Suppose a Generator is running at Load Angle Delta 0. Fault occurs and Output Electrical Power becomes Zero. Since Electrical Output Power (Pe) is less than Mechanical Input Power (Pm), Rotor accelerates. Load Angle increases to Delta 1.
Let the Fault clears at Delta 1 and Power is restored back. Pe becomes greater than Pm. Rotor decelerates and Angular Velocity (Omega R) decreases. However Angular Velocity of Rotor (Omega R) is still greater than Angular Velocity of Stator (Omega S). Load Angle increases to Delta 2 till Omega R = Omega S.
If at Load Angle Delta 2, Pe is greater than Pm, Rotor decelerates and reaches Steady State. There is Transient Stabilty.
If at Load Angle Delta 2, Pe is less than Pm, Rotor accelerates and never reaches to Steady State. Transient Stability is not there.
If at Load Angle Delta 2, Pe = Pm, Generator is Critically Stable.
By using Equal Area Criteria, Critical Clearing Angle is found. Fault should be cleared before Load Angle reaches Critical Clearing Angle, otherwise Generator becomes Unstable.
By using Swing Equation and knowing Critical Clearing Angle, Critical Clearing Time is found. Fault should be cleared before it otherwise Generator becomes Unstable.
Негізгі бет Critical Clearing Angle and Critical Clearing Time
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