The gas alarm or shut-down of a transformer induced by the Buchholz
relay is mostly caused by the oversaturation of the transformer
oil. Under specific conditions the oil releases a lot of relatively large
and stable bubbles which
migrate in the oil inventory of the main tank and finally they are collected
in the Buchholz relay (BR).
The relevant drop of the BR floater then induces either the warning signal or the shut-down of the transformer.
Most of BR warnings (more than 80%) are related to the oversaturation of the oil by air gases (N2 , O2). Air bubbles released from the oil rise through the transformer oil inventory, create areas of weak dielectric strength and strongly increase the probability of the failure due to a flashover. This phenomenon negatively affects the operational (instanteous) reliability of the transformer.
However, the rest of BR alarms or shut-downs are related to strong internal gas sources (e.g. hot spots) in the transformer and therefore mostly indicate not only a decrease of its operational reliability, but directly indicate internal fault(s) and consequently the decrease of the long-term reliability of the transformer.
The problem is that without the DGA reading it is very difficult to recognize
correctly what the given BR warnings represent.
In practice, just to be on the safe side, all BR actions are rated as potentially dangerous events.
The recommended treatment for non-combustible gases:
Buchholz Relay serves as an emergency sensor of your transformer. The frequent
BR action can be then interpreted as the omission of the proper screening
and a preventive maintenance of your transformer.