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Power System Protection MCQs

Power System Protection MCQs

Power System Protection MCQs with brief explanatory answers.

The essential components of power system protection are

The essential components of power system protection are:

  1. Protection relays & Breakers
  2. Current and Voltage transformers
  3. Batteries and communication channels
  4. Fuses
  5. All of these

Correct answer: 5. All of these

Explanation:

Essential components of power system protection

  • Protective relays & Breakers: Protection relays are used to sense the fault and initiate a trip, or disconnection. Once there is fault relays signals circuit breakers to open/close the system based on relay and auto-recloser commands.
  • Current and voltage transformers: CT & PTs are used to step down the high voltages and currents of the electrical power system to convenient and safe levels so that relays can deal with them
  • Batteries: They are responsible to provide power in case of power disconnection in the system
  • Communication channels: The allow analysis of current and voltage at remote terminals of a line. They also provide remote tripping of equipment.
  • Fuses: Fuses are capable of both sensing and disconnecting faults. They are generally employed for parts of distribution system.

The earth fault current is usually _______ the short-circuit current

The earth fault current is usually _______ the short-circuit current

  1. Lesser than
  2. More than
  3. Same as
  4. None of above

Correct answer: 1. Lesser than

Explanation:

  • Earth fault: It happens when the conductor of a cable comes in contact with earth. It is termed as fault or ground fault
  • Short circuit fault: It happens when two conductors of a multi-core cable come in electrical contact with each other due to insulation failure
  • The earth fault involves partial breakdown of winding insulation to earth. The leakage current due to earth fault is considerably lesser as compared to the short circuit current

Relay protection in an alternator is absolutely necessary against

Relay protection in an alternator is absolutely necessary against

  1. Overcurrent
  2. Stator winding faults
  3. Field failure
  4. None of the above

Correct answer: 2. Stator winding faults

Explanation: Stator winding faults occur mainly due to the insulation failure of the stator windings. Stator winding faults are most dangerous type of faults and they are very likely to result in considerable damage to the machinery. Relay protection in an alternator is therefore absolutely necessary against stator winding faults.

Overcurrent protection is not necessary since the modern alternators are designed with very high values of internal impedance and can stand a complete short-circuit at their terminals for sufficient time without any occurrence of serious overheating.

Compared to stator winding fault the chances of field failure of alternators are undoubtedly very rare and even that happens no immediate damage will be caused by permitting the alternator to run without a field for a short-period.

If stator winding fault is not cleared quickly, it can lead to

If stator winding fault is not cleared quickly, it can lead to

  1. Burning and welding-up of stator lami­nations
  2. Damage  to other  equipment
  3. Burning of stator  coils
  4. All of the above

Correct answer: 4. All of these

Arc in a circuit breaker is initiated by the process of

Arc in a circuit breaker is initiated by the process of

  1. Field emission of electrons
  2. Thermionic emission of electrons
  3. Both of these
  4. None of these

Correct answer: 4. Both of these

Explanation:

Thermionic emission is defined as discharge of electrons from heated materials, while field emission of electron is defined as electron emission using a strong electric field near to a metal surface. Arc in a circuit breaker is initiated by the process of field emission as well as thermionic emission of electrons.

If failure of alternator takes place in alternator, then it

If failure of alternator takes place in alternator, then it

  1. Draws some current from supply
  2. Runs as a synchronous motor
  3. Goes in inverted running mode
  4. All of these

Correct answer: 4. All of these

Which of following is a reason for using power system protection

Which of following is a reason for using power system protection:

  1. Minimize damage to equipment
  2. Protect against overloads
  3. Protect the public
  4. Improve system stability
  5. All of these

Correct answer: 5. All of these

Overheating of a transformer may be due to

Overheating of a transformer may be due to

  1. Overloads only
  2. Short-circuits only
  3. Cooling system failure
  4. All of these

Correct answer: 4. All of these

Explanation:

  • Overloading in transformers: Transformers are rated in kVA.  This kVA rating is also dependent on temperature. The rating of a transformer depends upon temperature rise, which intern depends on the transformer’s losses. Transformers can have two or more kVA ratings depending on temperature values.
  • Short circuits: Short circuits in transformer usually happen when two conductors of a multi-core cable come in electrical contact with each other due to insulation failure. Short circuits in transformers can result in overheating.
  • Cooling system failure: Malfunctioning of the cooling equipment such as blockages in the cooling oil circuit. Can result in overheating of transformers. Failure of the cooling system is however occasionally as compared to overloads and short circuits.

Buchholz relay is

Buchholz relay is:

  1. Oil actuated relay
  2. Gas actuated relay
  3. Both of these
  4. None of these

Correct answer: 2. Gas actuated relay

Explanation: Buchholz relay is a gas-actuated relay that is employed in oil-immersed transformers for protection against all kinds of faults. It is usually installed in the pipe connecting the conservator to the main tank.

Under normal conditions, a lighting arrestor conducts ___________ current to earth

Under normal conditions, a lighting arrestor conducts ___________ current to earth:

  1. Small
  2. Moderate
  3. Very high
  4. Zero

Correct answer: 4. Zero

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