An earthing transformer is usually associated with three-phase supply systems
On a three-phase system, the neutral would be earthed either directly or through some limiting impedance / resistance. When the neutral point is not available or does not exist with a delta secondary winding of the transformer, a neutral point needs to be created. This is the purpose of the earthing transformer, which could consist of a zig-zag winding, or a two-winding star delta transformer where the star winding of correct voltage supplies an accessible neutral point when connected to the supply system.
