152 HANDBOOK OF ELECTRICAL ENGINEERING
Figure 7.1 Short-circuit current decrement for a salient pole generator that has a high armature time
constantTa.
7.2.11 Fault breaking current
The fault current effects have been described above for the making duty. However, some further points
are appropriate for the breaking duty. The breaking duty root-mean-square duty is usually specified
to take place after a number of cycles of fault current have passed. (It may also be expressed as a
peak value of current, although this is less common.) This is usually taken to be the time given by the
manufacturer for the circuit breaker to open and clear the fault. This is typically 5 to 8 cycles of the
fundamental current. The engineer should specify the requirements for the particular power system
and the manufacturer should then confirm whether the equipment offered could meet the requirement.
Each power system should be considered on its own merits in this regard.
Equation (7.2) can be used to calculate the situation at the time given for the breaking duty.
Usually the sub-transient time decay term has fallen to zero, and the solution is in the transient
period. When an ‘external’ impedance is present its resistance can be included in theTatime con-
stant and its reactance added to the appropriate machine reactances. References 5 and 6 explain
how the ‘derived’ reactances and time constants are calculated and affected by the addition of the
external impedance.
If the lower envelop of the transient AC part and the DC part of (7.2) are separated out then
two functions can be presented as follows. At current zero the critical timetcin seconds occurs when,
t=tc,
Ia
Vpk=−
[
1
X′d−
1
Xd]
exp−t
T′d+[
1
Xd]
+
[
1
X′′d]
exp−t
Ta ( 7. 3 )