How to Calculate NGR Resistance Value?

Neutral Grounding Resistors (NGRs) play a critical role in electrical power systems by controlling fault currents and protecting equipment during ground faults. Proper calculation and selection of the NGR resistance value are essential to ensure the safety and reliability of transformers and alternators.

This article guides you through the key concepts, formulas, and practical examples needed to accurately determine the resistance for transformers and alternators in electrical installations.

What is NGR?

NGR-Neutral Grounding Resistance. NGR limits the ground fault current to the equipment’s safe operating current range to protect all the electrical equipment. The NGR is connected to the neutral point of the star point of the transformer and alternator.

Why is NGR Required for Transformers and Alternators?

When the supply system is balanced, the current flowing through all three phases will be balanced, and as a result, the neutral-to-ground current is zero.

However, when the three-phase supply system gets unbalanced because of an earth fault, arcing fault, or transient voltage, the current in all three phases gets unbalanced. The unbalanced current starts flowing from the neutral to the earth point.

If the neutral point is directly connected to earth(solid earth), a large current will flow through the transformer winding to neutral and earth. The large current can damage the equipment.

The neutral grounding resistor is connected between the neutral and earth point to limit the fault current to the safe value of the transformer. The NGR is connected at the star point of the transformer, as given in the figure below.

NGR Resistance Value Calculation

The NGR resistance value must be selected based on the following factors.

1. The system voltage
2. During an earth fault, the system capacitance current discharges through the NGR. The NGR should be capable of handling that current.
3. The resistance value must be selected such that the tripping relay must sense the fault current. If the NGR resistance value is too high, the relay may not sense the fault current.

The value of NGR resistance is;

R = V_L-N/ I_f
R= (V_L-L/√3) /I_f

Where:

• R = NGR resistance value (Ohms)
• V_L−N = Line-to-neutral voltage of the system (Volts)
• V_L−L = Line-to-line voltage of the system (Volts)
• I_f = Fault current to be limited (Amps)

Solved Problem on NGR Resistance Value Selection

Lets the transformer secondary voltage is 6.6 KV, and the transformer full load current is 1400 Amps.

The line to neutral voltage is 

VL-N = 6600/√3
= 6600/1.732
VL-N =3810 Volts

The transformer full load secondary current is 1400 amperes. Therefore, the resistance value required to limit the fault current to the transformer’s maximum current carrying capacity is;

R = V/I
R = VL-N/ I
R= 3810/1400
R= 2.72 Ω

NGR Rating

The rating of a Neutral Grounding Resistor (NGR) is an important parameter to ensure reliable and safe operation under fault conditions. The key ratings to consider include:

• Rated Voltage: This should match the system phase voltage at the neutral point where the NGR is connected, typically the transformer’s line-to-neutral voltage. The NGR must safely withstand this voltage during normal and fault conditions.
• Rated Current: This is the maximum continuous current the NGR can carry during a fault without damage. It is usually based on the transformer’s full load current or the maximum fault current that needs to be limited.
• Resistance Value: Calculated to limit the fault current to a safe and detectable level, balancing protection and equipment safety.
• Time Rating: The NGR must withstand the fault current for a specific duration, usually a short time (e.g., 10 seconds), until protective devices isolate the fault.
• Temperature Rise and Cooling: Ratings must account for heat generated during fault currents. Cooling methods (like Air Natural cooling) and temperature rise limits (e.g., up to 375°C) ensure the resistor operates safely.

Selecting an NGR with the correct ratings ensures the resistor limits fault currents effectively while maintaining durability and safety of the electrical system.

NGR Resistance Specifications

1. Location: Indoor/Outdoor
2. Cooling  : Air Natural(AN)
3. Ambient Temperature : 50° C
4. Rated Voltage: Specify the phase voltage of the system
5. Rated Current: Specify full load secondary current of the transformer
6. Rated Resistance
7. Allowable tolerance in NGR resistance : +/- 10 %
8. Temperature rise: 375° C
9. Time rating: 10 Seconds

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3. Why is the armature of the alternator connected in Star, not in Delta?
4. Parallel Operation of Alternators
5. Losses and Efficiency of an Alternator