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NHPC-JE’21 EE: Daily Practices Quiz 22-Sep-2021

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NHPC-JE’21 EE: Daily Practices Quiz

NHPC-JE’21 EE: Daily Practices Quiz 22-Sep-2021

Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 06
Time: 06 min.

Q1. In the HVDC system, the ac harmonics which get(s) effectively eliminated with 12 pulse bridge converters, are:
(a) Triplen harmonics
(b) Triplen and fifth harmonics
(c) Triplen, fifth and seventh harmonics
(d) Fifth and seventh harmonics

Q2. HVDC transmission systems generally employ:
(a) 3 pulse converters
(b) 6 pulse converters
(c) 12 pulse converters
(d) Either 12 or 6 pulse converters

Q3. In a transformer maximum flux density is not constant, the eddy current loss varies as
(a) f
(b) v
(c) f^2
(d) v^2

Q4. A 3-phase stepper motor has a 6 stator poles and 4 rotor poles, then the step angle is?
(a) 10⁰
(b) 30⁰
(c) 60⁰
(d) 40⁰

Q5. A mho relay is used for protection of:
(a) Protection of a transformer against external fault
(b) Long Transmission Line
(c) Protection of a transformer against all the internal faults and external fault
(d) Medium Length lines

Q6. Condition for maximum power delivered in D.C. Generator is_______:
(a) Back EMF = (2 × Supply voltage)
(b) Back EMF = Supply voltage
(c) Back EMF = (Supply voltage/4)
(d) Back EMF = (Supply voltage/9)

SOLUTIONS

S1. Ans.(d)
Sol. In the HVDC system, the ac harmonics which get(s) effectively eliminated with 12 pulse bridge converters, are (6N±1)….where N=1,2……………
i.e., N=1, harmonics eliminated: (6×1±1)=5^th and 7^th.

S2. Ans.(c)
Sol. HVDC transmission systems generally employ 12 pulse converters.

S3. Ans.(d)
Sol. Eddy current loss (Pe ) α ke Bm^2 f^2
And we know, Bm α v/f
If Bm=constant, then eddy current loss (Pe ) αf^2
If Bm is not constant, then eddy current loss (Pe ) α v^2

S4. Ans.(b)
Sol. Step angle(β)=((Ns-Nr)/(Ns×Nr ))×360^0=((6-4)/(6×4))×360^0=30⁰

S5. Ans.(b)
Sol. Mho relay is used for protection of long transmission line.
Important point of mho relay……
Mho relay is a high-speed relay and is also known as admittance relay.
Mho relay comes in the category of the distance relay protection scheme.
Mho Relay is less affected by the power swing.

S6. Ans.(a)
Sol. Condition for max. power transfer:
Eb=2V ….for generator
Eb=V/2…..for motor

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