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 MethodGround 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 GroundedHigh (> 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 ALow (≤ 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.*