Figure 4-23. - Three single-phase distribution transformers connected delta-wye.
standard secondary system three-phase voltages are 208Y/120, 240, 480Y/277, and 480 volts.
If one of the transformers from a delta connected bank is removed, the remaining two are said to be open- delta-connected. With one transformer removed, the remaining two transformers will still transform the voltages in all three phases and supply power to all three phases of the secondary mains. The proper connections, using two transformers to obtain three-phase service, for a delta primary circuit are shown in figure 4-24. The capacity of the two transformers is now, however, only 58 percent instead of 66 2/3 percent of what it would appear to be with two transformers.
The open-delta connection is often used where an increase in load is anticipated. The third unit is added when the load grows to the point at which it exceeds the capacity of the two transformers. Furthermore, if one transformer of the three-phase bank should become defective, the defective transformer can be removed and the remaining two transformers continue to render service to at least part of the load.
Power capacitors are used in distribution systems to supply reactive voltamperes (Vars) to the system. When applied to a system or circuit having a lagging power factor, you can obtain several beneficial results. These results include power factor increase, voltage increase, system loss reduction, and an increase of electric system capacity.
POWER FACTOR. - When an alternating voltage and the current, which it causes to flow, rise
Figure 4-24. - Two single-phase transformers connected open-delta.
and fall in value together in the same direction at the same instant, the two are said to be "in phase," and the power factor is unity or 1.0. This condition is shown in figure 4-25.
The current and voltage waves are not in phase in most cases. They do not rise and fall in value together, nor do they have the same direction at the same instant; but instead, the current usually lags behind the voltage. Figure 4-26 shows the usual condition in transmission
Figure 4-25. - Voltage and current waves are in phase; power factor is unity.
Figure 4-26. - Current wave lagging behind the voltage wave, usual condition in transmission and distribution systems.Continue Reading