What is Buchholz relay? Which equipment is protected by it?

A Buchholz relay is a safety device that is mounted on some oil-filled power transformers and reactors, equipped with an external overhead oil reservoir called a “conservator”.

It is used to detect the effects of dielectric failure inside the equipment, such as gas accumulation, oil leakage, or overheating.

¹²The Buchholz relay has two floats with mercury switches inside a metal chamber that is connected to the oil piping between the conservator and the main tank.

When a fault occurs inside the transformer, the oil temperature rises and the oil evaporates into gas. The gas moves upward and displaces the oil level in the chamber, causing the floats to tilt and activate the switches.

Depending on the severity of the fault, the switches can either trigger an alarm or disconnect the transformer from the power supply. ¹²³The Buchholz relay is named after its inventor, Max Buchholz, who patented it in 1921.

¹ It is a widely used protection device for oil-immersed transformers with a rating higher than 500 kVA. It can prevent further damage to the transformer and avoid fire hazards.