Friday, 21 February 2020

What is Buchholz Relay? Construction, Working Principle, Advantages, Limitations


  • Buchholz relay is a protection device used to detect transformer faults in oil filled transformers.
  • Short circuit faults such as inter turn faults, incipient faults, and core faults may occurs due to the impulse breakdown of the insulating oil or simply the transformer oil. 
  • Buchholz relay will sense such faults and close the alarm circuit.

Buchholz Relay

Buchholz Relay


  • Buchholz relay is placed in the pipe which connects to the conservator and transformer oil tank.
  • The oil filled chamber is present in it. Two hinged floats, one at the top of chamber and the other at the top of the chamber which is with a mercury switch each is present in the oil filled chamber. 
  • The mercury switch on the upper float is connected to an external alarm circuit and the mercury switch on the lower is connected to an external trip circuit.  
Construction of Buchholz Relay
Construction of Buchholz Relay

Working Principle

  • The buchholz relay works in transformer tank. It is mechanically actuated. Whenever there will be a minor internal fault in the transformer such as an insulation fault between turns, breakdown of core of transformer, core heating then it works.
  • When something fault occurs in  transformer tank then it accompanied with the generation of some type of gas and if the fault occurred is high enough it will be accompanied by a surge of oil from the tank to the conservator.
  • If some fault occurs inside the transformer, the oil in the transformer tank gets overheated and gases are generated.  In the intensity of fault occurs in transformer tank, the production of gases depends on it.
  • Also the heat will generates if the fault occurs high and it decomposes the transformer oil. And that gases produced in the can be used to detect the winding fault.
  •  So this is the basic principle of the working of buchholz relay.  

Buchholz Relay Placed in Transformer at-
  • Buchholz relay located in the pipe connecting the main oil tank and the conservator.
  • So it can sense both the oil level and the pressure of any gas generated by the thermal decomposition of the oil. 
Location of Buchholz Relay
Location of Buchholz Relay

Operating Conditions of Buchholz Relay
Buchholz relay operates during the three conditions

1. Whenever gas bubbles are formed inside the transformer due to severe fault.

2. When the level of transformer oil faulse.

3.When transformer oil flows rapidly from the conservation tank to the main or from main to the  conservation tank.


  • It indicates incipient faults.
  • Buchholz relay gives an alarm when the oil level reduces below a certain level due to leakage of oil from transformer.
  • It gives audible warning which informs the operator that there is some fault in transformer.
  • By testing the gas it also detects the failure of insulation.

Limitation of buchholz relay

  • It can be used only for oil immersed transformers having conservator tanks.
  • Only faults below oil level is detected.
  • Setting of mercury switches cannot be kept too sensitive otherwise the relay can operate due to bubbles, vibrations, earthquake, mechanical shocks etc.
  • The relay is slow to operate having minimum operation time of 0.1 seconds and average time 0.2 seconds.

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