A capacitor bank rated 50 MVAR at 400 kV is to be switched in a power system. Which device is most suitable for controlled switching to avoid transient overvoltages?
Answer: C
Pre-insertion resistors and reactors limit the inrush current and transient overvoltages when capacitor banks are switched, ensuring smooth energization.
Q.82Medium
In the Indian power system, the frequency regulation bandwidth for Load Frequency Control (LFC) is typically maintained within:
Answer: A
As per CEA guidelines and recent Grid Code specifications, LFC keeps the system frequency within ±0.2 Hz of 50 Hz nominal frequency to ensure quality power supply.
Q.83Medium
A 50 MW, 11 kV, 50 Hz synchronous generator has an inertia constant H = 6 seconds. Calculate the kinetic energy stored in the rotor at rated speed.
Answer: B
KE = H × S where S = rated MVA. KE = 6 × 50 = 300 MJ. (H is defined as H = KE/MVA)
Q.84Medium
In a multi-machine power system stability study, which parameter is used to measure relative rotor angle instability between two generators?
Answer: B
The stability of a multi-machine system depends on relative rotor angles between machines. Loss of synchronism occurs when relative rotor angle exceeds critical limits.
Q.85Medium
A STATCOM (Static Synchronous Compensator) is installed in a 220 kV network. In what mode should it operate to provide reactive power support during a voltage sag?
Answer: D
During voltage sag, STATCOM operates as a voltage source injecting capacitive reactive power (leading current) to support and restore the system voltage.
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Q.86Medium
In selective coordination of protective devices, what is the minimum time interval required between the operation of primary and backup fuses as per Indian standards?
Answer: C
IS 1885 and IEEE standards recommend a minimum coordination time interval of 0.5 seconds between protective device operations to ensure selectivity.
Q.87Medium
A transmission line is protected by a distance relay set with Zone 2 reach at 80% of line length. If the line is 150 km and has impedance of 0.4 Ω/km, what is the Zone 2 reach impedance?
Answer: C
Total line impedance = 150 × 0.4 = 60 Ω. Zone 2 reach = 80% × 60 = 48 Ω
Q.88Medium
A DC shunt motor is running at no load. If the field winding is suddenly short-circuited, the motor will:
Answer: B
Flux reduces drastically when field is short-circuited. As back EMF reduces, armature current increases causing motor speed to increase dangerously.
Q.89Medium
The efficiency of a transformer is maximum when:
Answer: A
Transformer efficiency is maximum when core loss (Pi) equals copper loss (Pcu), i.e., Pi = Pcu.
Q.90Medium
The starting current of a 3-phase induction motor is typically how many times the full-load current?
Answer: B
Induction motors draw 4-7 times full-load current during starting because rotor speed is zero and rotor resistance appears as reactance.
Q.91Medium
Which method is NOT used for starting a three-phase induction motor?
Answer: D
Standard starting methods are DOL, star-delta, autotransformer, and rotor resistance. Frequency variation is used for speed control, not starting.
Q.92Medium
A DC series motor is suitable for:
Answer: B
DC series motors have high starting torque (proportional to load current) and are used in electric vehicles, cranes, and hoists.
Q.93Medium
The power factor of an induction motor at full load is typically:
Answer: C
Induction motors operate at lagging power factor (0.85-0.95) due to reactive magnetizing current required for flux production.
Q.94Medium
What is the relationship between slip and rotor frequency in an induction motor?
Answer: A
Rotor frequency fr = slip × stator frequency = s × fs, where fs = 50 Hz for 50 Hz supply.
Q.95Medium
In a synchronous motor, the damper winding is used to:
Answer: B
Damper windings provide asynchronous starting torque to accelerate the rotor near synchronous speed before synchronous operation begins.
Q.96Medium
A 100 kVA transformer has 1000 primary turns and 100 secondary turns. The turns ratio and voltage ratio are:
Answer: A
Turns ratio k = N2/N1 = 1000100 = 0.1. Voltage ratio V1/V2 = N1/N2 = 110 or 10:1 (step-down).
Q.97Medium
A DC shunt motor shows increased speed. What could be the reason?
Answer: C
Speed = (V - IaRa)/(kΦ). Decreasing field current reduces flux Φ, increasing motor speed.
Q.98Medium
An induction motor operating at slip s = 0.04 has rotor frequency of 2 Hz. Find the supply frequency.
Answer: B
Rotor frequency = s × f. Therefore, 2 = 0.04 × f, giving f = 50 Hz
Q.99Medium
Which winding in a synchronous motor provides the initial torque for starting?
Answer: B
The damper winding (squirrel cage) in a synchronous motor provides the initial asynchronous torque for starting until synchronism is achieved.
Q.100Medium
A 50 kVA, 2302300 V transformer has a primary resistance of 2Ω. Calculate the equivalent primary resistance referred to secondary side.
