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Buchholz Relay Examination and Inspection
Inspection and testing of Buchholz relays: gas reading, contact and butterfly valve function tests, tripping verification, with engineer reports.
Buchholz relay examination and testing trials the protection element installed in the conservator line of oil-filled transformers — which detects gases released by internal faults and oil flow under abnormal surge, producing alarm and trip signals: gas accumulation reading, contact and mechanism functions, the electrical heating circuit and mounting integrity are examined periodically by authorised engineers. As the Buchholz catches the first outward gas echo of a tank fault (partial discharge, overheating, arcing), it counts as the most important internal protection of the oil transformer.
Operating Principle and Two Stages
The Buchholz works on two stages: slowly accumulated gases gather in the upper chamber, and when the level drops the float closes the alarm contacts. Sudden intense gas/oil flow (e.g. winding short circuit) rotates the butterfly valve on the lower stage closing the trip contacts and opening the breaker. The examination aims to prove both stages work mechanically and electrically.
Inspection Items
- Gas reading: During testing the accumulated gas is separated into air bubble versus fault gas — burnt off or sampled with a portable DGA kit and classified (the H₂/CH₄/C₂H₂ cocktail is a fault sign); volume, colour and odour notes enter the record.
- Alarm stage: Magnetic float movement with contact closure verified via test button or field kit; dirt-sludge accumulation and readability of the sight glass checked.
- Trip stage: Free butterfly valve movement, actuating flow velocity setting (typically 1.0-1.5 m/s — manufacturer value governs) and the trip signal dialogue with the control panel tested.
- Electrical and mounting: Cable gland watertightness, contact insulation resistance, body earthing; correct tank-to-conservator gradient (flow direction) and valve open-lock position verified.
- Special cases: Heated enclosure (winter freeze protection), broken glass-cracks, gasket ageing and leakage; on relay failure the risk of the transformer staying unprotected is evaluated.
Legislation and Standards
The control is tied to the periodic programme by Law No. 6331, the Regulation on High Current Electrical Installations and protection relay maintenance specifications; in practice relay examinations run annually, and relays showing gas are evaluated without waiting. Technical reference IEC 60599 (gas interpretation), relevant IEC 60255 series for protection devices and the manufacturer handbook. The examination joins with protection communication tests (breaker trip dialogue); results enter the plant protection coordination file.
Outcome
At the end of testing a signed examination report with the gas analysis note, contact-mechanism record and proposals is issued. Buchholz examinations are in our Transformer Tests and Analyses group; for gas evaluation after relay alarm or fault see our Dissolved Gas Analysis page, or contact us for your programme.
FAQ on Buchholz Relay Examination
The Buchholz relay, mounted on the pipe between the conservator and the body of an oil-insulated transformer, is a mechanical safety detector sensing gas bubbles and sudden oil flow from internal faults. Slowly accumulating gas (partial discharge-overheating) lowers the top float and closes the alarm contact; a sudden pressure wave (serious arc-interturn short) pushes the bottom float to trip the transformer via the breaker. These two stages catch the fault before it bursts.
Body-glass-level sight cleanliness-readability, accumulated gas volume-composition (visual-test), alarm-trip float-microswitch function trials (with magnet or flow simulation), contact-earth insulation control, correct direction arrow-pipe slope (so gas travels to the relay), gasket-tightness-oil leak and the integrity of the second-stage circuit (cable-panel) are checked. If gas has accumulated, a sample is drawn and the fault type is separated with DGA.
Quick observations like gas accumulation-sight-level run on the operating routine (e.g. monthly), while the function-microswitch-insulation examination is generally done in the yearly periodic control package. A relaying alarm is never silenced and passed; first the real fault (gas analysis) versus air-leak-moisture is separated. Since the relay is part of the protection regime, its records enter the test log and fault book.
The volume and the combustibility-odour of the gas are evaluated in the field; then a gas sample is drawn and, with laboratory DGA interpretation, the partial discharge-arcing-overheating separation is made. If noticeable combustible gas continues under load, the transformer is planned out for further test-examination. Constant air accumulation indicates a sealing-cooling-circuit problem. Pre-post relay level and panel records are reported.