Consider a protected bus that terminates four lines, as shown in Figure 10.51. A
ID: 2083083 • Letter: C
Question
Consider a protected bus that terminates four lines, as shown in Figure 10.51. Assume that the linear couplers have the standard X_ = 5 m ohm and a three-phase fault externally located on line 3 causes the fault currents shown in Figure 10.51. Note that the infeed current on line 3 to the fault is -j10 kA. (a) Determine V_0. (b) Let the fault he moved to an internal location on the protected bus between lines 3 and 4. Find V_0 and discuss what happens. (c) By moving the external fault from line 3 to a corresponding point on (i) line 2 and (ii) line 4, determine V_0 in each case.Explanation / Answer
Current entering into protected bus through bus 1 = -j16 kA
similarly through bus2 = -j7kA , bus3 = -j10kA , bus4 = -j13kA
A) a three phase fault occurs across bus3 = -j36kA
sum of the currents entering into the protected buses through 1,2,4 = -j16kA - j7kA - j13kA = -j36kA -----------1
amount of current through bus 3 from protected = -j36kA --------------2
direction of currents from equation 1, 2 are in opposite directions
there fore net voltages induced into the measurement instrument transformer = (-j36-(-j36))k * 5m = 0V
B) If fault occurs on protected bus between bus 3 and 4
total current entering into protected bus through all bus = -j16-j7-j10-j13 = -j46kA
voltage induced into instrument transformer = -j46k*5m = 230 v
C) similarly to the bus 1 , the fault at bus 2 , fault current = -j39kA
net voltage induced in instrument transformer = (-j39k-(-j39))*5m = 0v
for fault at bus 4 the fault current = -j33kA
net voltage induced in instrument transformer = (-j33-(-j33)) * 5m= 0 v
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