1 Minute to Calculate Neutral Grounding Resistors (6kV–35kV)

Writer： admin Time：2026-01-03 18:34:23 Browse：58℃

In medium-voltage (MV) power systems from 6 kV to 35 kV, the Neutral Grounding Resistor (NGR or NER) plays a critical role in system safety, overvoltage control, and protection coordination.

A properly designed NGR can suppress arcing ground overvoltage, limit earth fault current, and protect generators and transformers from severe damage.

This guide explains how to calculate neutral grounding resistance, choose the correct ground fault current, and define key technical parameters for reliable NGR selection.

What Is a Neutral Grounding Resistor (NGR)?

A Neutral Grounding Resistor is connected between the neutral point of a transformer or generator and ground to control single-line-to-ground fault current in MV systems.

Typical applications include:

1.   6.6 kV distribution networks

2.   10 kV industrial power systems

3.   11 kV utility feeders

4.   35 kV substation transformers

1. How to Calculate Neutral Grounding Resistance?

Core Design Principle: The resistive grounding current must be greater than the system capacitive ground current.

This ensures effective suppression of transient overvoltage during ground faults.

Neutral Grounding Resistor Formula

The resistance value is calculated using Ohm’s law:

R=U/I

Where:

• R = Neutral grounding resistance (Ω)

• U = Rated system line voltage (kV)

• I = Ground fault current through the resistor (A)

2. How to Select Ground Fault Current (HRG vs LRG)

The selected grounding current depends on the earthing method and protection philosophy.

High Resistance Grounding (HRG)

1. Ground fault current: ≤ 10 A

2. Purpose: Suppress arcing ground overvoltage and allow continued operation under single-line-to-ground fault

3. Common in: Industrial plants and continuous process systems

Low Resistance Grounding (LRG)

1. Ground fault current: 100 A to 1000 A

2. Purpose: Ensure fast relay operation and clear ground faults quickly

3. Common in: Utility substations and Generator grounding systems

Engineering rule:
Grounding current I ≥ total system capacitive current (Ic)

3. Key Technical Parameters for NGR Design

3.1 Insulation Level and BIL

The insulation level of the NGR must match system voltage requirements.

a. Rated voltage: Phase voltage (U / √3)

b. Impulse withstand (BIL): According to IEEE 32 or IEC standards

Example for a 10 kV NGR:

a. Power-frequency withstand: 42 kV

b. Lightning impulse withstand: 75 kV

3.2 Temperature Rise and Time Rating

Ground faults generate significant heat in resistor elements.

Common time ratings: 10 seconds/ 30 seconds/ 60 seconds/ Continuous duty

Application	Typical Time Rating
Low resistance grounding	10 s / 30 s
High resistance grounding	Continuous

Temperature rise limits (IEEE 32):

a. Stainless steel (short-time): up to 760°C

b. Continuous duty: typically ≤ 385°C

3.3 Neutral Grounding Resistor Material Selection

Material choice affects thermal stability, corrosion resistance, and service life.

1.Stainless steel (304 / 316 / 316L)

a.Most widely used

b.Stable resistance and good oxidation resistance

2.Nickel-chromium alloy (Cr20Ni80)

a.High-performance applications

b.Excellent corrosion resistance for coastal environments

3.Iron-chromium-aluminum (FeCrAl)

a. Cost-effective

b. Less suitable for extreme temperatures

3.4 IP Protection Class (Indoor vs Outdoor NGR)

Installation Location	Typical IP Rating
Indoor NGR	IP20 / IP23
Outdoor NGR	IP54 / IP55

Outdoor enclosures often use louvered panels and labyrinth ventilation to balance cooling and protection.

4. NGR Monitoring and Control Components

Modern neutral grounding resistor systems include intelligent monitoring:

1.Current Transformer (CT)

Measures ground fault current

2.Isolating switch or vacuum contactor

Allows maintenance disconnection

3.NGR Monitoring Unit

a. Resistance open-circuit detection

b. Temperature monitoring (PT100)

c. Ground fault event recording

These features significantly improve system reliability and safety.

5. Neutral Grounding Resistor Selection Table (Typical Values)

System Voltage	Grounding Type	Ground Current	Typical Resistance	Material
6.6 kV	LRG	400 A	—	SS 304
10 kV	LRG	200 A	—	SS 304
10 kV	HRG	5 A	—	NiCr
35 kV	LRG	600 A	—	SS 316

6. How to Choose the Right NGR – Quick Checklist

1. Define grounding objective: alarm or trip

2. Calculate system capacitive current

3. Select grounding current (HRG or LRG)

4. Calculate resistance value

5. Confirm time rating and thermal capacity

6. Select enclosure IP rating

7. Add monitoring and protection accessories

Frequently Asked Questions 

Q1: How do I calculate NGR resistance for a 10 kV system?
Use R = U / I, where U is system voltage and I is selected ground fault current.

Q2: What is the difference between HRG and LRG?
HRG limits current to a few amps for continuity; LRG allows higher current for fast fault clearing.

Q3: Why is neutral grounding resistor monitoring important?
An open-circuit NGR removes grounding protection and can cause dangerous overvoltage.

Detailed Calculation Example for Neutral Grounding Resistor (NGR) Selection

This example demonstrates how to select and calculate a Neutral Grounding Resistor (NGR) for a medium-voltage system, including resistance value, grounding current, thermal energy, and time rating.

1. System Data (Given Conditions)

Assume the following typical medium-voltage system:

a. System voltage: 10 kV (line-to-line)

b. System type: 3-phase, 50 Hz

c. Neutral grounding method: Low Resistance Grounding (LRG)

d. Fault clearing time: 10 seconds

e. Estimated capacitive ground current (Ic): 60 A

2. Selection of Ground Fault Current

Engineering Rule

To effectively suppress overvoltage:

I NGR ≥ IC

Selected Ground Fault Current

For reliable relay operation and margin:

• Selected ground fault current:
  I =200A
  

This value is commonly used in 10 kV LRG systems.

3. Calculation of Phase Voltage

Neutral grounding resistors are rated based on phase voltage, not line voltage.

4. Calculation of NGR Resistance Value

Using Ohm’s law:

✅ Selected Resistance

R ≈ 29 Ω

5. Thermal Energy (kJ) Calculation

The resistor must safely absorb the thermal energy generated during a ground fault.

Formula

E=I²×R×t

Where:

• I = grounding current (A)

• R = resistance (Ω)

• t = fault duration (s)

Calculation

E=200²×28.85×10$$E = 11,540,000 ext{ J}$$E=11540000 J$$E = 11,540 ext{ kJ}$$E=11540 kJ

✅ Required Thermal Capacity

≥ 11,540 kJ (10 s)

6. Time Rating Selection

Based on protection philosophy:

a. Low Resistance Grounding

b. Relay clearing time: < 10 s

✅ Selected Time Rating

10 seconds

(30 s can be selected for additional safety margin)

7. Resistor Material Selection

Based on thermal stress and environment:

1.Recommended material:
Stainless Steel 304 or 316

2. Advantages:

a. Stable resistance at high temperature

b. Good oxidation resistance

c. Long service life

8. Insulation Level Verification

For a 10 kV system, typical requirements are:

1. Rated voltage: 5.77 kV (phase voltage)

2. Power-frequency withstand: ≥ 42 kV

3. Lightning impulse withstand (BIL): ≥ 75 kV

Standards: IEEE 32 / IEC 60076 / IEC 61936

9. Enclosure and IP Rating

Assume outdoor installation:

1. Recommended IP rating: IP54 / IP55

2. Features:

a. Natural ventilation

b. Louvered or labyrinth airflow

c. Anti-corrosion coating

10. Final NGR Specification (Summary)

Parameter	Value
System voltage	10 kV
Grounding type	Low resistance
Ground fault current	200 A
Resistance	~29 Ω
Time rating	10 s
Thermal capacity	≥ 11,540 kJ
Material	Stainless steel 304/316
Installation	Outdoor
IP rating	IP54 / IP55
Standard	IEEE 32 / IEC

Conclusion

This example shows that proper NGR selection requires:

1. Determining grounding philosophy (HRG or LRG)

2. Selecting grounding current based on capacitive current and protection needs

3. Calculating resistance using phase voltage

4. Verifying thermal energy and time rating

5. Confirming insulation level and enclosure design