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Question

The positive , negative and zero sequence impedance of 3ϕ generator are 0.4 pu, 0.3 pu and 0.2 pu respectively. For a line to line fault with fault impedance Zf and fault current is If1=0.2If, where If is the fault current with zero fault impedance, then value of Zf will be ___ pu.
  1. 2.8

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Solution

The correct option is A 2.8
For given system, fault impedance Zf is given.

LL fault current will flow through Zf.

Including Zf in our impedance we can draw,


Fault current If1=3Ia1=3.VthZ1+Z2+Zf

Also, zero sequence impedance diagram,



Also, we know, fault current,

If=3Ia1=3.VthZ1+Z2

Also given, If1=0.2If

Substituting value, we get:3VthZ1+Z2+Zf=0.2.3VthZ1+Z2

Z1+Z2=0.2(Z1+Z2+Zf)

0.8(Z1+Z2)0.2=Zf

Zf=0.8(0.4+0.3)0.2=4(0.4+0.3)=2.8 pu

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