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Question
HOW CAN WE CALICULATE NGR RESISTANCE FOR A GENERATOR AND
TRANSFORMER ?
WHAT TYPE OF TRANSFORMER REQUIRES NGR ?
 Question Submitted By :: Electrical-Engineering
I also faced this Question!!     Answer Posted By  
 
Answer
# 1
NGR-Neutral Grounding Resistor.It is used to limit the
fault current in the neutral line.calculation based on fault
current in stator in generator and star side in trfr
 
Is This Answer Correct ?    13 Yes 2 No
Ramesh
 
Answer
# 2
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
 
Is This Answer Correct ?    7 Yes 1 No
Ravi Kumar Purohit
 
 
 
Answer
# 3
The value of the NGR to be inserted in the cicuit can be
calculated by the simple formula

R = V / (√3 * I)

Where V is the System Line voltage in Volts
I is the value of the fault current to be limited

In general practice the fault current is limited to the
full load rating of the largest equipment available in the
system.
 
Is This Answer Correct ?    6 Yes 4 No
Pavan
 
Answer
# 4
Ohmic value of neutral grounding resister for a transformer is calculated as follows:

R = Vph/ I where I - value of fault current to be limited to flow through the neutral path, normally it is 50A for 5 sec.

Based on above, for 6.6 kv star side of a transformer
resister value shall be 76.21ohm.
 
Is This Answer Correct ?    0 Yes 1 No
A S Bhandari
 

 
 
 
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