Answer: In case where the neutral point of a three phase system are not accessible or where transformer or generators are delta connected, an artificial neutral point is required to be created. This is accomplished by means of a zigzag transformer called the earthing transformer.
As compared to the power transformer the rating of Zig Zag transformer is quite different. A grounding transformer is usually specified by the single phase fault current that it handle the two most common interval specified for the fault current are 30 and 60 s. A 60 s grounding transformer will be more expensive as compared to a 30 one.
According to Wikipedia,
A grounding transformer or earthing transformer is a type of auxiliary transformer used in three-phase electric distribution systems to provide a neutral point where the main power source is unable to (a power transformer or generator with delta-connected winding).