81. Series reactors should have
A. low resistance
B. high resistance
C. low impedance
D. high impedance
Answer
B. High resistance
Explanation
- In a series reactor, high resistance is used.
- Series reactors are high impedance electrical components that are used to limit current, mostly in cases of inductive motors or powerful floodlights.
- Series reactors and shunt reactors are commonly used in power systems.
- Series reactors are mainly used to limit the short-circuit current.
82. Insulation resistance of high voltage circuit breakers is more than
A. 1 mega ohms
B. 10 mega ohms
C. 100 mega ohms
D. 500 mega ohms
Answer
C. 100 mega ohms
Explanation:
- The smaller current ratings will have higher contact resistance.
- As a general rule-of-thumb, low-voltage circuit breakers should have contact resistances of 100 µΩ or less, while readings above 300 µΩ are an indication of trouble.
- Insulation resistance of high voltage circuit breakers is more than 100 Mega ohms.
83. An efficient and a well designed protective relaying should have
A. good selectivity and reliability
B. economy and simplicity
C. high speed and selectivity
D. all of the above
Answer
D. all of the above
Explanation:
Requirements for Protection System
A basic requirement of a protective system is to clear the fault with sufficient speed to minimize the adverse effects of the fault. To fulfill this requirement, a protective system is required to possess the following properties.
Selectivity:-
- Selectivity is the ability of a protective system to isolate only a faulty section of a system from rest of the healthy power system.
- Selectivity is absolute if the protective scheme responds only to faults within its own zone.
- Whereas it is said to be relative if it is obtained by grading the settings of protective relays of different zones, all of them respond to a given fault.
- The absolutely selective protective systems are known as unit systems; whereas the protection systems in which selectivity is relative are known as non-unit systems.
- As for example, differential protection is said to be a unit protective system; whereas current time graded overcurrent protection or distance protection are said to be non-unit protective systems.
Reliability:-
- Reliability is the ability of a protective system to work properly during the period for which it is in service.
- The term reliability includes both the security in fault clearance and the security against undesired clearances.
- Quantitatively reliability can be expressed as an inverse of the probability of failure. Hence, it can be said that, less the probability of failure, better the reliability.
- Failure is confined not only to protective relays but also due to breaker defects, opened control circuits or due to an unfaithful transformation of inputs by system transducers, etc. Hence, each and every component of a protective system must be regarded as a potential source of failure.
Speed of operation:-
- Speed refers to the total fault clearing time of a protective system including the protective relay tripping time as well as time taken by the breaker to open and to extinguish the arc produced between its contacts.
- It is obvious that faster the speed of operation of the elements of a protective system (relay and breakers), lesser is the damage to the power system components.
- This also limits the ionization at the fault, increases the chances of a successful auto-reclosing and reduces the dead time interval of a power system.
- Moreover, quick disconnection of the faulty section for rest of the healthy system helps in maintaining the stability of the power system.
Discrimination:-
- A protection system should be able to discriminate between the faulty condition and the normal loading conditions, particularly, when the minimum fault current is less than the maximum load current.
- As for example, while protecting a power transformer, the protective system must be able to distinguish between a fault and a magnetizing inrush.
Stability:-
- The term stability is often used to describe the quality of a protective system by virtue of which it remains inoperative under all conditions associated with faults outside their own tripping zone.
- In other words, it can be said that the protective system should remain stable against any type of disturbance present at any point of the power system except for an in-zone electrical fault.
Sensitivity:-
- Sensitivity is the ability of a protective relay to react correctly to the relatively low values of fault current.
- Sensitivity also refers to the minimum level of fault current at which a protective relay operates.
- As for example, high resistance ground faults are very common for EHV and UHV transmission lines; and the protective system must be able to detect such faults before they develop further serious problems.
- In other words, it is the protective relay setting, usually expressed in terms of the operating quantity.
84. Distance relays are generally
A. split-phase relays
B. reactance relays
C. impedance relays
D. none of the above
Answer
C. impedance relays
Explanation:
- There is one type of relay which functions depending upon the distance of fault in the line.
- More specifically, the relay operates depending upon the impedance between the point of fault and the point where relay is installed.
- These relays are known as distance relay or impedance relay.
- A distance protection relay is a name given to the protection, whose action depends on the distance of the feeding point to the fault.
- The time of operation of such protection is a function of the ratio of voltage and current, i.e., impedance.
- This impedance between the relay and the fault depends on the electrical distance between them.
- Types of distance relays are impedance relays, reactance relays, and the mho relays.
85. For which of the following protection from negative sequence currents is provided?
A. Generators
B. Motors
C. Transmission line
D. Transformers
Answer
A. Generators
Explanation:
- It protects generators and motors from the unbalanced load by detecting negative sequence current.
- Negative sequence current may cause a dangerous situation for the machine.
- Phase to phase fault mainly occurs because of the negative sequence component.
- The negative sequence relay has earthing which protects from phase to earth fault but not from phase to phase fault.
86. To protect most of the electrical equipment handling low power, the types of relays used are
A. Thermocouple
B. Electronic and bimetallic
C. Both (A) and (B)
D. None of the above
Answer
C. Both (A) and (B)
Explanation:
- A thermocouple is a sensor that measures temperature. It consists of two different types of metals, joined together at one end.
- When the junction of the two metals is heated or cooled, a voltage is created that can be correlated back to the temperature.
- A bimetallic overload relay consists of a small heater element wired in series with the motor and a bimetal strip that can be used as a trip lever.
- The bimetal strip is made of two dissimilar metals bonded together.
87. A fuse in a motor circuit provides protection against
A. overload
B. short-circuit and overload
C. open circuit, short-circuit and overload
D. none of the above
Answer
B. short-circuit and overload
Explanation:
- A fuse is an electrical safety device that operates to provide over current protection of an electrical circuit
- Fuses are widely used for protection of electric motor circuits against small overloads and short circuit
- The motor protection circuit will open the controlling contactor automatically, and the fuse will only operate for short-circuits or extreme overload
88. Wave trap is used to trap waves of
A. power frequencies
B. higher frequencies entering generator or transformer units
C. either of the above
D. none of the above
Answer
B. higher frequencies entering generator or transformer units
Explanation:
- Wave trap is a device which prevents the high-frequency carrier signals to enter the substation side.
- It is also known as line trap.
- It is connected in series with the transmission line.
- It is designed to carry the rated power frequency (50 Hz or 60 Hz) current, as well as to withstand the substation fault currents.
- A line trap, also known as wave trap, or high-frequency stopper, is a maintenance-free parallel resonant circuit, mounted inline on high-voltage (HV) AC transmission power lines to prevent the transmission of high frequency (40 kHz to 1000 kHz) carrier signals of power line communication to unwanted destinations.
89. SF6 gas has
A. is yellow in colour
B. is lighter than air
C. is nontoxic
D. has pungent small
E. none of the above
Answer
C. is nontoxic
Explanation:
- SF6 gas has excellent insulating property and it has a high electronegativity.
- Due to high electronegativity, the recovery of dielectric strength after the arc interruption in the SF6 circuit breaker is very fast.
- It has a high affinity for absorbing free electrons.
- It absorbs electrons from a negative ion. The attachment of electron with SF6 molecules may occur in two different reactions as shown below,
- Ions are obviously much heavier than a free electron and the overall mobility of charged particles in SF6 gas is much less compared to the other common gases.
90. Air blast circuit breakers for 400 kV power system are designed to operate in
A. 100 microsecond
B. 50 millisecond
C. 0.5 sec
D. 0.1 sec
Answer
B. 50 millisecond
Explanation:
- Air blast circuit breakers are used for indoor services in the medium high voltage field and medium rupturing capacity.
- Generally up to voltages of 15 KV and rupturing capacities of 2500 MVA.
- The air blast circuit breaker is now employed in high voltage circuits in the outdoors switch yard for 220 KV lines.
- Air blast circuit breakers for 400 kV power system are designed to operate in 50 millisecond.
Power System MCQs With Explanation
- The advantages of high transmission voltage are
- The charging current drawn by the cable is
- The fact that current density is higher at the surface when compared to the center is known as
- In which of the following transmission lines capacitance effect is negligible?
- The distribution constant of the transmission line is
- Proximity effect is more in case of
- If the frequency is increased, then the skin effect will
- Bundled conductors in EHV transmission lines help in
- Which of the following produces radio interference in communication lines?
- Telecommunication lines are transposed to reduce the
- What is the function of economizer in Thermal Power Plant
- Which of the following effects are associated with transmission line
- The insulator thickness on the conductor, in cables, depends upon
- By using a guard ring in a transmission line, its string efficiency
- If sag in an overhead line increase, the tension in the line
- Corona is accompanied by
- The conductor carries more current on the surface as compared to the core. This is called as
- Critical disruptive voltage is a voltage at which
- Bundled conductors are used for EHV transmission lines primarily for reducing the
- Low power factor is usually not due to
- Bonding is done at both ends of the cable to reduce the
- For Improving power factor
- Stock bridge dampers are used to
- Which of the following insulator is practically used for railway crossings?
- Which of the following decides the insulation level of 400 kV, EHV line?
- When earth fault occurs
- The minimum length of pipe electrode used for earthing should not be less than
- A ground wire is used to
- Earth resistance in typical domestic wiring is
- By which of the following systems electric power may be transmitted?
- Which of the following equipment will be most economical for regulating the voltage in the distribution feeder?
Transformer MCQs With Explanation
- In a transformer the primary flux is
- The noise of the transformer is mainly due to
- Buchholz relay is used for the protection
- Iron loss in a transformer can be determined by
- Oil is provided in an oil-filled transformer for
- In a star-delta transformer, the delta side phase voltage leads the star sides phase voltage by an angle
- Minimum voltage regulation occurs in transformer when the power factor of the load is
- The chemical used in the breather of a transformer should have the property of
- The power transformer can be termed as a constant
- In a transformer, which of the following losses are zero?
- Which of the following Transformer can have a tap changer?
- If the supply frequency (f) of a transformer decreases, the effect of frequency on the transformer's secondary output voltage?
- The primary winding of a transformer has a 120 V ac supply. What is the value of secondary voltage if the turn ratio is 10?
- The efficiency of a power transformer is _____ at full load.
- In a transformer the relation between the input frequency and the output voltage on secondary winding is _____.
- In a current transformer, _____ winding is closed.
- A transformer is a _____ device.
- A step-down transformer has _____ number of turns on primary winding and _____ number of turns on the secondary winding.
- A step-up transformer has _____ number of turns on primary winding and _____ number of turns on the secondary winding.
- Transformer works on ______ principle.
- Shell Type Transformer is mainly used for
- A Magnetic Oil level Guage (MOG) is used in the Transformer to
- The Function Of Power Transformer is/are
- The Radiators are used in transformers for
Synchronous Motor MCQs With Explanation
- In 3 -Φ synchronous motor if one of the phases is short-circuited the motor will
- The speed regulation of a synchronous motor is
- The negative phase sequence in a three-phase synchronous motor exists when the motor is
- What happens if the field winding of the synchronous motor is short-circuited?
- For power factor correction, synchronous motors operate at