Correct Answer - Option 3 : Only 1 and 2
Power system stability is the capability of the power system to come back to its steady condition without affecting synchronism when subjected to any disturbances
Steady-State Stability:
It is the study that implies small and gradual variations or changes in the working state of the system. The purpose is to determine the higher limit of loading in the machine before going to lose the synchronism. The load is increased slowly
The highest power which can be transferred to the receiving end of the system without affecting the synchronism is termed as the Steady State Stability limit.
Transient Stability:
Transient stability limit is the maximum flow of power through a particular point in the power system without loss of stability when large and sudden disturbances occur
Value of transient stability limit will be lower than steady-state stability limit for maintaining the stability of the system
Critical clearing angle:
- It is related to the stability study of the power system.
- The maximum allowable value of the clearing time and angle for the system to remain stable are known respectively as critical clearing time and angle.
- The equal-area criterion method is useful in determining the critical clearing angle, i.e., the condition when the system will be stably provided the fault is cleared before the rotor angle exceeds the critical clearing angle.
- It is not required to know about the clearing angle for stable operation of the system, but we want to know the critical clearing time corresponding to the critical clearing angle.
- Critical clearing time is the time when the rotor would have moved to the critical clearing angle.
- It helps us to design the operating times of the relay and circuit breaker so that the total time taken by them should be less than the critical clearing time for stable operation of the system.
- Critical clearing angle is related to the transient stability limit.
Transient stability of the system can be improved by
- Increasing the system voltage
- Increase in the X/R ratio
- High speed circuit breakers help to clear the fault as quickly as possible
- By turbine fast valving
- High speed excitation
- Use of auto re-closing breakers some of the other ways to improve the transient stability are by employing lightning arresters, high neutral grounding impedance, single-pole switching, quick Automatic Voltage Regulators (AVRs).
