The Buchholz relay is one of the oldest and, at the same time, most reliable protective devices for oil-filled transformers with a conservator tank. It detects gas accumulation and sudden oil movements inside the tank that indicate incipient or acute internal faults, and triggers either a warning or immediate tripping, depending on the severity of the fault. Despite its purely mechanical operating principle, proven for over a century, it remains an indispensable part of the protection concept for every oil-cooled distribution or power transformer.
How does a Buchholz relay work?
The Buchholz relay is installed in the oil pipe between the transformer tank and the conservator. It operates on a two-stage principle: for smaller, slowly progressing faults such as partial discharges or insulation ageing, fine gas bubbles form in the oil, rise into the relay, and lower a float – this triggers the warning stage (gas alarm). In the case of severe faults, such as an internal arc or a winding short circuit, a sudden surge of gas is generated instead, producing a strong oil flow toward the conservator. This oil flow activates a baffle flap, which immediately triggers the tripping stage and disconnects the transformer from the grid on all poles.
Key Characteristics at a Glance
- Two-stage principle: warning stage on gas accumulation, tripping stage on strong oil flow
- Installed between the tank and the conservator in conventional oil-type transformers
- Detects arcing, partial discharges, and insulation faults at an early stage
- Purely mechanical operating principle – no external auxiliary power required
- Gas sampling via the sampling valve enables an initial fault analysis (flammable/non-flammable)
- Low-maintenance when correctly installed, though regular functional testing is still recommended
- Only usable with free-breathing or diaphragm conservator designs, not with hermetically sealed transformers
Frequently Asked Questions
Where does the name “Buchholz relay” come from?
The relay was named after the German engineer Max Buchholz, who patented it in the 1920s. Despite numerous technical refinements, the basic mechanical operating principle remains almost unchanged in use today.
What exactly is the difference between the warning and tripping stages?
The warning stage responds to slowly rising gas bubbles and merely raises an alarm without switching off the transformer – typical of incipient insulation ageing. The tripping stage, by contrast, responds to a sudden, strong oil flow caused by severe internal faults and immediately triggers all-pole disconnection to prevent consequential damage.
Do modern hermetically sealed transformers also need a Buchholz relay?
No. Hermetically sealed transformers without a conservator have no mounting location for a classic Buchholz relay. Here, alternative protection concepts such as pressure relief relays or pressure relief valves take over the corresponding protective function.
How is a Buchholz relay tested and maintained during operation?
Common practice includes a regular visual inspection for tightness and corrosion, a functional test of the contacts, and – if gas has been detected – a gas analysis to determine the fault. These checks are a fixed part of every annual maintenance service on oil-type transformers.
Are you planning an oil-type transformer with a well-thought-out internal protection concept? Our specialists will advise you on Buchholz relays, protection technology, and suitable accessories – talk to us.