Zigzag Transformer

Zigzag transformer Zig-zag connect on is sometimes desired as the disadvantages in star and delta connection can be overcome by the use of zigzag connection of transformer A Zigzag transformer is a special purpose transformer with a zigzag arrangement. It has primary windings but no secondary winding. One application is to derive an earth reference point for an ungrounded electrical system. Another is to control harmonic currents.

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.[2] 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. (Using symmetrical components, this is Ia0 = Ib0 = Ic0.) Zero sequence (earth fault) current exists between the transformers’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). Ensure the impedance is not too low for the desired fault limiting. Impedance can be added after the secondaries are summed (the 3Io path).[3] 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.

In older 600V and lower systems, and in many existing volt and higher systems, the system neutral may not be available. To be able to ground these systems, grounding transformers can be used to create a neutral, which in turn can be connected to ground either directly, or more commonly, through a Neutral Grounding Resistor (NGR). These combinations are known as artificial neutrals. The Zigzag grounding transformer is a commonly used option. It is a threephase, dry-type, air- cooled auto-transformer with no secondary winding. Each phase has two identical windings, which are wound in opposite directions to give the high impedance to normal phase currents. The windings are connected in a Wye configuration. The neutral point is then connected either directly or through a neutral grounding resistor (NGR) to ground. When a ground fault occurs downstream of the Zigzag transformer, ground fault current flows through the fault, back through ground and the NGR to the Zigzag where the current is divided equally in each leg of the Zigzag. Since these three currents are all equal and in time phase with each other (zero sequence), and because of the special Zigzag winding connections, they see a very low impedance. This allows the ground fault current to flow back into the system. It can be seen that the ground fault current is only limited by the resistance of the ground fault, the NGR, and the small reactance of the Zigzag. The Zigzag should be connected to the system on the line side of the main breaker, as close as possible to the power transformer secondary terminals. When more than one power transformer is involved, one Zigzag is required for each. Care should be taken not to have more than one Zigzag connected to the same section of the system at the same time. Short circuit protection should be provided on each of the three line connections of the Zigzag.