transformer is disconnected within cycles, before the fault energy destroys the active part.
The distinction matters in operation. An alarm is information: something is developing, investigate it. A trip is a decision already taken: the transformer has been disconnected to limit damage. Treating an alarm as a nuisance signal — resetting it and moving on — removes exactly the early warning the relay exists to provide.
Reading the Gas
When the alarm operates, the collected gas can be drawn off through the sampling cock at the top of the relay. Its composition identifies the fault — the same chemistry used in dissolved gas analysis, but sampled from the gas phase rather than from the oil.
| Dominant gas | Indicates |
|---|---|
| Hydrogen (H₂) | Partial discharge, corona in the insulation system |
| Methane (CH₄), ethane (C₂H₆) | Low-temperature overheating of oil, typically below 300 °C |
| Ethylene (C₂H₄) | High-temperature overheating, above roughly 300 °C |
| Acetylene (C₂H₂) | Arcing. The most serious indication — take the unit out of service and investigate |
| Carbon monoxide (CO), carbon dioxide (CO₂) | Degradation of cellulose — paper or pressboard insulation is involved |
| Nitrogen (N₂), oxygen (O₂) | Air ingress, not a fault. Usually after maintenance, an oil top-up or a leaking gasket |
Colour gives a first indication before laboratory analysis: colourless gas that does not burn is air; white or grey suggests paper insulation is decomposing; yellow points to wood or pressboard; black or dark grey indicates oil decomposition by arcing.
Faults Detected
| Fault condition | Relay response |
|---|---|
| Partial discharge, insulation deterioration | Alarm — slow gas accumulation over days or weeks |
| Core fault, loose clamping, local overheating | Alarm |
| Poor joint or lead connection | Alarm |
| Inter-turn or inter-layer short circuit | Alarm first, then trip as the fault develops |
| Major winding short circuit, flashover to earth | Trip — oil surge, operates within cycles |
| Loss of oil below relay level | Alarm, then trip if the level continues to fall |
| Air ingress after maintenance | Alarm — verify by gas analysis before assuming a fault |
Installation Requirements
A Buchholz relay that is installed incorrectly does not fail visibly — it simply stops detecting faults. Three geometric conditions matter.
Continuous rise
The pipe must rise continuously from the tank towards the conservator, typically 2–4°. Any dip creates a pocket where gas collects before reaching the relay, delaying or preventing the alarm.
Straight approach
A straight length of at least five pipe diameters is required on the tank side and three on the conservator side. Bends close to the relay disturb the flow and make the surge element behave unpredictably.
Correct pipe size
The relay bore matches the connecting pipe, which is sized to the oil volume. Undersized, the flow is restricted; oversized, the surge may not develop enough force to operate the trip element reliably.
The relay is also directional. An arrow on the housing indicates flow towards the conservator; fitted the wrong way round, the surge element faces the wrong direction and will not trip on a fault.
Typical Specifications
| Parameter | Typical value |
|---|---|
| Contacts | Two independent circuits — alarm and trip |
| Contact rating | 5 A |
| Nominal pipe diameter | 25 mm, 50 mm or 80 mm, according to transformer rating |
| Alarm volume | Typically 100–400 cm³ of accumulated gas, depending on relay size |
| Surge operating velocity | Commonly 0.65, 1.0 or 1.5 m/s — selected to suit the transformer |
| Pipe inclination | Rising 2–4° towards the conservator |
| Gas sampling | Sampling cock with sight glass and graduated scale |
| Test facility | Manual push button, testing alarm and trip elements independently |
| Enclosure protection | IP54 or higher for outdoor installation |
False Alarms and How to Tell Them Apart
Not every Buchholz alarm is a fault. Distinguishing the two takes a few minutes and avoids both unnecessary outages and — more dangerously — the habit of dismissing genuine warnings.
| Cause | How to recognise it |
|---|---|
| Air released after filling or top-up | Gas is colourless and non-flammable; analysis shows nitrogen and oxygen in atmospheric proportion |
| Air drawn in through a leaking gasket | Recurring alarms with the same gas composition; oil level falling slowly |
| Rapid oil level change during cooling | Coincides with a sharp load or ambient temperature change; no gas present in the relay |
| Mechanical shock, vibration, nearby works | Trip without gas accumulation and without any electrical disturbance recorded |
| Genuine developing fault | Combustible gas present; hydrogen, hydrocarbons or CO in the sample; alarm repeats at shortening intervals |
The pattern is often more informative than any single alarm. A one-off alarm shortly after maintenance is almost always trapped air. An alarm that returns after two weeks, then after ten days, then after five, describes a fault that is accelerating — and should be treated accordingly.
Conservator Transformers Only
A Buchholz relay needs a pipe between the tank and a conservator, and needs a gas space to collect into. Hermetically sealed transformers have neither: the tank is completely filled and sealed, with corrugated walls absorbing oil expansion. There is no conservator, no connecting pipe and therefore no place to install the relay.
Sealed units are instead protected by a compact protection device — a DMCR or RIS type unit mounted directly on the tank cover, combining gas detection, pressure sensing, oil level indication and temperature measurement in a single housing. The protective function is equivalent; the mechanism differs because the physical arrangement differs.
Note: Values in this article are typical for distribution and medium power transformers and are given for guidance. On conservator-type units, DATSAN fits Buchholz relays as standard above a defined rating, and on request below it. Relay size, surge velocity setting and contact configuration are determined at the design stage according to the transformer rating and the customer's protection scheme, and confirmed in the test report.