1. What are the functions of protective relays
To 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 defense
6. What is back up protection?
Is the second line of defense , 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 extendind 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?
Over current relays are those that operate when the current in a line exceeds a
predetermined value. (eg: Induction type non-directional/directional over current relay,
differential over current 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 bus bars
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
14. What is the need of relay coordination?
The operation of a relay should be fast and selective, i.e., 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 sufficiently 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
15. 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
differential 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 flow through the
relay under normal condition
16. What are the various faults to which a turbo alternator is likely to be subjected?
Failure of steam supply; failure of speed; over current; over voltage; unbalanced
loading; stator winding fault
17. 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
18. 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
19. 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
20. 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
21. 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
22. 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
23. Give the limitations of Merz Price protection.
Since neutral earthing resistances are often used to protect circuit from earth-fault
currents, it becomes impossible to protect the whole of a star-connected alternator. If an
earth-fault occurs near the neutral point, the voltage may be insufficient to operate the
relay. Also it is extremely difficult to find two identical CT’s. In addition to this, there
always an inherent phase difference between the primary and the secondary quantities
and a possibility of current through the relay even when there is no fault
24. What are the uses of Buchholz’s relay?
Bucholz relay is used to give an alarm in case of incipient(slow-developing)
faults in the transformer and to connect the transformer from the supply in the event of
severe internal faults. It is usually used in oil immersion transformers with a rating over
750KVA
25. What are the types of graded used in line of radial relay feeder?
Definite time relay and inverse-definite time relay
26. What are the various faults that would affect an alternator?
(a) Stator faults
1, 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
(c) 1, Over speed
2, Loss of drive
3, Vacuum failure resulting in condenser pressure rise, resulting in
shattering of the turbine low pressure casing
(d) 1, Fault on lines
2, Fault on bus bar
27. 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
28. What is the backup protection available for an alternator?
Over current and earth fault protection is the backup protections
29. What are faults associated with an alternator?
(a) External fault or through fault
(b) Internal fault
1, Short circuit in transformer winding and connection
2, Incipient or slow developing fault
30. What are the main safety devices available with transformer?
Oil level guage, sudden pressure delay, oil temperature indicator, winding
temperature indicator
31. What are the limitations of Buchholz relay?
(a) Only fault below the oil level are detected.
(b) Mercury switch setting should be very accurate, otherwise even for
vibration; there can be a false operation.
(c) The relay is of slow operating type, which is unsatisfactory
32. What are the problems arising in differential protection in power transformer and how are they overcome?
1. 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.
2. 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.
3. 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.
4. 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
33. 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 sufficient 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
34. 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.
35. Why bus bar protection is needed?
(a) Fault level at bus bar is high
(b) The stability of the system is affected by the faults in the bus zone.
(c) A fault in the bus bar causes interruption of supply to a large portion of the
system network.
36. What are the merits of carrier current protection?
Fast operation, auto re-closing possible, easy discrimination of simultaneous
faults
37. What are the errors in CT?
(a) Ratio error
Percentage ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100
The value of transformation ratio is not equal to the turns ratio.
(b) Phase angle error:
Phase angle _ =180/_[(ImCos _-I1Sin _)/nIs]
38. 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
39. 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.
40. 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
41. What is static relay?
It is a relay in which measurement or comparison of electrical quantities is made
in a static network which is designed to give an output signal when a threshold condition
is passed which operates a tripping device
42. What is power swing?
During switching of lines or wrong synchronization surges of real and reactive
power flowing in transmission line causes severe oscillations in the voltage and current
vectors. It is represented by curves originating in load regions and traveling towards relay
characteristics
43. What is a programmable relay?
A static relay may have one or more programmable units such as microprocessors
or microcomputers in its circuit
44. What is CPMC?
It is combined protection, monitoring and control system incorporated in the static
system.
45. What are the advantages of static relay over electromagnetic relay?
o Low power consumption as low as 1mW
o No moving contacts; hence associated problems of arcing, contact bounce,
erosion, replacement of contacts
o No gravity effect on operation of static relays. Hence can be used in
vessels ie, ships, aircrafts etc.
o A single relay can perform several functions like over current, under
voltage, single phasing protection by incorporating respective functional
blocks. This is not possible in electromagnetic relays
o Static relay is compact
o Superior operating characteristics and accuracy
o Static relay can think , programmable operation is possible with static
relay
o Effect of vibration is nil, hence can be used in earthquake-prone areas
o Simplified testing and servicing. Can convert even non-electrical
quantities to electrical in conjunction with transducers
