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Past Questions — 2016
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.
Best protection is provided by HRC fuses in case of
A)
Open circuits
B)
Short circuits
C)
Overloads
D)
None of these
HRC (High Rupturing Capacity) fuses are designed to interrupt very large fault currents safely and quickly. Their best protection is therefore against short-circuit faults. They are not the best choice for open circuits, and overload protection is generally better handled by other protective devices such as relays or circuit breakers with suitable settings.
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 dc
C)
The transformers required for testing cannot be used for long distances
D)
All of these
For testing AC transmission lines and cables, DC is preferred because if AC is used, the line/cable capacitance causes heavy charging current. This demands a very large testing transformer and makes AC testing impractical. Option (b) is incorrect because transmission lines and cables can be tested with DC; in fact, DC is commonly used for high-voltage testing. Option (c) is irrelevant to the reason for choosing DC. Therefore, the correct answer is (a).
Buchholz relay is:
A)
Located in the conservator tank
B)
Located in the transformer tank itself
C)
Connected in the pipe connecting main tank of transformer and conservator
D)
Installed in the circuit breaker
A Buchholz relay is a gas-actuated protective relay used in oil-immersed transformers. It is fitted in the pipeline between the transformer main tank and the conservator tank so that it can detect gases formed due to internal faults and also oil surge during serious faults. Hence, option (c) is correct. Options (a) and (b) are incorrect because the relay is not placed inside the conservator or the main tank. Option (d) is incorrect because it is not a circuit breaker device.
In air blast circuit breakers, the pressure of air is of the order of
A)
100mm 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 stored at a high pressure, typically in the range of about 20 to 30 kg/cm², and this air is used to blow out the arc. Option (a) is too low, (b) is much lower than the usual operating pressure, and (d) is excessively high for standard air-blast circuit breakers.
The instrument which is cheapest for dc measurement is:
A)
Moving iron
B)
PMMC
C)
Hot-wire
D)
Electro-dynamo
For DC measurements, the moving iron instrument is generally the cheapest among the given options. It is simple in construction and less expensive than PMMC instruments. PMMC is more accurate but costlier and mainly used for DC. Hot-wire and electro-dynamo instruments are more complex and costly, so they are not the cheapest choice.
Electrostatic type instruments are mainly used for measurement of:
A)
Heavy currents
B)
Low currents
C)
Low voltages
D)
High voltages
Electrostatic instruments work on the principle of electrostatic attraction/repulsion. Since the deflecting torque is very small, they are mainly suitable for measuring very high voltages and not currents. They are not used for heavy or low current measurement, and low voltages are usually measured by other instruments.
A system having connected load of 100 kW, peak load of 80 kW, base load of 20 kW and average load of 40 kW, will have a load factor of
A)
40%
B)
50%
C)
60%
D)
80%
Load factor = (Average load / Peak load) × 100. Here, average load = 40 kW and peak load = 80 kW. So, load factor = (40/80) × 100 = 50%. Connected load and base load are not needed for calculating load factor. Hence the correct option is (b) 50%.
If the height of transmission towers is increased, which of the following parameters is likely to change?
A)
Resistance
B)
Inductance
C)
Capacitance
D)
None of these
When the height of transmission towers is increased, the conductors are placed farther from the ground. This increases the effective distance between the line conductors and the earth, which changes the electric field distribution and reduces the shunt capacitance to ground. Among the given options, the parameter affected is capacitance. Resistance is not affected by tower height, and inductance is mainly determined by conductor spacing and geometry rather than tower height alone.
Which of the following is not the distribution system normally used?
A)
3 phase-4 wire
B)
3 phase-3 wire
C)
Single phase - 3 wire
D)
Single phase -4 wire
In distribution systems, the commonly used arrangements are 3-phase 4-wire, 3-phase 3-wire, and single-phase 3-wire systems. A single-phase 4-wire system is not normally used in standard distribution practice. Therefore, option (d) is the correct answer.
On what factor does the operating speed of the relay depend upon?
A)
Rate of flux built up
B)
Armature core air gap
C)
Spring tension
D)
All of these
The operating speed of an electromechanical relay depends on multiple factors: the rate at which flux builds up in the magnetic circuit, the air gap between the armature and core, and the spring tension opposing motion. A faster flux build-up and smaller air gap generally make the relay operate faster, while higher spring tension slows it down. Therefore, all the given factors affect relay operating speed.
Demand factor is the
A)
Maximum Demand / Average Demand
B)
Maximum Demand / Connected Load
C)
Average Demand / Maximum Demand
D)
Connected Load / Maximum Demand
Demand factor is defined as the ratio of maximum demand to the total connected load. So, Demand Factor = Maximum Demand / Connected Load. Option (a) is the load factor relation if reversed incorrectly, option (c) is the inverse of load factor, and option (d) is the reverse of demand factor.
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 may be designed to operate based on many electrical quantities depending on the protection scheme. They can respond to light intensity in special relays, impedance in distance relays, temperature in thermal relays, resistance/reactance in impedance-based relays, and also voltage and current in overvoltage, undervoltage, overcurrent, and related relays. Therefore, all the listed quantities are possible inputs for protective relays.
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.