{
  "language": "en",
  "interfaceLanguage": "en",
  "url": "https://voltformer.com/articles/grounding-transformers-zigzag-zn-and-neutral-earthing-resistors-ngr",
  "id": "art-19",
  "slug": "grounding-transformers-zigzag-zn-and-neutral-earthing-resistors-ngr",
  "title": "Grounding Transformers (Zigzag ZN) and Neutral Earthing Resistors (NGR): Sizing & Fault Duty",
  "summary": "Creating artificial neutral points in ungrounded delta systems, zero-sequence impedance (Z0) calculation, 10-second thermal rating of NGRs, and sensitive earth fault protection.",
  "readTime": "8 min read",
  "date": "2026-07-30",
  "author": "Voltformer Power System Protection Desk",
  "category": "Earthing & Grounding",
  "tags": [
    "Grounding Transformer",
    "Zigzag Winding",
    "NGR",
    "Zero Sequence",
    "IEEE 142"
  ],
  "contentMarkdown": "### 1. Artificial Neutral Grounding in Delta Networks\n\nUngrounded 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.\n\n### 2. Neutral Earthing Regime Comparison\n\n| Grounding Method | Ground Fault Current ($I_g$) | Transient Overvoltage ($V_{ph-g}$) | Protection Selectivity |\n|---|---|---|---|\n| **Ungrounded Delta** | $< 10\\text{ A}$ (capacitive) | High ($> 2.73 \\times V_{ph}$ arcing ground) | Hard to locate faulty feeder |\n| **Solid Grounded** | High ($> 10\\text{ kA}$) | Low ($1.0 \\times V_{ph}$) | Immediate trip, severe equipment damage |\n| **Low-Resistance Grounding (NGR)** | **Controlled ($100\\text{ A} - 1000\\text{ A}$)** | **Low ($\\le 1.73 \\times V_{ph}$)** | **Fast selective relay trip, minimal damage** |\n| **High-Resistance Grounding (HRG)** | $5\\text{ A} - 10\\text{ A}$ | Low ($\\le 1.73 \\times V_{ph}$) | Alarm only, system remains in service |\n\n### 3. Neutral Earthing Resistor (NGR) Resistance Formulation\n\nFor a desired ground fault current $I_{fault}$ (typically $200\\text{ A} - 400\\text{ A}$):\n\n$$ R_{NGR} = \\frac{U_{LL} / \\sqrt{3}}{I_{fault}} $$\n\n#### Example for $33\\text{ kV}$ Grid, $400\\text{ A}$ Fault Limit:\n$$ R_{NGR} = \\frac{33000 / \\sqrt{3}}{400} = 47.63\\text{ }\\Omega $$\n\n### 4. 10-Second Thermal Withstand Rating\n\nNGRs are rated for **10 seconds** full current duty per **IEEE 32**:\n\n$$ E_{thermal} = I_{fault}^2 \\cdot R_{NGR} \\cdot t_{fault} \\quad [\\text{Joules}] $$\n\nStainless steel grid elements withstand temperature rises up to **$760^\\circ\\text{C}$** without deformation.\n\n*Reference: IEEE Std 142-2007 (Green Book); IEEE Std 32-2020; IEC 60076-6.*",
  "contentLanguage": "en",
  "sources": []
}