Short Circuit Current Sources

When faults occurs in the power system the magnitude of the short circuit current will be in the order of kilo amperes where as the normal rated current will be in the order of few amperes to hundred amperes. This high amount of short circuit currents during faults are generated and delivered from various sources in the power system along with synchronous generators and feeds the fault.

While determining the magnitude of the short circuit currents some of the sources that are considered are:

• Generators (synchronous generators)
• Synchronous and asynchronous motors
• Induction motors

Generators:

Generators are driven the prime movers and when excitation is provided it delivers power during normal operation. When a fault occurs at a particular location in the power system, terminal voltage of the generator will drops. On seeing the dip in the voltage AVR of the generator will pump more exciter current in to the rotor windings of the generator. This results in the generation of large amount of armature current in the synchronous generator and this large current feeds the fault.

Synchronous motor:

By construction synchronous motors are similar to synchronous generators. Normally synchronous motors draws ac current to stator winding and field excitation is provided with dc current. 

When fault occurs the terminal voltage of the machine drops to a low value. During this period motor stops delivering energy to the mechanical load and starts retarding. However the inertia of the connected load and motor inertia tend to prevent the motor from slowing down. At this moment synchronous motor becomes synchronous generator and starts delivering the power and feeds the fault.

Induction Motors:

In industry 70% of the motors employed are induction motors. During fault condition induction motors also contributes and feeds the fault similar to synchronous motor as load inertia and motor inertia doesn't allow the motor to retard and acts as a generator. However the duration in which induction motor feeds the fault is less (few cycles) compared to synchronous generators and motors. This is because in synchronous motor field winding is supplied with dc current and during the fault condition field current is continuously fed to the synchronous motor. On the other hand induction motor works on the induction principle. In induction motor field windings are rotor windings which are short circuited (no supply is provided to field windings). Hence during the fault whatever flux trapped in the induction motor helps to operate induction motor as generator. This flux decays over a period of few cycles. Therefore induction motor operates as generator for few cycles after the fault and delivers the fault current.