These transformers have special windings, appropriate for special applications.
You can interconnect transformer windings in several ways to produce various output voltages. For power and distribution, you employ the conventional delta or wye connection. But for special applications, you can use anotherlesser-known configuration: the zigzag connection.
You will typically find the windings on each phase of a zigzag transformer in two halves. These winding halves interconnect to obtain a zigzag arrangement.
You can use the zigzag connection as an autotransformer, serving as the primary and secondary with no isolated circuits. The common portion of an autotransformer (low voltage) is the common winding, and the remainder is the series winding. (Together these make up the high voltage side of the transformer.) You can use the zigzag transformer in two winding transformer applications, where you obtain voltage transformation and isolation with the zigzag feature.
If you need a neutral for grounding or for supplying single-phase line to neutral loads when working with a 3-wire, ungrounded power system, a zigzag connection may be the answer. Due to its composition, a zigzag transformer is more effective for grounding purposes because it has less internal winding impedance going to the ground than when using a wye-type transformer.
In ungrounded systems, it's still necessary to detect a ground fault. With the zigzag transformer, you typically insert a resistor between the neutral point and ground to limit ground fault current on the system.
You can also use the zigzag connection in power systems to trap triplen harmonic (3rd, 9th, 15th, etc.) currents. Here, you install zigzag units near loads that produce large triplen harmonic currents. The windings trap the harmonic currents and prevent them from traveling upstream, where they can produce undesirable effects.
In the past, we used half-wave rectification circuits for producing DC voltage. However, these DC currents often caused transformer saturation. With the zigzag connection, the currents in the two halves of the windings on each leg of the transformer flow in opposite directions, avoiding saturation.
We rarely use zigzag configurations for typical industrial or commercial use, because they are more expensive to construct than conventional wye-connected transformers. But zigzag connections are useful in special applications where conventional transformer connections aren't effective.