Grounding Transformers (Zigzag ZN) and Neutral Earthing Resistors (NGR): Sizing & Fault Duty
Creating artificial neutral points in ungrounded delta systems, zero-sequence impedance (Z0) calculation, 10-second thermal rating of NGRs, and sensitive earth
1. Artificial Neutral Grounding in Delta Networks
Ungrounded medium-voltage systems (delta windings) experience severe transient overvoltages during arcing ground faults. Grounding transformers with Zigzag (ZN) or Wye-Delta (Yd) windings establish an artificial neutral point.
2. Neutral Earthing Regime Comparison
| Grounding Method | Ground Fault Current (Ig) | Transient Overvoltage (Vph-g) | Protection Selectivity |
|---|---|---|---|
| Ungrounded Delta | < 10 A (capacitive) | High (> 2.73 × Vph arcing ground) | Hard to locate faulty feeder |
| Solid Grounded | High (> 10 kA) | Low (1.0 × Vph) | Immediate trip, severe equipment damage |
| Low-Resistance Grounding (NGR) | Controlled (100 A - 1000 A) | Low (≤ 1.73 × Vph) | Fast selective relay trip, minimal damage |
| High-Resistance Grounding (HRG) | 5 A - 10 A | Low (≤ 1.73 × Vph) | Alarm only, system remains in service |
3. Neutral Earthing Resistor (NGR) Resistance Formulation
For a desired ground fault current Ifault (typically 200 A - 400 A):
RNGR = (ULL / √(3)) / (Ifault)
Example for 33 kV Grid, 400 A Fault Limit:
RNGR = (33000 / √(3)) / (400) = 47.63 Ω
4. 10-Second Thermal Withstand Rating
NGRs are rated for 10 seconds full current duty per IEEE 32:
Ethermal = Ifault2 · RNGR · tfault [Joules]
Stainless steel grid elements withstand temperature rises up to 760°C without deformation.
*Reference: IEEE Std 142-2007 (Green Book); IEEE Std 32-2020; IEC 60076-6.*