Determining the subtransient fault current in a power system involves representing transformers by their leakage reactances, transmission lines by their equivalent series reactances, and synchronous machines as constant voltage sources behind their subtransient reactances. In this analysis, certain elements are excluded, such as winding resistances, series resistances, shunt admittances, delta-Y phase shifts, armature resistance, saturation, saliency, non-rotating impedance loads, and small motors rated below 50 horsepower, because their impact on the subtransient fault current is negligible or complicates the analysis without significantly affecting the results.

When a three-phase short circuit occurs, the fault is modeled using two opposing voltage sources with equal phasor values. The superposition principle is applied to calculate the fault current. This involves using two opposing voltage sources with equal phasor values, which allows the system to be simplified by removing the phasor value, thereby isolating the fault current component for easier calculation.

The procedure to determine the subtransient fault current involves considering contributions from both the generator and the motor. By using the initial circuit conditions and applying the appropriate reactances, the subtransient fault current can be accurately calculated. This approach ensures that the analysis takes into account the immediate effects of the fault on the power system.

This method provides essential insights into the power system's behavior during faults, allowing engineers to design systems that can withstand such events. The ability to predict the subtransient fault current helps in maintaining system stability and minimizing potential damage. Understanding the contributions of different components during a fault and accurately modeling their reactances is crucial for effective fault management.

By focusing on key reactances and employing superposition, this analysis aids in the design and management of robust power systems. This ensures reliability and safety, allowing for effective mitigation strategies during faults. Predicting fault behavior and understanding its immediate impact is vital for maintaining the integrity of electrical networks, ensuring that power systems can continue to operate reliably under fault conditions.