Correct Answer - Option 3 : Greater than 90°
If a fault occurs in a system, δ begins to increase under the influence of positive accelerating power, and the system will become unstable if δ becomes very large.
Critical angle: There is a critical angle within which the fault must be cleared if the system is to remain stable and the equal area criterion is to be satisfied. This angle is known as the critical clearing angle.
The system will remain stable even though the rotor may oscillate beyond δ = 90°, so long as the equal area criterion is met. The condition of δ = 90° is meant for use in steady state stability only and does not apply to the transient stability case.
Important Points:
Equal area criterion:
- It is used to determine the limit on the load which can be acquired by the system without crossing the stability limit.
- The principle of this method consists of the basis that when δ oscillates around the equilibrium point with constant amplitude, transient stability will be maintained.
- It is employed for the determination of the transient stability limit of the single-machine-infinite bus system.
- It is also used for determining the maximum limit on the load that the system can take without exceeding the stability limit.
- In a single machine infinite bus system, if the system is unstable after a fault is cleared, δ(t) increases indefinitely with time, till the machine loses synchronism. In contrast, in a stable system, δ(t) reaches a maximum and then starts reducing.
