--- title: "Neutral grounding transformer sizing" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:neutral-grounding-transformer" source: https://www.bctengineer.com/q/semester_8/power_plant_design/neutral-grounding-transformer site: BCT Engineering --- # Neutral grounding transformer sizing Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2082 Shrawan, 4 marks; 2081 Chaitra, 2+4 marks (asked 2 times) - Video: [Grounding Transformer Sizing Calculation](https://www.youtube.com/watch?v=1urj8LMaYIQ) — ElectrikWave Online Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/neutral-grounding-transformer - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question What is neutral grounding transformer (NGT)? Also describe the steps to find optimal size of NGT. ## Short answer A NGT is a single-phase transformer with secondary resistor providing high-resistance generator neutral grounding to limit earth fault current to 5–15 A and prevent damage. Sizing: find capacitive current IC = 3ωC0Vph; choose IR ≥ IC and RN ≤ 1/(3ωC0); select voltage ratio; secondary resistor R2 = RN/n²; NGT kVA = Vph·IR (short-time); verify total fault current ≤10–15 A. ## Answer A **neutral grounding transformer (NGT)** is a single-phase distribution-type transformer connected between the **generator neutral and earth**, with a **resistor on its secondary**. The resistor, reflected to the primary as a high resistance, gives **high-resistance grounding** of the generator. ```text Generator (star) \ | / \|/ neutral | NGT primary (e.g. 11 kV/sqrt3) ||| NGT secondary (e.g. 240 V) | [R] loading resistor | + 59N relay across R === earth ``` **Purpose:** limits stator earth fault current to about 5–15 A, preventing core damage; limits transient overvoltages during arcing faults; allows sensitive stator earth fault protection (59N relay across the resistor). #### Steps to find optimal size of NGT 1. **Total capacitance to earth per phase** $C_0$: stator winding, generator bus duct/cables, GSU LV winding, surge capacitors and VT windings. 2. **Capacitive earth fault current:** $$ I_C = 3\omega C_0 V_{ph}, \quad V_{ph} = \frac{V_L}{\sqrt{3}} $$ 3. **Choose resistor current** $I_R \geq I_C$ (resistance not more than the capacitive reactance), so that transient overvoltage stays below about 2.6 p.u.: $$ R_N \leq \frac{1}{3\omega C_0} $$ 4. **NGT voltage ratio:** primary rated at generator phase-to-neutral voltage (often line voltage for safety margin), secondary e.g. 240 V; turns ratio $n = V_1/V_2$. 5. **Secondary resistor value:** $R_2 = R_N / n^2$. 6. **NGT kVA rating:** $S = V_{ph} \times I_R$ (short-time rating for 1 min or until protection trips), giving a smaller, cheaper transformer than a continuous rating. 7. **Check:** total fault current $I_f = \sqrt{I_R^2 + I_C^2}$ is within the allowed limit (typically below 10–15 A) and the relay sensitivity covers about 95% of the winding. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. An NGT is: A. A transformer between generator neutral and earth with a resistor on its secondary, giving high-resistance grounding B. A step-up transformer to the grid C. A station service transformer D. A voltage transformer for metering Answer: A. The secondary resistor reflected to the primary acts as a high neutral resistance. (Key point: What is NGT) 2. Why is the stator earth fault current limited to about 5-15 A? A. To raise ground potential B. To increase arcing C. To disable protection D. To prevent core damage and limit transient overvoltages, while allowing sensitive 59N protection Answer: D. Low fault current protects the stator core and limits overvoltages. (Key point: Purpose) 3. Which relation sizes the NGT resistor? A. R_N is at least 10 times the capacitive reactance B. R_N <= 1/(3 w C0), so that I_R >= I_C; secondary R2 = R_N/n^2; kVA = V_ph x I_R C. R2 = R_N x n^2 D. kVA = V_L x I_C with continuous rating Answer: B. Resistive current must be at least the capacitive current to keep transient overvoltage low; the secondary value is reflected by the turns ratio squared; short-time rating keeps it small. (Key point: Sizing) ## Related in Power Plant Design - [Types of generator neutral grounding](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agenerator-neutral-grounding-types) — Power Plant Design, chapter 4 - [Steps of grounding mat design](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agrounding-mat-design-steps) — Power Plant Design, chapter 4, asked 5× - [Design: 220 m head plant, optimal voltage](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2081-220m-optimal-voltage) — Power Plant Design, chapter 4 - [Isolated PV sizing: lamps, fan and TV](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Apv-sizing-lamp-fan-tv) — Power Plant Design, chapter 1 ## Related in other subjects - [Transformer sizing and 11 kV feeder SLD](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Atransformer-size-sld-11kv-230v) — Industrial Power Distribution and Illumination, chapter 4 - [Selecting distribution transformer size using Ku and Ks](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Atransformer-size-ku-ks) — Industrial Power Distribution and Illumination, chapter 4, asked 3× - [Transformer size for given industry (figure)](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Atransformer-size-attached-figure) — Industrial Power Distribution and Illumination, chapter 4, asked 2× - [Selecting distribution transformer size and location](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atransformer-size-and-location) — Transmission and Distribution System Design, chapter 7, asked 2×