1. Short circuit currents is due to
(a) Single phase to earth fault
(b) phase to phase fault
(c) all the three-phase to earth fault
(d) all three phases short-circuited
(e) any of the above
Ans: (e) any of above
Explanation:
- Short-circuits result from symmetrical (three-phase) faults, as well as unsymmetrical (LG, LL, LLG) faults.
- The occurrence of symmetrical fault brings the power transfer across the line to zero immediately, whereas the impact is only partial in case of unsymmetrical faults.
- The short-circuit currents may attain such high values that, if allowed to persist, they may result in thermal damage to the equipment.
- Therefore, the faulty section should be isolated as quickly as possible. Most of the short-circuit do not cause permanent damage.
2. The most serious consequence of a major uncleared short circuit fault could be
(a) blowing of fuse
(b) fire
(c) heavy voltage drop
(d) non of these
Ans: (b) fire
Explanation:
The most serious result of a major uncleared fault is fire which may not only destroy the equipment of its origin but may spread in the system and cause total failure. The most common type of fault which is also the most dangerous one is the short circuit.
3. Which of the following faults results in a symmetrical fault?
(a) single-phase-to earth.
(b) phase to phase fault
(c) All the three phases to earth
(d) two phases to earth
Ans: (c) All the three phases to earth
Explanation:
Symmetrical faults:
- A symmetrical fault is a fault where all phases are affected so that the system remains balanced
- A three-phase fault is a symmetrical fault
- As symmetrical faults result in balanced conditions, they may be analyzed using per-phase analysis
- LLLG fault is symmetrical fault
Unsymmetrical Faults:
- Unsymmetrical faults are the faults that lead unequal currents with unequal phase shifts in a three-phase system
- The unsymmetrical fault occurs in a system due to the presence of an open circuit or short circuit of transmission or distribution line
- It can occur either by natural disturbances or by manual errors
- The natural disturbances are heavy wind speed, ice loading on the lines, lightning strikes and other natural disasters.
- LG, LL, LLG faults are asymmetrical faults.
4. Which portion of the transmission system is more prone to fault ?
(a) Alternator
(b) Transformer
(c) Overhead line
(d) Underground Cable
Ans: (c) Overhead line
Explanation:
The overhead lines are open to the atmosphere and also pass through different environments viz. forests while other equipments are kept in substations where appropriate protection can be provided.
Additional Information
There are two types of faults:
Open Circuit Faults:
The open-circuit fault happens due to the failure of one or two conductors. These faults take place in series with the line so referred to as a series fault. Such types of faults have a strong impact on the reliability of the system. The open-circuit fault is classified as:
- Open Conductor Fault
- Two conductors Open Fault
- Three conductors Open Fault
Short Circuit Faults:
The short-circuit fault is commonly divided into symmetrical and asymmetrical types. These faults are further categorized as one of five types. In order of frequency of occurrence, they are:
Asymmetrical Faults:
Asymmetrical faults mandate the calculation of positive negative and zero sequence components separately.
Single Line to Ground Fault: This type of fault occurs when you have one of the phases (A, B or C) is shorted with the ground.
Line to Line Fault: This type of fault occurs when you have one of the phases (A, B or C) is shorted with the ground.
Double Line to Ground Fault: This type of fault occurs when two phases are shorted with the ground together (A-B-G, B-C-G or C-A-G)
Symmetrical Faults:
Symmetrical faults do not give rise to zero sequence or negative sequence components because they are perfectly balanced, symmetrical faults only have positive sequence values.
Three Phase Line to Ground Fault: The 3-phase to ground faults are faults in which all the phases (A, B, and C) are shorted together and are grounded.
Three Phase Line to Line Fault: The three-phase faults occur when you have A, B, and C phases are shorted together but the ground is not involved.
5. Which portion of power system is least prone to fault ?
(a) Alternators
(b) Switchgear
(c) Transformer
(d) Overhead line
Ans: (a) Alternator
Explanation:
- The Overhead lines have more prone to fault than other equipment and it is 50 %
- The Alternator has least prone to fault than the above option
- Majorly alternator affected by the internal fault like overloading, reverse power, Negative phase sequence
- The alternator protected by the use of "Merz prize differential relay ''
6. The magnitude of fault current depends upon
(a) Total impedance up to the fault
(b) Voltage at fault point
(c) load current being supplied before the occurrence of a fault
(d) both (a) and (b)
Ans: (d) both (a) and (b)
Explanation:
The amount of current that will flow through a short circuit depends on two values:
- The system voltage
- The connected total impedance of the path from the source to the point of the fault.
- This impedance normally includes the feeder conductor’s resistance and reactance, any transformers, impedances (going from the point of fault back to the energy source), and any other equipment connected in the path of current flow.
7. The most common type of fault is
(a) single phase to ground
(b) phase to phase
(c) two-phase to ground
(d) Three-phase to ground
Ans: (a) Single phase to ground
Explanation:
The different type of faults in power systems are:
- Single line to ground fault (LG)
- Line to line fault (LL)
- Double line to ground fault (LLG)
- Three-phase faults (LLL or LLLG)
Frequency of occurrence:
Among the given faults, LG or line to ground fault is most common and occurs frequently.
The order of frequency of occurrence is given below.
LG > LL > LLG > LLL
Severity of faults:
Among the given faults, LLLG or 3 phase faults are most severe. LG or line to ground fault is least severe.
Line to line fault is more severe than line to ground fault while double line to ground fault is one level severe than LL.
The order of Severity of faults is given below.
LLL > LLG > LL > LG
8. The maximum short circuit current occurs in the case of
(a) three phase bolted fault
(b) double line to ground fault
(c) line to line fault
(d) single line to ground fault
Ans:(c) Line to line fault
Explanation:
The different type of faults in power systems are:
- Single line to ground fault (LG)
- Line to line fault (LL)
- Double line to ground fault (LLG)
- Three-phase faults (LLL or LLLG)
Frequency of occurrence:
Among the given faults, LG or line to ground fault is most common and occurs frequently.
The order of frequency of occurrence is given below.
LG > LL > LLG > LLL
Severity of faults:
Among the given faults, LLLG or 3 phase faults are most severe. LG or line to ground fault is least severe.
Line to line fault is more severe than line to ground fault while double line to ground fault is one level severe than LL.
The order of Severity of faults is given below.
LLL > LLG > LL > LG
9. Series reactor are used to
(a) improve the transmission efficiency
(b) improve the power factor of the power system
(c) improve the voltage regulation
(d) bring down the fault level within the capacity of the switchgear
Ans: (d) bring down the fault level within the capacity of the switchgear
Explanation:
Fault current is inversely proportional to the reactance of the network. Therefore with the series reactor, the fault current magnitude is reduced. It is preferred in the generating stations in some special applications for limiting the current flowing through the low MVA rating circuit breakers.
Note:
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.
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 capacitors are used to compensate for the inductance of the 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.
10. Current limiting reactor maybe
(a) air-cored air-cooled
(b) oil-immersed magnetically shielded
(c) oil-immersed non magnetically shielded
(d) any of the above
Ans: (d) any of the above
11. The symmetrical components are used in the fault analysis because
(a) The number of equations become smaller
(b) the sequence networks do not have mutual couplings
(c) the results are required in terms of symmetrical components
Ans: (c) the results are required in terms of symmetrical components
12. In a star connected system without neutral grounding, Zero sequence current are
(a) Zero
(b) Phasor sum of phase currents
(c) same as RMS value of phase currents
(d) same as peak value of phase currents
Ans: (a) Zero
13. A balanced 3 phase system consist of
(a) Zero sequence current only
(b) positive sequence current only
(c) negative and zero sequences current
(d) Zero, negative and positive sequence currents
Ans: (b) positive sequence current only
14. The positive sequence current of a transmission line is
(a) always Zero
(b) 1/3 of negative sequence current
(c) equal to negative sequence current
(d) 3 times negative sequence current
Ans: (c) equal to negative sequence current
15. In case of an unbalanced star-connected load supplied from an unbalanced 3 phase, 3 wire system, load currents will consist of
(a) Positive sequence components
(b) Negative sequence components
(c) Zero sequence components
(d) only (a) and (b)
Ans: (d) only (a) and (b)
16. For an unbalanced fault, with paths for zero current, at the point of fault
(a) the negative and zero sequence voltages are minimum
(b) the negative and zero sequence voltages are maximum
(c) The negative sequence voltage is minimum and zero sequence voltage is maximum
(d) The negative sequence voltage is maximum and zero sequence voltage is minimum.
Ans: (c) The negative sequence voltage is minimum and zero sequence voltage is maximum
17. Zero sequence fault is absent when a fault is
(a) single line to ground fault
(b) line to line ground fault
(c) Double line to ground fault
(d) line to line
Ans: (d) line to line
18. The current of a single-phase load drawn from a 3- 3-phase system has:
(a) Zero sequence component of current as zero
(b) negative sequence component of current more than positive sequence component
(c) Positive, negative and zero sequence component equal
(d) Negative sequence component of current less than positive sequence component.
Ans: (c) Positive, negative and zero sequence component equal
19. Zero sequence currents can flow from a line to a transformer bank if the winding are in
(a) grounded star/delta
(b) delta/star
(c) star/grounded star
(d) delta/delta
Ans: (a) grounded star/delta
20. Negative sequence reactance of a transformer is
(a) equal to the positive sequence reactance
(b) larger than the positive sequence reactance
(c) smaller than positive sequence reactance
Ans: (a) equal to the positive sequence reactance
21. If all sequence voltages at the fault point in a power system are equal, then the fault is a
(a) three-phase fault
(b) the line to ground fault
(c) the line to line fault
(d) double line to ground fault
Ans: (d) double line to ground fault
22. Fuses have got advantages of
(a) cheapest type of protection
(b) inverse time-current characteristic
(c) no maintenance
(d) current limiting effect under short circuit conditions
(e) all of the above
Ans: (e) all of the above
23. The primary function fuse is
(a) open the circuit
(b) protect the appliance
(c) protect the line
(d) prevent excessive currents from flow through the circuit
Ans: (d) prevent excessive currents from flow through the circuit
24. The material used in the liquid fuse is
(a) SF6
(b) distilled water
(c) carbon tetrachloride
(d) transformer oil
Ans: (c) carbon tetrachloride
25. In a comparison of rewirable fuses HRC fuses have the advantage(s)
(a) high-speed operation
(b) high rupturing capacity
(c) no ageing effect
(d) all of the above
Ans: (d) all of the above
26.In HRC fuse the time between cut off and final current zero is called the
(a) Pre-arcing time
(b) arcing time
(c) total operating time
(d) none of these
Ans: (b) arcing time
27. If a combination of HRC fuse and a circuit breaker is employed, the circuit breaker operates for
(a) Short circuit current
(b) low overload currents
(c) Under all abnormal currents
(d) the combination is newer employed in the practice
Ans: (c) Under all abnormal currents
28. The least expensive protection for overcurrent in low voltage system is
(a) rewirable fuse
(b) isolator
(c) circuit breaker
(d) air break switch
Ans: (a) rewirable fuse
29. A circuit breaker normally operates
(a) When the power is to be supplied
(b) when the line is to be tested
(c) when the switch is to be put on
(d) whenever a fault occurs in the line
Ans: (d) whenever a fault occurs in the line
30. When a fault occurs in a high voltage transmission line, first the
(a) circuit breaker operates then the relay
(b) the relay operates then the circuit breaker
(c) the relay operates, then successively the isolator and the circuit breaker
(d) Isolator operates, then successively the relay and the circuit breaker
Ans: (b) the relay operates then the circuit breaker
31. The initiation of electric arc at the instant of contact separation is caused by
(a) Thermionic emission of electrons
(b) Field emission of electrons
(c) (a) and (b) both
(d) none of the above
Ans: (c) (a) and (b) both
32. Arcing voltage will be the least in case of
(a) carbon
(b) copper
(c) silver
(d) tungsten
Ans: (c) silver
33. The heat produced at the contact point owing to the flow of electric current is least affected by
(a) Temperature of the surrounding medium
(b) contact resistance
(c) the magnitude of electric current flowing
(d) duration of flow of current
Ans: (a) Temperature of the surrounding medium
34. Which of the following should have a low value for the contacts and their material?
(a) Thermal capacity
(b) Contact resistance
(c) thermal conductivity
(d) none of these
Ans: (b) Contact resistance
35. In a CB contact wipe is necessary
(a) to decrease the speed of opening of the CB
(b) to increase the speed of opening and reduce the arcing of contacts
(c) to increase the dielectric strength across the CB contacts
(d) none of the above
Ans: (c) to increase the dielectric strength across the CB contacts
36. Interrupting medium in a contactor may be
(a) air at atmospheric pressure
(b) SF6 gas
(c) oil
(d) compressed air
(e) any of these
Ans: (e) any of these
37. The fluids used in circuit breakers should be of
(a) High dielectric strength and thermal stability
(b) noninflammability
(c) arc extinguishing ability
(d) all of the above
Ans: (d) all of the above
38. Sparking between contacts can be reduced by inserting
(a) a capacitor in parallel with the contacts
(b) a capacitor in series with the contacts
(c) a resistor in the line
(d) a reactor in the line
Ans: (a) a capacitor in parallel with the contacts
39. For magnetic blow out of arc the magnitude field is produced
(a) in the load circuit
(b) at right angles to the axis of the arc
(c) in the line with the axis of the arc
(d) any of the above
Ans: (b) at right angles to the axis of the arc
40. In-circuit breaker the contact space is ionized by
(a) Field emission from the contact surface
(b) thermal emission from the contact surface
(c) thermal ionization of gas
(d) any of the above
Ans: (d) any of the above
41. In a circuit breaker ionization is facilitated by
(a) increase in field strength
(b) increase of mean free length
(c) high temperature of surrounding medium
(d) all of the above
Ans: (d) all of the above
42. The stability of arc in vacuum depends on
(a) The contact material only
(b) The contact material and its vapour pressure
(c) The circuit parameters only
(d) The combination of (b) and (c)
Ans: (d) The combination of (b) and (c)
43. The interrupting time of a circuit breaker is the period between the instant of
(a) initiation of short circuit and the arc extinction on an opening operation
(b) energizing of the trip circuit and the arc extinction on an opening operation
(c) initiation of short circuit and the parting of primary arc contatcs
(d) energizing of the trip circuit and the parting of primary arc contacts
Ans: (b) energizing of the trip circuit and the arc extinction on an opening operation
44. For a high-speed circuit breaker the total clearing time is around
(a) few minutes
(b) few seconds
(c) 1 to 2 cycles
(d) 5 to 20 cycles
Ans: (c) 1 to 2 cycles
45. The resistance of an electric arc can be increased by
(a) increasing the concentration of ionized particles
(b) reducing the arc length
(c) splitting the arc
(d) increasing the arc x-section
Ans: (c) splitting the arc
46. The arc voltage in a circuit breaker is
(a) In the phase with arc current
(b) lagging the arc current by 90 degree
(c) leading the arc current by 90 degree
(d) lagging the arc current by 180 degree
Ans: (a) In the phase with arc current
47. In a circuit breaker, the active recovery voltage depends upon
(a) circuit conditions
(b) Power factor
(c) amateur reaction
(d) all of these
Ans: (d) all of these
48. In a circuit breaker the current that exists at the instant of contact separation is called the .......... current.
(a) restriking
(b) breaking
(c) arc
(d) recovery
Ans: (b) breaking
49. The rate of rising of restriking voltage(RRRV) depends upon
(a) system voltage
(b) Circuit pf only
(c)switching condition only
(d) both (b) and (c)
50. The RRRV depends upon the
(a) type of the circuit breaker
(b) the capacitance of the system only
(c) the inductance of the system only
(d) Inductance and capacitance of the system
Ans: d) Inductance and capacitance of the system
Ans: (d) both (b) and (c)
