WAZIPOINT WAZIPOINT Engineering Science & Technology: Buchholz Relay for Power Transformer Protection Device

Thursday, June 4, 2020

Buchholz Relay for Power Transformer Protection Device

What Is A Buchholz Relay?

What Is A Buchholz Relay?

Buchholz relay is safety device to protect a transformer from internal faults, like- impulse breakdown of the transformer insulating oil or transformer coil turns insulation failure etc. Buchholz relay in the field of power transmission and distribution system is a kind of relay that is little different from other conventional relays. Buchholz relay  also a safety device that mounted on large size (normally more than 500kVA) oil immersed transformer reactor and equipped with conservator or overhead oil reservoir. Buchholz relay was first developed by Max Buchholz in 1921.

What is Buchholz in chess?  “The Buchholz system (also spelled Buchholtz) is a ranking or scoring system inchess developed by Bruno Buchholz (died ca. 1958) in 1932, for Swiss system tournaments (Hooper & Whyld 1992). ... It was probably first used in the 1932 Bitterfeld tournament.”

Working Principle Of Buchholz Relay

It is interesting that transformer is an electrical equipment but  protecting device Buchholz relay does use any electrical signal itself. Internal fault may occurs inside transformer due to-
  1. insulation oil overheating;
  2. insulation failure of coil turns;
  3. excess core heating or core breakdown;
  4. any fault may result in excess heat; 
Any fault inside the transformer causes the excess heat and heat decomposes the transformer oil and generate gas. Gas bubbles ahead upward direction to conservator, Buchholz relay collected this gas bubbles through connected pipe and operate the relay to safe the transformer from destructive damages.Buchholz Relay Constructional Diagram


Buchholz relay actually consists two floated hinged accomplished with mercury switch, one at the top and other at the bottom, both are in a oil filled chamber. The upper float mercury switch connected with an alarm circuit and the lower float mercury switch connected to an external trip breaker, the details working principle is shown in the constructional diagram of a Buchholz relay.

Major faults, like earth fault or  phase to earth short circuit, the generated heat is high to produce a large amount of gas inside the transformer that will similarly flow upwards, but its motion is high enough to tilt the lower float in the Buchholz relay. Finally the lower float will cause the lower mercury switch which will break or trip the transformer means isolated from the supply.
These  are typically connected to cause the transformer to trip and this relay also serves a third function, similar to the sudden pressure relay.

Why Buchholz Relay is Used In Transformer?

Its common for transformer to happen an internal fault due to short circuit to transformer coil turns, any incidental fault in transformer winding or any impulse breakdown of the transformer oil or insulating oil. Buchholz relay is used for the protection of transformers from such a kind of fault.

Know detail about safety relay in another post: 

Advantages and Disadvantages Of Buchholz Relay
The major advantages and disadvantages of Buchholz are as below:

      1. Buchholz relay indicates the internal faults due to heating and safe transformer from any major faults;
      2. To determine the fault severity no need to dismantling the transformer;
      3. Buchholz relay prevent accidents from any major fault isolating the transformer.
      1. Buchholz relay can only be used with oil immersed transformers equipped with conservators;
      2. Buchholz relay may operation may be actuated without any fault in the transformer;
      3. During oil fill in the transformer some air also get in, later this air may accumulate under the relay and false operation may happen;
      4. Buchholz relay mercury switch may locked it movement due to mechanical lock;
      5.  Buchholz relay can detect  faults only below oil leveling the transformer.
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