The currents flowing in the power system network during a fault is dependent on the machines connected to the system. Due to the effect of armature current on the flux that generates the voltage, the currents flowing in a synchronous machine differs immediately after the occurrence of the fault, a few cycles later, and under sustained or steady-state conditions.
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Further there is an
exponentially decaying D.C component caused by the instantaneous value at the
instant of fault occurring. These are shown in figure 2.8.
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Figure 2.9a and 2.9b show the steady state current waveform,
and the transient waveform of a simple R-L circuit, to show the decay in the DC
component. In addition to this, in the synchronous machine, the magnitude of
the AC current peak also changes with time as shown in figure 2.9c, with the
uni-direction component of the transient waveform removed.
Due to the initial low back EMF at the instant of fault
resulting in high current, the effective impedance is very low. Even when the DC
transient component is not present, the initial current can be several times
the steady state value. Thus three regions are identified for determining the reactance.
These are the sub-transient reactance Xd” for the first 10 to 20 ms
of fault, the transient reactance Xd' for up to about 500 ms, and the steady
state reactance Xd (synchronous reactance).
- The sub-transient must usually be used in fault analysis.