A Buchholz relay is a gas and oil operated device installed in the pipework between the top of the transformer main tank and the conservator. A second relay is sometimes used for the tapchanger selector chamber. The function of the relay is to detect an abnormal condition within the tank and send an alarm or trip signal. Under normal conditions the relay is completely full of oil. Operation occurs when floats are displaced by an accumulation of gas, or a flap is moved by a surge of oil. Almost all large oil-filled transformers are equipped with a Buchholz relay, first developed by Max Buchholz in 1921.
A - Gas Collection Chamber
B - Upper Float
C - Lower Float
D - Oil Surge Detector
A Buchholz relay will detect:
Gas produced within the transformer
An oil surge from the tank to the conservator
A complete loss of oil from the conservator (very low oil level) Fault conditions within a transformer produce gases such as carbon monoxide, hydrogen and a range of hydrocarbons (Tutorial T3). A small fault produces a small volume of gas that is deliberately trapped in the gas collection chamber (A) built into the relay. Typically, as the oil is displaced a float (B) falls and a switch operates - normally to send an alarm. A large fault produces a large volume of gas which drives a surge of oil towards the conservator. This surge moves a flap (D) in the relay to operate a switch and send a trip signal. A severe reduction in the oil level will also result in a float falling. Where two floats are available these are normally arranged in two stages, alarm (B) followed by trip (C). Gas and Oil Flows
Buchholz relays are equipped with a number of gas and oil inputs and outputs, including test and sampling facilities
Gas sampling - a graduated sight glass provides an indication of the volume of gas that has accumulated, typically 100-400cm3. After an alarm or trip signal has been received this must be collected...
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