Gas operated Buchholz relay

A Buchholz relay is a gas-operated relay mounted in the pipe connecting the main tank of an oil-immersed transformer to the conservator. It protects against all internal faults that produce gas or an oil surge.

Construction

• A domed, oil-filled chamber in the inclined connecting pipe.
• Upper float (hinged) with a mercury switch connected to an alarm circuit.
• Lower float or baffle flap in the path of oil flow, with a mercury switch connected to the trip circuit of the transformer breakers.
• A gas release cock at the top and an inspection window.

                     gas cock
                        │
  Tank  ════════╗  ┌────┴───────┐  ╔════════ Conservator
   oil  ───────►╠══│ (o)─upper  │══╣ (pipe rises gently
                ║  │   float→A  │  ║  towards conservator)
                ║  │ [flap]─lower│  ║
                ║  │   float→T  │  ║
                   └────────────┘
          A = alarm switch, T = trip switch

Working

1. Normal condition: the relay is full of oil; both floats stay up and their contacts are open.
2. Incipient faults (core insulation failure, inter-turn short, hot spot, bad joint): heat slowly decomposes the oil; gas bubbles rise to the relay chamber and collect at the top. The oil level falls, and the upper float tilts and closes the alarm circuit.
3. Severe faults (earth fault, phase-to-phase winding fault): gas forms rapidly, and the pressure drives a surge of oil through the pipe towards the conservator. This surge tilts the lower float/flap and closes the trip circuit, disconnecting the transformer from both sides.
4. Loss of oil (leak): the oil level falls first below the upper float (alarm) and then below the lower float (trip).
5. The collected gas is sampled; its colour and analysis show the nature of the fault.

Advantages

• Simplest form of transformer protection; no CTs or external supply needed.
• Detects incipient faults at an early stage, before they become serious, and gives an alarm.
• Detects inter-turn and core faults that differential protection may not see.
• Gas analysis helps to identify the type of fault.
• Also protects against loss of oil.

Limitations

• Usable only for oil-immersed transformers with a conservator.
• Protects only against faults inside the tank; it does not cover bushings, cables or external connections.
• Slow: the minimum operating time is about 0.1 s (average 0.2 s), so differential protection is still needed for fast clearance.
• Can maloperate on oil surges caused by heavy through faults, earthquakes or mechanical shocks, or on air entering after oil filling.
• Economical only for transformers above about 500 kVA.