Zig Zag Transformers

What is a Zig Zag Transformer?

The Zig Zag transformer, also known as "interconnected star winding," is a special-purpose transformer used in power systems. It acts as a barrier between the ground and the component, preventing the system component from being damaged by fault currents.

A zigzag transformer terminates the harmonics of the power system. It also safeguards the power system by lowering voltage stress during a fault event.

How a Zig Zag Transformer Functions

Although this sort of transformer connection isn't useful for converting electricity, it has a lot of advantages because it combines Star and Delta type winding connections.

As illustrated in the diagram, the Zig Zag transformer has six coils, three outside coils, and three inside coils. ZIG winding refers to the outside coil windings, while ZAG winding refers to the interior coil windings.

The zig winding of one phase is connected in series with the zag winding of another phase, resulting in interconnected star winding in which two star winding coils are linked together. To cancel the mismatched voltages, two coil windings with the same number of turns are wound in opposite directions in each phase.

The connections of zig zag winding coils are shown below.

1. The phase 'a' outer coil is coupled to the phase 'b' inner coil.
2. The phase 'b' outer coil is coupled to the phase 'c' inner coil.
3. The phase 'c' outer coil is coupled to the phase ‘a' inner coil.

The inner coil second terminals are joined together and connected to the neutral terminal to pass the zero sequence current components. A 30 phase shift occurs between the zag winding and the corresponding line to neutral voltage when distinct phase windings are connected.

To achieve the same magnitude of voltages, the zigzag winding has 15.47% more turns than traditional transformers. As a result, the zig zag transformer is expensive, but it is required in certain applications.

Applications of Zig Zag Transformers

Zig Zag transformers have applications in:

Grounding Transformer

It can be used as an earthing transformer in a delta connected (no neutral terminal) system or an ungrounded start connected (three terminal star) system where grounding is impossible. The earthing of the delta linked transformer is done with a zigzag transformer.

There will be no path to zero sequence components in a delta linked transformer, and no safety can be provided for these components, increasing stress and heating in the windings.

During a line to ground failure, the zig zag transformer acts as a neutral, providing a channel to zero sequence components and allowing the protection to activate.

In the absence of a grounded neutral, healthy voltages would increase line to line voltage, putting strain on the equipment's insulation. The Zig Zag transformer not only protects against symmetrical faults but also minimizes voltage stress.

Power Electronic Converters

The zig zag transformer is used in power electronic converters to reduce the DC magnetizing component caused by incorrect firing angles.

Due to the opposite direction of the DC magnetizing component of currents flowing in the windings on the same limb, inappropriate firing angles of power electronic components (SCR) may introduce DC magnetizing components. This is then negated in each limb of the zig zag transformer.

Earthing Reference or Earthing Transformer

Under fault conditions, the zig zag transformer provides a low impedance path to zero sequence components, making it ideal for an earthing transformer with an earthing reference.

An appropriate resistor can be installed in the zig zag neutral terminal if the earthing current must be limited under fault conditions.

Due to the opposing connection of winding coils, harmonic voltages in the system can be neutralized to some extent in zigzag windings.

Advantages of Zig Zag Transformers

• Compared to the Scott transformer, it is less expensive.
• It provides low impedance to zero sequence currents.
• It suppresses harmonic voltages.
• It ensures complete isolation between the ground and the component.
• It connects the delta connected transformer to the neutral and traps third harmonic current in the phase winding.
• Because there is no phase change between the main and secondary windings, delta – zig zag transformers are used similarly to star – star and delta – delta transformers.
• Zig zag transformers have a lower internal impedance than star connected transformers.