Zigzag transformer

Zig Zag transformers are a special-purpose three phase transformer with a ZigZag or "interconnected star" winding connection.   Zigzag transformers are used to create the missing ground in an ungrounded 3-phase system by providing a path for the neutral to ground.

Throughout the USA, many utility companies are requiring renewable power providers (Wind, Solar, Hydro) to install grounding transformers at their tie point to grounded, ungrounded or undergrounded systems to provide a ground reference.

The best way to resistance ground a power system is to obtain the system neutral through a generator or transformer with a wye-connected winding.

However, a system neutral may not be available, particularly in many older low voltage systems and a significant number of existing medium voltage systems. To avoid the high cost of replacing a source transformer, an existing delta-connected system can be grounded using a zigzag transformer to form an artificial neutral, then connecting a resistor between the newly created wye-point and ground.

In delta connected transformer, a zero sequence (i.e. an unbalanced current during the fault) is unavailable and therefore components are not protected which result in additional stress and increased heat on the system.  Zig zag transformer are a specialty transformer built with high end insulation materials, copper conductors and superior core steel to create a robust unit that provides a neutral during the ground fault.

Zig zag transformer are designed to offer a low impedance, creating a path for zero sequence components under fault conditions and prevents system voltage rise  in the unfaulted phases which makes a zigzag transformer a very good earthing transformer.  In the event that the current has to be limited during the fault condition, a suitable neutral grounding resistor can be added to the system.

Acutran Neutral Grounding Systems (single or three phase) can we combined with neutral ground fault relays and current transformers to automatic create an open circuit during the fault condition is the system can not automatically correct itself.[1]

A zigzag transformer is a special-purpose transformer with a zigzag or "interconnected star" winding connection, such that each output is the vector sum of two (2) phases offset by 120°.[2] It is used as a grounding transformer, creating a missing neutral connection from an ungrounded 3-phase system to permit the grounding of that neutral to an earth reference point; to perform harmonic mitigation, as they can suppress triplet (3rd, 9th, 15th, 21st, etc.) harmonic currents;[3] to supply 3-phase power as an autotransformer (serving as the primary and secondary with no isolated circuits);[4] and to supply non-standard, phase-shifted, 3-phase power.[2]

9-winding zigzag transformer

Zigzag transformer

Nine-winding, three-phase transformers typically have 3 primaries and 6 identical secondary windings, which can be used in zigzag winding connection as pictured.[2] As with the conventional delta or wye winding configuration three-phase transformer, a standard, stand-alone transformer containing only six windings on three cores can also be used in zigzag winding connection, such transformer sometimes being referred to as a zigzag bank.[2] In all cases, six or nine winding, the first coil on each zigzag winding 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 point, which can be tied to ground.[5]

Each of the three "limbs" are split into two sections. The two halves of each limb have an equal number of turns and are wound in opposite directions. With the neutral grounded, during a phase-to-ground short fault, a third of the current returns to the fault current, and the remainder must go through two of the three phases when used to derive a grounding point from a delta source.[6]

If one or more phases fault 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 I_a0 = I_b0 = I_c0.) Zero-sequence (earth fault) current exists between the transformer’s neutral to the faulting phase. The purpose of a zigzag transformer in this application is to provide a return path for earth faults on delta-connected systems. With negligible current in the neutral under normal conditions, providing the defective load will be automatically disconnected in a fault condition, an undersized transformer may be used only as short-time rating is required (i.e. the transformer can only carry full rated current for, say, 60 s). Impedance should not be too low for desired maximum fault current. Impedance can be added after the secondaries are summed to limit maximum fault currents (the 3I_o path).[7]

A combination of Y (wye or star), delta, and zigzag windings may be used 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 an option is to use a 110 kV/11 kV star/delta transformer. 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.[8]