61. The arc voltage produced in A.C. circuit breaker is always
A. In phase with the arc current
B. Lagging the arc current by 90°
C. Leading the arc current by 90°
D. None of the above
Answer
A. In phase with the arc current
Explanation:
- Arc voltage is the voltage that is responsible for maintaining the arc between the contacts of the circuit breaker.
- At the time of circuit breaker contacts opening, an arc is struck between the contacts due to ionization of medium surrounding the contacts.
- A small potential difference exists between the contacts and it is responsible for maintaining the arc. This voltage is called the Arc Voltage.
- The arc voltage produced in A.C. circuit breaker is always In phase with the arc current.
62. Circuit breakers are essentially
A. current-carrying contacts called electrodes
B. arc extinguishers
C. circuits to break the system
D. transformers to isolate the two systems
E. any of the above
Answer
A. current-carrying contacts called electrodes
Explanation:
- A circuit breaker is a switching device that interrupts the abnormal or fault current. It is a mechanical device that disturbs the flow of high magnitude (fault) current and in additions performs the function of a switch.
- The circuit breaker is mainly designed for closing or opening of an electrical circuit, thus protects the electrical system from damage.
- Circuit breaker essentially consists of fixed and moving contacts.
- These contacts are touching each other and carrying the current under normal conditions when the circuit is closed.
- When the circuit breaker is closed, the current carrying contacts, called the electrodes, engaged each other under the pressure of a spring.
63. The interaction between a transmission line and communication line is minimized by
A. transposing transmission as well as communication lines
B. increasing the height of the transmission line tower
C. increasing the distance between the two lines
D. all of the above
Answer
D. all of the above
Explanation:
- Transposition of overhead line conductors refers to exchanging the positions of power conductors at regular intervals along the line so that each conductor occupies the original position of every other conductor over an equal distance in order to reduce crosstalk and otherwise improve transmission.
- Transposing transmission as well as communication lines
- There are various ways that can reduce telephone interference. Some of them are as listed below
- The harmonics at the source can be reduced with the use of AC harmonic filters, DC harmonic filters, and smoothening reactors.
- Use greater spacing between power and telephone lines.
- The parallel run between the telephone line and the power line is avoided.
- Instead of using overhead telephone wires, underground telephone cables may be used
- If the telephone circuit is ground return then replace it with the metallic return.
- Use the microwave or carrier communication instead of telephone communication.
64. A transmission line is protected by
A. Inrush protection
B. Distance protection
C. Time graded and current graded over current protection
D. Both (B) and (C)
Answer
D. Both (B) and (C)
Explanation:
- Distance relay operates by using both voltage and current to determine if a fault is in its zone of protection.
- Distance relays provide high speed fault clearance and they are used where overcurrent relays become slow.
- They are the most widely used protective scheme for the protection of transmission line.
- A transmission line is protected by Distance protection & Time graded and current graded over current protection.
65. Which of the following is the least expensive protection for overcurrent in low voltage system ?
A. Rewireable fuse
B. Isolator
C. Oil circuit breaker
D. Air break circuit breaker
E. None of the above
Answer
A. Rewireable fuse
Explanation:
- Rewireable Fuses are the fuse carriers that contain a length of fuse wire, which is wrapped around the terminals, covering the space between carriers and pins, to complete the circuit.
- It is used in interrupting the circuit in conditions like over-current or short-circuits.
- Rewirable fuses are most commonly used in housing wiring and small current circuits. It is also known as a kit – kat fuse.
- It has a porcelain base which is carrying the fixed contact through which the live wires are connected. The fuse carrier is the independent part which is easily removed or stuck in the base.
- The least expensive protection for overcurrent in low voltage system is Rewireable fuse.
66. Large internal faults are protected by
A. merz price percentage differential protection
B. mho and ohm relays
C. horn gaps and temperature relays
D. earth fault and positive sequence relays
Answer
A. merz price percentage differential protection
Explanation:
- Merz price differential protection is used to protect the transformer from internal short circuit, Internal ground faults and inter-turn shorts.
- Merz price differential protection is nothing but a percentage differential protection Which works under the principle of circulating current scheme.
67. The short circuit in any winding of the transformer is the result of
A. mechanical vibration
B. insulation failure
C. loose connection
D. impulse voltage
Answer
D. impulse voltage
Explanation:
Internal faults in a transformer are classified into two groups:
Short circuits in the transformer winding and connections:
- These are electrical faults of serious nature and are likely to cause immediate damage.
- Such faults are detectable at the winding terminals by unbalances in voltage or current.
- This type of faults include line to ground or ground to line and interturn faults on HV and LV windings.
- These short circuit faults occur due to the impulse voltage by lighting.
Incipient faults:
- These faults include poor electrical connections, core faults, failure of the coolant, regular faults, and bad load sharing between transformers.
68. Overvoltage protection is recommended for
A. hydro-electric generators
B. steam turbine generators
C. gas turbine generators
D. all of the above
E. none of the above
Answer
D. all of the above
Explanation:
- There is always a chance of suffering an electrical power system from abnormal overvoltages.
- These abnormal overvoltages may be caused due to various reasons such as the sudden interruption of heavy load, lightning impulses, switching impulses, etc.
- These over-voltage stresses may damage the insulation of various equipment and insulators of the power system. Although all the overvoltage stresses are not strong enough to damage the insulation of the system, still these overvoltages are also to be avoided to ensure the smooth operation of the electrical power system.
- These all types of destructive and nondestructive abnormal overvoltages are eliminated from the system by means of overvoltage protection.
69. Overfluxing protection is recommended for
A. distribution transformer
B. generator transformer of the power plant
C. auto-transformer of the power plant
D. station transformer of the power plant
Answer
B. generator transformer of the power plant
Explanation:
- The over fluxing protection operates when the ratio of the terminal voltage to frequency exceeds a predetermined setting and resets when the ratio falls below 95 to 98% of the operating ratio.
- By adjustment of a potentiometer, the setting is calibrated from 1 to 1.25 times the ratio of rated volts to rated frequency.
- As per present-day transformer design practice, the peak rated value of the flux density is kept about 1.7 to 1.8 Tesla, while the saturation flux density of CRGD steel sheet of core of transformer is of the order of 1.9 to 2 Tesla which corresponds to about 1.1 times the rated value.
- If during operation, an electrical power transformer is subjected to carry rather swallow more than above mentioned flux density as per its design limitations, the transformer is said to have faced over fluxing problem and consequent bad effects towards its operation and life.
- Depending upon the design and saturation flux densities and the thermal time constants of the heated component parts, a transformer has some over excitation capacity.
70. Bus coupler is very essential in arrangement
A. single bus
B. double bus, double breaker
C. main and transfer bus
D. all of the above
Answer
C. main and transfer bus
Explanation:
Bus coupler is a device which is used to couple one bus to the other without any interruption in power supply and without creating hazardous arcs.
Bus coupler is a breaker used to couple two busbars in order to perform maintenance on other circuit breakers associated with that busbar.
n this type of main and transfer Bus arrangement, there is two busbar scheme, One is main bus and the other is transfer bus.
A main advantage is that whole of the load is transferred to the transfer busbar when maintenance or repair are being done on the main busbar.