Three phase power transformer secondaries that are set up in a delta configuration do not have an earthing or grounding point. By contrast, a wye configuration of windings would provide such a point, but delta windings are frequently the transformer designer’s choice (Figure 1).
Figure 1 Wye versus delta transformer secondaries.
Where the three coils of the wye configuration meet, a ground or earth connection can be established, but the three secondary coils of the delta configuration offer no such point.
In such cases, an earthing point can be established using a zig-zag transformer as in the following sketch in Figure 2.
Figure 2 A zig-zag transformer with an established earthing point.
The origin of the phrase “zig-zag” would seem to be self-evident. The underlying theory of zig-zag transformers and additional discussions of its characteristics have been written up extensively as shown in its Wikipedia page.
Looking at this device feeding just a single load (Figure 3), we can see how earthing can be achieved when power is fed from delta secondaries.
Figure 3 A zig-zag transformer with an earthed load with power fed from delta secondaries.
John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).
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