Interview Questions on Switchgear Protection
1. What are the functions of protective relaysTo detect the fault and initiate the operation of the circuit breaker to isolate the defective element from the rest of the system, thereby protecting the system from damages consequent to the fault.
2. Give the consequences of short circuit.
Whenever a short-circuit occurs, the current flowing through the coil increases to an enormous value. If protective relays are present , a heavy current also flows through the relay coil, causing it to operate by closing its contacts.The trip circuit is then closed, the circuit breaker opens and the fault is isolated from the rest of the system. Also, a low voltage may be created which may damage systems connected to the supply.
3. Define protected zone.
Are those which are directly protected by a protective system such as relays, fuses or switchgears.If a fault occurring in a zone can be immediately detected and or isolated by a protection scheme dedicated to that particular zone.
4. What are unit system and non unit system?
A unit protective system is one in which only faults occurring within its protected zone are isolated.Faults occurring elsewhere in the system have no influence on the operation of a unit system.A non unit system is a protective system which is activated even when the faults are external to its protected zone.
5. What is primary protection?
Is the protection in which the fault occurring in a line will be cleared by its own relay and circuit breaker.It serves as the first line of defence.
6. What is back up protection?
Is the second line of defence , which operates if the primary protection fails to activate within a definite time delay.
7. Name the different kinds of over current relays.
Induction type non-directional over current relay,Induction type directional over current relay & current differential relay.
8. Define energizing quantity.
It refers to the current or voltage which is used to activate the relay into operation.
9. Define operating time of a relay.
It is defined as the time period extended from the occurrence of the fault through the relay detecting the fault to the operation of the relay.
10. Define resetting time of a relay.
It is defined as the time taken by the relay from the instant of isolating the fault to the moment when the fault is removed and the relay can be reset.
11. What are over and under current relays?
Overcurrent relays are those that operate when the current in a line exceeds a predetermined value. (eg: Induction type nondirectional/ directional overcurrent relay, differential overcurrent relay)whereas undercurrent relays are those which operate whenever the current in a circuit/line drops below a predetermined value.(eg: differential over-voltage relay)
12. Mention any two applications of differential relay.
Protection of generator & generator transformer unit; protection of large motors and busbars .
13. What is biased differential bus zone reduction?
The biased beam relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected zone. It is essentially an over-current balanced beam relay type with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force.
The operation of a relay should be fast and selective, ie, it should isolate the fault in the shortest possible time causing minimum disturbance to the system. Also, if a relay fails to operate, there should be suficiently quick backup protection so that the rest of the system is protected. By coordinating relays, faults can always be isolated quickly without serious disturbance to the rest of the system.
14. Mention the short comings of Merz Price scheme of protection applied to a power transformer.
In a power transformer, currents in the primary and secondary are to be compared. As these two currents are usually different, the use of identical transformers will give di??erential current, and operate the relay under no-load condition. Also, there is usually a phase difference between the primary and secondary currents of three phase transformers. Even CT’s of proper turn-ratio are used, the differential current may ??ow through the relay under normal condition.
15. What is an under frequency relay?
An under frequency relay is one which operates when the frequency of the system (usually an alternator or transformer) falls below a certain value.
16. Define the term pilot with reference to power line protection.
Pilot wires refers to the wires that connect the CT’s placed at the ends of a power transmission line as part of its protection scheme. The resistance of the pilot wires is usually less than 500 ohms.
17. Mention any two disadvantage of carrier current scheme for transmission line only.
The program time (ie, the time taken by the carrier to reach the other end-upto .1% mile); the response time of band pass filter capacitance phase-shift of the transmission line .
18. What are the features of directional relay?
High speed operation; high sensitivity; ability to operate at low voltages; adequate short-time thermal ratio; burden must not be excessive.
19. What are the causes of over speed and how alternators are protected from it? Sudden loss of all or major part of the load causes over-speeding in alternators.
Modern alternators are provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime mover when a dangerous over-speed occurs.
20. What are the main types of stator winding faults?
Fault between phase and ground; fault between phases and inter-turn fault involving turns of the same phase winding.
21. What are the types of graded used in line of radial relay feeder? De??nite time relay and inverse-definite time relay.
What are the various faults that would affect an alternator?
Stator faults
Phase to phase faults 2, Phase to earth faults 3, Inter turn faults
(b)
1, Earth faults
2, Fault between turns
3, Loss of excitation due to fuel failure
1, Over speed
2, Loss of drive
3, Vacuum failure resulting in condenser pressure rise, resulting in shattering of the turbine low pressure casing
1, Fault on lines
2, Fault on busbars
22. Why neutral resistor is added between neutral and earth of an alternator?
In order to limit the flow of current through neutral and earth a resistor is introduced between them.
23. What are faults associated with an alternator?
External fault or through fault
Internal fault
1, Short circuit in transformer winding and connection 2, Incipient or slow developing faults
24. What are the problems arising in differential protection in power transformer and how are they overcome?
Difference in lengths of pilot wires on either sides of the relay. This is overcome by connecting adjustable resistors to pilot wires to get equipotential points on the pilot wires.
Difference in CT ratio error difference at high values of short circuit currents that makes the relay to operate even for external or through faults. This is overcome by introducing bias coil.
Tap changing alters the ratio of voltage and currents between HV and LV sides and the relay will sense this and act. Bias coil will solve this.
Magnetizing inrush current appears wherever a transformer is energized on its primary side producing harmonics. No current will be seen by the secondary. CT’s as there is no load in the circuit. This difference in current will actuate the differential relay. A harmonic restraining unit is added to the relay which will block it when the transformer is energized.
25. What is REF relay?
It is restricted earth fault relay. When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small, which may not be suffcient to activate the relay, unless the relay is set for a very low current.
Hence the zone of protection in the winding of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay.
26. What is over fluxing protection in transformer?
If the turns ratio of the transformer is more than 1:1, there will be higher core loss and the capability of the transformer to withstand this is limited to a few minutes only. This phenomenon is called over fluxing.
27. Why busbar protection is needed?
Fault level at busbar is high
The stability of the system is affected by the faults in the bus zone.
A fault in the bus bar causes interruption of supply to a large portion of the system network.
28. What is field suppression?
When a fault occurs in an alternator winding even though the generator circuit breaker is tripped, the fault continues to fed because EMF is induced in the generator itself. Hence the field circuit breaker is opened and stored energy in the field winding is discharged through another resistor. This method is known as field suppression.
29. What are the causes of bus zone faults?
Failure of support insulator resulting in earth fault
Flashover across support insulator during over voltage Heavily polluted insulator causing flashover Earthquake, mechanical damage etc.
30. What are the problems in bus zone differential protection?
Large number of circuits, different current levels for different circuits for external faults.
Saturation of CT cores due to dc component and ac component in short circuit currents. The saturation introduces ratio error.
Sectionalizing of the bus makes circuit complicated.
Setting of relays need a change with large load changes.
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