A zigzag transformer is a three-phase device that produces a ground reference point from a three-wire, three-phase power line. It falls under the category of three-phase transformers and makes use of a special design that derives a new set of three-phase vectors from the existing three-phase vectors that drive a star or Y-connection. To describe the three-phase zigzag transformer, it may be useful to look at a simple single-phase and double-phase analogy.
With a single-phase power line, there seems no need to derive the ground point because the neutral level is maintained at all condition. When the single-phase live wire is at fault or fault-free, the neutral wire is always at the same potential. Although the double-phase power feeder is rarely used, its principle is a good case for describing the role of the zigzag grounding transformer. In case it existed, the double-phase feeder would use two live wires each with a vector phase shift of 180 degrees from the other. If double-phase electrical transformers are considered, a 110 volts alternating current (VAC) double-phase distribution would make use of two 110 VAC live wires each with 110 VAC to neutral.
In the above example, it is next considered that the neutral or ground line has not been made available. It should be noted that the neutral point is accessible at the generator side where it is a center tap on the 110-0-110 VAC generator winding output. The challenge is to produce a neutral wire downstream in the distribution. Again, the double-phase counterpart of the zigzag transformer will have two windings with a common connection to neutral, and the other ends of the winding will be connected to the two live wires. There will be a zero potential difference between the zigzag neutral to ground if the load on each phase of the double-phase feeder is balanced; otherwise, there will be a non-zero voltage on the zigzag neutral to ground.
The zigzag transformer, being a star or Y-connection, regenerates a three-phase output by using voltages from two phases at a time. When all combinations of two phases have been made, the resulting three-phase vectors are convergent on a single reference neutral point. Inside the zigzag transformer are two sets of windings for each phase — an inner winding and an outer winding — that are oppositely wound for flux opposition. For each core, the inner winding is taken from the next phase in the sequence. When the loads are balanced on each of the three phases, the neutral point of the zigzag transformer is at the same potential as ground, but when there is a fault on a phase or an imbalance in the phase loads, there will be a voltage in the neutral point.
