← MES Jr. Grade
Past Questions
An isolator operates under
A)
no load condition
B)
full-load condition
C)
50% load condition
D)
5% load condition
An isolator (disconnect switch) is used to isolate a circuit from the supply after the circuit breaker has already opened the circuit. It is not meant to make or break current under load, so it operates only under no-load condition. The other options are incorrect because an isolator should never be opened or closed while carrying full load, 50% load, or even small load current.
The best method for determining the stability and transient response is
A)
Bode plot
B)
Nyquist plot
C)
root locus
D)
none of these
Root locus is the best method for determining both stability and transient response because it shows how the closed-loop poles move as system gain varies. Since stability depends on pole locations and transient response is strongly related to the dominant poles, root locus directly helps analyze both. Bode plot and Nyquist plot are mainly used for frequency-domain stability analysis, but root locus is more suitable for studying pole movement and transient behavior.
Z-Bus matrix is normally a
A)
null matrix
B)
sparse matrix
C)
full matrix
D)
unity matrix
The bus impedance matrix (Z-Bus) is generally a full matrix because each bus impedance term can be non-zero due to network interconnections. In contrast, the bus admittance matrix (Y-Bus) is usually sparse since most buses are directly connected to only a few others. Hence, Z-Bus is normally a full matrix. The other options are incorrect: a null matrix has all zero entries, a sparse matrix is characteristic of Y-Bus, and a unity matrix has 1s on the diagonal only.
The controlling medium in a pneumatic control system is
A)
liquid
B)
air
C)
oil
D)
helium
In a pneumatic control system, the controlling medium is compressed air. Pneumatic systems use air as the transmitting and controlling fluid, unlike hydraulic systems which use oil or other liquids. Therefore, the correct answer is air. Liquid and oil are used in hydraulic control systems, and helium is not used as the standard controlling medium.
A system with gain margin close to unity and phase margin close to zero is
A)
highly stable
B)
oscillatory
C)
relatively stable
D)
none of these
A gain margin close to unity and phase margin close to zero indicate the system is operating very near the stability limit. Such a system tends to exhibit sustained or lightly damped oscillations. Hence, it is considered oscillatory rather than highly stable or relatively stable. Option (a) is incorrect because high stability would require larger margins. Option (c) is incorrect because relatively stable systems have comfortable positive margins. Option (d) is incorrect because option (b) is the correct description.
In MHD generator, there are two types of e.m.f. induced, one is known as motionally induced e.m.f. and the other is known as
A)
stationary e.m.f.
B)
transformer action e.m.f.
C)
static e.m.f.
D)
Hall e.m.f.
In an MHD generator, the conducting fluid moves through a magnetic field, so one e.m.f. is due to motion of the conductor, called motionally induced e.m.f. The other is due to the time-varying magnetic flux associated with the changing field seen by the fluid, which is called transformer action e.m.f. Hence, the correct answer is transformer action e.m.f. The other options are not the standard term used in MHD generator theory.
A position control system is
A)
a process control system
B)
an automatic regulatory system
C)
a servomechanism
D)
a stochastic control system
A position control system is used to control the position of a mechanical element by comparing the actual position with the desired position and correcting the error automatically. Such systems are called servomechanisms. It is not a process control system, which regulates process variables like temperature or pressure, nor an automatic regulatory system in the general sense, and it is not a stochastic control system, which deals with random processes.
As the frequency increases, the capacity of the static condenser required for phase advancement
A)
increases
B)
decreases
C)
remains constant
D)
may increase or decrease
For a static condenser used for phase advancement, the reactive current it supplies depends on capacitive reactance, Xc = 1/(2πfC). As frequency increases, Xc decreases, so for the same voltage the condenser draws/supplies more current. Therefore, to achieve a given phase advancement, the required capacitance is less. Hence the capacity required decreases with increase in frequency.
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.
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.
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.
Transient stability of a power system means that the system is capable of
A)
maintaining a continuous balance between energy input and electrical output
B)
delivering power over tie lines to other systems during a disturbance
C)
remaining in synchronism, prior to governor action, following a system disturbance
D)
all of the above
Transient stability is the ability of a power system to remain in synchronism after a large disturbance such as a fault, line switching, or sudden loss of generation/load. The key point is the immediate post-disturbance period, before slower control actions like governor response take effect. Option (c) states this correctly. Option (a) describes general steady-state balance, not transient stability. Option (b) is unrelated to the definition. Therefore, (c) is correct.
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.
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.
Ferranti effect on overhead lines is experienced when
A)
the line is lightly loaded
B)
the power factor is unity
C)
the power factor is leading
D)
corona effect is dominant
Ferranti effect is the rise in receiving-end voltage above sending-end voltage in a transmission line, especially when the line is long and lightly loaded or open-circuited. Under light load, the line capacitance causes charging current, which leads to this voltage rise. It is not specifically due to unity power factor, leading power factor, or corona effect.
Error constants of system are a measure of
A)
steady-state response
B)
transient response
C)
relative stability
D)
steady-state as well as transient response
Error constants (position, velocity, and acceleration error constants) are used to determine the steady-state error of a control system for standard inputs like step, ramp, and parabolic inputs. Hence, they are a measure of steady-state response, not transient response or relative stability.
Steady-state stability limit of a power system means that the system is capable of maintaining
A)
the frequency at 50 Hz exactly
B)
a constant power flow throughout
C)
synchronism between machines and on external lines
D)
all of the above
Steady-state stability limit is the maximum power that can be transmitted without losing synchronism after small, gradual disturbances. So the system must be capable of maintaining synchronism between machines and on external lines. It does not mean frequency is exactly 50 Hz, and power flow is not necessarily constant.
If a transmission line is lossless, its characteristic impedance reduces to
A)
(LC)1/2
B)
(L/C)1/2
C)
LC
D)
none of the above
For a transmission line, the characteristic impedance is Z0 = sqrt((R + jωL)/(G + jωC)). For a lossless line, R = 0 and G = 0, so Z0 = sqrt(L/C). Hence the correct option is (b). Option (a) is incorrect because sqrt(LC) has wrong dimensions for impedance, and (c) is also dimensionally incorrect.
Transient stability of a power system depends on
A)
the short circuit ratio of the generating units
B)
the strength of the transmission network
C)
fault-clearing time
D)
all of these
Transient stability is influenced by multiple factors. The short-circuit ratio of generating units affects machine strength and stability margin, the strength of the transmission network affects power transfer capability and synchronizing torque, and fault-clearing time is critical because a longer clearing time allows the rotor angle to deviate more after a disturbance. Hence, all the given factors affect transient stability.
As compared to a closed-loop system, an open-loop system is
A)
more stable as well as more accurate
B)
less stable as well as less accurate
C)
more stable but less accurate
D)
less stable but more accurate
An open-loop system has no feedback, so it does not correct errors automatically. Because of this, it is generally simpler and more stable than a closed-loop system. However, it is less accurate since disturbances and parameter variations are not compensated. Therefore, the correct comparison is: more stable but less accurate.
Roots of characteristic equation are
A)
poles of closed loop
B)
poles of open loop
C)
zero of open loop
D)
zero of closed loop
In control systems, the characteristic equation is obtained from the closed-loop transfer function denominator, usually 1 + G(s)H(s) = 0. The roots of this equation are the closed-loop poles. Open-loop poles are the poles of G(s)H(s), not the characteristic equation. Open-loop zeros and closed-loop zeros are not the roots of the characteristic equation.
If the frequency increases, the characteristic impedance of a transmission line
A)
decreases
B)
increases
C)
remains constant
D)
first increases and then decreases
The characteristic impedance of a transmission line is given by Z0 = √((R + jωL)/(G + jωC)). As frequency increases, the inductive reactance term (ωL) and capacitive susceptance term (ωC) change, and for practical transmission lines the characteristic impedance generally increases with frequency. Hence, the correct option is (b) increases. The other options do not match the standard behavior of transmission lines.
Load flow study gives the information on
A)
magnitude and phase angles of load bus voltages
B)
reactive powers at generator buses
C)
real and reactive power flow on transmission lines
D)
all of these
Load flow (power flow) study determines the steady-state operating condition of a power system. It provides: (1) magnitude and phase angles of bus voltages, especially at load buses; (2) reactive power at generator buses, since generator buses regulate voltage and their Q output is computed; and (3) real and reactive power flow in transmission lines. Therefore, all the listed information is obtained from load flow analysis.
The transfer function of a system is the Laplace transform of its
A)
square wave response
B)
step response
C)
ramp response
D)
impulse response
The transfer function of a linear time-invariant system is defined as the Laplace transform of its impulse response, assuming zero initial conditions. Therefore, the correct option is (d). The step, ramp, and square wave responses are not the definition of transfer function.
For stability and economic reasons, the transmission line is operated with power angle in the range
A)
10°-25°
B)
30°-45°
C)
60°-75°
D)
75°-90°
For a transmission line, the power angle should normally be kept small for both stability and economic operation. A low power angle means the system operates well below the maximum power transfer limit, giving a good stability margin and reducing the risk of loss of synchronism. In practice, the preferred operating range is about 10° to 25°. The higher ranges given in the other options are too close to the stability limit and are not suitable for normal operation.