A Buchholz relay is a gas-operated protective relay fitted on the pipe connecting the transformer main tank to its conservator. When faults like arcing or insulation breakdown occur inside the oil, gases form and get trapped here. This device senses those gases and triggers alarms or trips the circuit to prevent damage.

Working Principle

The relay has two main chambers: an upper one for minor faults and a lower one for major faults.

  • Minor faults (like low oil overheating): Gases rise slowly, float to the upper chamber, and tilt a mercury switch or flap. This sends an alarm signal.
  • Major faults (like severe arcing): A sudden oil surge rushes oil and gas downward, hitting the lower chamber’s baffle. This activates a trip signal to cut off power instantly.

Key Components

  • Float chamber: Collects gas bubbles.
  • Mercury switches: Actuate alarms and trips.
  • Baffle plate: Responds to oil surges.
  • Equalizing pipes: Balance pressure between chambers.

These parts are housed in a weatherproof enclosure, typically mounted horizontally on the conservator pipe.

Applications in Transformers

Buchholz relays are widely used in large oil-immersed transformers (above 500 kVA) in power plants, substations, and industrial setups. They protect against:

  • Internal short circuits.
  • Insulation failure.
  • Overheating or partial discharges.

Not suitable for small dry-type transformers or outdoor ones without conservators. In modern systems, they integrate with digital relays for SCADA monitoring.

Advantages:

  • High sensitivity to slow faults.
  • No external power needed.
  • Cost-effective for large transformers.