In a delta-connected distribution system, to create a neutral point, a grounding transformer must be introduced. There are two grounding transformers: Z-type grounding transformers (ZN, ZNyn) and star or delta-connected transformers (YNd). Z-type grounding transformers are widely used, with the neutral point connected to an arc suppression coil.

Structurally, a Z-type grounding transformer is similar to an ordinary three-phase core-type power transformer. Still, the windings on each phase core are divided into two equal parts and connected in a zigzag manner. Depending on the wiring method, there are two forms: ZNyn1 and ZNyn11.

In a Z-type grounding transformer, the zero-sequence current direction in the two halves of the windings on the same core is opposite, resulting in very low zero-sequence reactance and no throttling effect on the zero-sequence current. When the neutral point of a Z-type grounding transformer is connected to an arc suppression coil, the compensation current of the arc suppression coil can freely flow through it, making Z-type transformers widely used as grounding transformers.

A Z-type grounding transformer can also be equipped with low-voltage windings, connected in a star configuration with a grounded neutral point (yn), serving as a station transformer.

There are two types of insulation for Z-type grounding transformers: oil-immersed and dry-type insulation, with resin-cast being a kind of dry-type insulation.

Applicable Range: Suitable for oil-immersed Z-type grounding transformers with capacities up to 220 kVA and voltages up to 35 kV.

For 35 kV and 66 kV distribution networks, transformer windings usually adopt a Y connection with a drawn neutral point, thus not requiring a grounding transformer.

For 6 kV and 10 kV distribution networks, transformer windings typically use a delta connection with no drawn neutral point, necessitating a grounding transformer to provide a neutral point.

The role of a grounding transformer is to draw out the neutral point in systems with delta or unextracted neutral point Y connections, allowing connection to an arc suppression coil.

A grounding transformer adopts a Z-type (or zigzag) connection, where each phase coil is wound on two magnetic cores, and the zero-sequence magnetic flux generated by the two-phase windings cancels each other out. Consequently, the zero-sequence impedance of a Z-type grounding transformer is very low (generally less than 10Ω), has a low no-load loss, and the transformer capacity can utilize over 90%.

In contrast, the zero-sequence impedance of an ordinary transformer is much higher, and the capacity of the arc suppression coil generally should not exceed 20% of the transformer's capacity. Hence, a Z-type connected transformer is a preferable choice for a grounding transformer.

When the system's unbalanced voltage is relatively high, the three-phase windings of a Z-type transformer can be balanced to meet measurement requirements. When the system's unbalanced voltage is relatively low (e.g., in a full cable network), the neutral point of the Z-type transformer must generate an unbalanced voltage of 30V to 70V to meet measurement requirements.

Besides being able to carry an arc suppression coil, a grounding transformer can also carry a secondary load, replacing the station transformer. When carrying a secondary load, the primary capacity of the grounding transformer should be the sum of the arc suppression coil capacity and the secondary load capacity.