Modeling High-Leg (Open Delta) Transformer

High-leg delta (also called “red-leg” or “wild-leg” delta) configuration are commonly used in older
three-phase electric power installations in which it supplies both line-to-line and line-to-neutral
services in the same distibution system. Typically, it supplies lighting loads connected phase-to-neutral
and 3-phase motor loads at line-to-line voltage.

One common method to create this type of service is by utilizing two transformers: one set as a
single phase connection and the other is set as a single phase mid-tap.

High-Leg Delta Model:
Voltages for each phase:
Phase A: 120V
Phase B: 208V (High Leg)
Phase C: 120V

(Note: 208V is obtained from 120V * √3 )

PTW Model:
To model the high-leg delta in PTW, two parallel single-phase transformers are connected to a
common bus. One is set as single-phase midtap (XF2-0001) and the other as single-phase (XF2-
0002). The high leg (phase B) is the one set as single-phase. In PTW, the transformers will need to
be connected to a fictitious cable for the parallel connection to be valid.

Once the Unbalanced/Single-phase Load Flow is ran, the voltages for each phase shows the
corresponding line-to-neutral voltages as seen on BUS-0001.