Tutorial T5 Buchholz Relay

5/15/2011

utorial T5

Buchh Buch holz Re lays A Buchholz relay is a gas and oil operated device installed in the pipework between the top of  the transformer main tank and the cons conserv ervator. ator. A second sec ond relay is sometimes somet imes used us ed for the tapchanger select or chamber. The function of the relay relay is to detect an abnormal abnormal condit condition ion within the tank and send an alarm or trip signal. Under normal conditi conditions ons the relay is completely complet ely full of oil. Operation occurs occ urs when when floats are displaced by an acc accumulation umulation of gas, or a flap is mov moved ed by a surge of of oil. Almos Almostt all large oil-filled transformers are equipped with with a Buchholz relay, first developed by Max Buchholz in 1921. General Arrangement Front Fro nt View Vie w

Rear View Vi ew (Cov (Cover er Remov Re moved) ed)

A - Gas Collection Chamber

B - Upper Float C - Lower Float D - Oil Surg e Detector

Conditions Detected

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 ( Tutorial T3). T3). A small fault produces produces a smal smalll volume volume of gas that is deliberately delibe rately trapped trapped in the gas collection collect ion chamber chamber (A) built built into the relay. Typically, as the t he oil is displac di splaced ed a float (B) falls and a swit switch ch operates - normally normally to send an alarm. A large fault fault produces a large volume volume of gas gas which whic h drives a surge of oil towards the conserv cons ervator. ator. This surge moves moves a flap flap (D) in the relay to operate a switch switc h and send a trip signal. A severe severe reduction in the oil level level will also result resul t in a float falling. Where two floats floats are av available ailable these t hese 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

5/15/2011

utorial T5

Gas sampling - a graduated sight glass provides an indication of the volume of gas that has accumulated, typically typic ally 100-400cm 100-400cm3. After an alarm or trip signal has been been received this must mus t be collected collect ed and and analysed before before the transformer is returned to service. service. Gas collection collec tion can be done at the relay, or at ground lev level el if suitable arrangements exist. exis t. Clearly the latter latt er 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 trick le of air is equiv equivalent alent to a gradual acc accumulation umulation of gas. A blast simulates sim ulates an oil surge. The These se tests are somet sometimes imes referred referred to as 'blowing 'blowing the Buc Buchholz'. hholz'. 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. drained. As with gas sampling, sampli ng, this facility facili ty can c an be brought brought down to ground lev level el for  enhanced operator safety and convenience. Accessories

A range of accessories and services are available to assist with the safe and correct operation of Buchholz Buchholz relays including incl uding:: Ground level oil and gas sampling kits Gas sampling devices - automatic Gas sampling devices - manual On-site gas testers - simple air/fault gas analysis On-site gas testers - complex fault gas composition Relay test kits Recalibration

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