← MES Jr. Grade
Past Questions — 2012
Corona loss occurs in
A)
distribution transformer
B)
transmission lines
C)
generators
D)
none of these
Corona loss occurs when the electric field around a conductor in air becomes strong enough to ionize the surrounding air. This phenomenon is mainly seen in high-voltage transmission lines, especially on overhead lines. It is not a typical loss in distribution transformers or generators. Therefore, the correct answer is transmission lines.
In the system of the above question, if ξ > 1, the system is
A)
under-damped
B)
over-damped
C)
absolutely damped
D)
critically damped
In a standard second-order control system, the damping ratio is denoted by ξ (or ζ). If ξ > 1, the system is overdamped. Such a system returns to equilibrium without oscillation, but more slowly than a critically damped system. For reference: ξ < 1 is underdamped, ξ = 1 is critically damped, and ξ > 1 is overdamped. Hence, option (b) is correct.
The transient response of a system is mainly due to
A)
internal forces
B)
inertia forces
C)
friction
D)
stored energy
The transient response is the part of the output that appears immediately after a change in input and dies out with time. It is mainly caused by the energy already stored in the system elements such as inductors, capacitors, masses, and springs. This stored energy produces the temporary behavior before the system settles to steady state. Hence, the correct answer is stored energy. The other options are not the main cause: internal forces and friction may affect motion, and inertia forces are related to dynamics, but the fundamental reason for transient response is stored energy.
Nyquist criterion enables us to find
A)
absolute stability only
B)
relative stability only
C)
both
D)
either of the two
The Nyquist criterion is used to determine the stability of a closed-loop control system from the open-loop frequency response. It tells us whether the system is stable or unstable, so it helps in finding absolute stability. It also gives information about how close the system is to instability through gain margin and phase margin, which indicates relative stability. Therefore, it enables us to find both absolute and relative stability.
Which of the following is used for Nyquist plot?
A)
Pole zero plot
B)
Closed-loop function
C)
Open-loop function
D)
Characteristic equation
A Nyquist plot is drawn for the open-loop transfer function G(s)H(s) evaluated along the Nyquist contour in the s-plane. It is used to assess closed-loop stability from the open-loop frequency response. The pole-zero plot is different, the closed-loop function is not what is directly plotted, and the characteristic equation is used for stability analysis but not for constructing the Nyquist plot.
Bandwidth is used as means of specifying performance of a control system related to
A)
relative stability of the system
B)
the speed of response
C)
the constant gain
D)
all of these
Bandwidth in control systems indicates the range of frequencies over which the system can respond effectively. A larger bandwidth generally means the system can follow input changes more quickly, so it is used as a measure of the speed of response. It is not a direct measure of relative stability or constant gain, so options (a) and (c) are incorrect, and therefore (d) is also incorrect.
The maximum short circuit current occurs in the case of:
A)
a dead short circuit
B)
a double line to ground fault
C)
a line to line fault
D)
a single line to ground fault
A dead short circuit means the fault impedance is practically zero, so the current is limited only by the system impedance. Therefore, it produces the maximum short-circuit current. In comparison, double line-to-ground, line-to-line, and single line-to-ground faults all involve some fault path conditions that generally result in lower current than a solid (dead) short circuit.
HVDC transmission is preferred to EHV-AC transmission, because
A)
HVDC terminal equipment are inexpensive
B)
VAR compensation is not required in HVDC systems
C)
system stability can be improved
D)
harmonics problem is avoided
HVDC transmission is often preferred over EHV-AC for long-distance bulk power transfer because it improves system stability and allows controlled power flow. In AC systems, stability limits can restrict the amount of power transmitted over long distances. HVDC avoids these AC stability constraints.
Why the other options are incorrect:
- (a) HVDC terminal equipment are inexpensive: False. HVDC converter stations are generally expensive.
- (b) VAR compensation is not required in HVDC systems: False. HVDC converters require reactive power support, so VAR compensation is needed.
- (d) harmonics problem is avoided: False. HVDC systems actually introduce harmonics and need filters.
Hence, the correct answer is system stability can be improved.
The temperature of resistance furnance can be controlled by changing the
A)
applied voltage
B)
number of heating elements
C)
circuit configuration
D)
all of the above
In a resistance furnace, temperature can be controlled by varying the applied voltage, changing the number of heating elements in use, or altering the circuit configuration. Since all three methods are valid control methods, the correct answer is 'all of the above'.
A system has some roots with real parts equal to zero, but none with positive real parts. The system is
A)
absolutely stable
B)
relatively stable
C)
marginally stable
D)
absolutely unstable
A system is **marginally stable** when it has **no roots with positive real parts** and **some roots lie exactly on the imaginary axis** (real part = 0). Such a system is not asymptotically stable, but it does not diverge either.
- **Absolutely stable**: all roots have negative real parts.
- **Relatively stable**: stability with a certain degree of damping/relative stability margin, not the standard classification here.
- **Marginally stable**: some roots on the imaginary axis, none in the right half-plane.
- **Absolutely unstable**: at least one root has positive real part.
Hence, the correct answer is **(c) marginally stable**.
Shuttle valve is a pneumatic
A)
OR gate
B)
NOR gate
C)
AND gate
D)
NAND gate
A shuttle valve in pneumatics allows air from either of two input lines to pass to the output, while blocking the other line. This is the same logical behavior as an OR gate: output is obtained if input A OR input B is present. Hence, the correct answer is OR gate. NOR, AND, and NAND do not match this function.
The steady state error for unit ramp input to the characteristic equation s^2 +2s +8 = 0 is
A)
0.20
B)
0.25
C)
0.30
D)
0.50
For a unit ramp input, the steady-state error is found using the velocity error constant Kv. The given characteristic equation s^2 + 2s + 8 = 0 is treated as a standard second-order closed-loop characteristic equation, and the corresponding steady-state ramp error is 1/Kv = 0.20. Hence, the correct answer is 0.20. The other options do not match the computed steady-state error.
Shunt capacitors in a substation
A)
consume lagging VAR
B)
deliver lagging VAR
C)
consume active power
D)
deliver active power
Shunt capacitors are connected in parallel with the load or busbar to supply reactive power locally. A capacitor draws a leading current, which means it provides reactive power (VAR) to the system and helps improve power factor. Hence, in substation practice, shunt capacitors are said to deliver VAR, specifically leading reactive power. Among the given options, the intended answer is 'deliver lagging VAR' only if the paper uses the common but imprecise wording; however, electrically capacitors actually deliver leading VAR. Since the option set is limited, the exam’s expected choice is option (b).
The two windings of a transformer is
A)
conductively linked
B)
inductively linked
C)
not linked at all
D)
electrically linked
In a transformer, the primary and secondary windings are not electrically connected; instead, energy is transferred from one winding to the other through the alternating magnetic flux in the core. Therefore, the two windings are inductively linked. Option (a) is wrong because they are not conductively connected, option (d) is wrong because there is no direct electrical link, and option (c) is wrong because the windings are linked through mutual induction.
An electromechanical device which actuates a train of step angular movements in response to a train of input pulses on one to one basis is
A)
synchro control transformer
B)
LVDT
C)
stepper motor
D)
tachogenerator
A stepper motor is an electromechanical device that converts a train of input pulses into discrete angular steps on a one-to-one basis. Each pulse causes the rotor to move by a fixed step angle. The other options do not match this function: a synchro control transformer is used for position error detection, an LVDT measures linear displacement, and a tachogenerator produces a voltage proportional to speed.
Transient stability of a power system can be improved by
A)
increasing the system voltage
B)
decreasing the system voltage
C)
keeping the system voltage constant
D)
none of these
Transient stability is the ability of a power system to remain in synchronism after a large disturbance. Increasing the system voltage improves power transfer capability and strengthens the electrical coupling between generators, which helps maintain synchronism during disturbances. Hence, transient stability can be improved by increasing the system voltage. Decreasing the voltage would reduce power transfer capability and worsen stability, while keeping it constant does not specifically improve transient stability.
For the interruption of high voltages and low currents, the circuit breaker preferred is
A)
air blast circuit breaker
B)
oil circuit breaker
C)
vacuum circuit breaker
D)
all of these
For interrupting high voltages with low currents, a vacuum circuit breaker is preferred because the vacuum has very high dielectric strength and excellent arc-quenching properties, especially for small current interruption. Air blast circuit breakers are generally used for high-current, high-voltage applications, while oil circuit breakers are older and less preferred for this specific duty. Hence, the correct answer is vacuum circuit breaker.
For LV applications (below 1KV) _______ cables are used.
A)
paper insulated
B)
plastic
C)
single core
D)
oil filled
For low-voltage (LV) applications below 1 kV, plastic insulated cables are commonly used because they are economical, flexible, and suitable for low-voltage distribution. Paper-insulated and oil-filled cables are generally used for higher-voltage applications, while single-core refers to the number of conductors, not the insulation type.
The transfer function of a control system depends on
A)
initial condition of input and output
B)
system parameters alone
C)
nature of input
D)
nature of output
A transfer function is defined as the ratio of the Laplace transform of output to input with all initial conditions assumed zero. Therefore, it depends only on the system parameters, not on the initial conditions or the specific nature of input/output. Hence, option (b) is correct. Options (a), (c), and (d) are incorrect because initial conditions and signal shapes do not determine the transfer function.
Corona causes
A)
communication interference
B)
power loss
C)
both (a) and (b)
D)
none of these
Corona in transmission lines produces ionization of air around conductors. This leads to power loss in the form of corona loss and also generates electromagnetic noise that causes communication interference. Therefore, both (a) and (b) are correct.
Skin effect
A)
increases the resistance and effective internal resistance
B)
decreases the effective resistance and effective resistance
C)
decreases the effective resistance and effective internal reactance
D)
increases the effective internal reactance
Skin effect causes alternating current to concentrate near the surface of a conductor, reducing the effective cross-sectional area available for current flow. This increases the AC resistance compared to DC resistance. It also increases the effective internal resistance of the conductor. Hence, option (a) is correct. The other options are incorrect because skin effect does not decrease resistance, and it is not primarily described as increasing internal reactance.
The characteristic equation of a second order system is given by s^2+2ξω0s+ω0^2 = 0. If ξ=1, the poles of the transfer function will be
A)
real and equal
B)
imaginary and equal
C)
complex and conjugate
D)
equal to -1
For a second-order system, the characteristic equation is s^2 + 2ξω0 s + ω0^2 = 0. When ξ = 1, the system is critically damped. The roots become s = -ω0 and s = -ω0, i.e., two real equal poles. Hence the poles are real and equal. The other options are incorrect because imaginary and equal or complex conjugate poles occur when ξ < 1, and 'equal to -1' is not generally true unless ω0 = 1.
In a flapper-nozzle mechanism, if flapper moves close to nozzle, the nozzle back pressure will
A)
decrease
B)
increase
C)
remain constant
D)
depend upon the construction
In a flapper-nozzle mechanism, when the flapper moves closer to the nozzle, the nozzle opening becomes smaller. This restricts air flow through the nozzle, so the pressure builds up on the nozzle side. Therefore, the nozzle back pressure increases. Option (a) is wrong because the pressure does not decrease; (c) is wrong because the pressure changes with nozzle gap; and (d) is wrong because the basic effect is not dependent on construction in this standard mechanism.
In a force balance type pneumatic controller, the number of bellows required for PI- action is
A)
1
B)
2
C)
3
D)
4
In a force-balance pneumatic controller, PI action requires two bellows: one for the proportional action and one for the integral action. The proportional bellows provides immediate response to error, while the integral bellows accumulates the error over time to eliminate steady-state offset. Hence, the correct answer is 2.
The most common type of fault on the overhead transmission line is
A)
single line to ground fault
B)
line to line fault
C)
double line to ground fault
D)
three-phase fault
The most common fault on overhead transmission lines is the single line-to-ground fault. This happens because one conductor may come in contact with the ground due to lightning, insulation failure, tree contact, or mechanical damage. It is more frequent than line-to-line, double line-to-ground, or three-phase faults. Three-phase faults are the least common but the most severe.