71. The power loss is an important factor for the design of
A. transmission line
B. motor
C. generator
D. feeder
Answer
A. transmission line
Explanation:
- The transmission over long distances creates power losses.
- The major part of the energy losses comes from Joule effect in transformers and power lines. The energy is lost as heat in the conductors.
- The overall losses between the power plant and consumers is then in the range between 8 and 15%.
- That part of power system which distributes electric power for local use is known as distribution system.
- In general, the distribution system is the electrical system between the substation fed by the Transmission system and the consumer’s meters. It generally consists of feeders, distributors, and service mains.
Feeder:
- A feeder is a conductor which connects the sub-station (or localized generating station) to the area where power is to be distributed.
- Generally, no tapings are taken from the feeder so that current in it remains the same throughout.
- The main consideration in the design of a feeder is the current carrying capacity.
Distributors:
- A distributor is a conductor from which tapings are taken for supply to the consumers. In Fig. AB, BC, CD and DA are the distributors.
- The current through a distributor is not constant because tapings are taken at various places along its length.
- While designing a distributor, voltage drop along its length is the main consideration since the statutory limit of voltage variations is ± 6% of rated value at the consumers’ terminals.
Service mains:
- A service mains is generally a small cable which connects the distributor to the consumers’ terminals.
72. The single phasing relays are used for the protection of
A. single-phase motors only
B. two-phase motors only
C. two single-phase motors running in parallel
D. three-phase motors
Answer
D. three-phase motors
Explanation:
- Single phase preventer is used to protect the induction motor from single phasing fault. Single phasing is a very dangerous fault to the electrical motor and Which damages the motor stator winding rapidly.
- Generally single phasing is nothing but a motor runs when one of the supply is disconnected due to open circuit or improper contact in switch or other electrical equipment failure.
- Normally, the motor runs with the three phase supply and which takes balance current in each phase winding. Consider one of the fuse has blown. But the motor still in a rotating position which tries to rotates at the same speed.
- At that same time, the absence phase current will be shifted to the remaining live phases. Therefore, the current in the other phases increases up to 3 times its normal value instantly. This is called a single phasing fault.
- The single phasing leads to an unbalanced current in the motor stator. The component which is present in this unbalanced current called negative sequence component.
73. The contacts of high voltage switches used in the power systems are submerged in oil. The main purpose of the oil is to
A. lubricate the contacts
B. insulate the contacts from switching body
C. extinguish the arc
D. all of the above
E. none of the above
Answer
C. extinguish the arc
Explanation:
- The contacts of high voltage switches used in the power systems are submerged in oil. The main purpose of the oil is to extinguish the arc.
- As the name implies, the contacts of switches are opened under oil.
- Usually transformer oil is used whose purpose is to cool and quench the arc formed on account of opening the contacts.
- These switches are used for circuits of high voltage and large current carrying capacities.
74. To limit short-circuit current in a power system are used
A. earth wires
B. isolators
C. H.R.C. fuses
D. reactors
Answer
D. reactors
Explanation:
Current Limiting Reactor:
- It is also known as a Fault limiting reactor.
- The current limiting reactor is an inductive coil having large inductive reactances in comparison to their resistance.
- It is used for limiting short circuit currents during fault conditions.
- Current-voltage reactors also reduced the voltage disturbances on the rest of the system.
- It is installed in feeders and ties, in generators leads, and between bus sections.
- Therefore used for reducing the magnitude of short circuit currents and the effect of the respective voltage disturbance.
- The current reactor allows free interchange of power under normal conditions.
- But when the fault occurs the disturbance is restricted by the current reactor to the faulty section.
- As the resistance of the system is very small as compared to their reactance. Hence, the efficiency of the system is not much affected.
Functions of Current Limiting Reactor:
- The current limiting reactor reduces the flow of short circuit current so as to protect the appliances from mechanical stress and overheating.
- The current reactor reduced the magnitude of voltage disturbances which is caused by short circuits.
- Therefore they protect the circuit breakers which have an inadequate rating
- It limits the fault current to flow into the healthy feeders or parts of the system, thereby avoiding the fault from spreading.
- This increases the chances of continuity of supply.
Drawbacks of the current limiting reactor:
- When the reactor is installed on the network, the total percentage reactance of the circuit increases.
- It decreases the power factor and thus the regulation becomes poorer.
Ground Wire:
- Ground wires or earth wires are bare conductors supported at the top of transmission towers.
- They serve to shield the line and intercept lightning stroke before it hits the current carrying conductors below i.e. power lines.
- Ground wires normally do not carry current. Therefore, they are often made of steel.
- The ground wires are solidly connected to ground at each tower in transmission and distribution system.
Isolator
- An isolator is a device used for isolating a circuit or equipment from a source of power.
- An isolator is a mechanical switching device that, in the open position, allows for isolation of the input and output of a device.
HRC Fuse
- HRC fuses are used to protect high voltage switch gears from short circuits.
- Used for backup security. This type of fuse is also used in motor stators.
- Used for protection of electrical appliances such as motors, transformers, automobiles.
75. Series capacitors are used to
A. compensate for line inductive reactance
B. compensate for line capacitive reactance
C. improve line voltage
D. none of the above
Answer
A. compensate for line inductive reactance
Explanation:
- Series capacitors are used to compensate the inductance of transmission line. They will increase the transmission capacity and the stability of the line. These are also used to share the load between parallel lines.
- Whenever an inductive load is connected to the transmission line, power factor lags because of lagging load current. To compensate this, a shunt capacitor is connected which draws current leading the source voltage. The power factor can be improved.
- Series reactors are used as current limiting reactors to increase the impedance of a system. They are also used to limit the starting currents of synchronous electric motors and to compensate reactive power in order to improve the transmission capacity of power lines.
- A shunt reactor is an absorber of reactive power, thus increasing the energy efficiency of the system.
76. In a single bus-bar system there will be completely shut down when
A. fault occurs on the bus itself
B. fault occurs on the neutral line
C. two or more faults occur simultaneously
D. fault occurs with respect to earthing
Answer
A. fault occurs on the bus itself
Explanation:
- The arrangement of such type of system is very simple and easy.
- The system has only one bus bar along with the switch. All the substation equipment like the transformer, generator, the feeder is connected to this bus bar only.
The advantages of single bus bar arrangements are
- It has low initial cost.
- It requires less maintenance
- It is simple in operation
Drawbacks of Single Bus-Bars Arrangement
- The only disadvantage of such type of arrangement is that the complete supply is disturbed on the occurrence of the fault.
- The arrangement provides the less flexibility and hence used in the small substation where continuity of supply is not essential.
77. A differential relay measures the vector difference between
A. two currents
B. two voltages
C. two or more similar electrical quantities
D. none of the above
Answer
C. two or more similar electrical quantities
Explanation:
- Among them differential relay is very commonly used relay for protecting transformers and generators from localised faults.
- Differential relays are very sensitive to the faults occurred within the zone of protection but they are least sensitive to the faults that occur outside the protected zone.
- The main function of differential relay was to send trip signal to circuit breaker when there is a certain difference current value between primary side and secondary side of the transformer.
- A differential relay measures the vector difference between two or more similar electrical quantities.
78. A switchgear is device used for
A. interrupting an electrical circuit
B. switching an electrical circuit 111
C. switching and controlling an electrical circuit
D. switching, controlling and protecting the electrical circuit and equipment
Answer
D. switching, controlling and protecting the electrical circuit and equipment
Explanation:
- In an electric power system, switchgear is composed of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment.
- Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream.
- The apparatus used for controlling, regulating and switching on or off the electrical circuit in the electrical power system is known as switchgear.
- The switches, fuses, circuit breaker, isolator, relays, current and potential transformer, indicating instrument, lightning arresters and control panels are examples of the switchgear devices.
79. With which of the following, a circuit breaker must be equipped for remote operation?
A. Inverse time trip
B. Time-delay trip
C. Shunt trip
D. None of the above
E. All of the above
Answer
C. Shunt trip
Explanation:
- Inverse time tripping is a characteristic of circuit breakers in which the breaker trips in more time with lower overcurrent, and less time with higher overcurrent.
- The trip time delay is the length of time it takes for the moving metal core inside the current sensing coil to move to the fully "in" position, thereby tripping the circuit breaker.
- A shunt trip is an optional accessory device that electricians and/or manufacturers install to a circuit breaker.
- This is known as a shunt trip circuit breaker. Some circuit breakers allow shunt trip kits to be installable in the field. Others require factory installation when ordering the circuit breaker.
- A shunt trip device is an optional accessory in a circuit breaker that mechanically trips the breaker when power is applied to the shunt trip terminals.
- The power for the shunt trip does not come from within the breaker, so it must be supplied from an external source.
80. A _______ is used to measure the stator % winding temperature of the generator.
A. thermocouple
B. pyrometer
C. resistance thermometer
D. thermometer
Answer
C. resistance thermometer
Explanation:
- The resistance thermometer or resistance temperature detector (RTD) uses the resistance of electrical conductor for measuring the temperature.
- The resistance of the conductor varies with the time. This property of the conductor is used for measuring the temperature.
- The main function of the RTD is to give a positive change in resistance with temperature.
- A resistance thermometer is used to measure the stator % winding temperature of the generator.