Important Short Questions Answers: Stability Analysis

1. Define stability.

The stability of the system is defined as the ability of the system to return to stable operation when it is subjected to a disturbance.

2. Define steady state stability.

Steady state stability is defined as the ability of the power system to remain stable i.e,without loosing synchronism for small disturbances.

3. Define transient stability.

Steady state stability is defined as the ability of the power system to remain stable i.e,without loosing synchronism for large disturbances.

4.    Define synchronizing or stiffness coefficient. For what value of synchronizing coefficient the system remains stable.

The term P_max Cos ᵟ_o is called synchronizing or stiffness coefficient. The system is stable if the synchronizing or stiffness coefficient is positive.It is positive when 0 < ᵟ_o < π/2.

5. Give an expression for swing equation.

(H/ πf)(d²ᵟ/dt²) = P_m – P_e

6. Define swing curve.

The swing curve is the plot or graph between the power angle and time.

7. What is the use of swing curve?

It is usually plotted for a transient state to study the nature of variation in power angle for a sudden large disturbance. From the nature of variation in power angle the stability of a system for any disturbance can be determined.

8. Define power angle.

The power angle or torque angle is defined as the angular displacement of the rotor from synchronously rotating reference frame.

10.            Name the two ways by which transient stability study can be made in a system where one machine is swinging with respect to an infinite bus.

a. Equal area criterion

b. Point by Point method.

11.            Define critical clearing angle and critical clearing time.

The critical clearing angle is the maximum allowable change in the power angle before clearing the fault, without loss of synchronism.

The critical clearing time is defined as the maximum time delay that can be allowed to cleara fault, without loss of synchronism.

12.            List the methods of improving the transient stability limit of a power system.

a. Increase of system voltage and use of Automatic Voltage Regulation (AVR)

b. Use of high speed excitation systems.

c. Reduction in system transfer reactance.

d. Use of high speed reclosing breakers.

13.            State Equal area criterion.

Equal area criterion for stability states that the system is stable if the area under P_a - ᵟ curve reduces to zero at some value of ᵟ.

This is possible only if the positive area under P_a - ᵟ curve is equal to the negative area under

P_a - ᵟ curve for a finite change in ᵟ.hence this stability criterion is called Equal area criterion.