# Power Electronics Questions and Answers – Applications of Power Electronics-2

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “Applications of Power Electronics-2”.

1. HVDC transmission lines are __________ as compared to HVAC lines.
a) difficult to erect
b) more expensive for long distances
c) more expensive for short distances
d) less expensive for short distances

Explanation: HVDC lines require additional cost of the converting and inverting equipments, hence they are more expensive for short distances as compared to HVAC lines.

2. In HVDC transmission lines
a) both the stations operate as an inverter
b) both the stations operate as a converter
c) one acts as a converter and other as an inverter
d) depends upon the type of the load

Explanation: The supply side station is the converting station, power is transmitted in dc then again converted back to ac at the inverting or receiving station further which it is distributed in ac to houses.

3. Two six pulse converters used for bipolar HVDC transmission system, are rated at 1000 MW, +- 200 kV. What is the dc transmission voltage?
a) 200 kV
b) 400 kV
c) 500 kV
d) 100 kV

Explanation: As the link is bipolar, the total voltage transmitted will 200 + 200 kV.

4. Two six pulse converters used for bipolar HVDC transmission system, are rated at 1000 MW, +- 200 kV. Find the dc current in the transmission line.
a) 500 A
b) 25 A
c) 2500 A
d) 5 A

Explanation: Transmission voltage = 200 + 200 = 400 kV.
I = P/V = 1000 mW/ 400 kV = 2500 A.

5. Two six pulse converters used for bipolar HVDC transmission system, are rated at 1000 MW, +- 200 kV. Find the rms current rating required for the SCRs.
a) 2500 A
b) 1350 A
c) 1445 A
d) none of the mentioned

Explanation: Transmission voltage = 200 + 200 = 400 kV.
I = P/V = 1000 mW/ 400 kV = 2500 A.
As in 3-phase full converter each SCR conducts for 120° for a total period of 360°.
Irms = 2500 x √(120/360) = 1443.4 A. Nearly about 1445 A as it is a standard current rating.
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6. For high power applications _________ are used as static switches whereas for low power applications __________ are used.
a) Transistors, SCRs
b) SCRs, transistors
c) Diodes, transistors
d) SCRs, diodes

Explanation: As SCR are of higher rating they are preferred in high power applications.

7. _________ can be used as a single phase static ac switch.
a) Diode
b) SCR
c) DIAC
d) TRAIC

Explanation: SCR cannot be used, as it is unidirectional. Diode isn’t a switch nor is the DIAC.

8. ___________ can be used as a dc static switch.
a) GTO
b) Transistor
c) Both GTO and transistor
d) TRIAC

Explanation: Both GTO and transistor can be used as a dc static switch.

9. A single-phase ac switch is used in between a 230 V source and load of 2 kW and 0.8 lagging power factor. Determine the rms current rating required by the SCR. Use the factor of safety = 2.
a) 10.87 A
b) 87 A
c) 21.74 A
d) 32 A

Explanation: I = (200√2)/(230 x 0.8) = 15.37 A.
I(rms) = 15.37/1.414 = 10.87
Required rating = 10.87 x factor of safety = 15.37 x 2 = 21.74 A.

10. Solid State Relays (SSRs) have
a) moving parts
b) no moving parts
c) a coil
d) a contactor

Explanation: SSRs have no moving parts, they simply consist of a LED and a transistor or photo diode.

Sanfoundry Global Education & Learning Series – Power Electronics.

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