Power System Analysis - Stability Analysis - 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)(d^{2}ᵟ/dt^{2})
= 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.

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Power System Analysis : Stability Analysis : Important Short Questions Answers: Stability Analysis |

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