I can give a couple of examples, which would fall under the classification of an Earthing Transformer:
[*]Zig Zag Transformers,
[*]3 Wire Wye Secondary Transformers, with common center point Earth Grounded.
For the Zig Zags, these could be used to either derive a Grounded Circuit Conductor from an Ungrounded supply, or simply to have an "Earth Ground Reference. These are typically Wye connected.
There may be Schematics of Zig Zags posted in the Technical Reference area (???).
For the 3 wire Wye Secondary types, these are typically used on Power Transmission circuits from where the Power was Generated. The point where the 3 Secondary coils are connected - forming the Wye / Star point - is driven to Earth Ground. No Conductor is tapped off this common point.
This assists in Lightning Protection of the Transmission Feeder circuitry.
The Transformer also is used to increase the AC Voltage.
Example: Output from Generator = 18 KV 3 Phase 3 Wire, which is fed into the Wye Secondary Step-Up Transformer. For long distance Transmission, Voltage is Stepped Up to 345 KV.
The Step-Up Transformer is either a Wye / Wye or Delta / Wye 3 Phase 3 Wire Transformer, with Primary Voltage of 18 KV and Secondary Voltage of 345 KV. The Star Point / Common Center Point on the Secondary is Earth Grounded.
Hope this helps a little!
Let me know if anyone would like to have Schematics of these Transformers added to the Technical Reference area.