Chapter 7: Q21MCQ (page 464)
A typical fusible link metal in fuses is _____, and a typical filler material is _____.
Answer
A typical fusible link metal in fuses is Silver strip or wire, and a typical filler material is sand.
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For the power system given in Problem 7.14, a three-phase short circuit occurs at bus 4, where the prefault voltage is. Prefault load current is neglected. Determine (a) the Thévenin equivalent at the fault, (b) the subtransient fault current in per unit and in, and (c) contributions to the fault from transformer and from line 3–4.
Two identical synchronous machines, each rated 60 MVA and 15 kV with a sub transient reactance of 0.1 pu., are connected through a line of reactance on the base of the machine rating. One machine is acting as a synchronous generator, while the other is working as a motor drawing at 0.8 pf leading with a terminal voltage of 14.5 kV , when a symmetrical three phase fault occurs at the motor terminals. Determine the sub transient currents in the generator, the motor, and the fault by using the internal voltages of the machines. Choose a base of 60 MVA and 15 kV in the generator circuit.
Solve Example 7.3 parts (a) and (c) without using the superposition principle. First calculate the internal machine voltages and localid="1656758482853" using the prefault load current. Then determine the subtransient fault, generator, and motor currents directly from Figure 7.4(a). Compare your answers with those of Example 7.3.
As shown in Figure 7.21, a, synchronous generator with is connected through a transformer to a bus that supplies four identical motors. The rating of the three-phase transformer is with a leakage reactance of. Each motor has a sub-transient reactance on a base of .A three-phase fault occurs at point P, when the bus voltage at the motors is.
(a) the sub-transient fault current,
(b) the sub-transient current through breaker A, and
(c) the symmetrical short-circuit interrupting current (as defined for circuit breaker applications) in the fault and in breaker A.
Even though the fault current is not symmetrical and not strictly periodic, the rms asymmetrical fault current is computed as the rms ac fault current times an “asymmetry factor,” which is a function of _____.
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