← MES Jr. Grade
Past Questions — 2012
The equal area criterion can be used to
A)
find critical reclosure time
B)
solve the swing equations for a three machine case
C)
find rate of change of velocity of rotors
D)
all of these
The equal area criterion is used in power system stability studies to determine transient stability of a single-machine infinite bus or equivalent two-machine system. It helps find the critical clearing time and, in reclosing problems, the critical reclosure time. It does not directly solve swing equations for a three-machine case, nor does it find the rate of change of rotor velocity. Hence, option (a) is correct.
If the system has multiple poles on the jw- axis, the system is
A)
stable
B)
unstable
C)
marginally stable
D)
conditionally stable
A system with multiple poles on the jω-axis is unstable. For marginal stability, poles on the imaginary axis must be simple (non-repeated) and all other poles must lie in the left half-plane. Repeated poles on the jω-axis cause the response to grow without bound, so the system is not stable. Hence, the correct option is (b) unstable.
Earthing of transformer neutral through reactance will improve its
A)
steady-state stability
B)
transient stability
C)
both
D)
neither
Earthing the transformer neutral through reactance limits the earth-fault current and helps maintain system voltage during disturbances. This improves the system’s transient stability, not steady-state stability. Steady-state stability is mainly affected by power-angle characteristics and system impedance, while neutral grounding reactance is used to control fault currents and transient overvoltages.
Load frequency control can be achieved by properly matching the individual machines ‘
A)
reactive powers
B)
generated voltages
C)
turbine inputs
D)
turbine and generator ratings
Load frequency control is achieved by adjusting the mechanical power input to the generators, i.e., the turbine inputs. When load changes, the system frequency deviates; governors act to change turbine input so that generation again matches demand. Reactive power mainly affects voltage, not frequency. Generated voltages and machine ratings are not the direct control variable for frequency regulation.
The system in the above question, when ξ=1, is
A)
under-damped
B)
over-damped
C)
absolutely damped
D)
critically damped
For a standard second-order control system, the damping ratio ξ (or ζ) determines the nature of response. When ξ = 1, the system is **critically damped**. This is the boundary between under-damped (0 < ξ < 1) and over-damped (ξ > 1) behavior. Hence, the correct option is (d).
A second order system is always
A)
stable
B)
unstable
C)
marginally stable
D)
none of these
A second-order system is not always stable, unstable, or marginally stable. Its stability depends on the coefficients/poles of the characteristic equation. For example, a second-order system can be stable if both poles lie in the left half-plane, unstable if any pole lies in the right half-plane, or marginally stable in special cases. Therefore, the correct choice is 'none of these'.
If synchronous capacitors are used to provide VAR, leading VAR will be produced by
A)
increasing the speed of the machine
B)
increasing the field current
C)
reducing the field current
D)
none of these
A synchronous capacitor (synchronous condenser) supplies leading reactive power when it is under-excited. Under-excitation is obtained by reducing the field current, which makes the machine draw lagging magnetizing current from the system and thus deliver leading VAR to the network. Increasing field current over-excites the machine and tends to produce lagging VAR instead. Therefore, the correct answer is reducing the field current.
Which of the following acts as a protection against high voltage surges due to lightening and switching?
A)
Breather
B)
Conservator
C)
Horn gaps
D)
Thermal overload relays
Horn gaps are used as a protective device against high-voltage surges caused by lightning and switching. They provide a path for the surge to flash over safely, protecting the equipment. Breather and conservator are parts of a transformer used for moisture control and oil expansion, respectively. Thermal overload relays protect against overcurrent/overheating, not surge voltages.
A system has poles at -1 and -5 and zeros at 1 and -2, the system is
A)
stable
B)
unstable
C)
marginally stable
D)
highly stable
For a continuous-time LTI system, stability depends on the locations of the poles. A system is stable only if all poles lie in the left half of the s-plane. Here the poles are at -1 and -5, which are both in the left half-plane, so the pole locations alone indicate stability. However, the question includes zeros at 1 and -2, and zeros do not determine stability. Since the only relevant factor is the poles, the system is stable. But among the given options, if the intended interpretation is based on pole-zero cancellation or transfer function behavior, the question is ambiguous. With the information provided, the correct classification is stable.
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.
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.
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.
The open loop transfer function of a unity feedback control system is given by G(s) = K/(s(s+1)), if the gain K is increased to infinity, then the damping ratio will tend to become
A)
1/√2
B)
1
C)
0
D)
infinity
For a unity feedback system, the closed-loop characteristic equation is 1 + G(s) = 0, so
s(s+1) + K = 0 => s^2 + s + K = 0.
Comparing with the standard second-order form s^2 + 2ζω_n s + ω_n^2 = 0, we get
ω_n^2 = K and 2ζω_n = 1.
Hence, ζ = 1/(2√K).
As K → ∞, ζ → 0.
So the damping ratio tends to zero, meaning the system becomes more oscillatory.
Options (a), (b), and (d) are incorrect because the damping ratio does not approach a finite positive value or infinity as K increases without bound.
If the system specifications are given in time domain, best approach for designing is
A)
Nyquist plot
B)
Bode plot
C)
root locus
D)
any of these
When system specifications are given in the time domain, the most suitable design method is the root locus technique. Time-domain requirements such as rise time, peak overshoot, settling time, and damping ratio are directly related to the location of closed-loop poles, which root locus helps determine. Nyquist and Bode plots are mainly frequency-domain methods, so they are less direct for time-domain specification-based design.
In the load flow analysis, the load at a bus is represented as
A)
a constant current drawn from the bus
B)
a constant impedance connected at the bus
C)
constant real and reactive powers drawn from the bus
D)
a voltage-dependent impedance at the bus
In load flow (power flow) analysis, loads are usually modeled as constant power loads, meaning the specified active power (P) and reactive power (Q) are drawn at the bus. This is the standard assumption because the load demand is treated as known and fixed for the operating condition being studied. Constant current or constant impedance models may be used in some special studies, but the basic load flow formulation uses constant P and Q.
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.
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.
The main application of transfer function is in the study of
A)
steady-state behaviour of systems
B)
steady state as well as transient behaviour of systems
C)
only transient behaviour of systems
D)
none of these
A transfer function represents the input-output relationship of a linear time-invariant system in the Laplace domain. It is mainly used to analyze both transient response and steady-state response of control systems. Hence, option (b) is correct. Option (a) is incomplete, option (c) is incorrect because transfer function is not limited to transient behavior, and option (d) is therefore wrong.
In a force balance type pneumatic controller, the number of bellows required for PID- action is
A)
1
B)
2
C)
3
D)
4
In a force-balance pneumatic controller, PID action requires three separate functional elements: proportional, integral, and derivative. These are implemented using three bellows. Hence, the number of bellows required is 3. Option (a) 1 and (b) 2 are insufficient for full PID action, and (d) 4 is not the standard arrangement.
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.
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.
Loading of a transmission line leads to an increase in
A)
inductance of the line
B)
capacitance of the line
C)
resistance of the line
D)
conductance of the line
When a transmission line is loaded, the conductor temperature rises due to increased current. Higher temperature increases the resistivity of the conductor, so the effective resistance of the line increases. Inductance, capacitance, and conductance are not increased by loading in this context.
Bundled conductors are used to
A)
reduce inductance of the line
B)
reduce both inductance and capacitance
C)
reduce corona loss
D)
reduce corona loss and the line inductance
Bundled conductors are used in EHV transmission lines to increase the effective conductor diameter. This reduces the electric field intensity at the conductor surface, thereby reducing corona loss. At the same time, the larger effective radius lowers the line inductance. Hence, the correct answer is both corona loss and line inductance. Option (a) is incomplete, and option (b) is incorrect because capacitance does not reduce; it generally increases.
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.