For the rotor angle stability measurment i'm simulating a three phase fault on a transmission line in ETAP, because this scenario has the biggest impact on the rotor angle deviation due to the large short circuit current.

Out of my simulation results i have seen that the frequency, rotor angle, and rotor speed increases and the bus voltage and the electric torque and the electric power decreases during the fault.
I have some questions about this phenomenon.

Why does the rotor of a synchronous generator accelerate when a three phase fault occurs on a transmission line?

What happens with the flux and the stator and rotor field currents?

what leads to an increasing frequency?

What is its effect on the electrical torque and the electric power?

why does it decrease?

 

Wikash...

A1) At the time of the fault, no generator power can be transmitted to the load. In addition the generator's terminal-voltage sharply decreases. Subsequently, within a fraction of a second, generator (and prime-mover) start to accelerate because of imbalance between mechanical-input power to the electrical-output power. This results in a reduction of Torque-Angle(**) in a direction counter to rotor rotation.

A2) Driven by pre-fault air-gap voltage (taking armature-reaction into account, armature-current peaks then decreases following a transitory-path based on the generator's internal impedances, namely sub-transient, transient, and steady-state! Generally, field-current increases due to AVR action.

A3) Rotational-velocity, hence frequency, increases because the generator is no-longer 'Synchronized' to line-frequency. Its synchronizing-torque is simply gone.

A4) Contrary to the Ampere-to-Torque theory, electrical-torque is of course zero, while Amperes increase.

A5) Answered in A4).

(**) IEEE-Std-100, "Angular Displacement, at rated-load, of the center-line of a field-pole from the axis of the armature mmf force pattern."

Regards,
Phil Corso