The Buchholz relay has 3 functions, gas accumulation operated by a float in the upper casting chamber, low oil operated by a float in the lower casting chamber and oil flow operated by flow over a vane attached to the lower float assembly. The relay is installed in the pipeline connecting the transformer tank to the underside of the conservator and is the only primary protection relay which anticipates a fault rather than only react to a fault condition. The relays presently stocked and sold are manufactured by Elettrindustria (ETI), Milan, and were introduced to Australia in 1994.
ETI relays are nominated series “GQ”, succeeding the ESP Gas Pressure relay series “GP”.
To choose the correct relay, the following considerations apply : model code, nominal bore (25, 50 or 80 mm pipeline diameter), whether air injection is required for ground level testing of the low float switch, switch configuration and oil flow rate expressed in cm/sec. For complicated switch configuration, the ETI switch nomenclature may be added.
All casting flanges are drilled to Australian Table D or 6 hole metric thread, see GQ casting
By default, unless otherwise described, all relays are switched 04-101C (1 N/O upper float switch, 1 N/O lower float / oil flow switch) with a flow rate of 1 m/sec.
Example : GQ80A4SFR120 04-505C describes a 4 switch 80 mm relay with air injection, flow rate 120 cm/sec and switch configuration 505C, see GQ switch
Evolved gas in the upper chamber can be collected at ground level see drawing RAGA B , known as the GLGR (ground level gas receiver). A variant design allows for the positioning of a valve connected to the underside of the casting for air injection onto the face of the oil flow vane, see drawing RAGA S AUS02 . For connection instructions see drawing RAGA B CCC .
The full ETI Buchholz relay catalogue is available here: GQ manual
Analysis of the gas collected in the relay can indicate an incipient fault or progress of insulation aging. A device for extracting gas from the receiver is described here: Gas Sampler