What Is Zigzag Transformer? What Is Zigzag Connection Of Transformers?

The zigzag connection is also called the interconnected star connection. This connection has some of the features of the Y and the Δ connections, combining the advantages of both. The zigzag connection is a three-phase connection and is constructed as shown in Figure 2.14.

There are three pairs of windings, each having a 1:1 turns ratio. The left-hand set of windings shown in the
figure is a conventional Y connection, a′-b′-c′, with the neutral N brought out.

The open ends of the Y are electrically connected to the right-hand set of windings as follows: a′ connects to the right-hand winding paired with to the b′-N winding, b′ connects to the right-hand winding paired to c′-N winding, and c′ connects to the right-hand winding paired to the a′-N winding.

The opposite ends of the right-hand windings are brought out as the phase terminals a, b, and c. The vector diagram shown on the right of Figure 2.14 makes it is obvious why this is called a zigzag connection. It operates on the following principle:

If three currents, equal in magnitude and phase, are applied to the three terminals, the ampere-turns of the a′-N winding cancel the ampere-turns of the c′- c winding, the ampere-turns of the b′-N winding cancel the ampere turns of the a′-a winding, and the ampere-turns of the c′-N winding cancel the ampere turns of the b′-b winding. Therefore, the transformer allows the three in-phase currents to easily flow to neutral.

If three currents, equal in magnitude but 120° out of phase with each other, are applied to the three terminals, the ampere-turns in the windings cannot cancel and the transformer restricts the current flow to the negligible level of magnetizing current.

Therefore, the zigzag winding provides an easy path for in-phase currents but does not allow the flow of currents that are 120° out of phase with each other.

The ability to provide a path for in-phase currents enables us to use the zigzag connection as a grounding bank, which is one of the main applications for this connection. If a zigzag winding is used as a secondary winding with a Δ winding used as a primary winding, the Δ-zigzag connection is created, as show nin Figure 2.15.

AΔ-zigzag transformer is technically not a two-winding transformer but rather a three-winding transformer because three separate windings are wound around each core leg.  Since two of the sets of windings are interconnected, we treat the Δ-zigzag as if it were a two-winding transformer.

As usual, the sets of windings that are magnetically linked on common core legs are drawn in parallel to each other, as shown in Figure 2.15.

The Δ-zigzag connection provides the same advantages as the Δ-Y connection, like third harmonic suppression and ground current isolation. One added advantage is that there is no phase angle displacement between the primary and the secondary circuits with this connection; therefore, the Δ-zigzag connection can be used in the same manner as Y-Y and Δ-Δ transformers without introducing any phase shifts in the circuits.

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