Power System Analysis PYQs

 Q1 >> A 3-phase, 50 Hz overhead transmission line has inductance per phase 

$L$ and capacitance per phase $C$. The surge impedance $Z_0$ of the line is:
A) $\sqrt{\dfrac{L}{C}}$
B) $\sqrt{\dfrac{C}{L}}$
C) $L/C$
D) $C/L$
ANS: A

Q2 >> In per-unit system, if you change base MVA from $S_{base1}$ to $S_{base2}$ (same base kV), the p.u. reactance $X_{pu2}$ relates to $X_{pu1}$ as:
A) $X_{pu2} = X_{pu1}$
B) $X_{pu2} = X_{pu1}\dfrac{S_{base1}}{S_{base2}}$
C) $X_{pu2} = X_{pu1}\dfrac{S_{base2}}{S_{base1}}$
D) $X_{pu2} = X_{pu1}^2$
ANS: B

Q3 >> For balanced three-phase short-circuit fault at a bus, sequence networks required are:
A) Positive only
B) Positive and negative in series
C) Positive, negative and zero in parallel
D) Positive, negative and zero all separate (not connected)
ANS: A

Q4 >> The Y-bus matrix of a power network is:
A) Symmetric and sparse for passive networks
B) Always diagonal
C) Skew-symmetric
D) Dense and non-symmetric
ANS: A

Q5 >> In Newton-Raphson load flow using polar form, the Jacobian size (number of rows) for an n-bus system with 1 slack bus is:
A) (n−1) × (n−1)
B) 2(n−1) × 2(n−1)
C) (2n) × (2n)
D) n × n
ANS: B

Q6 >> A single line-to-ground (L-G) fault at the receiving end of a line (solid fault) is best modeled using sequence networks connected as:
A) Positive, negative and zero in series
B) Positive and negative parallel, zero separate
C) Positive only
D) All three in parallel to source
ANS: A

Q7 >> The equal-area criterion is applicable to:
A) Small-signal (linear) stability only
B) First-swing transient stability for a single machine infinite bus model
C) Frequency control steady state
D) Load flow convergence
ANS: B

Q8 >> A transmission line is classified as a medium line when its length is approximately:
A) < 80 km (short line)
B) 80–250 km (medium line)
C) > 250 km (long line)
D) Any length (classification irrelevant)
ANS: B

Q9 >> In economic dispatch without losses, the necessary condition for optimality is:
A) All units produce equal real power
B) Incremental cost ($dC/dP$) are equal for all committed units
C) Largest unit loaded to maximum
D) Minimum spinning reserve used
ANS: B

Q10 >> Ferranti effect (voltage rise at receiving end) is most significant for:
A) Short heavily loaded line
B) Long lightly loaded or open-circuited line
C) Transformer with tap changer
D) Underground cable at low voltage
ANS: B

Q11 >> The zero-sequence impedance of a buried neutral overhead line depends mainly on:
A) Line inductance only
B) Earth return path and earth resistivity
C) Positive-sequence impedance only
D) Transformer neutral grounding only
ANS: B

Q12 >> For a synchronous generator connected to infinite bus, steady synchronous operation requires:
A) Mechanical torque = electrical torque (on average)
B) Rotor angle = 0 always
C) Field current = mechanical torque
D) No damping torque required
ANS: A

Q13 >> The per-unit impedance of a transformer referred to secondary side when primary base voltage is different is changed using:
A) Turns ratio (or voltage ratio) only
B) Current ratio only
C) Impedance is invariant under reference change
D) Temperature correction factor
ANS: A

Q14 >> The short-circuit MVA at a bus is inversely proportional to:
A) The per-unit reactance at that bus
B) Base voltage at that bus
C) Base MVA only
D) None of these
ANS: A

Q15 >> In Z-bus building algorithm, adding a new shunt element at bus k updates Z-bus by:
A) A rank-one update (Sherman-Morrison type)
B) Complete re-inversion of Y-bus required every time
C) No change to Z-bus
D) Changing only diagonal element by subtraction
ANS: A

Q16 >> A single generator connected to an infinite bus with small disturbance — the small-signal stability is determined by:
A) Eigenvalues of linearized swing and control equations
B) Equal-area criterion only
C) Load flow Jacobian only
D) Y-bus sparsity
ANS: A

Q17 >> The X/R ratio of a transmission network influences:
A) The magnitude and DC offset of fault currents (symmetrical)
B) Only steady-state load flow results
C) Transformer tap settings exclusively
D) Conductor sag
ANS: A

Q18 >> In sequence network analysis, negative sequence components arise due to:
A) Unbalanced loads or asymmetrical faults
B) Balanced three-phase operation only
C) Zero-impedance paths to earth
D) High power factor loads only
ANS: A

Q19 >> A STATCOM (voltage-source converter based) connected at a bus primarily provides:
A) Steady active power injection only
B) Rapid, controllable reactive power support to regulate bus voltage
C) Permanent frequency control for grid
D) Steady state harmonic elimination only
ANS: B

Q20 >> The maximum power transferable (Pmax) between two equal voltage magnitude nodes through reactance X occurs at δ = 90°, and equals:
A) $\dfrac{V_1 V_2}{X}$
B) $\dfrac{V_1 V_2}{2X}$
C) $\dfrac{V_1^2}{X}$
D) $\dfrac{V_1+V_2}{X}$
ANS: A

Q21 >> In load flow, a PV bus is one where:
A) Active power (P) and voltage magnitude (V) are specified
B) Reactive power (Q) and voltage magnitude (V) are specified
C) P and Q are specified
D) None of these
ANS: A

Q22 >> The Newton-Raphson method converges faster than Gauss-Seidel because:
A) It uses second-order derivatives
B) It uses linear approximation (Jacobian) leading to quadratic convergence near solution
C) It avoids solving linear equations
D) It uses fewer buses
ANS: B

Q23 >> The effect of series compensation (capacitor) on a transmission line is to:
A) Increase the effective series reactance
B) Reduce the effective series reactance and increase power transfer capability
C) Increase corona loss
D) Reduce thermal rating of line
ANS: B

Q24 >> For a three-phase balanced system, symmetrical components consist of:
A) Positive, negative and zero sequences; only positive exists for balanced system
B) Positive and negative only always present
C) Zero sequence only for balanced system
D) None of the above
ANS: A

Q25 >> The breaker fault interrupting rating selection primarily requires knowledge of:
A) Symmetrical and asymmetrical (peak) fault current at point of fault
B) System load factor only
C) Line capacitance only
D) Average daily load
ANS: A

Q26 >> A two-area AGC (automatic generation control) scheme is primarily designed to:
A) Maintain scheduled tie-line power exchange and system frequency after load changes
B) Control reactive power at buses only
C) Reduce harmonic distortion in HVDC links
D) Protect transformers from overload
ANS: A

Q27 >> HVDC back-to-back links between two AC systems are used mainly to:
A) Transfer active power while allowing asynchronous operation and control of power flow
B) Supply only reactive power between systems
C) Replace single phase AC transmission lines
D) Reduce the need for circuit breakers in AC system
ANS: A

Q28 >> In a long transmission line, distributed-parameter model is required because:
A) Shunt capacitance is negligible and can be ignored
B) Line parameters (R, L, C) are distributed continuously along length and wave effects matter
C) Computation is simpler than lumped models
D) Line is always lightly loaded
ANS: B

Q29 >> The participation factor and eigenvalue analysis in small-signal stability help to:
A) Identify modal frequencies, damping and which states (machines/controllers) participate in an oscillatory mode
B) Replace load flow studies entirely
C) Compute switching transients only
D) Determine insulation coordination
ANS: A

Q30 >> Transmission line corona onset voltage reduces if:
A) Conductor radius increases
B) Surface roughness or irregularities increase or air pressure decreases
C) Bundle spacing increases always
D) Frequency decreases only
ANS: B

Q31 >> The Kron reduction technique is mainly used to:
A) Eliminate load buses from network equations
B) Compute symmetrical components
C) Reduce line length in short line models
D) Estimate transmission losses
ANS: A

Q32 >> For a synchronous generator connected to an infinite bus, the rotor angle δ increases suddenly due to a disturbance. For stability, the accelerating area (Pa > Pm) must be followed by:
A) Decelerating area (Pm > Pe) of smaller size
B) Decelerating area of equal size
C) Increasing angle without deceleration
D) Constant power transfer
ANS: B

Q33 >> In per-unit system, transformer impedance remains almost:
A) Invariant with change of base values
B) Dependent on turns ratio
C) Different for primary and secondary
D) Zero if referred to low-voltage side
ANS: A

Q34 >> Which of the following is true for Gauss-Seidel load flow?
A) Requires Jacobian matrix at each iteration
B) Slow convergence but simple implementation
C) Quadratic convergence
D) Best suited for large, sparse systems
ANS: B

Q35 >> Newton-Raphson load flow in polar form is widely used because:
A) Jacobian matrix elements are simple derivatives of P and Q wrt |V| and δ
B) No need of matrix inversion
C) It always converges in one iteration
D) It does not require initial guess
ANS: A

Q36 >> In a short transmission line, which component is neglected?
A) Resistance
B) Inductance
C) Shunt capacitance
D) None
ANS: C

Q37 >> The swing equation is:
A) Nonlinear second-order differential equation
B) Linear first-order equation
C) Algebraic equation
D) Static equation
ANS: A

Q38 >> The inertia constant (H) of a machine is defined as:
A) Kinetic energy stored at synchronous speed per MVA rating
B) Maximum accelerating torque per MVA
C) Electrical energy generated per hour
D) Ratio of rotor mass to torque angle
ANS: A

Q39 >> Which of the following FACTS device is series connected?
A) SVC
B) STATCOM
C) TCSC
D) UPFC (only shunt)
ANS: C

Q40 >> A PV bus in load flow may switch to PQ bus when:
A) Generator reactive power limit is violated
B) Bus voltage becomes zero
C) Active power limit is violated
D) Slack bus fails
ANS: A

Q41 >> Symmetrical components were introduced by:
A) Fortescue
B) Kron
C) Kirchhoff
D) Gauss
ANS: A

Q42 >> Zero sequence current does not flow in:
A) Star-connected system with isolated neutral
B) Delta-connected system
C) Balanced three-phase system
D) All of these
ANS: D

Q43 >> Critical clearing time is:
A) Maximum time allowed to clear a fault without losing synchronism
B) Time to restore steady state
C) Time of oscillation cycle
D) None
ANS: A

Q44 >> The surge impedance loading of a line is equal to:
A) $V^2/Z_c$
B) $V/Z_c$
C) $V^2 \times Z_c$
D) None
ANS: A

Q45 >> In steady-state stability, maximum power transfer occurs when:
A) δ = 90°
B) δ = 180°
C) δ = 0°
D) δ = 45°
ANS: A

Q46 >> Equal area criterion is valid only for:
A) Single machine infinite bus system
B) Multi-machine systems
C) Short transmission line
D) Underground cables
ANS: A

Q47 >> Fault current in a power system is limited by:
A) System reactance
B) System resistance
C) Circuit breakers
D) Relays
ANS: A

Q48 >> Which fault is least severe in terms of fault current magnitude?
A) L-G
B) L-L
C) L-L-G
D) 3-phase fault
ANS: A

Q49 >> In Y-bus matrix, off-diagonal elements are:
A) Negative of line admittance between buses
B) Positive of line impedance
C) Always unity
D) Zero
ANS: A

Q50 >> The diagonal dominance of Y-bus ensures:
A) Stability of load flow solution
B) Symmetry
C) Sparsity
D) None
ANS: A

Q51 >> The iterative methods for load flow are preferred because:
A) Networks are sparse and nonlinear
B) Y-bus is always diagonal
C) Computation cost is higher otherwise
D) None
ANS: A

Q52 >> In Newton-Raphson method, convergence is:
A) Linear
B) Quadratic
C) Cubic
D) Logarithmic
ANS: B

Q53 >> Which of the following affects steady-state stability limit?
A) Line reactance
B) Generator excitation
C) Infinite bus voltage
D) All of these
ANS: D

Q54 >> In economic dispatch, penalty factor > 1 indicates:
A) Transmission loss present
B) No transmission loss
C) Generation at unity power factor
D) None
ANS: A

Q55 >> The DC offset in fault current is due to:
A) X/R ratio of the system
B) Reactive power
C) Fault resistance
D) Corona
ANS: A

Q56 >> For a 3-phase balanced system, negative sequence current produces:
A) Double frequency torque pulsations in rotating machines
B) Constant torque only
C) Zero torque
D) None
ANS: A

Q57 >> The main objective of AGC (Automatic Generation Control) is:
A) Maintain frequency and tie-line power within limits
B) Improve load factor
C) Increase plant efficiency
D) Reduce transformer loss
ANS: A

Q58 >> Which HVDC control mode is generally used?
A) Constant current control at rectifier and constant extinction angle at inverter
B) Constant voltage at both ends
C) Constant power at inverter
D) Constant reactive power
ANS: A

Q59 >> Bundled conductors are used in EHV lines to:
A) Reduce corona and inductance, increase surge impedance loading
B) Reduce resistance
C) Reduce skin effect only
D) Increase losses
ANS: A

Q60 >> Load factor is defined as:
A) Average load / Maximum demand
B) Maximum demand / Connected load
C) Average load / Plant capacity
D) None
ANS: A

Q61 >> Ferranti effect is prominent in:
A) Short transmission lines under heavy load
B) Long lightly loaded transmission lines
C) Medium lines with compensation
D) Cable networks only
ANS: B

Q62 >> Surge impedance of a line is:
A) √(L/C)
B) √(C/L)
C) L/C
D) C/L
ANS: A

Q63 >> The per-unit reactance of large synchronous machines lies typically between:
A) 0.1 to 0.3 pu
B) 0.5 to 1.0 pu
C) 1.0 to 2.0 pu
D) 2.0 to 3.0 pu
ANS: A

Q64 >> The main advantage of per-unit system is:
A) Values are independent of voltage level
B) Easy to compare different machines
C) Reduces calculation complexity
D) All of the above
ANS: D

Q65 >> Infinite bus concept means:
A) Constant voltage and frequency source
B) Large generator with varying frequency
C) Small isolated system
D) System with infinite reactance
ANS: A

Q66 >> For stability studies, the system is represented by:
A) One machine equivalent model
B) Detailed winding model
C) Per-phase equivalent
D) Kron reduction always
ANS: A

Q67 >> In symmetrical fault analysis, sequence networks are connected:
A) In series for 3-phase fault
B) In parallel for L-L fault
C) Positive, negative, zero in series for L-G
D) Positive and negative in parallel for L-L
ANS: A

Q68 >> For an unloaded transmission line, receiving end voltage is:
A) Greater than sending end (Ferranti effect)
B) Less than sending end
C) Equal
D) Zero
ANS: A

Q69 >> The transient stability limit is always:
A) Greater than steady state limit
B) Equal to steady state limit
C) Less than steady state limit
D) Independent of steady state limit
ANS: C

Q70 >> Fast decoupled load flow neglects:
A) Coupling between P–V and Q–δ
B) Coupling between P–δ and Q–V
C) All active power
D) All reactive power
ANS: A

Q71 >> If the fault level at a bus is 2000 MVA, the Thevenin equivalent reactance at that bus (base 100 MVA) is:
A) 0.05 pu
B) 0.1 pu
C) 0.2 pu
D) 0.5 pu
ANS: A

Q72 >> Equal area criterion neglects:
A) Damping
B) Mechanical power
C) Inertia constant
D) Fault clearing
ANS: A

Q73 >> The governor’s primary role in power system is to control:
A) Frequency
B) Voltage
C) Power factor
D) Load factor
ANS: A

Q74 >> The phase shift produced by a 400 kV, 100 km line under load is proportional to:
A) Line reactance
B) Line resistance
C) Line capacitance
D) Surge impedance
ANS: A

Q75 >> Maximum power transfer capability of a line increases with:
A) Higher X
B) Lower X
C) Higher R
D) Lower R
ANS: B

Q76 >> Zero sequence network of a transformer depends on:
A) Winding connections and grounding
B) Core type
C) Voltage ratio
D) Load type
ANS: A

Q77 >> In power flow studies, slack bus is also called:
A) Swing bus
B) Generator bus
C) Load bus
D) Reference bus
ANS: A

Q78 >> In an overhead line, resistance and reactance per unit length:
A) R decreases with conductor size, X depends on spacing
B) Both decrease with conductor size
C) Both depend only on length
D) None
ANS: A

Q79 >> A synchronous condenser is used for:
A) Power factor improvement
B) Active power generation
C) Harmonic filtering
D) Frequency control
ANS: A

Q80 >> If the bus admittance matrix is singular, the system is:
A) Islanded
B) Stable
C) Balanced
D) Overloaded
ANS: A

Q81 >> In transient stability analysis, critical clearing angle is:
A) The maximum rotor angle permissible after fault clearance
B) Angle of voltage drop
C) Zero
D) Infinity
ANS: A

Q82 >> Short-circuit capacity of a bus is equal to:
A) Reciprocal of Thevenin reactance at the bus
B) Inverse of Thevenin resistance
C) Y-bus diagonal term
D) None
ANS: A

Q83 >> In Gauss-Seidel method, acceleration factor is used to:
A) Improve convergence rate
B) Reduce memory
C) Change Jacobian size
D) Eliminate mismatch
ANS: A

Q84 >> Line charging susceptance is due to:
A) Shunt capacitance
B) Series inductance
C) Resistance
D) Skin effect
ANS: A

Q85 >> The base MVA selected in per-unit system affects:
A) Numerical value of impedance but not % impedance
B) Actual current
C) Real power only
D) Frequency
ANS: A

Q86 >> In power system stability studies, damping torque helps in:
A) Reducing oscillations
B) Increasing synchronizing power
C) Reducing load angle
D) Increasing fault level
ANS: A

Q87 >> Load frequency control loop mainly consists of:
A) Speed governor, turbine, generator
B) AVR
C) Synchronous condenser
D) Transmission line
ANS: A

Q88 >> A 3-phase line has self GMR = r and mutual GMR = d. Inductance per conductor depends on:
A) log(d/r)
B) log(r/d)
C) d/r²
D) None
ANS: A

Q89 >> Kron reduction reduces:
A) Size of Y-bus by eliminating load buses
B) Impedance of lines
C) Fault level
D) Swing equation order
ANS: A

Q90 >> HVDC transmission is preferred for:
A) Long distance bulk power transfer
B) Short distance rural feeders
C) Low power only
D) Substation auxiliary supply
ANS: A

Q91 >> The main disadvantage of overhead lines compared to cables is:
A) More corona and line losses
B) Higher cost
C) Difficult maintenance
D) All of the above
ANS: A

Q92 >> The reactance of transmission line is proportional to:
A) log(D/r)
B) log(r/D)
C) D/r²
D) 1/D
ANS: A

Q93 >> Which type of bus does NOT have specified P and Q?
A) Slack bus
B) PV bus
C) PQ bus
D) None
ANS: A

Q94 >> In economic load dispatch without considering transmission losses, λ (incremental cost) is:
A) Same for all units
B) Different for each unit
C) Zero
D) Maximum for largest unit
ANS: A

Q95 >> The surge impedance loading of a 400 kV line is approximately:
A) 600 MW
B) 1000 MW
C) 2000 MW
D) 4000 MW
ANS: B

Q96 >> Corona loss decreases with:
A) Smooth conductors and higher spacing
B) Rough conductors
C) Increased frequency
D) Higher humidity
ANS: A

Q97 >> A line has resistance R and reactance X. For maximum power transfer, load power factor angle should be:
A) Equal to line impedance angle
B) Zero
C) Unity
D) 90° lagging
ANS: A

Q98 >> Under transient conditions, rotor angle stability is mainly affected by:
A) Acceleration power (Pm–Pe)
B) Reactive power only
C) Damping
D) Excitation alone
ANS: A

Q99 >> The iterative load flow solution terminates when:
A) Power mismatch is below tolerance
B) Y-bus becomes singular
C) All bus voltages are unity
D) Slack bus is eliminated
ANS: A

Q100 >> For a given system, if the inertia constant doubles, the frequency of rotor oscillations will:
A) Decrease
B) Increase
C) Remain unchanged
D) Become infinite
ANS: A