Final answer:
The transient reactance (X'd) in a generator represents the reactance that accounts for the initial response to a fault. It helps to limit sudden changes in current and voltage following a fault and maintain generator stability.
Step-by-step explanation:
The transient reactance (X'd) in a generator represents the reactance that accounts for the initial response to a fault. It is the reactance that comes into play during the transient period immediately following a fault in the electrical system. During this period, faults cause sudden changes in current and voltage, and the transient reactance helps to limit these changes and maintain the stability of the generator.