Conducting a three-phase short circuit test on an unloaded synchronous machine helps understand its impact on the system. The AC fault current's oscillogram, with the DC offset removed, reveals that the waveform amplitude decreases from an initially high value to a steady-state level for one phase of the machine.
This behavior occurs due to the magnetic flux produced by the short-circuit armature currents. Initially, these currents follow high-reluctance paths but eventually shift to lower-reluctance paths, thereby increasing the armature inductance. This dynamic can be represented by a time-varying series resistor-inductor (R-L) circuit.
According to standard machine theory, specific reactances, such as sub-transient, transient, and steady-state reactances, are used to calculate the instantaneous AC fault current. This calculation is based on the RMS line-to-neutral prefault terminal voltage. The RMS sub-transient fault current at the moment of the fault is determined by the direct axis short-circuit sub-transient reactance and the associated time constant. Over time, the RMS AC fault current stabilizes to its steady-state value.
Each phase of the machine experiences a different DC offset, with the peak offset occurring when the initial current angle is zero. The reactances and time constants of the machine, provided by manufacturers or derived from tests, are crucial for predicting the machine's behavior during faults. This information aids in managing potential damage and maintaining system stability during faults.
Understanding these characteristics is essential for designing robust power systems. The data obtained from these tests allow engineers to predict how the system will behave under fault conditions, facilitating the development of strategies to control damage and ensure system stability. This approach ensures that power systems can withstand faults and continue operating reliably, thereby maintaining the integrity and safety of electrical networks.
