ABSTRACT
In designing protective circuits for generators, transformers, transmission lines, and other equipment, it is important to determine their terminal currents which flow due to local abnormal operating conditions such as line-to-line faults, or a fault from line to ground, and other device failures. Protective circuit breakers are selected according to voltage-withstanding capability, magnitude of fault currents to be interrupted, and response time from sensing the excessive current to the instant of opening the circuit. The calculations are performed in the symmetrical component frame, then converted to abc-phase per-unit values, then ultimately to engineering units for physical voltages and currents. Generator transient reactances plus transmission line impedances limit the current in a fault. Synchronous condensors are treated similarly to generators. Large motor loads, which are on the same order of magnitude as the generators and operate at a low power factor, contribute to fault currents because of their inductance.
