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Past Questions — 2016
Steel poles for transmission lines need protection against
A)
termites
B)
borer
C)
corrosion
D)
all of these
Steel poles used in transmission lines are mainly exposed to atmospheric moisture, rain, and pollution, so they require protection against corrosion. Termites and borer attack timber poles, not steel poles. Therefore, the correct answer is corrosion.
In order to increase the limit of distance of transmission line
A)
series resistances are used
B)
synchronous condensers are used
C)
shunt capacitors and series reactors are used
D)
series capacitors and shunt reactors are used.
For increasing the transmission distance of a line, compensation is used to reduce the effective series reactance and to control voltage. Series capacitors compensate the line inductive reactance, improving power transfer capability, while shunt reactors help control overvoltage due to line charging, especially on long lines. Hence, the correct choice is series capacitors and shunt reactors.
Why others are incorrect:
- (a) Series resistances increase losses and reduce efficiency, so they are not used.
- (b) Synchronous condensers are mainly used for reactive power support and voltage regulation, but this option is not the standard combined method for increasing transmission line length.
- (c) Shunt capacitors and series reactors is the opposite of what is generally used for long-distance transmission; series reactors would further increase reactance.
During load shedding
A)
system voltage is reduced
B)
system frequency is reduced
C)
some loads are switched off
D)
system power factor is changed
Load shedding means intentionally disconnecting part of the load from the power system when demand exceeds available supply. This is done to prevent system collapse and maintain stability. It does not primarily mean reducing voltage, frequency, or power factor; those may change as a consequence of system conditions, but the defining action is switching off some loads.
The relay best suited for phase fault relaying for medium transmission lines is:
A)
Mho relay
B)
Reactance relay
C)
Impedance relay
D)
None of these
For phase-fault protection on medium transmission lines, the **reactance relay** is best suited because it is less affected by **fault resistance**. Medium transmission lines often have appreciable fault resistance, and a reactance relay responds mainly to the reactive component of line impedance, making it more reliable for such faults.
Why not the others?
- **Mho relay**: Better suited for **long transmission lines** and directional protection.
- **Impedance relay**: Can be affected by fault resistance and is less suitable than reactance relay for phase faults on medium lines.
- **None of these**: Incorrect because reactance relay is the correct choice.
In the below figure, the relay circuit is divided into three parts. What does the first part consist of?
A)
Primary winding of a current CT which is connected in series with the line to be protected
B)
Secondary of the CT and the operating coil
C)
Tripping circuit
D)
All of these
In a protective relay circuit, the first part is the current transformer (CT) primary connected in series with the line to be protected. This senses the line current and feeds a proportional current to the relay circuit. The secondary of the CT and the operating coil form the next part, while the tripping circuit is the final part. So only option (a) correctly describes the first part.
The protection against direct lightening strokes and high voltage steep waves is provided by
A)
earthing of neutral
B)
lightening arresters
C)
ground wires
D)
lightning arresters and ground wires
Protection against direct lightning strokes and high-voltage steep waves is provided by both lightning arresters and ground wires. Lightning arresters divert surge voltage safely to earth, while ground wires intercept direct lightning strokes and protect overhead lines. Earthing of neutral is for system grounding and fault protection, not direct lightning protection.
The equipment installed in power plants to reduce air pollution due to smoke is
A)
Induced draft fans
B)
De-super heaters
C)
Electrostatic precipitators
D)
Re-heaters
Electrostatic precipitators are used in power plants to remove fine dust and ash particles from flue gas before it is released into the atmosphere, thereby reducing air pollution due to smoke. Induced draft fans only help move flue gases through the boiler system, de-superheaters control steam temperature, and reheaters increase steam temperature between turbine stages. They do not clean smoke.
Which power plant is free from environmental pollution problems?
A)
Thermal power plant
B)
Nuclear power plant
C)
Hydro-power plant
D)
Geothermal energy power plant
Hydro-power plants use the energy of flowing or falling water, so they do not burn fuel and therefore do not cause air pollution like thermal power plants. Nuclear power plants have radioactive waste and safety concerns, and geothermal plants may release gases and other environmental issues. Hence, among the given options, hydro-power plant is considered free from environmental pollution problems.
The operation speed of a relay depends on:
A)
Rate of flux buildup
B)
Armature core air gap
C)
Spring tension
D)
All of these
The operating speed of a relay is influenced by several mechanical and magnetic factors. A faster rate of flux buildup helps the relay pick up quicker, a smaller armature core air gap reduces the time needed for attraction, and spring tension affects how much force must be overcome for operation. Since all three affect the relay’s operation speed, the correct answer is "All of these".
The breakdown voltage in gases depends on
A)
distance between the electrodes
B)
relative air density
C)
humidity
D)
all of these
The breakdown voltage of a gas depends on multiple factors: (1) distance between the electrodes, because a larger gap generally requires a higher voltage to ionize the gas; (2) relative air density, since gas density affects ionization and dielectric strength; and (3) humidity, which changes the ionization characteristics of air. Therefore, all the given factors affect breakdown voltage.
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 heavy through-fault currents and trips only for internal faults within the protected zone. Sensitive earth fault protection is mainly for earth faults, and inter-turn fault protection is for faults between turns of the same winding, not specifically for external faults. Therefore, the correct answer is biased-differential protection.
The instrument which is cheapest for dc measurement is:
A)
Moving iron
B)
PMMC
C)
Hot-wire
D)
Electro-dynamo
For DC measurement, the moving iron instrument is generally the cheapest. It is rugged, simple in construction, and less costly than PMMC instruments. PMMC is more accurate but more expensive, while hot-wire and electro-dynamo instruments are costlier and used for special purposes. Hence, the correct answer is moving iron.
Thickness of the layer of insulation on the conductor, in cables, depends upon
A)
Reactive power
B)
Power factor
C)
Voltage
D)
Current carrying capacity
The thickness of insulation in a cable is mainly decided by the operating voltage. Higher voltage requires thicker insulation to prevent breakdown and leakage. Current carrying capacity affects the size of the conductor, not the insulation thickness. Reactive power and power factor are not direct factors for insulation thickness.
The emf generated due to nth harmonic component of flux in an alternator will be
A)
n times the fundamental emf
B)
same as fundamental emf
C)
less than the value of fundamental emf
D)
none of these
In an alternator, the induced emf due to the nth harmonic flux depends on the harmonic order and winding factors. Higher harmonics generally produce lower effective emf because the winding and distribution factors reduce the contribution of harmonics compared with the fundamental component. Therefore, the emf generated by the nth harmonic component is less than the fundamental emf. Option (a) is incorrect because harmonic emf is not simply n times the fundamental emf in practical alternators, and option (b) is also incorrect because harmonics do not produce the same emf as the fundamental.
A Ohmmeter is basically:
A)
A ammeter
B)
A voltmeter
C)
A multimeter
D)
None of these
An ohmmeter measures resistance by passing a small current through the unknown resistance and using the current-deflection principle. Structurally, it is basically a sensitive ammeter with an internal battery and suitable series resistance. Hence, the correct answer is A ammeter. It is not a voltmeter, and while a multimeter can measure resistance among other quantities, an ohmmeter itself is essentially based on an ammeter.
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 soil's ability to conduct current, and higher salt concentration generally increases conductivity. Since all three factors influence earthing conductivity, the correct answer is 'All of these'.
The insulating material for a cable should have
A)
low cost
B)
high dielectric strength
C)
high mechanical strength
D)
all of these
An insulating material used in cables should ideally be economical, able to withstand high electric stress without breakdown, and mechanically strong enough to resist damage during installation and service. Hence, it should have low cost, high dielectric strength, and high mechanical strength. Therefore, the correct option is 'all of these'.
The phenomenon of corona is generally accompanied by :
A)
a bang
B)
a hissing sound
C)
magnetic hum
D)
all of these
Corona on transmission lines ionizes the surrounding air and is generally accompanied by audible effects such as a hissing sound, a crackling/banging noise, and a magnetic hum. Therefore, the correct option is 'all of these'.
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. Lower ranges like 1000-4000 kcal/kg are too low for coal and are more typical of low-grade fuels, while 9000-10,500 kcal/kg is too high and closer to premium fuels or certain refined petroleum products. Therefore, option (c) is correct.
Protective relays can be designed to respond to
A)
Light intensity, impedance
B)
Temperature, resistance, reactance
C)
Voltage and current
D)
All of these
Protective relays are designed to sense abnormal conditions in a power system. Depending on the type of relay, they may respond to light intensity, impedance, temperature, resistance, reactance, voltage, and current. Since all the listed quantities can be used in relay operation, the correct answer is "All of these".
The protection against direct lightening strokes and high voltage steep waves is provided by
A)
earthing of neutral
B)
lightening arresters
C)
ground wires
D)
lightning arresters and ground wires
Protection against direct lightning strokes and high-voltage steep waves is provided by both lightning arresters and ground wires. Lightning arresters divert surge currents safely to earth, while ground wires intercept direct lightning strokes and carry them to ground. Earthing of neutral is for system grounding and does not provide this specific protection.
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 a fault occurs inside the protected equipment, the currents become unequal and the relay operates. Therefore, differential relays are used for protection against internal faults. Over-current protection is handled by overcurrent relays, while reverse current and reverse power are associated with reverse current/power relays, not differential relays.
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 operates only during periods of high demand for a short duration. Therefore, the main requirement is quick starting and meeting sudden load demand, not high efficiency. Hence efficiency is a secondary consideration. In contrast, base load plants run continuously for long periods, so efficiency is very important there.
Potentiometer is a/an___ instrument:
A)
Indicating
B)
Comparison
C)
Calibrating
D)
Recording
A potentiometer is a comparison instrument because it measures an unknown voltage by comparing it with a known standard voltage under a null-balance condition. It does not directly indicate the value like an indicating instrument, nor does it record data or primarily serve as a calibrating instrument. Hence, the correct answer is Comparison.
Direct current is preferred over alternating current for testing of ac transmission lines and cables because:
A)
Heavy charging currents will be drawn and so a large sized transformer is required if tested with ac
B)
Transmission lines and cables should not be tested with de
C)
The transformers required for testing cannot be used for long distances
D)
All of these
DC is preferred for testing AC transmission lines and cables because with AC, the line/cable capacitance causes heavy charging current. This would require a much larger testing transformer and makes AC testing impractical. Option (b) is false because DC testing is commonly used. Option (c) is not the main reason and is irrelevant. Therefore, only option (a) is correct, so (d) cannot be correct.