Main Accessories of Three Phase Transformer


The following are main accessories of the three phase transformer in addition to the core and windings.

  • Transformer Tank
  • Conservator
  • Oil level indicator
  • Breather
  • Explosion vent
  • Buchholz relay
  • Thermometer
  • Bushings

Transformer Tank

  • It provides protection for core and windings of the transformer. 
  • It is made from rolled steel for small transformers and boiler plates for large transformers. 
  • The cooling tubes or separate radiators are provided with tank for cooling of transformer windings and core. 
  • The aluminum is used for transformer tank in order to reduce weight. It reduces stray magnetic fields and also stray load loss.



  • It is mounted on top of the transformer and connected to it through a small pipe as shown in the Figure. 
  • The oil level in the transformer tank changes with the change in load condition.  
  • The oil expands at higher load and air comes out of the transformer tank. Similarly at light load, load contracts and air drawn in the transformer tank. This is known as breathing of transformer. 
  • The oil may be vaporizing due to short circuits, temperature rise and over – load. 
  • The oil vapours mixture with air which ignites and can cause damage the transformer windings therefore the function of the conservator is to take up expansion and contraction of the transformer oil without allowing it to come in contact with ambient air. 
  • The transformer tank is completely filled with oil whereas conservator is about half fill with oil.

Oil level indicator

  • It indicates level of oil in the conservator tank.


  • Due to breathing of transformer oil, it comes in direct contact with air and resulting reduces the dielectric strength of the oil.  
  • The breather consists of small container connected to the vent pipe and contains material like silica gel with cobalt chloride.  
  • The function of the breather is to absorb moisture of the air when entering into transformer tank. 
  • The silica gel becomes blue when dry and a whitish pink when damp. A glass is provided in the front of container to observe colour of silica gel.

Explosion vent

  • It is a thin glass relief diaphragm placed at the top of tank. 
  • A fault in the transformer tank may produce a pressure in the tank. 
  • The diaphragm burst if the pressure inside the tank becomes excessive therefore protecting the tank.

Buchholz Relay

  • It is used for detecting fault conditions such as incipient winding faults, inter-turn faults, core bolt etc. 
  • It is a gas operated relay, connected between transformer tank and conservator.



  • It is completely filled with oil. 
  • It consists of two floats with mercury switches. 
  • The contacts of the upper and lower mercury switches are connected to alarm circuit and trip circuit respectively. 
  • A release cock is provided at the top of the Buchholz relay so that the pressure of the evaporated gas can be released. 
  • A small window in the wall of the relay is provided, to observe the gas and its colour.


  • When minor fault occurs, gas bubbles are generated in the oil due to heating effect by fault current. 
  • As the tank of transformer is completely filled with oil, gas rise and go towards the conservator through the pipe. 
  • As soon as gas bubbles enter into the buchholz relay, oil level falls and upper float is tilted. 
  • As a result contacts of upper mercury switch are closed and alarm circuit is closed. 
  • In the case of severe faults such as short circuits between phases or faults in the tap changing mechanism, large amount of gas is produced. 
  • As a result contacts of lower mercury switch is closed and trip coil is energized the relay of a circuit breaker which open the circuit. 
  • The colour of the gas indicates the nature of the fault and amount of gas indicates the severity of the fault.


  • The hot oil temperature is indicated by dial type thermometer. 
  • The dial of the thermometer is kept outside the tank and bulb of the thermometer is mounted in the oil. 
  • The bulb is heated by heaters which carry a current proportional to the winding current and hence the thermometer indicates the winding temperature.


  • The bushings are used in order to reduce flash over from the high voltage connection to earthed tank. 
  • It consists of a current carrying conducting rod, bus or cable at the center and porcelain cylinder surrounded on it.

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