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A zigzag transformer creates a ground reference point from a three-wire three-phase power line. It uses a special design to derive a new set of three-phase vectors and has two sets of windings for each phase. The neutral point is at the same earth potential when loads are balanced, but there will be a voltage at the neutral point in case of a fault or imbalance.
A zigzag transformer is a three-phase device that produces a ground reference point from a three-wire three-phase power line. It falls into the category of three-phase transformers and uses a special design that derives a new set of three-phase vectors from existing three-phase vectors driving a star or Y connection. To describe the three-phase zigzag transformer, it may be helpful to look at a simple single-phase analogy and two-phase.
With a single-phase power line, it does not seem necessary to derive the earth point because the neutral level is maintained under all conditions. When the single-phase live wire is faulty or faultless, the neutral wire is always at the same potential. Although the dual-phase power supply is rarely used, its principle is a good case to describe the role of the zigzag grounding transformer. If it existed, the dual-phase power supply would use two live wires each with a vector phase shift of 180 degrees from the other. When considering two-phase power transformers, a two-phase distribution of 110 volt alternating current (VAC) would make use of two live wires rated 110 VAC each with 110 VAC at the neutral.
In the example above, it is then assumed that the neutral or earth line has not been made available. It should be noted that the neutral point is accessible from 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 of the distribution. Again, the dual-phase counterpart of the zigzag transformer will have two windings with a common connection to the neutral, and the other ends of the winding will be connected to the two live wires. There will be zero potential difference between the zigzag neutral and ground if the load on each phase of the two-phase power supply is balanced; otherwise, there will be a non-zero voltage on the neutral zigzagging to ground.
The zigzag transformer, being a wye or Y connection, regenerates a three-phase output using voltages of two phases at a time. When all two-phase combinations have been made, the resulting three-phase vectors converge on a single reference neutral point. Inside the zigzag transformer there are two sets of windings for each phase – an inside and an outside winding – which are oppositely wound for flux opposition. For each nucleus, the inner coil is taken from the next step 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 earth potential, but when there is a fault on one phase or an imbalance in the phase loads, there will be a voltage at the point neutral.
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