Figure 1.1 shows how a short circuit current varies on occurrence of a short circuit fault. During the first moment of the fault the current attains its maximum value and then decays to some steady-state value.
For checking the thermal and dynamic stability of the equipment in electrical installations with respect to the fault currents which may arise in the power networks, it is necessary to know the following values of the short circuit currents:
1. Sustained Current, I∞:
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Sustained current is the fault current that will flow in the circuit if it is allowed to persist after the transient process ends. This is the value used in designing of electrical equipment for thermal stability.
2. First Peak Short-Circuit Current ifp:
This is the maximum instantaneous value of the current that will reach during the first moment of a short circuit. The value of this current is used in designing electrical equipment for dynamic stability.
It is determined from the following equation (Eq. 1.7):
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First peak short circuit current, ifp = 1.8 (√2Ipc) …(1.7)
where Ipc is the rms value of the periodic component of the short circuit current 2-3 periods after the short circuit is initiated.
The values of the sustained current I∞ and the initial periodic component current Ipc are required in carrying out the calculations for a given protective relay scheme.
Thus we observe that during the fault, the current undergoes a continuous change and the phenomenon observed is called the transient phenomenon. The word ‘transient’ refers to a ‘temporary happening’ which lasts for a short duration of time. The zone in which current is very high but falls very rapidly is called the sub-transient state.
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After the first few cycles when the decrease in current is less rapid, the region is called the transient state. The transient state lasts for several cycles and after the transient state, steady-state is reached. During the steady-state the rms value of the short circuit current remains almost constant.