SESSION:15

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1)       What are the main elements of current carrier protection?

The main elements of the carrier current protection are: Transmitter, Receiver, Coupling equipment and Line trap.

2)       Why Merz-Price Protection is not suitable beyond 33kV?

Difficulties are experienced in balancing the secondaries of the CTs and this is why Merz-Price protection is not employed beyond 33kV.

3)       What types of relays are suitable for the protection of radial feeders?

Induction type IDMT relays are more suitable for protection of the radial feeders because the time current characteristics are similar in shape and in no case they cross each other at any time.

4)       What is the purpose of Line trap and Coupling capacitors in carrier channels?

Line trap is an LC network inserted between the busbar and connection of coupling capacitors to the line and tuned to resonance at the high frequency and is used to confine the carrier currents to the protected section so as to avoid interference with or from adjacent carrier current channel.

Coupling capacitors is used to connect the high frequency (carrier) equipment to one of the line conductors and simultaneously serves to isolate the carrier equipment from high power line voltage.

5)       What is Unit Protection?

Unit system of protection is one in which the protection responds to the faults in the protected zone alone and it does not respond to through faults (faults beyond the protected zone). None unit systems does not have zone boundary.

6)       What is the minimum time interval set for two adjacent circuit breakers to operate in radial feeders?

It will be usually around 0.4 sec.

7)       What are the type of faults that are likely to occur in operation of 3 phase induction motor?

The faults which are likely to occur in operation of 3-phase induction motors are: 

Stator faults (phase to phase, phase to earth, and inter turn faults), Rotor fault (earth fault and inter turn fault), prolonged overloading, stalling, unbalanced system voltages, single phasing, under voltage and reverse phase.

8)       Why prolonged overloading protection is required for Induction motor?

Prolonged overloading on induction motor can cause excessive temperature rise in the stator and rotor windings of induction motor and also it damages the insulation resulting in the winding fault. Hence overloading protection is provided irrespective size and rating of the motor. Overload protection for the motor should not operate during starting of the motor.

Thermal overload relays or inverse over current relays are provided for protecting motors against prolonged overloading conditions.

9)       Why Negative sequence current protection is provided for Induction Motor?

Negative sequence currents flow in to the motor when the motor is supplied with unbalanced supply voltage. This flow of negative sequence currents will cause the overheating of the motor.

10)    What is Stalling in Induction motor and how to protect motor during Stalling?

Due to the mechanical problem in the motor or due to high overloading during starting of motor induction motor fails to start. This condition where the motor fails to start is called Stalling and it is undesirable as motor draws heavy currents. Therefore motor should be immediately isolated from the supply. 

Instantaneous over-current relay is employed to protect the motor against Stalling.

11)    What is Single Phasing and problems associated with it in Induction Motor?

Single Phasing in an Induction motor arises when one of the supply lines out of three phase open circuits. In this condition, motor continue to operate delivering load not exceeding 57.7% of the normal rating with the same temperature rise compared to three phase supply delivering full load. 

Some of the problems associated with single phasing are it may cause extreme magnetic unbalance, reduction in the torque of the motor, and overheating due to negative phase sequence currents. Operation of the motor under this condition is not advisable at it damages the motor. Therefore Thermal overload relays are provided for the protection of motor against single phasing.