A Zig-Zag Transformer engineered for reliable system grounding, low zero-sequence impedance, and stable fault current control in medium-voltage power networks
01 General
1.1 Project Description
In 2025, a 6900 kVA earthing transformer was delivered to Australia for a 132/22 kV substation project, where stable operation and fault control were critical; within this system, it operates in coordination with the main power transformer, not for energy conversion but to enable reliable neutral earthing of transformer networks under continuous load conditions.
The main transformer transfers power. The earthing transformer defines grounding.
Designed in accordance with IEC 60076, this three-phase unit (22 kV, 50 Hz, vector group ZN) establishes an artificial neutral point while maintaining low zero-sequence impedance, allowing effective neutral earthing of transformer systems; with ONAN cooling and mineral oil insulation, it also supports a short-time neutral current rating of 6350 A for 10 seconds, ensuring safe fault current dissipation and stable system grounding performance.
1.2 Zig-Zag Transformer in Grounding Systems
In neutral earthing of transformer systems, zig-zag transformers are used to create a stable artificial neutral point in ungrounded or delta networks. Their windings are split into two equal parts per phase and connected in a zig-zag pattern, allowing zero-sequence current to flow with low impedance-typically around 10 Ω-without choking. This makes them suitable for transformer neutral earthing, especially when connected to an arc suppression coil, ensuring effective fault current compensation and reliable system grounding.
1.3 Technical Specification
6900kVA zig zag transformer specification and data sheet
| Delivered to | Australia |
| Year | 2025 |
| Type | Earthing Transformer |
| Standard | IEC 60076 |
| Rated Power | 6900KVA |
| Frequency | 50HZ |
| Short Time Rating | 6350A, neutral current for 10 seconds |
| Phase | three |
| Cooling Type | ONAN |
| Liquid insulant | Petro China 25# Pcb-Free Mineral Oil |
| Rated Voltage | 22 kV |
| Vector Group | ZN |
| Insulation Level：H.V. line | LI125 AC50kV |
| H.V. neutral | LI125 AC50kV |
| No Load Loss | 2.675 kW |
| NO Load Current | 0.08% |
| Accessories | Standard Configuration |
1.4 Drawings
6900kVA zig zag transformer drawing and nameplate
02 Manufacturing
2.1 Core
The core of this zig zag transformer is based on a three-phase limb structure using grain-oriented silicon steel, carefully assembled to ensure uniform magnetic flux distribution; unlike conventional earthing transformers where zero-sequence flux tends to circulate through leakage paths, this design confines flux within the core, supporting stable operation under system grounding conditions and fluctuating electrical load. |  |
2.2 Winding
The winding adopts a zig zag transformer configuration, each phase split into two equal sections and wound in opposite directions on the same core limb; this structure allows zero-sequence currents to cancel magnetically, while still maintaining electrical continuity for fault current circulation-key operating principle of a zig zag transformer used in grounding applications. Compared with conventional winding layouts, zero-sequence impedance is significantly reduced in the zig zag transformer design.
2.3 Tank
The earthing transformer tank is made of welded steel plates with reinforced structural design, suitable for transportation, installation, and fault conditions; it is designed for outdoor service where mechanical strength and thermal stability are critical for zig zag transformer operation. Key mechanical features include: The tank is sealed and structurally reinforced for long-term reliability. It also helps limit circulating currents and localized heating, supporting stable zig zag transformer performance during continuous operation.
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2.4 Key Components
To ensure safe and reliable operation, the zig zag transformer integrates several protective and monitoring components, as shown in the configuration:
• Buchholz relay for internal fault detection and gas accumulation monitoring
• Transformer body control terminal box for signal interface and protection wiring
• Oil level indicator for main conservator oil monitoring
• H.V. bushings and H.V. neutral bushing (24 kV class) for system connection
• Earthing bushings for core and clamp grounding
• Pressure relief device for overpressure protection
Each component is arranged to support stable operation of the zig zag transformer in grounding applications, while ensuring safe fault response and long-term reliability in field conditions.
03 Packing and transportation
3.1 Packing
- Insulation Oil Weight: 1750 kg
- Insulation Oil Volume (at 25°C): 1966 L
- Active Parts Weight: 3700 kg
- Total Weight (Including Oil): 6300 kg
- Transportation Weight: 6850 kg
3.2 Shipping
04 Site and Summary
The zig zag transformer delivered for the 132/22 kV substation project in Australia serves as a dedicated grounding solution within the wider power system architecture, ensuring stable system behavior where a defined neutral point is required in medium-voltage networks.
It operates as a structural support element of system grounding design, complementing the main power transformer and maintaining operational stability under fault and normal load conditions.
