29.2 : 三相短路 - 空载同步电机

A three-phase short circuit test on an unloaded synchronous machine is conducted to study its impact.

The AC fault current's oscillogram with its DC offset removed, corresponding to one phase of an unloaded synchronous machine, shows a decreasing waveform amplitude from an initial high to a steady-state value.

This is attributed to the magnetic flux caused by short-circuit armature currents, shifting from high-reluctance paths to lower ones, increasing armature inductance. This can be modeled with a time-varying series R-L circuit.

Standard machine theory provides specific reactances used to calculate the instantaneous AC fault current comprising the RMS line-to-neutral prefault terminal voltage.

The RMS sub-transient fault current is determined at time equals zero by the direct axis short-circuit sub-transient time constant, and as time progresses, the RMS AC fault current nears its steady-state value.

Each phase has a different DC offset, peaking when alpha equals zero.

Machine reactances and time constants, provided by manufacturers or obtained from tests, assist in predicting power system behavior during faults, enabling damage control and system stability.