BigJohn,
The posts aleady made to this thread should be adequate for covering the topic, but I will add a few more points.
1: The 3 Phase 3 Wire Grounded Delta System is an optional type system, which offers the "Stability" of a Grounded AC System, and the use of a Delta Connected Secondary.
2: The "Stability" of a Grounded AC System refers to the issues of Stresses on the Conductor's Insulation, which are common on Ungrounded AC Power Systems.
With one Grounded System Conductor in use, the Voltage to Ground will be somewhat "Stable", as viewed by the Capacitive Reactance of the System.
3: Use of a Delta Connected Secondary: This would allow for the temporary use of an Open Delta Vee connection, incase of an emergency.
Also, the Permanent System may be setup as an Open Delta Vee, Open Delta Tee, or a full "Closed" Delta.
There are a few ways to achieve a Delta arrangement - as listed below:
A: Open Delta Vee, Corner Grounded: Using Two Single Phase Transformers, connected in a traditional "Vee" arrangement, with one of the phase lines physically Grounded,
B: Open Delta Tee, Corner Grounded: Using Two Single Phase Transformers, connected in a non-traditional "Tee" arrangement, with one of the phase lines physically Grounded,
C: Open Delta Vee, Center Tap Grounded: Using Two Single Phase Transformers, connected in a traditional "Vee" arrangement, with the Center Tap of one Transformer physically Grounded,
D: Open Delta Tee, Center Tap Grounded: Using Two Single Phase Transformers, connected in a non-traditional "Tee" arrangement, with the Center Tap of one Transformer physically Grounded,
E: Closed Delta, Corner Grounded: Using Three Single Phase Transformers, connected in a traditional "Parallel" Delta fashion, with one phase line physically Grounded,
F: Closed Delta, Center Tap Grounded: Using Three Single Phase Transformers, connected in a traditional "Parallel" Delta fashion, with the Center Tap of one Transformer physically Grounded,
G: 3 Phase 3 Wire Delta, Ungrounded: Any version of the Open Deltas (Vee or Tee), or a Closed Delta, where there is no Main Bonding Jumper between the Local Grounding Electrode System, and the AC Power System (i.e. no Grounded Conductor).
Please review the Technical Reference section's schematics, and check out the various types of Delta Systems.
The link provided by Larry will take you to a slightly detailed view of a basic "Corner Grounded 3 Wire Delta", with 2 views of Panelboard applications, 1 view of use with HID Lighting, and 1 view of use with a basic Motor application.
Feel free to make additional comments if needed.
Scott35
