Zig Zag Transformer

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A zigzag transformer is a special purpose transformer. It has primary windings but no secondary winding.

As with other three phase transformers, the zigzag transformer contains six coils on three cores. The first coil on each core is connected contrariwise to the second coil on the next core. The second coils are then all tied together to form the neutral and the phases are connected to the primary coils. Each phase, therefore, couples with each other phase and the voltages cancel out. As such, there would be negligible current through the neutral pole and it can be tied to ground.

If one phase, or more, faults to earth, the voltage applied to each phase of the transformer is no longer in balance; fluxes in the windings no longer oppose. Zero sequence (earth fault) current exists between the transformer's neutral to the faulting phase. Hence, the purpose of a zigzag transformer is to provide a return path for earth faults on delta-connected systems. With negligible current in the neutral under normal conditions, engineers typically elect to under size the transformer; a short time rating is applied (i.e., the transformer can only carry full rated current for, say, 60 s).

An application example: Occasionally engineers use a combination of Y (wye or star), delta, and zigzag windings to achieve a vector phase shift. For example, an electrical network may have a transmission network of 110 kV/33 kV star/star transformers, with 33 kV/11 kV delta/star for the high voltage distribution network. If a transformation is required directly between the 110 kV/11 kV network the most obvious option is to use 110 kV/11 kV star/delta. The problem is that the 11 kV delta no longer has an earth reference point. Installing a zigzag transformer near the secondary side of the 110 kV/11 kV transformer provides the required earth reference point

You can find more information at http://en.wikipedia.org/wiki/Zigzag_transformer

Simon,

I do not think the statement "It has primary windings but no secondary winding." is correct.

The purpose of any transformer is to change voltages. It has to have 2 (normally galvanic isolated) windings; in case of an autotransformer, one winding (part of it) is used by both the primary side (input) and the secondary side(output) (not galvanically separated).

This is not the case for a zig-zag connection transformer, who has 2 windings (primary/input and secondary/output). One winding (usually the primary side) can be connected in delta or Y/star, while the other winding (usually the secondary) is divided into 2 halves and are interconnected as described by you (and shown in the image).

But it definitely has 2 windings. The description given in Wikipedia is confusing (in my opinion) since it refers to the 2 halves (of the secondary winding) as primary and secondary.

Sir thanks for your information, But I don't get a clear idea about this transformer can you please explain me this in a simple way with a simple winding connection digram or please tell me a site address or E-book address to know more about this transformer

Regards

JK

JK,

In the first picture in Simon's response you have a connection (star/Y). This is symbolically representing 3 cores with 3 coils around them (A,B,C). Each coil has an input end (at the junction connection) and an output end (at the output/letters).

This represents any (Y.wye.star) connected winding, regardless if primary or secondary.

In case of a zig-zag winding, each coil is divided into 2 parts (let's call them A1 and A2, ...).

The second diagram shown in Simon's response represents this connection. Basically, the voltage for phase A there is obtained by adding the voltages A1-C2, B1-A2 and C1-B2, where A1, B1,C1 are the first half-coils on the respective legs while A2,B2,C2 are the second half-coils on the same legs. The sign minus (-) denotes that that coil is connected in opposite direction (output of A1 connected to output of C2).

There are a lot of web sites about transformers, at different levels. Just type in "Zig-zag transformer diagram). One of them is this (diagrams on page 45)

Power Quality in Power Systems and Electrical Machines - Google Books Result

by Ewald F. Fuchs, Mohammad A. S. Masoum - 2008 - Technology & Engineering - 638 pages
Problem 1.3: Current Phasor Diagrams of a Three-Phase Transformer in Delta/Zigzag Connection With Line-To-Line Load The delta/zigzag, three-phase ...
books.google.com/books?isbn=0123695368...

Thanks Sir Thanks A lot for helping me, if you don't mind may I ask u more doubt about transformers after checking google link

sir thanks for your information, but its a bit difficult for me to understand, can you please explain me this in a simple way with a simple digram.

Regards

JK