← MES Jr. Grade
Past Questions — 2012
Carrier-current protection scheme is normally used for
A)
HV transmission line only
B)
HV cables only
C)
HV transmission lines and cables
D)
none of these
Carrier-current protection is a communication-based protection scheme mainly used for high-voltage overhead transmission lines. It is not normally used for cables because cable capacitance and construction make carrier transmission impractical. Therefore, the correct answer is HV transmission line only.
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.
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.
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.
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.
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.
Mho relay is used to protect
A)
long transmission lines
B)
medium length lines
C)
short transmission lines
D)
all of these
A mho relay is a distance relay with a directional characteristic, and it is especially suitable for protecting long transmission lines. Long lines have higher line impedance, so mho relays provide stable and reliable protection. Medium and short lines are more commonly protected by impedance or reactance relays, not specifically mho relays.
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.
For a line to line fault, the sequence of network is comprised of
A)
only positive sequence
B)
only negative sequence
C)
only zero sequence
D)
positive and negative sequence
In a line-to-line fault, there is no path for zero-sequence current because the fault does not involve ground. The fault current is represented by the positive-sequence and negative-sequence networks connected together. Hence, the sequence network involved is positive and negative sequence only. Option (a) is wrong because positive sequence alone cannot represent the unbalanced fault. Option (b) is wrong because negative sequence alone is not sufficient. Option (c) is wrong because zero sequence is absent in a line-to-line fault.
In a force balance type pneumatic controller, the number of bellows required for P- action is
A)
1
B)
2
C)
3
D)
4
In a force-balance pneumatic controller, proportional (P) action is obtained using a single feedback bellows. The input signal acts against the feedback force from this one bellows to establish balance. Integral and derivative actions require additional bellows, but for P-action alone only 1 bellows is needed. Hence, the correct answer is (a) 1.
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.
Maximum power transfer will take place between stations through the interconnector if the angle between the station voltages is
A)
0°
B)
60°
C)
90°
D)
180°
For two AC stations connected by an interconnector, the power transferred is proportional to sin(δ), where δ is the phase angle between the station voltages. The sine function reaches its maximum value at 90°, so maximum power transfer occurs when the angle is 90°. At 0° the transfer is zero, and at 180° it is again zero.
Which of the following types of power plant is hardly used for supplying base load?
A)
hydro
B)
thermal
C)
nuclear
D)
solar
Base load plants are those that can supply power continuously and reliably for long periods. Thermal and nuclear plants are commonly used for base load, and hydro can also be used depending on water availability. Solar power is intermittent and depends on sunlight, so it is hardly used for supplying base load.
If two synchronous generators connected in parallel loose synchronism, it will lead to
A)
stalling of generators
B)
wild fluctuations in current
C)
wild fluctuations in voltage
D)
both (b) and (c)
If two synchronous generators operating in parallel lose synchronism, their rotor positions drift relative to each other. This causes large oscillations in power transfer, which appear as wild fluctuations in current and also in terminal voltage. Hence both current and voltage become unstable. Stalling is not the direct effect here.
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.
Which one of the following is an example of an open-loop system?
A)
Household refrigerator
B)
Respiratory system of an animal
C)
Stabilization of air pressure entering into a mask
D)
Execution of a program by a computer
An open-loop system operates without feedback; the output does not affect the control action. Execution of a program by a computer is typically open-loop because the steps are carried out according to the instructions without using output feedback to modify the process. In contrast, a household refrigerator uses a thermostat (feedback), the respiratory system of an animal uses physiological feedback, and stabilization of air pressure entering into a mask also involves feedback control.
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.
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.
With the feedback system, the transient response
A)
decays slowly
B)
decays rapidly
C)
rises slowly
D)
rises quickly
In a feedback control system, negative feedback generally improves stability and increases damping, so the transient oscillations die out faster. Hence the transient response decays rapidly. The other options are incorrect because feedback does not usually make the response decay slowly or primarily describe the rise as the key effect here.
The electric potential due to an electric dipole of length L at point distance r away from it will be doubled if the
A)
L is doubled
B)
r is doubled
C)
r is halved
D)
L is halved
For an electric dipole, the potential at a point on the axial line is proportional to the dipole moment p = qL, and inversely proportional to r^2. Thus, V ∝ L/r^2 (for fixed charge q and distance r). To double the potential, L must be doubled. If r is doubled, potential becomes one-fourth; if r is halved, potential becomes four times; if L is halved, potential becomes half.
With the increase of temperature, insulating property of insulators
A)
weakens
B)
strengthens
C)
remains same
D)
none of these
As temperature increases, the insulating resistance of most insulating materials decreases, so their insulating property weakens. Higher temperature increases leakage current and reduces dielectric strength. Hence option (a) is correct. Options (b) and (c) are incorrect because insulation does not improve or remain unchanged with rising temperature.
Hydrometer is used to determine
A)
specific gravity of liquids
B)
relative humidity
C)
specific gravity of solids
D)
specific gravity of gases
A hydrometer is an instrument used to measure the specific gravity (relative density) of liquids. It works by floating at different levels depending on the liquid’s density. It is not used for humidity, solids, or gases.
A capacitor of 40 µF is charged to a potential difference of 500V. The energy stored in the electric field between the plates of this condenser is
A)
4.5 joules
B)
4 joules
C)
5 joules
D)
40 joules
Energy stored in a capacitor is given by U = 1/2 CV^2. Here, C = 40 µF = 40 × 10^-6 F and V = 500 V. So, U = 1/2 × 40 × 10^-6 × (500)^2 = 1/2 × 40 × 10^-6 × 250000 = 5 J? Let's calculate carefully: 40 × 10^-6 × 250000 = 10, and half of that is 5 J. Therefore the correct energy is 5 joules, which corresponds to option (c).
An air gap is usually inserted in magnetic circuits to
A)
increase the flux
B)
prevent saturation
C)
increase mmf
D)
none of these
An air gap increases the reluctance of the magnetic circuit, so for a given magnetizing force the flux is reduced and the core is less likely to reach magnetic saturation. Hence, air gaps are commonly inserted to prevent saturation. Option (a) is wrong because an air gap does not increase flux; option (c) is wrong because mmf is not increased by the gap itself; it only requires more mmf to produce the same flux.