The classic book by C. Russell Mason "The Art and Science of Protective Relaying" explains the Buchholz relay. This can be found on page 228 (Gas Accumulator and Pressure Relays) of:
The Buchholz relay is a mechanical device that is used to detect gas accumulation and surge in the main oil supply line of conservator type transformers. The device is essentially an oil filled chamber, with a float switch for the gas accumulation alarm function and a baffle operated switch for the surge (trip) detection.
The position of the relay in the conservator oil supply line ensures that any gas generated in the tank is trapped in the relay; there is a sampling line from the accumulation chamber to allow drawing off gas samples for analysis.
In case of a heavy internal fault, the resulting oil surge will operate the trip element due to to high flow through the relay. The trip is usually wired to a lockout relay to trip the transformer breakers; the Buchholz is considered a primary protection trip and would normally be wired to a separate lockout to the differential relay if duplicate tripping is provided for the transformer.
Suggestion: Try
1. Say M. G. "Alternating Current Machines," A Halsted Press Book, John Wiley & Sons, New York, 1978
Reference 1 includes Buchholtz Protection on page 187. The Buchholtz Relay gives alarm or initiates isolation on internal faults in liquid immersed transformers.
The some present IEEE Standards including the dictionary do not include the term "Buchholtz Relay or Protection."
