Switchgear and Protection MCQs Part 10

Switchgear and Protection MCQs Part 10

An arc is produced when switch having high voltage and large current circuit is

An arc is produced when switch having high voltage and large current circuit is

1. Closed
2. Opened
3. Either of above
4. None of above

Correct answer: 2. Opened

A circuit breaker is rated as 1500 A, 1000 MVA, 33 kV, 3 second, 3 phase oil circuit breaker. The short time rating is

A circuit breaker is rated as 1500 A, 1000 MVA, 33 kV, 3 second, 3 phase oil circuit breaker. The short time rating is:

1. 15236.66 A for 3 seconds
2. 20994.55 A for 3 seconds
3. 22342.48 A for 3 seconds
4. 25669.55 A for 3 seconds

Correct answer: 2. 20994.55 A for 3 seconds

Solution: Rated symmetrical breaking current = (1200 * 10^6)/(√3 * 33 * 10^3) = 20994.55 A for 3 seconds

The rated secondary current of current transformer for relays is usually

The rated secondary current of current transformer for relays is usually:

1. 2.5 A
2. 5 A
3. 10 A
4. 20 A

Correct answer: 2. 5 A

Which of following is correct about back-up protection

Which of following is correct about back-up protection:

1. It operates when primary protection fails
2. It is second line of defense
3. When back-up protection operates a large portion of system disconnects
4. All of these

Correct answer: 4. All of these

A 60 Hz, 33 kV, 3-phase alternator with earthed neutral has reactance of 5 ohms per phase and is connected to bus-bar through a circuit breaker

A 60 Hz, 33 kV, 3-phase alternator with earthed neutral has reactance of 5 ohms per phase and is connected to bus-bar through a circuit breaker. Distributed capacitance upto circuit breaker between phase and neutral is 0.01 µF. The frequency of oscillations is:

1. 12500 Hz
2. 13852 Hz
3. 14990 Hz
4. 15672 Hz

Correct answer: 2. 13852 Hz

Solution:

Inductance per phase L = XL/2πf = 5/(2π * 60) = 0.0132 H

Capacitance per phase, C = 0.01 µF = 0.01 * 10^-6 F

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Frequency of oscillations

f = 1/2π√LC

f = 1/2π√0.0132 H * 0.01 µF = 13852 Hz