# Power Electronics Questions and Answers – Interconnecting Thyristors

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This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “Interconnecting Thyristors”.

1. SCRs are connected in parallel to fulfill the ___________ demand
a) high voltage
b) high current
c) size
d) efficiency

Explanation: Number of devices connected in parallel can carry huge amounts of current.

2. The term used to measure the degree of utilization of SCRs connected in series & parallel is
a) tuf
b) string efficiency
c) voltage/current utilization ratio
d) rectification efficiency

Explanation: String Efficiency = Rating of the whole string/(rating of one SCR x number of SCRs)

3. To have maximum possible string efficiency
a) SCRs of same rating must be used
b) SCRs with similar V-I characteristics must be used
c) SCRs with the same dimensions must be used
d) SCRs with similar thermal characteristics must be used

Explanation: Having similar ratings does not mean they have similar charc.
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4. For a string voltage of 3300 V, let there be six series connected SCRs each of voltage 600V. Then the string efficiency is
a) 99.36 %
b) 91.7 %
c) 98.54 %
d) 96 %

Explanation: String efficiency = 3300/(6 x 600) = 98.54.

5. The measure of reliability of string is given by the factor
a) DRF = 1 – String efficiency
b) DRF = 1 + String efficiency
c) DRF = String efficiency – 1
d) DRF = String efficiency x 2

Explanation: DRF is de-rating factor given by the above expression.

6. When an extra SCR is connected in series with a string
a) DRF decreases
b) DRF increases
c) DRF remains constant
d) None of the mentioned

Explanation:
DRF = 1 – String efficiency
String Efficiency = Rating of the whole string/(rating of one SCR x number of SCRs)
Extra SCR will reduce the string efficiency which in turn increase the DRF.

7. The most practical way of obtaining a uniform distribution of series connected SCRs is to
a) connect a resistor of value R in series with each of the series connected SCRs
b) connect a resistor of value R in parallel with each of the series connected SCRs
c) connect a resistor of value R in series with one of the series connected SCRs
d) connect a resistor of value R in parallel with one of the series connected SCRs

Explanation: For uniform distribution of voltage across series connected SCRs, a resistor of value R in parallel with each series connected SCR.

8. 3 SCRs are connected in series. The string efficiency is 91%. SCRs 1, 2 & 3 have leakage currents 4 mA, 8 mA & 12 mA. Which SCR will block more voltage?
a) SCR-1
b) SCR-2
c) SCR-3
d) All the three will block equal voltage

Explanation: The SCR with lower leakage current block more voltage.

9. Two parallel connect SCRs have same voltage drop (Vt) having rated current = 2I1. SCR-1 carries a current of I1=2.6 A whereas SCR-2 carries a current of I2=1.4 A. Find the string efficiency.
a) 45 %
b) 77 %
c) 92 %
d) 84 %

Explanation: The total current would be I1+I2 & rated current is 2I1
String efficiency = (I1+I2)/2I1.

10. SCRs with a rating of 1000 V & 200 A are available to be used in a string to handle 6 KV & 1 KV. Calculate the number of series & parallel units required in case the de-rating factor is 0.1. (Round off the fraction to the greatest & nearest integer)
a) Series = 7, Parallel = 6
b) Series = 6, Parallel = 7
c) Series = 6, Parallel = 6
d) Series = 7, Parallel = 7

Explanation: DRF = 1-S.E
Therefore
0.1 = (1-6000/1000Ns) = (1-1000/200Np)
Ns = 6.6 = 7(say)
Np = 5.5 = 6(say)

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