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NHPC-JE’21 EE: Daily Practices Quiz
NHPC-JE’21 EE: Daily Practices Quiz 24-Sep-2021
Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 06
Time: 06 min.
Q1. Parallel branch elements can be found out in a 1-phase transformer by conducting
(a) Open circuit test
(b) Short circuit test
(c) Both SC and OC test
(d) None of the above
Q2. 100 % string efficiency means
(a) Zero potential across each disc
(b) One of the insulator discs shorted
(c) Equal potential across each insulator disc
(d) None of the above
Q3. What is the approximate efficiency of a normal thermal power station?
(a) 20-25 %
(b) 30-40 %
(c) 45-55 %
(d) 60-70 %
Q4. To minimize the current chopping tendency, the SF6 gas is used at
(a) Low velocity and low pressure
(b) High velocity and high pressure
(c) Low velocity and high pressure
(d) High velocity and low pressure
Q5. As per Indian Electricity rule in overhead systems, the recommended span in case of RCC pole is….
(a) 40-50 meters
(b) 50-80 meters
(c) 80-100 meters
(d) 100-300 meters
Q6. Voltage regulation due to the Ferranti effect may be:
(d) All of the above
Sol. For determining shunt branch elements (R_c,x_m), we perform OC test at rated voltage.
Sol. Due to formation of stray capacitance, unequal potential distribution occurs across discs which is expressed in terms of string efficiency.
If the string efficiency is 100 % then it means that equal potential difference occurs across each insulator disc.
Sol. Efficiency of a normal thermal power station≃ 30-40 %.
Sol. Due to low velocity and pressure employed in SF_6 CBs, current chopping is prevented and capacitive currents are interrupted without restriking.
Sol. RCC poles: span=80-100 meters
|Line support||Recommended span|
|Wooden pole||40-50 meters|
|Steel pole||50-80 meters|
|RCC pole||80-100 meters|
|Steel tower||300 meters or above|
Sol. Voltage regulation: (VS-VR)/VR
Due to Ferranti effect: VS<VR
Hence, voltage regulation is negative in case of Ferranti effect.