Subtransient Reactance (Generator Fault Contribution)

The internal reactance level of a synchronous generator (denoted X''d) that applies immediately after a short circuit occurs, during the first few tens of milliseconds, before the damper- and field-winding effects decay. Because the subtransient reactance is considerably lower than the later transient reactance (X'd) and the eventual steady-state synchronous reactance (Xd) that applies after a few seconds, a generator delivers a much larger contribution to the fault current immediately after a short circuit than its steady-state values would suggest — an effect that is crucial when determining the required breaking capacity of circuit breakers and the settings of fast protection in installations with on-site generation (standby generators, CHP plants). Under IEC 60909 (the standard for short-circuit current calculation), the calculation of the initial symmetrical short-circuit current (Ik'') in a network with synchronous generators must explicitly use this subtransient value rather than the steady-state generator impedance, because using the latter would lead to a dangerous underestimation of the actual peak fault current.