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Past Questions
In outdoor substation, the lightning arrester is placed nearer to
A)
the isolator
B)
the current transformer
C)
the power transformer
D)
the current breaker
In an outdoor substation, the lightning arrester is installed as close as possible to the equipment to be protected, especially the power transformer. This minimizes the length of the lead and reduces the surge voltage reaching the transformer. The isolator, current transformer, and circuit breaker are not the primary reason for placing the arrester nearby.
Advantage of passive instrument is
A)
It does not need power supply
B)
Cheap
C)
Sensitive
D)
Accurate
Passive instruments do not require an external power source for operation, so their construction is simpler and they are generally less expensive. Hence, the main advantage is that they are cheap.
Option (a) is incorrect because it is a feature of passive instruments, not the usual stated advantage in such MCQs. Option (c) is incorrect because passive instruments are generally less sensitive than active instruments. Option (d) is incorrect because high accuracy is not their primary advantage.
A thermo-couple instrument can be used for the measurement of:
A)
Direct current only
B)
Alternating current only
C)
Both direct current and alternating current
D)
dc/ac voltage only
A thermo-couple instrument works on the heating effect of current and responds to the RMS value of current, independent of whether the current is DC or AC. Therefore, it can measure both direct current and alternating current. It is not limited to only DC, only AC, or voltage measurement only.
Measurement range of a voltmeter can be extended by using:
A)
High current resistance
B)
High series resistance
C)
Low shunt resistance
D)
Low series resistance
A voltmeter measures potential difference and is connected in parallel. To extend its measurement range, a large resistance is added in series with the meter so that higher voltages can be measured without drawing excessive current. A shunt resistance is used to extend ammeter range, not voltmeter range. Therefore, the correct answer is high series resistance.
A lightning arrester connected between the line and earth in a power system
A)
Protects the terminal equipment against travelling surges
B)
Protects the terminal equipment against lightning strokes
C)
Suppresses the high frequency oscillations in the line
D)
Reflects back the travelling waves approaching it
A lightning arrester is connected between the line and earth to divert surge current to ground and limit the surge voltage appearing across equipment. Its main purpose is to protect terminal equipment from travelling surges caused by lightning or switching. Option (b) is too narrow because the arrester does not stop lightning strokes themselves; it only protects against the resulting overvoltage. Option (c) is incorrect because suppression of high-frequency oscillations is not the primary function. Option (d) is incorrect because a lightning arrester does not reflect waves back; it shunts the surge to earth.
Earth fault relays are:
A)
Directional relay
B)
Non-directional relay
C)
Short operate time relay
D)
Under voltage relay
Earth fault relays generally respond to the presence of earth leakage or ground fault current regardless of the direction of power flow, so they are classified as non-directional relays. Directional relays depend on current direction, short operate time relay is a time-based classification, and under voltage relay operates on voltage drop, not on earth fault current.
Any winding of a power transformer develops short circuit owing to:
A)
Loose connections
B)
Insulation failure
C)
Impulse voltage
D)
Mechanical vibration
A short circuit in any winding of a power transformer is mainly caused by breakdown of insulation between turns, layers, or windings. When insulation fails, adjacent conducting parts come into direct contact, creating a short circuit. Loose connections may cause overheating, impulse voltage may stress insulation and eventually lead to failure, and mechanical vibration can damage components, but the direct cause of winding short circuit is insulation failure.
Power plants using coal work closely on known which of the following cycle?
A)
Otto cycle
B)
Binary vapor cycle
C)
Brayton cycle
D)
Rankine cycle
Coal-based thermal power plants generate steam in a boiler, expand it in a turbine, condense it, and pump it back to the boiler. This is the Rankine cycle, which is the standard vapor power cycle used in steam power plants. Otto cycle is for spark-ignition engines, Brayton cycle is for gas turbines, and binary vapor cycle is a specialized cycle using two working fluids, not the basic coal plant cycle.
Which component connects the substation to the area where power is to be distributed?
A)
Distributors
B)
Service mains
C)
Feeders
D)
All of these
A feeder is the conductor that carries power from the substation to the distribution area. It connects the substation to the locality where electricity is to be distributed. Distributors are used for supplying consumers from the distribution area, and service mains connect the distributor to an individual consumer. Therefore, the correct answer is feeders.
In case of transmission line conductors with the increase in atmospheric temperature
A)
length increase but stress decreases
B)
length increases and stress also increases
C)
length decreases but stress increases
D)
both length as well as stress decreases
With increase in atmospheric temperature, a transmission line conductor expands, so its length increases. Due to thermal expansion, the conductor sags more and the mechanical tension/stress in it decreases. Therefore, the correct option is (a).
The fault clearing time of a circuit breaker is usually
A)
few minutes
B)
few seconds
C)
one second
D)
few cycles of supply voltage
A circuit breaker is designed to interrupt fault current very quickly, typically within a few cycles of the AC supply frequency (for example, around 2 to 5 cycles). This rapid operation limits equipment damage and maintains system stability. Few minutes, few seconds, or even one second are far too slow for fault clearing in power systems.
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
In a cable, the thickness of insulation is mainly determined by the operating voltage. Higher voltage requires thicker insulation to withstand the electric stress and prevent breakdown. Current carrying capacity affects the conductor size, not the insulation thickness. Reactive power and power factor are not the deciding factors for insulation thickness.
For low head and high discharge, the hydraulic turbine used is
A)
Kaplan turbine
B)
Francis turbine
C)
Pelton wheel
D)
Jonual turbine
Hydraulic turbines are selected based on head and discharge. For low head and high discharge conditions, the appropriate turbine is the Kaplan turbine, which is a reaction turbine designed for such operating ranges. Francis turbine is used for medium head and medium discharge, while Pelton wheel is used for high head and low discharge. 'Jonual turbine' is not the correct standard turbine for this condition.
For low head and high discharge, the hydraulic turbine used is
A)
Kaplan turbine
B)
Francis turbine
C)
Pelton wheel
D)
Jonual turbine
Hydraulic turbines are selected based on head and discharge. For low head and high discharge conditions, a Kaplan turbine is used because it is an axial-flow reaction turbine designed for large flow rates at low heads. Francis turbine is mainly for medium head and medium discharge, while Pelton wheel is for high head and low discharge. 'Jonual turbine' is not the standard correct turbine for this condition.
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'.
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 relay circuit, the first part is the current transformer (CT) side connected in series with the line to be protected. This primary winding carries the line current and supplies a reduced current to the relay circuit. The secondary of the CT and operating coil form the second part, while the tripping circuit is the third part. Therefore, option (a) is correct.
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.
In air blast circuit breakers, the pressure of air is of the order of
A)
100 mm Hg
B)
1 kg/cm2
C)
20 to 30 kg/cm2
D)
200 to 300 kg/cm2
In an air blast circuit breaker, compressed air is used at a high pressure, typically around 20 to 30 kg/cm2, to extinguish the arc quickly. This high-pressure air blast cools and de-ionizes the arc path. Option (a) 100 mm Hg and option (b) 1 kg/cm2 are far too low for effective arc interruption, while option (d) 200 to 300 kg/cm2 is excessively high and not the usual operating range.
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 generation of power, the average load should be as close as possible to the maximum demand. A high load factor means better utilization of the installed capacity, lower fixed cost per unit, and hence more economical generation. Diversity factor also helps the system economically, but the standard direct condition for economy in generation is a high load factor. Option (a) is not the main criterion, and (d) is incorrect because low load factor and low diversity factor both worsen economy.
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.