It is very important to consider all sources of fault current and to know the impedance characteristics of the fault current sources when calculating the magnitudes of short circuit fault currents. Following equipment feeds fault current into a short circuit:
Generators are driven by turbines, water wheels, diesel engines or other types of prime movers. When a short circuit occurs on the system powered by a generator, the generator continues to produce voltage at the generator terminals as the field excitation is maintained and the prime mover drives the generator at normal speed. The generated voltage causes a large magnitude fault current flow from the generator to the short circuit. The flow of fault current is limited only by the generator impedance and the impedance of circuit between the generator and short circuit. In case of a short circuit at the generator terminals, the fault current is limited by generator impedance only.
Synchronous motor design is very similar to generator design. Both have a magnetic field produced by direct current and stator carrying alternating current. During normal operation, synchronous motors consume ac power from power line and convert the electric energy into mechanical torque. When a short circuit happens on the system feeding a synchronous motor, the voltage at motor terminals drops drastically. As a result, motor stops delivering mechanical torque to the load and starts to decelerate. Still, the inertia of the load and motor wheel drives the synchronous motor. The synchronous motor turns to generate and deliver fault current for prolonged period of time after the initiation of short circuit. The fault current is limited by motor impedance and impedance of the system from short circuit to the motor terminals.
Same as with synchronous motor, the inertia of an induction motor mechanical load drives the motor after voltage drops at the motor terminals. Although the induction motor does not have direct current winding exciting the motor magnetic field, there is a flux induced by stator current and penetrating the induction motor during normal operation. The flux in the rotor can not change momentarily right after the power supply has been disconnected. Therefore, a voltage is produced at the motor terminals causing fault current flow to the short circuit until the rotor flux drops to zero. Since there is no sustained flux in the stator, the fault current will drop to zero in about four cycles. The fault current generated by induction motor must be considered in calculations involving momentary duty of circuit breakers and interrupting rating of electric fuses. The fault current is limited by motor impedance and impedance of the system from short circuit to the motor terminals. The initial values of fault current fed by the induction motor is close to the locked rotor start current of the motor.
The fault current delivered by a utility system depends on the impedance of the generators and the impedance of power system to the terminals of the supply transformer. Supply transformers deliver fault current from the power system generators. Transformers change the system voltage and magnitude of current. The fault current delivered by a transformer depends on the transformer secondary voltage and impedance, the impedance of the upstream section of power system to the terminals of the transformer and the impedance of the circuit from the transformer to the short circuit.
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