Earthing Transformer : Construction & Working

• 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.

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.

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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