What is the critical clearing angle in transient stability analysis?
Answer: A
Critical clearing angle is the maximum rotor angle at fault clearing beyond which the system loses synchronism. It determines the maximum fault clearing time.
Q.22Hard
What is the primary mechanism of voltage instability in power systems?
Answer: A
Voltage instability occurs when reactive power demand exceeds generation and compensation capabilities, leading to progressive voltage decline and potential collapse.
Q.23Hard
Which formula represents the swing equation for a synchronous machine during a disturbance?
Answer: C
The swing equation 2H(d²δ/dt²) = Pm - Pe - D(dδ/dt) describes rotor angle dynamics, where H is inertia constant, Pm is mechanical power, Pe is electrical power, and D is damping coefficient.
Q.24Hard
As per IEEE 1588-2019 standard, what is the typical accuracy of a Phasor Measurement Unit (PMU) for frequency measurement in power systems?
Answer: B
IEEE 1588-2019 specifies ±0.01 Hz accuracy for steady-state frequency measurement in PMUs used for power system monitoring.
Q.25Hard
A 132 kV transmission line has a shunt capacitance of 0.008 µF/km and length of 200 km. Calculate the total reactive power generated by the line capacitance at steady state (assuming 132 kV operating voltage).
Answer: C
QC = (V²/Xc) where Xc = 1/(ωC), Total C = 0.008 × 200 = 1.6 µF, Qc = (132000)² × 2π × 50 × 1.6×10^-6 = 256.7 MVAR
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Q.26Hard
In the context of transient stability, a generator loses synchronism when:
Answer: B
Loss of synchronism occurs when the generator rotor continuously accelerates because decelerating torque never becomes greater than accelerating torque, preventing the rotor from returning to synchronous speed.
Q.27Hard
Which of the following correctly represents the relationship between the steady-state stability limit and transient stability limit?
Answer: B
The transient stability limit is always greater than the steady-state stability limit. A system can maintain transient stability even after a disturbance that would violate steady-state stability.
Q.28Hard
In a power system experiencing voltage instability, the reactive power support margin becomes critical. At what reactive power reserve margin should emergency measures be initiated according to Indian Grid Code?
Answer: C
Indian Grid Code 2023 specifies that when reactive power reserves fall below 15% of the available reactive power limit, emergency corrective measures must be initiated to prevent voltage collapse.
Q.29Hard
The torque developed in a 3-phase induction motor is maximum at a slip of:
Answer: C
Maximum torque occurs at slip sm = R2/X2. This is crucial for motor design and performance analysis.
Q.30Hard
An 8-pole induction motor running at 720 rpm with 50 Hz supply has a slip of approximately:
Answer: A
Ns = 120×850 = 750 rpm. Slip s = (750-720)/750 = 75030 = 0.04 or 4%.
Q.31Hard
In a DC machine, armature reaction causes:
Answer: B
Armature reaction is caused by armature MMF which distorts the main magnetic field, reduces net flux, and causes commutation issues. Compensation is needed through commutating poles.
Q.32Hard
The equivalent circuit of an induction motor includes a series resistance R2/s in the rotor branch. The power dissipated in this resistance represents:
Answer: B
In equivalent circuit, voltage across R2/s is rotor voltage and current through it gives mechanical power = I2² × (R2/s - R2) = mechanical power developed.
Q.33Hard
In a synchronous motor, the pull-out torque depends on:
Answer: C
Pull-out torque τmax = 1.5(PΦIf)/(ωs), depends on pole flux and field current (excitation) and supply voltage.
Q.34Hard
A transformer is operating at 110% of rated voltage. The core loss will be approximately:
Answer: C
Core loss Pc ∝ B² ∝ V². At 1.1V, loss = (1.1)² × rated loss = 1.21 × rated ≈ 21% increase
Q.35Hard
The efficiency of a DC generator is maximum when:
Answer: C
Maximum efficiency occurs when variable losses (copper) equal constant losses (friction, windage, and core losses).
Q.36Hard
A 100 kW, 3-phase induction motor has a full-load slip of 0.04. The rotor copper loss at full load is:
Answer: B
Rotor copper loss = s × Pg, where Pg is air gap power. At full load, assuming 96% efficiency, losses ≈ 4.17 kW ≈ 4 kW
Q.37Hard
The reluctance torque in a synchronous motor is maximum when the load angle is:
Answer: B
Reluctance torque τrel ∝ sin(2δ), which is maximum when sin(2δ) = 1, i.e., when δ = 45°
Q.38Hard
Which statement is correct regarding the torque-slip characteristic of a squirrel-cage induction motor?
Answer: C
For induction motor, maximum torque occurs at slip s_max = R2/X2, where R2 and X2 are rotor resistance and reactance. This is derived from dT/ds = 0.
Q.39Hard
A 3-phase, 50 Hz, 4-pole synchronous motor operates at rated speed. The excitation is reduced by 20%. What happens?
Answer: B
Synchronous motor speed depends only on frequency and poles. Reducing excitation causes the motor to draw more lagging reactive current to maintain synchronism, operating at lagging power factor.
Q.40Hard
In a synchronous generator connected to an infinite bus, if the excitation is increased beyond rated value:
Answer: C
In an infinite bus system, frequency and terminal voltage are fixed by the bus. Increasing excitation beyond rated value causes the generator to draw leading reactive power (operating as a synchronous condenser).
