24 March 2018

Earthing Transformer : Construction, Working and Application

  • It is a special transformer which provides return path for earth fault on Delta connected transformer.

Construction

  • It has primary winding but no secondary winding. 
  • It is a core type transformer consists of three limbs. 
  • Each limb carries two windings which have equal number of turns. 
  • These two windings are connected such that one winding provides star point and current flow in the two windings are opposite direction.
earthing transformer

Working

  • When an earthing transformer is connected to a healthy system, only small magnetizing current flows between the neutral point and earth. 
  • This is due to fact that the magnetic fluxes in the split windings cancel each other out. 
  • The zero sequence current flows in the transformer earth connection in the case of earth fault in the one phase. 
  • The impedance of the earthing transformer is very low therefore the fault current will be high. 
  • The magnitude of the fault current is limited by inserting resistance either in the neutral path or in series with winding of the transformer. 
  • The earth fault current flows for few seconds therefore the size of it is much small than the ordinary three phase transformer of same kVA rating. 
  • The kVA rating of the earthing transformer is given by the product of the rated neutral current and the rated line to neutral voltage.
connection of earthing transformer with star delta transformer
Application
  • It is used to trap harmonic currents.  
  • The DC voltage is produced by half wave rectification. However the direct current produces saturation of transformer.  
  • The earthing transformer avoids saturation because the current in each half winding of the transformer flows in the opposite direction.
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