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Hi There Can anybody help with the following question An 11 kV motor is fed by cables from a transformer via switchgear, having a phase impedance of 0.3 + j0.3 ohm. The earth return path to the...

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Hi There

Can anybody help with the following question

An 11 kV motor is fed by cables from a transformer via switchgear, having a phase impedance of 0.3 + j0.3 ohm. The earth return path to the transformer neutral has a resistance of 0.42 ohm. Determine a suitable value of neutral resistance if the voltage rise at the motor in the event of an earth fault at the motor is not to exceed 430 V. Neglect transformer winding resistance.

Many Thanks in advance

Answered Same Day Dec 23, 2021

Solution

Robert answered on Dec 23 2021
123 Votes
Maximum Fault cu
ent in this case is the cu
ent if no ground resistance in motor is provided i.e.
cu
ent through the ground return path in the transformer.
Therefore, Maximum fault cu
ent IG= 430/ 0.42 = 1023.8 A
Required ground resistance = VLL/ √ IG
= 11X103/√ * 1023.8
...
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