Forgot to mention:
LADWP (Los Angeles Department of Water & Power) used Open Delta setups quite commonly in areas zoned for Commercial + Residential Customers.
This would be in areas where there are both Commercial Customers + Residential Customers on the same Transformer(s).
There were some areas where the physical sizes of the Transformers on the Open Deltas, were noticeably different.
For example, the "Main" Transformer (the one with the Center Tap of the Secondary being used), would be very large, in comparison to the "Kicker" Transformer (the Transformer used for the "Top Of The Delta").
Sizes such as 75 KVA "Main" / 15 KVA "Kicker" were common in certain areas.
Some of the LADWP Open Delta Pole arrays exist today. Most have been eliminated during a Building's Alteration, in favor of a 4 Wire Wye derived from a Pad Mounted transformer.
Additionally, where Primary Distribution + Secondary Feeders have been placed in Underground Ducts, the resulting affects call for Pad Mounted Transformers, or large Transformers in Underground Structures.
UGS's are very common for Primary Voltages of 34,500V, with Secondary Voltages of 480Y/277 - 600V, and Capacity of >750 KVA (2500 KVA, 3750 KVA,).
So, to sum up the response to your question; the Open Delta "Vee" (as shown in Greg's image), is a Cost-Effective method for Pocos to deliver 3 Phase Service to Customers, where the Demand is low.
The Demand may be 3 Phase 3 Wire, Single Phase, or 3 Phase 4 Wire.
Specifically, the concept was to deliver 3 Phase Service to a Customer, whom had a small 3 Phase Demand, and a nominal 1 Phase Demand.
Example:
* (2) 5 HP 3 Phase Motors,
* (1) 7.5 HP 3 Phase Motor,
* 20 KVA @ 240V 1 Phase 3 Wire Demand.
* 3 Phase Motors are Non-Coincidental Loads: One normally runs alone, however, Two may run at same time for less than 15 Minutes.
Any Single Motor may run >180 Minutes at a given time.
Another use of an Open Delta is the "TEE" connection.
These are used for 2 Phase Systems, and where a low power 208Y/120V 3 Phase 4 Wire SDS (Separately Derived System) is installed.
The 2 Phase Systems are illustrated in the Technical Reference section, according to the type of SDS obtained.
These are "TEE" Connections for Open Delta setups.
For Closed setups, they are "Taylor" and similar.
Per the 208Y/120V 3 Phase 4 Wire "Open Delta TEE", many of the Dry-Type Transformers rated 15 KVA and less, will be Open Delta TEE arrangements.
This eliminates one full Primary & Secondary winding, yet still yields an output consisting of any of the following:
* 208V 3 Phase - 3 Wire;
* 208V 1 Phase - 2 Wire: from L1-L2, L2-L3, or L1-L3;
* 120V 1 Phase 2 Wire: from L1-N, L2-N, or L3-N;
* 120V 1 Phase 3 Wire (Multiwire): from L1/L2-N, L2/L3-N, or L1/L3-N;
* 120V 1 Phase 4 Wire (Multiwire): from L1/L2/L3-N.
Input may be from any 3 Phase 3 Wire available, such as:
* 200V (208V),
* 230V (240V),
* 380V,
* 416V,
* 460V (480V),
* 575V (600V),
* 120V,
* 220V.
Scott
