Answer / ramesh

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

Answer / 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

Answer / pavan

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.

Answer / madivalanagaraja@gmail.co

It is similar to the ground resistor calculation for transformer

The typical example was answered earlier for transformer dt.19-06-2009

Neutral of transformer can be grounded solidly earthed OR with Neutral Grounded with Resistance.

Typical shunt calculations fro 5 MAV 11/6.6 kv transformer neutral are as :_

Transformer rating = 5 MVA

Voltage ratio = 11/6.6 KV

Vector Group = Dyn11 (6.6 KV ground through Resistor)

During Earth fault voltage between Neutral

Answer / a s bhandari

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.

mostly the questions were from machines... d.c. machines, transformer, synchronous and induction all theory paper........ few from power system... but the level of paper was easy.....

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