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

This set of FACTS Multiple Choice Questions & Answers (MCQs) focuses on “Stability Factors of AC Transmission System – 1”.

1. The line loads on an electrical power transmission system ________
a) do not vary at all
b) vary continuously
c) vary at sinusoidal cycles
d) vary at discrete signals
View Answer

Answer: b
Explanation: The line loads on an electrical power transmission system vary continuously. There remains a wide range of load conditions changing continuously and generation schedules in such system. Line outages also contribute to this continuously changing line loads.

2. When mechanically controlled, a series capacitor inserted in an ac transmission network may lead to ________
a) synchronous resonance
b) sub-synchronous resonance
c) super-synchronous resonance
d) asynchronous resonance
View Answer

Answer: b
Explanation: When mechanically controlled, a series capacitor inserted in an ac transmission network may lead to sub-synchronous resonance. This resonance happens at a frequency less than the synchronous frequency of the generators and the network. This sub-synchronous resonance, if remains unclear in the ac power transmission system, may damage the shaft of the turbine-generator units.

3. How can a sub-synchronous resonance be damped resulting from the insertion of a series capacitor in a transmission line of a mesh connected network?
a) Series capacitor is mechanically controlled
b) Series capacitor is taken out of the network
c) Series capacitor is uncontrolled
d) Series capacitor is thyristor-controlled
View Answer

Answer: d
Explanation: The sub-synchronous resonance resulting from the insertion of a series capacitor in a transmission line of a mesh connected network can be damped if the series capacitor is thyristor-controlled. If the series capacitor is taken out of the network, the purpose of series compensation for which it was inserted will no more persist; hence it is the wrong option. Unlike that of a mechanically controlled one, the thyristor-controlled series capacitor can be modulated to rapidly damp any sub-synchronous resonance conditions.
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4. Which one among the following is preferred for a practical ac transmission system?
a) All mechanically controlled series capacitor
b) All thyristor-controlled series capacitor
c) Partly mechanically controlled and partly thyristor-controlled series compensation
d) Uncontrolled series capacitor
View Answer

Answer: c
Explanation: Among the given options, partly mechanically controlled and partly thyristor-controlled series compensation is preferred for a practical ac transmission system. Thyristor-controlled series capacitor by its inherent property can enhance the system stability by reducing the risk of system collapse and damage to the generator shafts. However, because of financial implication partly mechanically controlled and partly thyristor-controlled series compensation is preferred to attain the system stability at an affordable price.

5. While enhancing the system stability of an ac transmission system of mesh network, a FACT controller can be a thyristor-controlled ________
a) series inductor
b) series capacitor
c) phase angle -regulator
d) series inductor or a series capacitor or a phase angle -regulator
View Answer

Answer: d
Explanation: To enhance the system stability of an ac transmission system of mesh network, a FACT controller can be a thyristor-contolled series inductor or a series capacitor or a phase angle –regulator. A series capacitor added to a line in a mesh network helps to remove overload from the other overloaded line. A series inductor added to a line in a mesh network helps to remove overload from the same overloaded line to which it is connected.

6. What happens when a series capacitor is inserted to a line to enhance the system stability of an ac transmission system of mesh network?
a) The line with the series capacitor shares more loads than earlier
b) The line with the series capacitor shares lesser loads than earlier
c) The line with the series capacitor shares same load as earlier
d) The original overloaded line gets more loaded
View Answer

Answer: a
Explanation: When a series capacitor is inserted to a line to enhance the system stability of an ac transmission system of mesh network, its effective line impedance reduces by the inherent property of a capacitor. The line with the series capacitor shares more loads than earlier. This happens because electric power-flow is proportional to inverse of transmission line impedance of transmission lines.

7. What happens when a series inductor is inserted to a line to enhance the system stability of an ac transmission system of mesh network?
a) The line with the series inductor shares more loads than earlier
b) The line with the series inductor shares lesser loads than earlier
c) The line with the series inductor shares same load as earlier
d) The original overloaded line gets more loaded
View Answer

Answer: b
Explanation: When a series inductor is inserted to a line to enhance the system stability of an ac transmission system of mesh network, its effective line impedance increases by the inherent property of a inductor. The line with the series inductor shares less loads than earlier. This happens because electric power-flow is proportional to inverse of transmission line impedance of transmission lines.
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8. To enhance the system stability of an ac transmission system of mesh network, a FACT controller can be used taking into consideration ________
a) the continuous ratings (in kW/MW) of all the transmission lines
b) the emergency ratings (in kW/MW) of only the overloaded line
c) the continuous ratings (in kW/MW) of only the overloaded line
d) the continuous ratings (in kW/MW) of only the under loaded line
View Answer

Answer: a
Explanation: To enhance the system stability of an ac transmission system of mesh network, a FACT controller can be used taking into consideration the continuous ratings (in kW/MW) of all the transmission lines; not only the overloaded one or the under rated one. Also, the emergency ratings (in kW/MW) of all the transmission lines along with the allowable time for such conditions, is necessary. This is done to avoid loading of any transmission line beyond its allowable continuous rating while selecting a FACT controller for any line in the mesh network.

9. Which of the following is to be followed to enhance the system stability of the mesh network given below?

a) A series inductor of +2Ω in the BC line
b) A series inductor of +2Ω in the AB line
c) A series inductor of +2Ω in the CD line
d) A series inductor of +2Ω in any line
View Answer

Answer: a
Explanation: To enhance the system stability of the mesh network, it has to avoid loading of any transmission line AB, BC, CD or DA beyond its allowable continuous rating. When a series inductor is inserted to line BC, the effective line impedance of that line increases by the inherent property of an inductor and shares lesser loads than earlier. So out of the given options, BC line is one which can reduce its load sharing within its allowable continuous rating with a series inductor of +2Ω inserted in it.
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10. Which of the following is to be followed to enhance the power transfer capability and improve the stability of the mesh network given below?

a) A series capacitor of -2Ω in the BC line
b) A series capacitor of -2Ω in the AB line
c) A series capacitor of -2Ω in the CD line
d) A series capacitor of -2Ω in any line
View Answer

Answer: c
Explanation: To enhance the power transfer capability and improve the stability of the mesh network, it has to avoid loading of any transmission line AB, BC, CD or DA beyond its allowable continuous rating. When a series capacitor is inserted to line CD, the effective line impedance of that line reduces by the inherent property of a capacitor and shares more loads than earlier. So out of the given options, CD line is one which can enhance its load sharing within its allowable continuous rating with a series capacitor of -2Ω inserted in it.

11. Which of the following is to be followed to enhance the power transfer capability of the mesh network given below?

a) a phase angle regulator in the BC line
b) a phase angle regulator in the AB line
c) a phase angle regulator in the CD line
d) a phase angle regulator in any line
View Answer

Answer: d
Explanation: To enhance the power transfer capability of the mesh network, it has to avoid loading of any transmission line AB, BC, CD or DA beyond its allowable continuous rating. This can be achieved with a phase angle regulator inserted in any line, such that the total phase angle difference along the transmission line is reduced to distribute the load sharing among transmission lines AB, BC, CD or DA within its allowable continuous rating.

12. In the mesh network given below, one FACTs controller is NOT sufficient to improve power transfer capability.

a) True
b) False
View Answer

Answer: b
Explanation: In the mesh network given in the question, one FACTs controller is sufficient to improve power transfer capability. This is because here a FACT controller can be a thyristor-controlled series capacitor or a thyristor-controlled series inductor or a thyristor-controlled phase-angle regulator. Taking into considerations all the necessary parameters we compute the required capacitance or inductance or the phase-angle difference required to incorporate into this system.

13. Does the loading capability of a transmission line depend on weather conditions of the area where it will be erected?
a) Yes
b) No
View Answer

Answer: a
Explanation: The loading capability of a transmission line does depend on weather conditions of the area where it will be erected. This is because the real-time loading capability of a transmission line is limited by its thermal capability which is affected by the ambient temperature of the area where it will be erected, subject to seasonal fluctuations and other weather conditions.

14. The loading capacity of a transmission line is limited by its dielectric.
a) True
b) False
View Answer

Answer: a
Explanation: The loading capacity of a transmission line is limited by its dielectric. This is from the point of view of insulation of the transmission line which is subject to various dynamic and transient over voltages during the power flow. Generally, a given nominal voltage rating of any transmission line is allowed with a fluctuation of ±10% voltage change and accordingly the insulation is computed.

15. Voltage injection method finds its eminent presence in a FACT controller.
a) True
b) False
View Answer

Answer: a
Explanation: Voltage injection method finds its eminent presence in a FACT controller. With the appropriate voltage injection accompanied by its varying magnitude and phase angle, both the active and reactive current flow can be controlled. This principle is widely used in a FACT controller controlling the power transfer capability and stability of a transmission system.

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

To practice all areas of Flexible AC Transmission System (FACTS), here is complete set of 1000+ Multiple Choice Questions and Answers.

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