When short circuit faults occur in the power system some machines in the power system will act as source to the short circuit current and delivers the short circuit current to the fault location. Mainly there are three sources which can feed into the short circuit; they are:
- Synchronous generators or alternators
- Synchronous motors and synchronous condensers
- Induction motors
Synchronous generators or alternators:
Synchronous generators or alternators in the power system contribute major short circuit current to the fault location. Alternators are driven by the prime movers (turbines) ans when short circuit occurs, generators continue to run at normal synchronous speed and the excitation which is provided by the dc source will also be maintained normal. Thus the normal voltage is continued to be maintained at the generator terminals. Due to this generated voltage an enormous armature current or line current is fed to the fault location. This fault current fed by the synchronous generator is limited by the impedance of the generator and that of the circuit up to the fault location. For example, in a circuit containing the generator and transformer followed by a transmission line, if fault occurs on the transmission line short circuit current contribution by the synchronous generator is less when compared to fault occur at generator terminals.
Synchronous motor or Synchronous condenser:
Synchronous motors are similar to synchronous generators or alternators in construction. They have dc excitation and draws ac power from the power system and converts it into mechanical power. On occurrence of the short circuit, the system voltage considerably reduced (system voltage almost becomes zero during short circuit) and so the supply of electrical power to the synchronous motor is stopped. As a result, the motor starts slowing down. But the fly-wheel and the inertia of the motor prevents it from slowing down and drives the rotor as prime mover (synchronous motor undergoes synchronous generator operation). As the dc excitation is maintained, the synchronous motor starts operating as synchronous generator or alternator. Mechanical energy stored in the flywheel and the rotor is thus converted into electrical energy and fed to the fault location. The magnitude of the short circuit current which it can supply to the fault point depends in the output power (kW), reactance and voltage rating of the machine. Synchronous motor delivers current to the fault location up to few cycles until rotor shaft halts.
In industries, 70% of the motors and drives employed are induction motors. Induction motors unlike alternators and synchronous motors delivers the fault current to the source point for the first few cycles.
Like synchronous motors, the induction motors also behaves like a generator during short circuit. The difference between the both the machines is that induction motors does not have dc excitation, but during normal operation the flux from the stator appears to be as if is being produced by the dc winding.
On occurrence of the short circuit in the power system, the terminal voltage across the motor terminals drops to almost zero suddenly The flux cutting the rotor does not change instantaneously and the inertia of the motor continues to drive it, thus behaving as generator. This starts feeding the fault location by the induction motor until the stator flux decays
This short circuit contributed by the synchronous generators, motors and induction motors will be for shorter duration and decays with the time.