FERRORESONANCE AND DISTRIBUTION TRANSFORMER CONTRIBUTION BASICS AND TUTORIALS

FERRORESONANCE AND DISTRIBUTION TRANSFORMER CONTRIBUTION BASIC INFORMATION
What Is The Contribution Of Distribution Transformers On Ferroresonance?

Ferroresonance is an overvoltage phenomenon that occurs when charging current for a long underground cable or other capacitive reactance saturates the core of a transformer.

Such a resonance can result in voltages as high as five times the rated system voltage, damaging lightning arresters and other equipment and possibly even the transformer itself.

When ferroresonance is occurring, the transformer is likely to produce loud squeals and groans, and the noise has been likened to the sound of steel roofing being dragged across a concrete surface.

A typical ferroresonance situation is shown in Figure 2.2.10 and consists of long underground cables feeding a transformer with a delta-connected primary.

FIGURE 2.2.10 is a typical ferroresonance situation. (From IEEE C57.105-1978, IEEE Guide for Application of Transformer Connections in Three-Phase Distribution Systems, copyright 1978 by the Institute of Electrical and Electronics Engineers, Inc. The IEEE disclaims any responsibility or liability resulting from the placement and use in the described manner. Information is reprinted with the permission of the IEEE.)


The transformer is unloaded or very lightly loaded and switching or fusing for the circuit operates one phase at a time.

Ferroresonance can occur when energizing the transformer as the first switch is closed, or it can occur if one or more distant fuses open and the load is very light. Ferroresonance is more likely to occur on systems with higher primary voltage and has been observed even when there is no cable present.

All of the contributing factors — delta or wye connection, cable length, voltage, load, single-phase switching —must be considered together. Attempts to set precise limits for prevention of the phenomenon have been frustrating.

For more on ferroresonance click the link.

ZIGZAG CONNECTION OF TRANSFORMER BASICS AND TUTORIALS

ZIGZAG TRANSFORMER CONNECTION BASIC INFORMATION
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.