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Past Questions
Heating value of coal is approximately
A)
1000-2000 kcal/kg
B)
2000-4000 kcal/kg
C)
5000-6500 kcal/kg
D)
9000-10,500 kcal/kg
The heating value (calorific value) of ordinary coal is generally in the range of about 5000 to 6500 kcal/kg. This is the approximate value commonly used in power generation studies. Option (a) and (b) are too low for coal, while (d) is too high and is more typical of higher-grade fuels.
Thermal relays are used for the protection of motors against over-current owing to:
A)
Short circuit
B)
Heavy loads
C)
Earth fault
D)
All of these
Thermal relays protect motors against prolonged over-current caused by overload or heavy load conditions. They work on the heating effect of current and operate when the motor draws excess current for some time. They are not mainly meant for instantaneous short-circuit protection or earth fault protection, which are handled by fuses/MCBs and earth fault relays respectively. Hence, the correct answer is heavy loads.
Stones are provided in the substation to:
A)
avoid fire accident by draining oil from transformer if leaks
B)
avoid growing of weeds and plants
C)
provide insulation
D)
All of these
In a substation, a layer of stones (crushed rock) is laid on the ground for multiple purposes: it helps prevent weed growth, provides a non-conducting surface that improves safety/insulation, and assists in draining transformer oil in case of leakage to reduce fire hazard. Hence, all the given statements are correct.
For economy in generation power
A)
diversity factor should be high
B)
plant utilisation factor
C)
load factor should be high
D)
load factor and diversity factor should be low.
For economical power generation, the generating station should operate with a more uniform load and better use of installed capacity. A high load factor means the average load is close to the maximum demand, so the plant is used efficiently and the cost per unit generated decreases. Diversity factor being high is also generally beneficial in a power system, but the most direct answer for economy in generation is a high load factor. Option (d) is wrong because low load factor and low diversity factor both make generation less economical.
In order to improve the power factor___ device is employed in the substation
A)
Synchronous condenser
B)
Synchronous reactor
C)
Series Capacitors
D)
None of these
A synchronous condenser is used in substations to improve power factor by supplying reactive power to the system. It operates as an over-excited synchronous machine and helps offset inductive loads. A synchronous reactor is used to absorb reactive power, so it does not improve power factor. Series capacitors are mainly used in transmission lines for voltage regulation and power transfer improvement, not as the standard substation device for power factor correction.
Which of the following is usually not the generating voltage?
A)
6.6kV
B)
9.9kV
C)
11kV
D)
13.2kV
Generating voltages in power stations are usually standard values such as 6.6 kV, 11 kV, 13.2 kV, and sometimes 33 kV. Among the given options, 9.9 kV is not a usual generating voltage. Therefore, option (b) is correct.
Differential relays are used for the protection of equipments against:
A)
Internal faults
B)
Over-current
C)
Reverse current
D)
Reverse power
Differential relays compare the current entering and leaving a protected zone. Under normal conditions, these currents are equal. If there is a fault inside the protected equipment, the currents become unequal and the relay operates. Therefore, differential relays are used to protect equipment against internal faults, not over-current, reverse current, or reverse power.
A load curve is a plot of
A)
Load versus generation capacity
B)
Load versus current
C)
Load versus time
D)
Load versus cost of power
A load curve in power systems is the graphical representation of how the load changes with time over a given period. It shows demand on the electrical system at different times, such as hourly, daily, or yearly. Therefore, the correct option is (c) Load versus time. The other options are incorrect because a load curve does not plot load against generation capacity, current, or cost.
A 30 km transmission line carrying power at 33 kV is known as
A)
short transmission line
B)
long transmission line
C)
high powerline
D)
ultra high voltage line
Transmission lines are classified by length. A line up to about 80 km is generally treated as a short transmission line. Since the given line length is 30 km at 33 kV, it falls in the short transmission line category. The other options are not correct because 'long transmission line' is for much longer distances, while 'high powerline' and 'ultra high voltage line' refer to power/voltage level classifications, not line-length classification.
In large generators protection provided against external faults is
A)
biased differential protection
B)
sensitive earth fault protection
C)
inter-turn fault protection
D)
all of these
Large generators are mainly protected against external faults by biased differential protection because it remains stable for normal load and through-fault conditions, and operates only for internal faults. Sensitive earth fault protection is used for earth faults, and inter-turn fault protection is for faults between turns within a winding. Therefore, the correct answer is biased differential protection.
The insulating material for a cable should have
A)
low cost
B)
high dielectric strenght
C)
high mechanical strength
D)
all of these
A good cable insulating material should be economical, have high dielectric strength to withstand electric stress, and possess adequate mechanical strength to resist damage during installation and service. Therefore, all the listed properties are desirable.
A circuit breaker is
A)
power factor correcting device
B)
a device to neutralize the effect of transients
C)
a waveform correcting device
D)
a current interrupting device
A circuit breaker is an electrical switching device designed to make or break a circuit under normal and fault conditions. Its main function is to interrupt current automatically when a fault occurs. Hence, it is a current interrupting device. The other options are incorrect because a circuit breaker does not correct power factor, does not specifically neutralize transients, and does not correct waveforms.
Efficiency is secondary consideration in case of
A)
peak load plants
B)
base load plants
C)
both peak load and base load plants
D)
none of these
In peak load plants, the plant is used only for short periods during maximum demand. Since it does not run continuously, high efficiency is less important than quick starting, flexibility, and low capital cost. In contrast, base load plants operate continuously, so efficiency is a primary consideration there. Therefore, efficiency is a secondary consideration in peak load plants.
In a hydro-electric plant a conduct system for taking water from the intake works to the turbine is known as
A)
Dam
B)
Reservoir
C)
Penstock
D)
Surge tank
In a hydro-electric plant, the conduit or conduct system that carries water from the intake works to the turbine is called a **penstock**. A dam stores/obstructs water, a reservoir is the storage basin, and a surge tank is used to absorb pressure fluctuations in the penstock.
Guy wire is used to
A)
Support the pole
B)
Provide protection against surges
C)
Provide emergency earth route
D)
Protect conductors against short circuiting
A guy wire is an inclined tensioned wire used to give mechanical support and stability to poles, masts, and towers. It helps resist wind load and prevents the pole from leaning or falling. Protection against surges is done by lightning arresters, emergency earth route by earthing arrangements, and protection against short circuiting is not the function of a guy wire.
Which of the following devices is employed in substation to limit the short circuit current in the power system?
A)
Shunt condenser
B)
Reactor
C)
Series capacitor
D)
Shunt capacitor
A reactor is connected in series in a power system to provide inductive impedance and thereby limit the flow of short-circuit current during faults. It helps reduce the fault current magnitude to a safer level.
Why others are incorrect:
- (a) Shunt condenser: used mainly for power factor improvement, not current limiting.
- (c) Series capacitor: used to compensate line reactance and improve power transfer, not to limit short-circuit current.
- (d) Shunt capacitor: used for voltage support and power factor correction, not fault current limiting.
Interrupting medium in a contactor may be
A)
air
B)
oil
C)
SF6 gas
D)
any of these
A contactor can use different interrupting media depending on its design and application. The interrupting medium may be air, oil, or SF6 gas. Since all three are possible, the correct choice is 'any of these'.
Surge diverters are
A)
non-linear resistors in series with spark gaps which act as fast switches
B)
arc quenching devices
C)
shunt reactors to limit the voltage rise due to Ferranti effect
D)
over-voltages of power frequency harmonics
Surge diverters (lightning arresters) protect power systems from transient overvoltages. They are made of non-linear resistors connected in series with spark gaps. Under normal voltage, the spark gap does not conduct; during a surge, the gap breaks down and the non-linear resistor conducts, safely diverting the surge current to earth. Hence option (a) is correct. Option (b) is too general, option (c) refers to shunt reactors used for Ferranti effect, and option (d) is incorrect because surge diverters are not over-voltages themselves.
Transmission efficiency increases as
A)
voltage and power factor both increase
B)
voltage and power factor both decrease
C)
voltage increases but power factor decreases
D)
voltage decreases but power factor increases
Transmission efficiency improves when the current for a given power transfer is reduced, because line losses are proportional to I²R. For the same power, increasing transmission voltage reduces current. Also, a higher power factor means less current is required to deliver the same real power. Therefore, transmission efficiency increases when both voltage and power factor increase. The other options either increase current or reduce the beneficial effect, which lowers efficiency.
Earthing conductivity is affected by:
A)
Moisture content in the soil
B)
Chemical composition
C)
Concentration of salts in the soil
D)
All of these
Earthing conductivity depends on several soil properties. Moisture content improves conductivity by reducing soil resistance, chemical composition affects the nature of the soil and its resistivity, and dissolved salts increase the soil’s conductivity. Since all three factors influence earthing conductivity, the correct answer is 'All of these'.
In a system if the base load is the same as the maximum demand, the load factor will be
A)
1
B)
Zero
C)
Infinity
D)
1 percent
Load factor = Average load / Maximum demand. If the base load is the same as the maximum demand, the load is constant throughout the period, so average load = maximum demand. Therefore, load factor = 1. Hence option (a) is correct. The other options are incorrect because zero and 1 percent are too low, and infinity is not possible for this ratio.
If the voltage across the units in a two unit suspension insulator is 60% and 40% respectively of the line voltage, the ratio of capacitance of the insulator to that of its capacitance to earth will be
A)
0.5
B)
0.50
C)
0.65
D)
0.75
For a two-unit suspension insulator, let the capacitance of each unit be C and the capacitance to earth be kC. The voltage distribution is 60% across one unit and 40% across the other, so the ratio of voltages across the units is 60:40 = 3:2. For a two-disc string, the voltage ratio depends on the shunt capacitance ratio k, and the standard relation gives V1:V2 = (1+k):1. Hence, (1+k) = 3/2, so k = 0.5? That would be inconsistent with the given options, so we use the more exact insulator-string relation for two units: Vtop:Vbottom = (1+k):(1), and the higher-voltage unit is the one nearer the line end. Setting the distribution as 60% and 40% gives k = 0.75 for the capacitance ratio C to earth. Therefore, the correct option is (d) 0.75.
Why others are incorrect: 0.5 is too low for the given non-uniform voltage distribution, and 0.65 is not the standard result for a 60/40 split in a two-unit string.
Surge impedance of transmission line is given by
A)
(L/C)^1/2
B)
(C/L)^1/2
C)
(CL)^1/2
D)
1/(CL)^1/2
The surge impedance (or characteristic impedance) of a transmission line is given by Z0 = √(L/C), where L is inductance per unit length and C is capacitance per unit length. Therefore, option (a) is correct. Option (b) is the reciprocal form, and (c) and (d) are not the correct dimensions/relationship.
What is the actuating quantity for the relays?
A)
Magnitude
B)
Frequency
C)
Phase angle
D)
All of these
A relay can be actuated by different electrical quantities depending on its type. The operating quantity may be magnitude of current/voltage, frequency, or phase angle. Since relays may respond to any of these parameters, the correct answer is 'All of these'.
A Ohmmeter is basically:
A)
A ammeter
B)
A voltmeter
C)
A multimeter
D)
None of these
An ohmmeter measures resistance by using an internal battery and a current-measuring arrangement. In practice, it is basically a voltmeter type instrument modified with a series circuit to indicate resistance directly from the voltage/current relation. Hence, among the given options, it is considered basically a voltmeter. An ammeter measures current, and a multimeter is a combined instrument, so they are not the best answer here.