Why Bucholz relay is used for transformer incipient fault detection?

Shahar, If your incipient fault is generating dissolved gas the Bucholz will detect it. Alan

Also called a "sudden pressure relay," this form of fault detection is as fast or faster than a current differential relay applied to primary and secondary, while costing much less. The disadvatage of it, compared to current differential, is that it can only detect faults within the tank, and not faults within the bushings or the connecting conductors between the transformer terminals and breakers on primary and secondary.

Gary, Bucholz relay and SPR (Sudden Pressure Relay) are two different types of transformer relays. The Bucholz has an accumulator where the rate of the gas production is measured while an SPR does not, it simply responds to the pressure waves caused by the rapid change in the oil pressure due to a fault.

The two are often used on the same transformer, Bucholz for alarming (Hey, there's a lot of gas being produced), SPR for tripping (Oops, too late). There are some Bucholz's that provide both functions.

Alan

Sirs.
dissolved gasses is a different story.
buchholz is for free gas.
Sudden Pressure Relay will detect the pressure raise.
may be just a matter of semantics.
Well why we see simple questions that can be googled from top contributors?

Dissolved gases is follow up of the Buchholz relay Alarm or Trip...
from the characteristics of dissolved gases one can know what kind of fault is being developed or is developing....
Baljinder Bansal

Buchholz device operates on the physical disturbance of gas or oil in the transformer tank as a result of an event (fault) in the tank

Buchholz operates on gas released and found the way up in to the pipeline to conservator. generally the gas is produced because of Arcing in the transformer but rarely due to other phenomena. If the arcing is severe the gasses released may not dissolve and indicates severe fault. In a new transformer some times the air trapped in various parts inside get released and find the way to Buchholz.
Dissolved gasses are generated at all temperatures above 90 degrees the gas is generally indicative of the temperatures inside. hydrogen is rarely seen in excess doe to water content in the oil getting dissociated.
Even the above information and gas temperature relation ship can be googled.