The use of Buchholz relay for the protection of oil immersed transformers of rating less than __________ is generally uneconomical.

Buchholz relay

• Buchholz relay is an oil and gas actuated relay located in the pipeline connecting the transformer main tank and the conservator.
• It provides early warning and protection against internal faults such as short circuits, insulation failures, and overheating by detecting gas accumulation and oil flow disturbances within the transformer tank.
• The use of a Buchholz relay for the protection of oil-immersed transformers is generally considered uneconomical for transformers with a rating of less than 750 kVA. This is because smaller transformers are less prone to internal faults that require gas accumulation detection, and alternative protection methods (such as fuses and circuit breakers) are more cost-effective for lower ratings.

Operation:

1. In case of incipient faults within the transformer, the heat due to the fault causes the decomposition of some transformer oil in the main tank. The products of decomposition contain more than 70% of hydrogen gas. The hydrogen gas, being light, tries to go into the conservator and in the process gets entrapped in the upper part of the relay chamber. When a predetermined amount of gas gets accumulated, it exerts sufficient pressure on the float to cause it to tilt and close the contacts of the mercury switch attached to it. This completes the alarm circuit to sound an alarm.
2. If a serious fault occurs in the transformer, an enormous amount of gas is generated in the main tank. The oil in the main tank rushes towards the conservator via the Buchholz relay and in doing so tilts the flap to close the contacts of mercury switch. This completes the trip circuit to open the circuit breaker controlling the transformer