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.
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
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 and analysed before the transformer is returned to service. Gas collection can be done at the relay, or at ground level if suitable arrangements exist. Clearly the latter is a safer and more convenient option.
Functional Tests - a test petcock enables dry air to be admitted into the relay to check correct operation. A trickle of air is equivalent to a gradual accumulation of gas. A blast simulates an oil surge. These tests are sometimes referred to as 'blowing the Buchholz'. On completion it is important that the relay is bled to remove the air that has been introduced.
Draining - a valve in the bottom of the relay enables an oil sample to be taken or the relay to be drained. As with gas sampling, this facility can be brought down to ground level for enhanced operator safety and convenience.
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