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7.
Consider a power system shown below
Given that: Vs1 = Vs2=1.0 + j0.0 pu;
The positive sequence impedance are Zs1= Zs2= 0.001 + j0.01 pu and ZL = 0.006 + j0.06 pu
3-phase Base MVA = 100
Voltage base = 400 kV(Line to Line)
Nominal system frequency = 50 Hz
The reference Voltage for phase ‘a’ is defined as v(t)=Vm cos(ωt).
A symmetrical three phase fault occurs at centre of the line, i.e. point 'F at time t0. The positive sequence impedance from source S1to point ‘F’ equals 0.004 + j0.04 pu. The waveform corresponding to phase 'a' fault current from bus X reveals that decaying dc offset current is negative and in magnitude at its maximum initial value. Assume that the negative sequence impedances are equal to positive sequence impedances, and the zero sequence impedances are three times positive sequence impedances.
[1] The instant (t0) of the fault will be [2 marks]
(A) 4.682 ms
(B) 9.667 ms
(C) 14.667 ms
(D) 19.667 ms[2] The rms value of the ac component of fault current (Ix) will be
[2 marks]
(A) 3.59 kA
(B) 5.07 kA
(C) 7.18 kA
(D) 10.15KA[3] Instead of the three phase fault, if a single line to ground fault occurs on phase 'a' at point ‘F’ with zero fault impedance, then the rms value of ac component of fault current(Ix)for phase 'a' will be [2 marks]
(A) 4.97pu
(B) 7.0pu
(C) 14.93pu
(D) 29.85puasked in Electrical Engineering, 2008
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