How to Calculate NGR Resistance Value?

What is NGR ?

NGR-Neutral Grounding Resistance. NGR is used to limit the ground fault current to the equipment 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 NGR required for Transformer and Alternator?

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

However, when the three phase supply system get unbalanced because of earth fault or arcing fault or transient voltage, the current in all three phases get unbalanced.The unbalance current starts flowing from neutral to 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 below figure.

NGR Resistance Value Calculation

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

The system voltage
During earth fault, the the system capacitance current will also gets discharged through the NGR. The NGR should be capable to handle that current.
The resistance value must be selected such that the tripping relay must sense the fault current. If NGR resistance value is selected too high the relay may not sense the fault current.

The value of NGR resistance is;

R = VL-N/ If

= (VL-L/√3) /If

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 =3810 Volts

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

R = V/I

R = VL-N/ I

R= 3810/1400

R= 2.72 Ω