From the Law of Cosines
Figure 24 Open-Delta Unbalanced phase-to-phase
Fault
Voltages
)
V
2
V
(
=
12
23
*
arccos
θ
23
2
12
2
2
V = (
V
2
) + (
3
2
* 120 )
Settings For Typical Phase-to-Phase Fault Voltages
V
12
= V
f
V
12
1
5 10 15 20 25 30 35 40 45 50 55 60 65 70
V
23
104 104 104 104 104 105 105 105 106 106 106 108 108 109 110
At
θ
°
Lag
270 271 273 274 275 277 278 280 281 282 284 285 286 287 289
3.4.3.2 T Connection
The second method of obtaining a three-phase, three-wire voltage source is the so-called T-
Connection. The method, shown in the following figure, is easier to use when obtaining an
unbalanced, phase-to-phase fault simulation since it eliminates calculations. To reduce confusion
when using the T-Connection, the voltage output #1 is designated V
a
and its phase angle set at
0
°
, voltage output #2 is designated V
b
and its phase angle set for 180
°
, and voltage output #3 is
designated V
c
and its phase angle is set for 270 Any combination of balanced three phase faults
or unbalanced phase-to-phase fault conditions can be easily simulated. The following figure
indicates these phase relationships.
NOTE: This method should not be used for very low fault voltages (i.e. 5 volts or less, or for
testing ABB or Westinghouse type SKD relays).
37
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