11. Short-circuit currents are due to
A. single phase to ground faults
B. phase to phase faults
C. two phase to ground faults
D. three phase faults
E. any of these
Answer
E. any of these
Explanation:
- Short circuit protection is protection against excessive currents or current beyond the acceptable current rating of equipment and it operates instantly.
- As soon as an overcurrent is detected, the device trips and breaks the circuit.
- Short-circuit currents are due to single phase to ground faults, phase to phase faults, two phase to ground faults, three phase faults.
12. Resistance grounding is used for voltage between
A. 33kV to 66kV
B. 22kV to 33kV
C. 3.3kV and 11kV
D. none of the above
Answer
C. 3.3kV and 11kV
Explanation:
- For circuits between 3.3 kV and 33 kV the earth fault currents are likely to be excessive, if solid grounding is employed.
- Either resistance or reactance is connected in neutral to ground connection in those circuits.
- There is no rule as regards which grounding should be used—resistance or reactance.
- So that the prescribed voltage level for resistance or reactance grounding is 3.3 kV to 33 kV.
- In this type of neutral grounding, the neutral of the system is connected to the ground through one or more resistance. Resistance grounding limits the fault currents.
- It protects the system from transient overvoltages.
13. Discrimination between main and back up protection is provided by the use of relays which are
A. fact
B. sensitive
C. slow
D. none of the above
Answer
C. slow
Explanation:
Types of Protection:
- The fault occurs on any part of the electric power system, it must be cleared as soon as possible to avoid damage or interference with the rest of the system.
- The protection scheme divided into two-part that is primary protection and back-up protection.
Primary Protection:
- It is the protection scheme that is designed to protect the component parts of the power system.
- The fault occurs on any line, it will be cleared first by its relay and circuit breaker.
- This forms the primary or main protection and serves as the first line of defense.
- The service record of primary relaying is very high with well over ninety percent of all operations being correct.
- Sometimes faults are not cleared by the primary relay system because of trouble within the relay, wiring system, or breaker. Under such conditions, back-up protection used.
Back-up protection:
- It is the second line of defense in case of failure of the primary protection.
- It is designed to operate with sufficient time delay so that primary relaying will be given enough time hence it is slower than primary protection.
- Definite time delay is used in backup protection.
14. Thermal overload relays are used to protect the motor against over current due to
A. short-circuits
B. heavy loads
C. grounds
D. all of the above
Answer
B. heavy loads
Explanation:
- Thermal protection switch protects against the overload.
- Thermal breakers are used where overloads can be tolerated for an extended time, such as in commercial wiring and in the protection of motors, transformers, and tungsten lamp loads.
- The long temperature dependent delay means the thermal breakers are not applicable to electronic systems and power conversion equipment protection.
- A thermal overload relay works on the heat produced by the excessive overload current.
- The heat produced by the overload current is utilized to trip the motor circuit.
- These are mostly used for protection of low-voltage squirrel cage induction motors or DC motors of lower output rating.
15. When a transmission line is energized, the wave that propagates on it is
A. current wave only
B. voltage wave only
C. both (A) and (B)
D. power factor wave only
Answer
C. both (A) and (B)
Explanation:
- When a transmission line is energized, the wave that propagates on it is current wave and voltage wave.
- When the transmission line is energized by a voltage source , a forward surge is created (the magnitude: , where is the input circuit impedance).
- The surge reaches the other end in the transit time , being the line length. Then, a backward surge is generated by the reflection factor , being the output impedance.
16. Overload relays are of type.
A. induction
B. solid state
C. thermal
D. electromagnetic
E. all above
Answer
E. all above
Explanation:
overload relay:
An overload relay is an electrical device used to protect an electric motor from overheating. It is mainly three types.
Thermal relay:
Thermal overload relay is a protective device, and that is mainly designed to cut the power whenever the motor uses too much current for an extended time period.
Once extreme current supplies throughout the motor circuit, then the relay gets open because of the improved temperature of the motor.
Electro Magnetic relay:
Magnetic overload relay can be operated by detecting the magnetic field strength which is generated by the flow of current towards the motor. This relay can be built with a variable magnetic core within a coil that holds the motor current.
Solid-state relay:
Solid-state relays are semiconductor equivalents of the electromechanical relay and can be used to control the electrical loads without the use of moving parts.
17. H.R.C. fuse, as compared to a rewireable fuse, has
A. no ageing effect
B. high speed of operation
C. high rupturing capacity
D. all of the above
Answer
D. all of the above
Explanation:
HRC fuse VS Rewireable Fuse
18. The inverse definite mean time relays are used for over current and earth fault protection of transformer against
A. heavy loads
B. internal short-circuits
C. external short-circuits
D. all of the above
Answer
B. internal short-circuits
Explanation:
- It's operation is based on the principle of discrimination between both ' operating time ' and ' fault current '.
- Operating time is inversely proportional to fault current.
- Widely used for the protection distribution lines.
- Used in the backup protection of the transformer against external short circuits.
- Used in the backup protection of distance relay in the protection of transmission lines
- Overcurrent really cannot distinguish between overload, internal fault, or external short circuit condition. It will trip C.B. in all these cases. So, an Earth Fault relay is used for backup protection.
- Backup protection can be placed in load side or primary side of the transformer but it should trip C.B. of both primary as well as secondary sides of the transformer.
- This scheme protects the windings of transformers from external short circuit faults.
Important Points:
- The protection of a transformer against overloads is performed by thermal overload relay.
- Buchholz relay is used to protect the transformer against internal faults.
19. Which of the following devices will receive voltage surge first travelling on the transmission line ?
A. Lightning arresters
B. Relays
C. Step-down transformer
D. Switchgear
Answer
A. Lightning arresters
Explanation:
- A lightning arrester or a surge diverter is a protective device which conducts the high voltage surges on the power system to the ground.
- It consists of a spark gap in series with a non-linear resistor.
- The length of the gap is so set that normal line voltage is not enough to cause an arc across the gap, but a dangerously high voltage will break down the air insulation and form an arc.
- The property of the non-linear resistance is that its resistance decreases as the voltage (or current) increases and vice-versa.
- One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded.
- The lightning arrester should not absorb any current during the normal operation, but during overvoltages, it must provide an easy path to earth.
- The impulse spark-over voltage of the arrester must safeguard the insulation of the terminal apparatus.
- The diverter must be capable of carrying the discharge current for a short duration without being damaged.
- The arrester must, after discharge, cease to catty any current, i.e., it must seal-in itself.
- After an operation, the arrester must be in a condition to accept and deal with ensuing surges i.e., there must be no failure of the arrester itself.
Important Points:
- The earthing screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against traveling waves that may reach the terminal apparatus.
- The lightning arresters or surge diverters provide protection against such surges caused by indirect lightning stroke.
20. The relay with inverse time characteristic will operate within
A. 1.5 sec
B. 5 to 10 sec
C. 5 to 20 sec
D. 20 to 30 sec
E. none of the above
Answer
B. 5 to 10 sec
Explanation:
- The relay with inverse time characteristic will operate within 5 to 10 sec.
- An inverse-time relay is one in which the operating time is approximately inversely proportional to the magnitude of the actuating quantity.