Representative imageNeutral / Earthing / Zig-Zag Transformers
Neutral / earthing / zig-zag transformers create an artificial neutral (ground reference) on a three-phase system that has no accessible neutral — most often a delta-connected or otherwise ungrounded supply — so the system can be earthed and ground faults detected and cleared safely.
Overview
Neutral / earthing / zig-zag transformers create an artificial neutral (ground reference) on a three-phase system that has no accessible neutral — most often a delta-connected or otherwise ungrounded supply — so the system can be earthed and ground faults detected and cleared safely. The classic form is the zig-zag (interconnected-star) transformer: each limb carries interleaved sections of two phases so the winding presents high impedance to normal balanced (positive/negative-sequence) currents but a low zero-sequence impedance path to ground, letting it pass earth-fault current while drawing only small magnetizing current in healthy operation. The derived neutral is usually taken to ground through a Neutral Grounding Resistor (NGR) or reactor to limit fault current to a chosen value. These units are short-time rated (sized to carry full fault current for a defined time such as 10 s or 30 s) and are specified by system voltage, fault current, duration and zero-sequence impedance. A buyer needs one wherever an ungrounded/delta system must be solidly or impedance-earthed for protection.
Specifications
| System voltage / voltage class | LV up to MV (e.g. 415 V to 33 kV) |
|---|---|
| Connection | zig-zag (interconnected star) or grounded wye-delta |
| Continuous neutral current rating and ground-fault (short-circuit) current | |
| Short-time rating / duration | e.g. 10 s, 30 s, or continuous |
| Zero-sequence impedance | engineered to set fault-current division |
| Phases | three-phase |
| Frequency | 50 / 60 Hz |
| NGR / NGT (resistor or reactor) integration to limit fault current | |
| Cooling/construction | dry-type or oil-filled; indoor/outdoor |
| Insulation/thermal class and BIL per voltage class | |
| Standards | IEC 60076-6 / IEEE C57.32 / IEEE C62.92; IS where applicable |
Applications
- Deriving a neutral on delta / ungrounded three-phase systems
- Resistance- or reactance-earthing (with NGR/NGT)
- Ground-fault detection and protection coordination
- Generator and transformer-secondary grounding
- Industrial, distribution and utility earthing schemes
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