Answer Posted / ravi kumar purohit

DESIGN AND SELECTION CRITERIA OF
NGR

Neutral Grounding Resistor:

The power transformer secondary 6.6kV neutral shall be
medium resistance earthed in order to limit the 6.6kV
system earth faults to low values such as to limit
excessive damage to 6.6kV equipments.
Also that the earth faults are not too low value to cause
earth fault relay non-operation.

System voltage = 6.6kV
System voltage per phase = 6.6/√3
Vph = 3810.6Volts
Earth fault current (I) =50A for 5 sec

NGR calculation is done on the basis of following formula,

Vph= I X R

R= Vph / I

R= 3810.6/50

Resistance= 76.21 Ohms

NGR Selected is 6.6kV , 50A, 76.2Ohms, 5sec.

Normally 50 A

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