COREN Engineering Practice Exam — Electrical/Electronic Engineering Specialisation (Nigeria)

A series RLC circuit has resistance R = 10 ohms, inductance L = 0.1 H, and capacitance C = 10 microfarads. What is the resonant frequency (f_r) of the circuit and its quality factor (Q)?

• A159.2 Hz, 10.0
• B159.2 Hz, 100.0
• C503.3 Hz, 10.0
• D1000.0 Hz, 100.0

In a linear active bilateral network, the open-circuit voltage across two terminals is 24 V. When a 12 ohm resistor is connected across these terminals, the voltage drops to 16 V. Find the Thevenin equivalent resistance (R_th) of the network.

• A4 ohms
• B6 ohms
• C8 ohms
• D12 ohms

An AC voltage source with an internal impedance of Z_s = (8 + j6) ohms delivers power to a load. For maximum power transfer to the load, what must be the load impedance (Z_L)?

• A8 + j6 ohms
• B8 - j6 ohms
• C10 ohms
• D14 ohms

A DC voltage source of 10 V is applied to a series RL circuit with R = 5 ohms and L = 2 H at t = 0. What is the expression for the current i(t) for t >= 0?

• Ai(t) = 2(1 - e^(-2.5t)) A
• Bi(t) = 2(1 - e^(-0.4t)) A
• Ci(t) = 2 * e^(-2.5t) A
• Di(t) = 10(1 - e^(-0.4t)) A

A symmetric T-network has series arm resistances R1 = R2 = 4 ohms, and a shunt arm resistance R3 = 10 ohms. Find the open-circuit impedance parameters Z_11 and Z_12 of this two-port network.

• AZ_11 = 4 ohms, Z_12 = 10 ohms
• BZ_11 = 14 ohms, Z_12 = 10 ohms
• CZ_11 = 14 ohms, Z_12 = 4 ohms
• DZ_11 = 10 ohms, Z_12 = 14 ohms

Three identical resistors of 18 ohms are connected in a delta configuration. What is the value of each equivalent resistor if they are transformed into a star (Y) configuration?

• A54 ohms
• B9 ohms
• C6 ohms
• D18 ohms

A node in a circuit has a voltage V1. Connected to this node are a 2 A independent current source flowing into the node, a 4 ohm resistor connected to ground, and a dependent current source of value 0.5 * V1 flowing out of the node. Calculate V1.

• A1.6 V
• B2.67 V
• C8.0 V
• D4.0 V

In a linear circuit containing an independent voltage source V = 12 V and an independent current source I = 3 A, the current through a branch is measured. When V acts alone, the branch current is 1.5 A. When I acts alone, the branch current is -0.5 A. What is the total branch current when both sources act simultaneously?

• A1.0 A
• B2.0 A
• C1.8 A
• D3.0 A

A load consumes 100 kW at a power factor of 0.8 lagging from a 240 V, 50 Hz single-phase supply. What value of shunt capacitor is required to raise the power factor to unity?

• A4.14 millifarads
• B1.38 millifarads
• C2.76 millifarads
• D0.69 millifarads

In a balanced three-phase system, two wattmeters are used to measure the total power. The readings of the two wattmeters are W1 = 5 kW and W2 = 2.5 kW. What is the power factor of the system?

• A0.500
• B0.866
• C0.707
• D0.982

A periodic voltage waveform is defined as v(t) = 10 + 5 * sin(314t) V. Calculate the root-mean-square (RMS) value of this voltage.

• A15.0 V
• B10.6 V
• C11.2 V
• D12.5 V

A second-order RLC series circuit has a damping ratio (zeta) defined by its components. If R is doubled while L and C remain constant, what happens to the damping ratio?

• AIt is halved
• BIt is doubled
• CIt increases by a factor of 4
• DIt remains unchanged

Two coils with inductances L1 = 4 H and L2 = 9 H have a coupling coefficient of k = 0.5. What is the mutual inductance (M) between the coils?

• A1.5 H
• B3.0 H
• C6.5 H
• D13.0 H

According to Norton's theorem, any linear active two-terminal network can be replaced by an equivalent circuit consisting of a current source I_n in parallel with an admittance Y_n. If the open-circuit voltage is V_oc and the short-circuit current is I_sc, how is Y_n calculated?

• AY_n = V_oc * I_sc
• BY_n = V_oc / I_sc
• CY_n = I_sc / V_oc
• DY_n = 1 / (V_oc * I_sc)

In network graph theory, if a graph has N nodes and B branches, what is the number of fundamental loops (mesh equations) required to analyze the circuit?

• AB - N + 1
• BB - N
• CN - 1
• DB + N - 1

A 100 kVA, 11000/400 V, 50 Hz single-phase transformer has a full-load copper loss of 1200 W and an iron loss of 800 W. At what percentage of full-load kVA will the transformer operate at maximum efficiency?

• A66.7%
• B81.6%
• C50.0%
• D90.5%

Which of the following statements correctly identifies the parameters obtained from the open-circuit and short-circuit tests on a transformer?

• AOpen-circuit test gives winding resistance; short-circuit test gives core loss.
• BOpen-circuit test gives core loss and magnetizing admittance; short-circuit test gives eq…
• COpen-circuit test gives leakage reactance; short-circuit test gives shunt branch paramete…
• DBoth tests give identical parameters and are used only for redundancy.

A three-phase transformer is connected in a Yd11 (Star-Delta) configuration. What is the phase displacement of the low-voltage line voltage with respect to the high-voltage line voltage?

• A0 degrees
• B30 degrees lagging
• C30 degrees leading
• D180 degrees

A DC shunt motor is running at 1200 rpm from a 220 V supply. The armature resistance is 0.2 ohms and the armature current is 50 A. If the load torque is doubled and the field flux remains constant, calculate the new steady-state speed of the motor.

• A1143 rpm
• B1200 rpm
• C1171 rpm
• D1085 rpm

Why is a starter required to launch a large DC motor instead of connecting it directly to the supply line?

• ATo limit the high starting current caused by the absence of back EMF.
• BTo increase the initial starting torque by increasing field current.
• CTo compensate for the high reactance of the armature windings.
• DTo prevent the motor from running in the reverse direction.

A 4-pole, three-phase induction motor operates from a 50 Hz supply. If the motor runs at a steady-state speed of 1440 rpm, what is the slip (s) of the motor?

• A2.0%
• B4.0%
• C5.0%
• D6.0%

For a three-phase induction motor, how does the maximum torque developed (T_max) and the slip at which it occurs (s_max) change if the rotor resistance is doubled?

• AT_max is halved; s_max is doubled.
• BT_max remains unchanged; s_max is doubled.
• CT_max is doubled; s_max remains unchanged.
• DBoth T_max and s_max are doubled.

A synchronous motor is connected to an infinite bus. If the excitation (field current) is adjusted so that the motor operates at an over-excited state, what is the power factor and the nature of the reactive power?

• ALagging power factor; absorbs reactive power.
• BLeading power factor; delivers reactive power.
• CUnity power factor; zero reactive power.
• DLeading power factor; absorbs reactive power.

What is the primary effect of armature reaction in an alternator (synchronous generator) supplying a load with a purely lagging power factor (zero power factor lagging)?

• ACross-magnetizing effect only.
• BDemagnetizing effect only.
• CMagnetizing effect only.
• DDemagnetizing and cross-magnetizing effect.

A variable reluctance stepper motor has 8 rotor teeth and 6 stator poles. What is the step angle of this stepper motor?

• A15 degrees
• B30 degrees
• C7.5 degrees
• D45 degrees

Why cannot a single-phase induction motor start on its own when connected to a single-phase AC supply?

• ABecause single-phase AC produces a pulsating magnetic field instead of a rotating one.
• BBecause the starting winding has too high resistance compared to the running winding.
• CBecause there is no slip at starting condition.
• DBecause the rotor conductor bars are short-circuited.

A long straight conductor of radius 'a' carries a steady current I distributed uniformly across its cross-section. According to Ampere's Law, what is the magnetic field intensity H at a radius r inside the conductor (r < a)?

• AH = I / (2 * pi * r)
• BH = I * r / (2 * pi * a^2)
• CH = I * a^2 / (2 * pi * r)
• DH = 0

A magnetic circuit has a mean core length of 0.5 m, cross-sectional area of 10 cm^2, and a relative permeability of 2000. Calculate the reluctance (S) of this magnetic path.

• A1.99 * 10^5 A-t/Wb
• B1.99 * 10^4 A-t/Wb
• C1.99 * 10^6 A-t/Wb
• D3.98 * 10^5 A-t/Wb

Which of Maxwell's equations is modified by the introduction of displacement current to satisfy the equation of continuity for time-varying fields?

• AGauss's Law for Electricity
• BGauss's Law for Magnetism
• CFaraday's Law of Induction
• DAmpere's Circuital Law

At the boundary between two linear magnetic media with permeabilities mu1 and mu2, what are the boundary conditions for the magnetic field intensity H and magnetic flux density B?

• ANormal H is continuous; tangential B is continuous.
• BTangential H is continuous (with zero surface current); normal B is continuous.
• CBoth tangential and normal components of H are continuous.
• DBoth tangential and normal components of B are continuous.

Which of the following power stations in Nigeria is a hydro-electric power plant, and what is its approximate installed capacity?

• AEgbin Power Station, 1320 MW
• BKainji Power Station, 760 MW
• CAfam Power Station, 960 MW
• DGeregu Power Station, 414 MW

A three-phase transmission line has a configuration that causes an inequality in the inductance of the individual phases. What is the primary purpose of transposing transmission line conductors at regular intervals along the route?

• ATo reduce the skin effect and proximity effect in the line.
• BTo balance the phase inductances and capacitances, minimizing voltage imbalance and inter…
• CTo increase the surge impedance loading (SIL) capacity of the line.
• DTo prevent corona discharge at high operating voltages.

A medium-length transmission line (100 km) is represented using the Nominal-Pi model. If the total series impedance is Z and the total shunt admittance is Y, how is the ABCD parameter 'A' defined?

• AA = 1 + Y*Z
• BA = 1 + Y*Z / 2
• CA = Z
• DA = Y * (1 + Y*Z / 4)

A 330 kV three-phase transmission line has a positive sequence inductance of 1.2 mH/km and a capacitance of 0.012 microfarads/km. Calculate the Surge Impedance Loading (SIL) of this line.

• A344 MW
• B100 MW
• C363 MW
• D272 MW

Which of the following factors increases the critical disruptive voltage of a transmission line, thereby reducing the occurrence of corona loss?

• ADecreasing the spacing between conductors.
• BIncreasing the conductor radius (diameter).
• CUsing conductors with rough or stranded surfaces.
• DOperating at high altitudes with low atmospheric pressure.

In symmetrical component analysis, if the operator 'a' is defined as a complex number of unit magnitude and 120-degree phase shift (1 |__ 120), what is the value of 1 + a + a^2?

• Aj * sqrt(3)
• B1.0
• C0
• D-1.0

A generator rated 100 MVA, 11 kV has a subtransient reactance of 15% on its own base. What is its per-unit subtransient reactance on a new base of 200 MVA and 22 kV?

• A0.30 pu
• B0.15 pu
• C0.075 pu
• D0.60 pu

For a single line-to-ground (LG) fault on an unloaded alternator, how must the sequence networks (positive, negative, and zero sequence) be connected to represent the fault condition?

• AAll three sequence networks connected in parallel.
• BAll three sequence networks connected in series.
• CPositive and negative sequence networks in parallel, zero sequence disconnected.
• DPositive and zero sequence networks in series, negative sequence bypassed.

In power flow (load flow) studies, what are the known and unknown variables at a PQ bus (load bus)?

• AKnown: Active Power (P) and Voltage Magnitude (V); Unknown: Reactive Power (Q) and Phase…
• BKnown: Active Power (P) and Reactive Power (Q); Unknown: Voltage Magnitude (V) and Phase…
• CKnown: Voltage Magnitude (V) and Phase Angle (delta); Unknown: Active Power (P) and React…
• DKnown: Reactive Power (Q) and Phase Angle (delta); Unknown: Active Power (P) and Voltage…

The swing equation governs the rotor dynamics of a synchronous machine during power system transients. If the inertia constant is H, electrical frequency is f, input mechanical power is P_m, and output electromagnetic power is P_e, how is the swing equation written in terms of rotor angle delta?

• A(H / (pi * f)) * d^2(delta)/dt^2 = P_m - P_e
• B(2H / (pi * f)) * d^2(delta)/dt^2 = P_m - P_e
• C(H / (2 * pi * f)) * d^2(delta)/dt^2 = P_e - P_m
• D(2H / f) * d^2(delta)/dt^2 = P_m * P_e

An industrial facility has a load with an impedance of Z = 3 + j4 ohms. If the base impedance of the system is 10 ohms, what is the per-unit impedance (Z_pu) of this load?

• A30 + j40 pu
• B0.3 + j0.4 pu
• C3.0 + j4.0 pu
• D0.03 + j0.04 pu

What is a primary advantage of High Voltage Direct Current (HVDC) transmission over conventional High Voltage Alternating Current (HVAC) transmission for long-distance power grids?

• AHVDC lines require more conductors per circuit, enhancing reliability.
• BHVDC is cheaper to construct for very short distances due to low converter station costs.
• CHVDC has no charging current losses and no skin effect, allowing lower transmission losse…
• DHVDC transformers are much smaller and cheaper than AC transformers.

Which type of distance protective relay is less affected by arc resistance and is therefore preferred for the protection of short transmission lines?

• AImpedance relay
• BReactance relay
• CMho relay
• DOvercurrent relay

Why is 'harmonic restraint' integrated into the differential protection scheme of power transformers?

• ATo prevent false tripping during high magnetizing inrush current which is rich in second…
• BTo filter out third harmonics caused by core saturation under overvoltage conditions.
• CTo detect inter-turn faults within the windings.
• DTo stabilize the relay during external through-faults.

For a high-voltage circuit breaker, what is the relation between the rated making current (symmetrical/asymmetrical peak) and the rated breaking current (symmetrical RMS)?

• AMaking current is equal to breaking current.
• BMaking current is approximately 2.55 times the breaking current.
• CBreaking current is approximately 2.55 times the making current.
• DMaking current is exactly 1.414 times the breaking current.

An overcurrent relay has a plug setting of 125% and is connected to a 400/5 A current transformer (CT). If a fault current of 4000 A flows through the primary of the CT, what is the Plug Setting Multiplier (PSM) of the relay?

• A8.0
• B10.0
• C6.4
• D12.5

In substation earthing design under IEEE Std 80, what is the definition of 'step voltage'?

• AThe potential difference between a person's hands touching a grounded structure and their…
• BThe potential difference between two feet of a person standing 1 meter apart on the earth…
• CThe voltage drop across the grounding grid impedance during a fault.
• DThe maximum voltage a cable insulation can withstand before flashover.