Many older industrial facilities use single-phase, oil-filled transformers made for pole-mounting but installed at ground level to create 3-phase power distribution systems. Usually, primary and secondary “bus” conductors are strung open between wood poles. Beneath these conductors are the transformers or “tubs.” Open conductor drops are run from the bus conductors to the tubs' primaries and secondaries, and the appropriate transformer connections are made using split-bolt connectors.
Suppose your electric utility is providing 2,400/4,160V wye, 3-phase, 4-wire power to the facility, and you need a lower voltage for lighting and power. What are your options?
One option, as shown in Fig. 1 (click here to see Fig. 1), is a delta-connected transformer system. Here, three 4,160V, single-phase transformers are needed, with their primaries connected in series. The electric utility neutral is not needed. The banking of these units results in a 240/120V, 3-phase, 4-wire secondary.
Note how a center-tap terminal is used on the secondary of one transformer to ground the entire system. On a 240/120V, 3-phase, 4-wire system, there's a potential of 120V between this center-tap terminal and each ungrounded terminal on either side of it (Phases A and C), as shown in Fig. 2 (click here to see Fig. 2). A high leg (“red” or “wild” leg) results at Phase B that has a higher voltage-to-ground than the other two phases. This voltage is found by multiplying the voltage-to-ground of either of the other two legs by the square root of 3. Thus, the voltage between Phase A and ground is 208V (120V × √3).
Wye-wye connected system
A second option, as shown in Fig. 3 (click here to see Fig. 3), is a wye-wye connected transformer system. Here, the primary neutral must be available. With this system, you can use 2,400V, single-phase transformers, resulting in a cost savings over 4,160V, single-phase transformers. The secondary voltage is 208 wye/120V, 3-phase, 4-wire.
Note that the neutrals of the primary system and the transformer bank are tied together. As such, part of the load current will flow on the primary neutral should the 3-phase load be unbalanced. That's why it's very important the neutrals be tied together as shown. If this is not done, the line-to-neutral voltages on the secondary will be very unstable. In other words, if the load on one phase were heavier than on the other two, the voltage on this phase would drop excessively, and the voltage on the other two phases would rise.
Another point of concern is that varying voltages would appear between lines and neutral, both in the transformers and the secondary system, in addition to the 60-Hz component of voltage. Thus, for a given rms voltage value, the peak voltage would be higher than for a pure 60-Hz voltage. This would overstress the insulation of the transformers and connected equipment.