# Power Systems Questions and Answers – Effect of Synchronous Machine Excitation – 2

This set of Power Systems MCQs focuses on “Effect of Synchronous Machine Excitation – 2”.

1. How can we check upon the transient stability of a power system?
a) By checking variation in load angle
b) By checking variation of real power with load angle
c) Checking variation in load angle and real power
d) Checking variation in load angle or real power

Explanation: Checking the variation in load angle and real power flow helps in deciding the transient stability.

2.To maintain the transient stability for a power system _____________

Explanation: The rate of change of load angle must be positive and so the change in real power variation in order to supply the demand.

3. How can we improve the steady state stability of the synchronous generator for a better performance?
a) Increasing the excitation
b) Increasing reactance
c) Decreasing moment of inertia
d) Increasing moment of inertia

Explanation: P(max) = EV/X.

4. There is 2-machine model having losses too, with their transfer impedance being resistive. The maximum value of sending power P1max and maximum receiving end power P2max will take place with power angle ‘δ’ such that
a) Both P1max and P2max will occur at δ<90
b) Both P1max and P2max will occur at δ>90
c) P1max occurs at δ > 90 and P2max will occur at δ<90
d) P1max occurs at δ < 90 and P2max will occur at δ>90

Explanation: Even after connecting parallel machines, their maximum power will occur at load angle lesser than 90 degrees.

5. As per the system given, machine B will receive the active and reactive power from machine A when ______________
a) δ is positive and Va is more than Vb
b) δ is positive and Va is less than Vb
c) δ is positive and Va is same as Vb
d) δ is negative and Va is more than Vb

Explanation: Real power flows from higher to lower δ while the reactive power flows from higher voltage to lower.
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6. If there are two machines having moment of inertia M1 and M2, where M1<M2; The possible number of equivalent moment of inertia will be _______________
a) 1
b) 2
c) 3
d) 4

Explanation: M1+M2, M1||M2 and M1-M2.

7. If there are two machines having moment of inertia M1 and M2, where M1<M2; Then one is proposing to add more inertia to the system. Then it will be added to _____________

a) Machine 1
b) Machine 2
c) Machine 1 and Machine 2
d) Either of Machine 1 and Machine 2

Explanation: Since the inertia is less for machine 1.

8. The measure to improve the transient stability of the power system during the unbalanced or unsymmetrical fault can be taken as ________
a) Single pole switching of CB
b) Excitation control
c) Phase shifting transformer
d) Increasing turbine valve opening

Explanation: Single pole switching of CB allows the immediate isolation of the fault frim the system and thus safeguarding the healthy part.

9. Use of high speed circuit breaker will actually increase the system stability.
a) True
b) False

Explanation: High speed CB immediately cuts off the fault and saves the system to undergo transient oscillations.

10. What does the steady state stability of a power system signify?
a) Maintaining the rated voltage
b) Maintaining rated frequency
c) Maintaining a synchronism between machines and tie-lines
d) All of the mentioned

Explanation: Steady state stability means to maintain the stable system inside and outside too with the external tie lines.

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