MIL-DTL-3124E(SH)
6.5.11 Per-unit system. In this system, the rating quantity is regarded as unity. Any other amount of the
quantity is expressed as a fraction of the rated amount. The per-unit system is the same as the percentage system,
except that unity is used as a base instead of 100.
6.5.12 Short-circuit ratio. The short-circuit ratio is the ratio of the field current for rated open-circuit armature
voltage at rated frequency to the field current required to produce rated armature current for a sustained symmetrical
short circuit at rated frequency.
6.5.13 Subtransient reactance. The subtransient reactance is the rated voltage direct-axis subtransient
reactance. This value of reactance will be that obtained from the sudden application of a three-phase short-circuit at
the terminals of the generator at rated speed and rated armature voltage and is defined as:
Subtransient reactance per-unit (X"d) = Et
I"
Where:
Et = The terminal voltage of the generator, per-unit, at the instant before the short circuit is applied.
I" = The value of the short-circuit current, per-unit, is determined by the extrapolation of the envelope
of the AC component of the current wave to the sudden application of short-circuit.
6.5.14 Synchronous reactance. The synchronous reactance is the direct-axis unsaturated synchronous
reactance. This value is defined as:
Synchronous reactance in per-unit (Xd) = Ifec
Ifeg
Where:
Ifec = The field current required to produce rated armature current under a sustained symmetrical
short-circuit at the terminals of the generator.
6.5.15 Transient open-circuit time constant. The transient open-circuit time constant is the direct-axis transient
open-circuit time constant, and is defined as the time in seconds required for the rms AC value of the slowly
decreasing component present in the direct-axis component of symmetrical armature voltage on open circuit to
decrease to 0.368 of its initial value when the field winding is suddenly short-circuited with the machine running at
rated speed.
6.5.16 Transient reactance. The transient reactance is the rated voltage direct-axis transient reactance. This
value of reactance will be that obtained from the sudden application of a three-phase short-circuit at the terminals of
the generator at rated speed and rated armature voltage, and is defined as:
Transient reactance in per-unit (X d) = Et
I
Where:
Et = The terminal voltage of the generator, per-unit, at the instant before the short-circuit is applied.
I = The value of the short-circuit current, per-unit, determined by the extrapolation on the envelope
of the AC component of the current wave to the instant of the sudden application of short circuit,
neglecting the high decrement currents during the first few cycles.
6.6 Maintainability test schedule. The contracting activity and NAVSEA should be notified at least 30 days in
advance of scheduled maintainability demonstration tests so that arrangements can be made to witness the
demonstration, if desired.
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