# FACTS Questions and Answers – Stability Factors of AC Transmission System – Set 3

This set of FACTS Multiple Choice Questions & Answers (MCQs) focuses on “Stability Factors of AC Transmission System – 3”.

1. In order to prevent overvoltages at light loads, which of the following should be connected to a transmission system?
a) Shunt reactors
b) Resistors
c) Capacitors
d) Voltmeters

Explanation: In order to prevent overvoltages at light loads, shunt reactors should be connected to a transmission system. It is needed to absorb reactive power at light loads. Its use in case of heavily loaded transmission lines may prove detrimental.

2. The effective SIL of a capacitor compensated line is ________ an uncompensated line.
a) higher than
b) lower than
c) is equal to
d) higher or lower than

Explanation: The effective SIL of a capacitor compensated line is higher than an uncompensated line. This happens because lumped capacitors when connected in series or shunt to a system, change the effective line parameters of the system. They reduce the line reactance of the system and increase its shunt susceptance, thereby decreasing the surge impedance effectively.

3. The thermal limit is the main limiting factor for deciding loadability of short transmission lines.
a) True
b) False

Explanation: Thermal limit is the main limiting factor for deciding loadability of short transmission lines (< 100 km). Here voltage constraints are not violated even for large deviations from SIL. The line loadability actually behaves as a function of distance and accordingly gives characteristic preference for considering thermal or voltage limits as its key limiting factor.

4. The thermal limit is the main limiting factor for deciding loadability of EHV transmission lines.
a) True
b) False

Explanation: Thermal limit is NOT the main limiting factor for deciding loadability of EHV transmission lines (long). Here the cross-sectional area for EHV lines is mainly decided by electric field considerations (corona). So by default these lines have large thermal capabilities, much in excess of the SIL. Long lines are limited by voltage constraints, stability constraints. The line loadability actually behaves as a function of distance and accordingly gives characteristic preference for considering thermal, voltage or stability limits as its key limiting factor.

5. A power transmission system is said to be stable when it ________
a) can withstand disturbances and come to an equilibrium
b) cannot withstand disturbances
c) cannot come to an equilibrium after small disturbances
d) cannot come to an equilibrium after large disturbances

Explanation: A power transmission system is said to be stable when it can withstand disturbances and come to an acceptable equilibrium. Disturbances can be small or large. They can occur at any moment resulting from faults causing line trips, dynamic load changes, generator tripping and many more other causes.

6. Problems related to angular stability is one of the major stability problems in interconnected transmission network.
a) True
b) False

Explanation: Problems related to angular stability is one of the major stability problems in interconnected transmission network. The synchronous generators ultimately powering the interconnected transmission grid generate restoring torques, when subjected to any disturbance. This ensures the system to stay in equilibrium keeping the phase angular differences at various points constant, ultimately stabilizing the system.

7. Angular stability problem means ________
a) loss of synchronism
b) loss of capacitive compensation
c) loss of power
d) loss of inductive compensation

Explanation: Angular stability problem means loss of synchronism among the synchronous machines of the interconnected transmission grid. When machines lose synchronism due to failure of the system to attain equilibrium, the phase angular difference between ac buses in the system cannot settle to constant values. This makes the system unstable and unsecure for operation.

8. Voltage stability is triggered by having loads which attempt to draw power which is ________ the maximum capability of generation and transmission.
a) beyond
b) equal to
c) below
d) much below

Explanation: Voltage stability is triggered by having loads which attempt to draw power which is beyond the maximum capability of generation and transmission. Normally, a power system possesses connected loads which are lesser than the maximum power transfer capability of the generation and transmission network. However, loss of lines and generator tripping may significantly increase transmission reactance. This increase of effective reactance results in decrease of effective voltage seen by loads.

9. Limited power transfer capability occurs due to a weak network aggravated by generators hitting their ________ power capability limits
a) reactive
b) resistive
c) capacitive
d) inductive

Explanation: Limited power transfer capability occurs due to a weak network aggravated by generators hitting their reactive power capability limits. This results in inability to maintain the required voltage at key points in the network. Thus a stressed power system with inadequate reactive power capability can lead to voltage instability.

a) draws reactive power
b) draws capacitive power
c) generates reactive power
d) generates inductive power

Sanfoundry Global Education & Learning Series – Flexible AC Transmission System (FACTS).

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