Please wait...

Power Systems Test 2
Result
Power Systems Test 2
  • /

    Score
  • -

    Rank
Time Taken: -
  • Question 1/15
    1 / -0.33

    What is the advantage of the static capacitors?
    Solutions

    Static capacitor:

    • The power factor can be improved by connecting capacitors in parallel with the equipment operating at lagging power factor
    • The capacitor draws a leading current and partly or completely neutralises the lagging reactive component of load current
    • This raises the power factor of the load, for three-phase loads, the capacitors can be connected in delta or star
    • Static capacitors are invariably used for power factor improvement in factories

    Advantages:

    • They have low losses
    • They require little maintenance as there are no rotating parts
    • They can be easily installed as they are light and require no foundation
    • They can work under ordinary atmospheric conditions

    Disadvantages:

    • They have short service life ranging from 8 to 10 years
    • They are easily damaged if the voltage exceeds the rated value
    • Once the capacitors are damaged, their repair is uneconomical
  • Question 2/15
    1 / -0.33

    ​If the load factor is varied; in two-part tariff
    Solutions

    Two-Part Tariff: When the rate of electrical energy is charged based on maximum demand of the consumer and the units consumed, it is called a two-part tariff

    In this method, the total charge is divided into two components: fixed charges and running charges

    The fixed charges depend upon the maximum demand of the consumer while the running charges depend upon the number of units consumed by the consumer.

    Running charges are affected by varying load factor as these depend on the number of units consumed.

    The consumer is charged at a certain amount per kW of maximum demand plus a certain amount per kWh of energy consumed.

    This type of tariff is mostly applicable to industrial consumers who have appreciable maximum demand

    Total charges = Rs (b × kW + c × kWh)

    b = charge per kW of maximum demand

    c = charge per kWh of energy consumed

  • Question 3/15
    1 / -0.33

    Phase advancers are used for which among the following machines?
    Solutions

    Phase advancers:

    • These are used to improve the power factor of induction motors
    • The low power factor of an induction motor is since its stator winding draws exciting current which lags the supply voltage by 90°
    • If the exciting ampere turns can be provided from some other AC source, then the stator winding will be relieved of exciting current and the power factor of the motor can be improved
    • This can be done by using the phase advancer which is simply an AC exciter
    • The phase advancer is mounted on the same shaft as the main motor and is connected in the rotor circuit of the motor
    • It provides exciting ampere turns to the rotor circuit at slip frequency
    • By providing more ampere turns than required, the induction motor can be made to operate on leading power factor like an over-excited synchronous motor
  • Question 4/15
    1 / -0.33

    Power factor can be improved by connecting which among these?
    Solutions

    We can improve the power factor by connecting the following equipment

    • Static capacitors
    • Synchronous condenser
    • Phase advancers
  • Question 5/15
    1 / -0.33

    In balanced three phase system,
    Solutions
    • The positive sequence currents involved in all type of systems
    • Negative sequence currents involved in only unbalanced systems
    • Zero sequence currents involved in systems involving earth
    • Thus, in balanced three phase system both negative and zero sequence currents are zero
  • Question 6/15
    1 / -0.33

    On which among the following factors does the magnitude of the fault current depend?
    Solutions

    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.
  • Question 7/15
    1 / -0.33

    Which among these is the advantage of synchronous condensers?
    Solutions

    Synchronous condenser:

    • A synchronous motor takes a leading current when over-excited and, therefore, behaves as a capacitor
    • An over-excited synchronous motor running on no load is known as synchronous condenser
    • When such a machine is connected in parallel with the supply, it takes a leading current which partly neutralises the lagging reactive component of the load
    • Thus, the power factor is improved

    Advantages:

    • By varying the field excitation, the magnitude of current drawn by the motor can be changed by any amount, this helps in achieving stepless control of power factor
    • The motor windings have high thermal stability to short circuit currents
    • The faults can be removed easily

    Disadvantages:

    • There are considerable losses in the motor
    • The maintenance cost is high
    • It produces noise
    • Except in sizes above 500 kVA, the cost is greater than that of static capacitors of the same rating
    • As a synchronous motor has no self-starting torque, therefore, an auxiliary equipment has to be provided for this purpose
  • Question 8/15
    1 / -0.33

    In which of the following given faults, all the sequence currents are equal?
    Solutions

    The following circuit shown is a system with line to ground fault.

     

     

     

     

     

     

     

     

     

     

    Here, Va = 0, Ib = 0, Ic = 0

    Now, 

    By substituting Iand Ic values,

    Hence in LG faults, all of the three components la0, la1 and la2 are equal
  • Question 9/15
    1 / -0.33

    Minimum faults occur in which of the following power system equipment?
    Solutions

    In transmission system we use different power system equipment. The occurrence of faults is as follows.

    • The occurrence of faults is more common in transmission lines
    • Minimum faults occur in alternators
  • Question 10/15
    1 / -0.33

    ​In flat rate tariffs, the rate is fixed by considering their
    Solutions

    Flat Rate Tariff: When different types of consumers are charged at different uniform per unit rates, it is called as a flat-rate tariff.

    • The consumers are grouped into different classes and each class of consumers is charged at a different uniform rate
    • The different classes of consumers are made considering their diversity and load factors
    • It is fairer to a different type of consumers and quite simple in calculations
    • The flat rate for power load is generally lower than the lighting load

    Disadvantages:

    • Separate meters are required for lighting load, power load etc. This makes expensive and complicated
    • A class of consumers is charged at the same rate irrespective of the magnitude of energy consumed
  • Question 11/15
    1 / -0.33

    Consumers pay less fixed charges in
    Solutions

    Two-Part Tariff: When the rate of electrical energy is charged based on maximum demand of the consumer and the units consumed, it is called a two-part tariff

    In this method, the total charge is divided into two components: fixed charges and running charges

    The fixed charges depend upon the number of units consumed by the consumer.

    The consumer is charged at a certain amount per kW of maximum demand plus a certain amount per kWh of energy consumed.

    This type of tariff is mostly applicable to industrial consumers who have appreciable maximum demand

    Total charges = Rs (b × kW + c × kWh)

    b = charge per kW of maximum demand

    c = charge per kWh of energy consumed

    Advantages:

    • It is very easy to understand
    • It recovers the fixed charges which depend upon the maximum demand of the consumer but independent of the units consumed

    Disadvantages:

    • The consumer must pay the fixed charges irrespective of the fact that whether he has consumed or not
    • There is always an error in assessing the maximum demand of the consumer

    Maximum Demand Tariff:

    • It is similar to the two-part tariff with the only difference that the maximum demand is actually measured by installing maximum demand meter in the premises of the consumer
    • It is mostly applied to big consumers
    • It is not suitable for the small consumer as a separate maximum demand meter is required
    • Consumers pay less fixed charges in
  • Question 12/15
    1 / -0.33

    In which portion of the transmission system is the occurrence of the fault more common?
    Solutions

    In transmission system we use different power system equipment. The occurrence of faults is as follows.

    • The occurrence of faults is more common in transmission lines
    • Minimum faults occur in alternators
  • Question 13/15
    1 / -0.33

    Which of following are used to reduce short circuit fault currents?
    Solutions
    • Current limiting reactors are used to reduce short circuit fault currents
    • The main motive of using current limiting reactors is to reduce short-circuit currents so that circuit breakers with lower short circuit breaking capacity can be used
    • They can also be used to protect other system components from high current levels and to limit the inrush current when starting a large motor
  • Question 14/15
    1 / -0.33

    When a line-to-line fault occurs, the short circuit current of an alternator depends upon its
    Solutions

    When a line-to-line fault occurs, the short circuit current of an alternator depends upon its synchronous reactance.

    The synchronous reactance (XS) is the imaginary reactance employed to account for the voltage effects in the armature circuit produced by the actual armature leakage reactance and by the change in the air gap flux caused by the armature reaction.
  • Question 15/15
    1 / -0.33

    Which of the following fault results into a three phase faults?
    Solutions

    Three phase involves two kind of faults and they are,

    • Line to line to line to ground (LLLG fault) - fault due to all the three phases to earth
    • Line to line to line (LLL fault) - fault due to all the three phases
User Profile
-

Correct (-)

Wrong (-)

Skipped (-)


  • 1
  • 2
  • 3
  • 4
  • 5
  • 6
  • 7
  • 8
  • 9
  • 10
  • 11
  • 12
  • 13
  • 14
  • 15
Get latest Exam Updates
& Study Material Alerts!
No, Thanks
Click on Allow to receive notifications
×
Open Now