Calculating subtransient fault currents for three-phase faults in an N-bus power system involves using the positive-sequence network. When a three-phase short circuit occurs at a specific bus, the analysis uses the superposition method to evaluate two separate circuits.

In the first circuit, all machine voltage sources are short-circuited, leaving only the prefault voltage source at the fault location. The positive-sequence bus impedance matrix can be determined by solving the nodal equations, which incorporate the positive-sequence bus admittance matrix. With a single voltage source at the faulted bus, the matrix solution of the nodal equations provides the fault current and the voltage at any bus.

The second circuit reflects the prefault conditions. By neglecting the prefault load currents, all voltages are assumed to equal the prefault voltage. The equivalent circuit for short-circuit currents includes both self-impedances and mutual impedances. By neglecting the prefault load currents, all synchronous machines are assumed to have equal internal voltage sources, which can be represented by a single equivalent source.

Initially, with the switch open, all currents are zero, and each bus has an equal voltage. When the switch closes, a short circuit occurs at the designated bus, causing its voltage to drop to zero.

This approach provides a comprehensive method to determine the subtransient fault currents and their impact on the power system. By focusing on the positive-sequence network and utilizing the superposition principle, engineers can accurately predict the behavior of the system during a fault. This method ensures that potential disruptions and damage can be minimized, contributing to the stability and reliability of the power system.

Understanding the dynamics of subtransient fault currents is crucial for effective power system design and management. By accurately modeling and analyzing fault conditions, engineers can develop strategies to mitigate the effects of faults, ensuring continuous and stable operation of electrical networks. This analysis forms the foundation for designing robust systems capable of withstanding and recovering from faults efficiently.