Zigzag earthing transformer — an artificial star point for networks without a natural neutral

The guide on transformer vector groups covers the winding connection and phase shift of ordinary power transformers. This article covers a specialised type of transformer that does not transfer power between two networks, but serves one specific purpose: creating a star point where none naturally exists.

The problem: no accessible neutral point

A delta-connected network, or a network with an isolated star point, has no physically accessible neutral point. Without such a point, there is nowhere to connect an earthing resistor, a Petersen coil (arc-suppression coil), or a neutral conductor — something needed to let earth-fault current flow in a controlled, measurable way and to enable earth-fault detection (as covered in the [guide on IT system and insulation monitoring](/guides/nen-1010/it-stelsel-imd-eerstefoutdetectie)).

How the zigzag winding creates an artificial star point

A zigzag earthing transformer has three winding limbs; each limb carries two winding halves, each connected to a different phase and wound in opposite sense relative to each other (hence "zigzag"). This has two consequences that are exactly what an earthing transformer needs:

For balanced current (positive or negative sequence — the normal operating current of a healthy, symmetrical network), the magnetic fields of the two winding halves on each limb largely cancel each other out. The transformer then behaves as a high (magnetising) impedance and draws hardly any current.
For zero-sequence current (three phases flowing in the same direction and in phase, as during a phase-to-earth fault), the two winding halves do not oppose each other, and the transformer presents a low impedance. Earth-fault current can then flow via the zigzag transformer's artificial star point to earth and back to the fault.

This makes the zigzag transformer the mirror image of an ordinary power transformer in terms of sequence impedances: high impedance to balanced current, low impedance to zero-sequence current — exactly the opposite of what an ordinary power transformer is optimised for.

Typical applications

Creating a star point for a generator that is itself wound in delta or ungrounded star, so that earth-fault protection can function on the generator.
Deriving a neutral point in a medium-voltage network that has no naturally accessible star point, for connecting an earthing resistor, Petersen coil, or earth-fault relay.

Sizing: short-time fault current, not continuous full-load power

Because a zigzag earthing transformer draws virtually no current in normal operation (only a small magnetising current and any unbalance current), it is not sized on continuous full-load power like an ordinary power transformer, but on the short-time zero-sequence fault current it must be able to carry during an earth fault, for the time the protection needs to clear the fault. A zigzag transformer sized on a continuous full-load basis is typically grossly oversized for this application, and therefore needlessly expensive.

Note: the exact zero-sequence impedance value and the current/duration combination for which a specific zigzag earthing transformer is sized follow from the network's system study (desired earth-fault current, protection times) and the manufacturer's specification per IEC 60076-6 (reactors) and the application guide IEC 60076-8; this article covers the principle, not a specific design.

Practical relevance

When assessing the earthing scheme of a network fed from a delta-wound source — for example the generator side of an emergency generator, or an MV network without an accessible star point — it is worth checking whether the earthing transformer (and any neutral earthing resistor or coil in series with it) is matched to the desired earth-fault current and the protection settings, rather than only looking at the transformer's power rating label.

Common mistakes

Sizing a zigzag earthing transformer on continuous full-load power as if it were an ordinary power transformer, instead of on its short-time zero-sequence fault current.
Assuming a zigzag transformer can transfer power between two networks — unless a separate secondary winding has been added, this is purely an earthing device without a power-transfer function.
Confusing the zigzag earthing transformer with the star point of an ordinary Yyn or Dyn power transformer — the latter already has an accessible star point and does not need a separate zigzag unit.
Selecting the earthing resistor or coil without matching it to the zero-sequence impedance of the zigzag transformer, resulting in an earth-fault current that is too high or too low for the protection settings.