Operation and Principle of Buchholz Relay in transformer


Buchholz Relay in transformer is an oil container housed the connecting pipe from main tank to conservator tank. It has mainly two elements. The upper element consists of a float. The float is attached to it in such a way that it can move up and down depending upon the oil level in the Buchholz Relay Container. One mercury switch is fixed on the float. The alignment of mercury switch hence depends upon the position of the float.

Working Principle-


  1. Whenever there will be a minor internal fault in the transformer such as an insulation faults between turns, break down of core of transformer, core heating, the transformer insulating oilwill be decomposed in different hydrocarbon gases, CO2 and CO. The gases produced due to decomposition of transformer insulating oil will accumulate in the upper part the Buchholz Container which causes fall of oil level in it.2
  2. Fall of oil level means lowering the position of float which cause  tilting of  the mercury switch. The contacts of this mercury switch are closed and an alarm circuit energized. Sometime due to oil leakage on the main tank air bubbles may be accumulated in the upper part the Buchholz Container which may also cause fall of oil level in it and alarm circuit will be energized. By collecting the accumulated gases from the release pockets which is on the top of the relay and by analyzing them one can predict the type of fault in the transformer.                                                                                          
  3. More severe types of faults, such as short circuit between phases or to earth and faults in the tap changing equipment, are accompanied by a surge of oil which strikes the baffle plate and causes the mercury switch of the lower element to close. This switch energized the trip circuit of the Circuit Breakers associated with the transformer and immediately isolate the faulty transformer from the rest of the electrical power system by  tripping the Circuit Breakers associated with both LV and HV sides of the transformer.                                             buchholz_relay
  4. The Buchholz Relay few times starts working without any fault in the transformer. For example, when oil is added to a transformer  air may get in together with oil, accumulated under the relay and thus cause a false Buchholz Relay operation.
  5. That is why mechanical lock is provided in that relay so that one can lock the movement of mercury switches when oil is topping up in the transformer. This mechanical locking also helps to prevent unnecessary movement of mercury switches during transportation of the Buchholz Relays.

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