Direct-axis transient reactance (Xd’)is the quotient of the initial value of a sudden change in that fundamental AC component of armature voltage, which is produced by the total direct-axis primary flux, and the value of the simultaneous change in fundamental AC component of direct-axis armature current, the machine running at rated speed. Unsaturated reactance values can be determined by application of short-circuit tests to synchronous generators, excited to reduced terminal voltages of up to 20%
