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An unloaded synchronous machine's three-phase short-circuit current undergoes a series of sub transient and transient episodes as the amplitude of the sinusoidal symmetrical ac component decreases from a high beginning value to a lower steady-state value.
First off, the term "subtransient" describes how quickly this quantity changes—faster than even "transient" itself. Technically speaking, the subtransient time period lasts from 16.7 milliseconds to 50 milliseconds, or around one to three cycles (assuming a power frequency of 60 Hz). There are three time periods: During the sub-transient period, the AC current is very big and falls quickly after the fault; during the transient period, the current falls more slowly; and during the steady-state period, the current reaches its steady value. In the computation of fault current, transient and sub-transient reactance are often the words that are used. Subtransient reactance, which is generally represented by the symbol X"d, is the reactance that is used to estimate the current during the first cycle after the occurrence of the fault.
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