This set of FACTS Multiple Choice Questions & Answers (MCQs) focuses on “Power Semiconductor Devices – Thyristor, GTO and MTO”.
1. What is the full form of GTO?
a) Gate turned on
b) Gate turned off
c) Gallium transistor open
d) Gallium transistor off
View Answer
Explanation: The full form of GTO is gate turned off. Actually it indicates gate turned off thyristor. Based on gate turnoff (GTO) thyristors, the Static Compensator (STATCOM) is constructed. On the other hand the Static VAR Compensator (SVC) is assembled based on conventional thyristors.
2. Thyristor with no gate turn-off capability is employed in ________
a) current sourced converter
b) voltage sourced converter
c) both current and voltage sourced converter
d) neither current nor voltage sourced converter
View Answer
Explanation: Thyristor with no gate turn-off capability is employed in current sourced converter which is economical for HVDC schemes. Such current sourced converters employing thyristor with no gate turn-off facility only consume reactive power. They are incapable of supplying reactive power.
3. Thyristor with no gate turn-off capability is employed in ________
a) current sourced converter
b) voltage sourced converter
c) both current and voltage sourced converter
d) neither current nor voltage sourced converter
View Answer
Explanation: Thyristor with gate turn-off capability is employed in voltage sourced converter which finds applications in FACTS. Such voltage sourced converters employing thyristor with gate turn-off facility are also capable of supplying reactive power.
4. MTO is the abbreviation of ________
a) Metal-Oxide Semiconductor Turn-on Thyristor
b) Metal-Oxide Semiconductor Turn-off Thyristor
c) Metal-Oxide Semiconductor Turn-off Transistor
d) Metal-Oxide Semiconductor Turn-on Transistor
View Answer
Explanation: MTO is the abbreviation of Metal-Oxide Semiconductor Field Effect Transistor. MTO is basically a combination of a GTO and MOSFET. This underlying principle is to overcome the limitations of the GTO. like snubber circuits, dv/dt limitations, etc.
5. A turn-on ________ signal is applied to the gate electrode of a thyristor for conduction.
a) time
b) current
c) voltage
d) current or voltage
View Answer
Explanation: A turn-on current or voltage signal is applied to the gate electrode of a thyristor for conduction. Thus it displays inherently gate controlled turning–on characteristics. The thyristor starts conduction immediately in a forward direction when a trigger current pulse is passed from gate to cathode.
6. A thyristor is ________ device.
a) only symmetrical
b) either symmetrical or asymmetrical
c) only asymmetrical
d) neither symmetrical nor asymmetrical
View Answer
Explanation: A thyristor may be a symmetrical or asymmetrical device. The thyristor on getting a trigger current pulse from gate to cathode, quickly starts full conduction with a low forward voltage drop. When the circuit current drops to zero, the thyristor recovers quickly in reverse blocking voltage; thereafter it can block the forward voltage until the next turn-on pulse is applied.
7. Thyristors are designed with ________ capability.
a) only gate-controlled turn-on
b) only gate-controlled turn-off
c) only time-controlled turn-on or turn-off
d) gate-controlled turn-on or turn-off
View Answer
Explanation: Some thyristors are designed with only gate-controlled turn-on capability. Also some thyristors are designed with both gate-controlled turn-on and turn-off capability. This choice of controllability depends on the requirement of the design.
8. The SCR cannot force its current back to zero.
a) True
b) False
View Answer
Explanation: The SCR or conventional thyristor cannot force its current back to zero. To come to its non-conduction state from being latched to full conduction, it has to rely on the circuit connected to it. When the current in the connected circuit comes to zero, it gets turn-off.
9. GTO will not turn off when the current in the circuit naturally comes to zero.
a) True
b) False
View Answer
Explanation: GTO (like a conventional thyristor) will definitely turn off when the current in the circuit naturally comes to zero. Actually GTO also has gate-controlled turn-off capability. So when a turn-off pulse is applied to the gate in reverse direction GTO turns off also.
10. GTO does not turn on in a fully conducting mode, when a turning-on current pulse is given to its gate with respect to its cathode.
a) True
b) False
View Answer
Explanation: GTO (like a conventional thyristor) turns on in a fully conducting mode when a turning-on current pulse is given to its gate with respect to its cathode. In its latched mode, it also has a low forward voltage drop. In addition to its gate-controlled turn-off capability, the GTO will also turn off when the current in the associated circuit attains a zero crossing.
11. MTO uses ________ to assist itself in turning-off.
a) transistors
b) diodes
c) resistors
d) capacitors
View Answer
Explanation: MTO uses transistors to assist itself in turning-off. By doing so, MTO succeeds to achieve fast turning-off ability. Also transistor thus empowers MTO with low turning-off switching losses.
ETO incorporates low voltage transistors in series with a high voltage GTO to achieve fast turn-off and low turn-off switching losses.
12. CMOS devices produce much waste heat than NMOS.
a) True
b) False
View Answer
Explanation: CMOS devices DO NOT produce much waste heat than NMOS. Actually in CMOS technology one transistor of the MOSFET pair is always off. Therefore, the series combination configuration draws major power only for a short time during between switching-on and switching-off.
13. A diode is characterized by its latching current.
a) True
b) False
View Answer
Explanation: A diode is NOT characterized by its latching current. A thyristor is characterized by its latching current. It indicates the minimum current above which the thyristor can start to conduct in forward direction.
14. A diode has npnp layers.
a) True
b) False
View Answer
Explanation: A diode DOES NOT have npnp layers. Structurally a thyristor has npnp layers. Diode is basically a pn junction device. Instead of having these n and p layers, they are electrically neutral fundamentally.
15. A transistor is characterized by its holding current.
a) True
b) False
View Answer
Explanation: A diode is NOT characterized by its holding current. A thyristor is characterized by its holding current. It indicates the maximum current below which the thyristor ceases to conduct and blocks in forward direction.
Sanfoundry Global Education & Learning Series – Flexible AC Transmission System (FACTS).
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