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. </OL>
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.
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!
#133942 - 10/10/0209:16 PMRe: Does anyone know what an Earthing transformer is?
Too add to Scott's observations, from IEEE transformer standards... The zigzag-connected winding has each electrical phase linked equally by two magnetic phases as shown. This winding connection is most commonly used in a single-winding transformer to perform grounding duty. The connection presents a low-impedance path to ground for zero-sequence currents (that is, current components of equal magnitude and phase angle in each leg), and therefore gound current flows with any shift of the system neutral.
Grounding transformers may be sized so as to furnish only the magnitude of ground current required for relaying or may be large enough to stabilize the neutral of an otherwise ungrounded system. In the latter case, the grounding transformer supplies the unbalanced phase-to-neutral load as well as ground fault current.
#133943 - 10/10/0210:28 PMRe: Does anyone know what an Earthing transformer is?
An interconnection diagram is shown that was used for deriving a 277V neutral connection for lighting in a research highbay during construction. There was 480V 3ø 3w service for welding machinery, without a neutral conductor. Three 1.5kVA 240:240V drytype transformers and a 20-amp circuit breaker were assembled on framing channel in a zigzag configuration and fed a length of 10/5 SO cord fed the 277V HID light fixtures. This arrangement was smaller, significantly less expensive and more readily available than a similar-capacity 480∆-480Y/277V transformer.
[This message has been edited by Bjarney (edited 10-11-2002).]
#133944 - 10/11/0210:37 PMRe: Does anyone know what an Earthing transformer is?