The document discusses various methods and types of protection for transmission lines, including over-current protection and distance protection, detailing their principles, operation, and classifications. It emphasizes the need for auto-reclosing schemes to minimize outage time and improve system reliability while addressing factors affecting their effectiveness. Additionally, it outlines the importance of bus bar protection to prevent widespread outages and ensure system stability during faults.

1. Unit-III
Apparatus Protection:
Md Irshad Ahmad
Irshad.ahmad@jit.edu.in
Electrical Engineering Department
Subject: NEE –602: SWITCHGEAR AND PROTECTION

2. Unit-III
Protection of Transmission Line:
 Over current protection
 Distance protection,
 Pilot wire protection
 Carrier current protection
 Protection of bus
 Auto re-closing,

3.  As the fault impedance is less than load impedance,
the fault current is more than load current. If a short
circuit occurs the circuit impedance is reduced to a
low value and therefore a fault is accompanied by
large current.
 Over-current protection is that protection in which
the relay picks up when the magnitude of current
exceeds the pickup level.
 The Over-current relays are connected to the system,
normally by means of CT's
Over current protection

4.  1. High speed Over-current protection.
 2. Definite time Over-current protection.
 3. Inverse minimum time Over-current protection.
 4. Directional Over-current protection
(of above types).
Over-current relaying has following
types:

5.  Construction and working principle of instantaneous over current
relay quite simple.
 Magnetic core is wound by current coil. A piece of iron is so fitted by
hinge support and restraining spring in the relay, that when there is
not sufficient current in the coil, the NO contacts remain open. When
current in the coil crosses a present value, the attractive force
becomes sufficient to pull the iron piece towards the magnetic core
and consequently the no contacts are closed.
Instantaneous Over Current Relay

6.  Definite Time Over Current Relay
 This relay is created by applying intentional time delay after crossing
pick up value of the current. A definite time over current relay can
be adjusted to issue a trip output at definite amount of time after it
picks up. Thus, it has a time setting adjustment and pick up
adjustment.
 Inverse Time Over Current Relay
 Inverse time is a natural character of any induction type rotating
device. This means the speed of rotation of rotating art of the device
is faster if input current is increased.
 Inverse Definite Minimum Time Over Current Relay or IDMT O/C
Relay
 Inverse time characteristics can not be achieved, in an over current
relay. As the current in the system increases, the secondary current
of the current transformer is increased proportionally. The secondary
current is fed to the relay current coil. But when the CT becomes
saturated

7. Classification of Transmission Lines
Short Lines Length < = 80kM
Medium Lines Length < = 240kM
Long Lines Length > 240kM

8. Types of faults in the transmission
system
Short circuit faults Frequency
Phase – Ground faults 85%
Phase- Phase faults 8%
Phase – Phase –Ground faults 5%
3 Ph faults 2%
Open circuit faults
Broken conductor
Open jumper

9. Protection Scheme
Protection Scheme for Transmission lines as per CBIP guidelines
• Should have two independent high speed main protection schemes
• Two stage over voltage protection
• Sensitive IDMT directional E/F relays
• Auto reclose relay suitable for 1 ph/3ph (with deadline charging and
synchro check) reclosure.
Types of main Protections:
• Over Current Protection.
• Differential or Phase Comparison or Unit Protection.
• Distance Protection.

10. • Shall have min. of three independent zones with directional characteristics.
• Shall be non switched type with separate measurement for both earth faults
and phase faults
• Capable of 1phase and 3 phase tripping.
• Capable of operation for close up faults and switch on to faults
• Accuracy of better than 5% of reach setting for Zone 1, 10% for Zone-2 &3.
• Shall have variable residual compensation.
• Shall include power swing detection feature for selectively blocking.
• Shall include fuse failure feature to monitor all types of fuse failures and block
distance protection.
Requirements of distance protection:

11. If = E/(ZS+ZL)
The reach of over current relay is function of Source
Impedance which varies considerably, making it difficult
to get fast and Selective tripping .
E ZS ZL
If
XXXXX
Over Current Protection

12. Phase Comparison Protection
 Current Phase comparison type
 Suitable for operation with PLCC
 High sensitivity and selectivity for all types of faults
 Capable of single and three pole tripping.
 Un effected By:
 Heavy load transfer
 Power swings
 CT saturation
 CT Phase errors


13. Distance Protection
Type of distance relays
Reactance
 Suitable for short lines
 Not effected by fault resistance
 Effected by power swings
 Non directional
Impedance
 Suitable for medium lines
 Non directional
 Effected by fault resistance
Mho
 Directional
 Least effected by power swings
 Less effected by fault resistance

14. Impedance Relay Characteristics
Load
Area
X
R
Z1
Z2
Z3

15. MHO relay characteristic
The characteristic of a mho impedance element , when plotted
on a R/X diagram, is a circle whose circumference pass through
the origin .
Y
Y = relay characteristic angle
R
X

16. OFF set MHO characteristic
Under close up faults, when the voltage is near to zero then MHO will
not operate. The mho characteristic can be shifted towards origin for
operation of close up faults. This is know as OFF set MHO.
Y
Y = relay characteristic angle
R
X

17. Lenticular characteristics
The characteristic of lenticular mho will be useful to provide
maximum load transfer condition with maximum fault
resistance coverage.
Y
Y = relay characteristic angle
Z-1
Z-2
Z-3
Z-3
R

18. Quadrilateral characteristic
It is a basically a reactance relay superseded with
controlled resistive reach.
Y
Y = relay characteristic angle
Z-1&2
Z-1
Z-2
Z-3
Z-3

19. Zones of Distance Protection:
Z1
Z2
Z3
BASIC SETTING PHILOSOPHY
ZONE –1 : 80 % of protected line
ZONE –2 : 100 % of protected line + 20 % of shortest adj. line
section or 100% + 50% of transformer impedance
ZONE –3 : 100% of protected line + 100 % of longest adj. line
or 100 % + 100% of transformer impedance.
ZONE -4 : To cover close up back-up non-directional faults generally
reverse reach will be provided in relays (10%).
X X X X X X

20. Parallel Compensation
Necessity of parallel compensation:
For the fault on the parallel line, fault current also fed from healthy line and
 this current pass through ground. This current changes the mutual
 inductance and in turn causes relay measuring impedance to increase and
 is more than actual fault impedance.
This effect will be compensated by connecting neutral current of the line to
 parallel line.
This compensation will not work, if the parallel line neutral current is more
 than line neutral current.

21. ZONE-II CHARACTERISTICS
Delayed tripping and non selective phase tripping.
Provide back up protection for part of adjacent line.
Trip the faulty line instantaneously using carrier aided tripping.
Time delay is normally 500ms
ZONE-III CHARACTERISTICS
This provides back up protection for the adjacent lines or
transformer
Time delay is normally 1500ms
ZONE –IV CHARACTERISTICS
This provides back up protection for the station faults
It is normally in the reverse direction
Time delay is normally 1500ms

22. Distance Schemes:
1 . P . U . R -- Permissive under reach scheme
2. P . O . R -- Permissive Over Reach scheme
3. BLOCKING SCHEME
4. WEAK END FEED

23. AUTORECLOSE – PHILOSOPHY
NEED FOR AUTO RECLOSE
1. REDUCING OUTAGE TIME
2. IMPROVED RELIABILITY
3. RESTORATION OF NETWORK STABILITY AND
SYNCHRONISM
TYPES OF FAULTS
1. TRANSIENT FAULTS
2. SEMI PERMANENT FAULTS
3. PERMANENT FAULTS

24. TRANSIENT FAULTS -CHARACTERISTIC
Chracterised by disappearnance after Short dead time and are
disapper without any action being taken.
TYPES OF TRANSIENT FAULTS
1. Lightning strokes resulting in fashovers
2. Conductor swinging due to high winds
3. Bird fault
4. Temporary contact with foreign objects like tree etc.
About 85 % of faults on transmission lines are transient in nature

25. DEAD TIME :
The time between the autoreclose scheme being energised
and the operation of the contacts which energise the CB closing
Circuit.
RECLAIM TIME :
The time following a successful closing operation measured
from the instant the A/R relay closing contacts make, which
must elapse before the autoreclose relay will initiate reclosing
sequence in the event of a further fault.

26. CHOICE OF RECLAIM TIME
The reclaim time must not be set to such a low value that
the intended operating cycle of the breaker is exceeded
when two fault incidents occurs close together.
for example the reclaim time for a air blast circuit breakers
must allow time for air pressure to recover to its normal
value.
CHOICE OF DEAD TIME
Dead time for EHV system lower limit is decided by
de-ionising time, upper limit is decided by transient stability
and synchronism

27. Power Swing
Power Swings are disturbances in system due to various reasons
such as sudden load throw, bad synchronization etc
Power swings are characterized by slow power flow oscillations,
resulting in swinging of voltages and currents, resulting in
operating point movement into distance relay characteristics,
in turn can cause tripping of distance relays.
Tripping during power swings is undesirable since no actual fault
is present and moreover a line outage during power swing may
cause further deterioration to system stability.
Detection of power swing will block the distance protection
Zones 2,3,4. Normally tripping in Zone-I is not blocked even after
detection of power swing.

28. X
R
Z1
Z2
Z3
Power
swing
detection
zones
Power Swing detection
Time taken by fault locus to cross the power swing
detection zones is more than 40-50ms, then it is called
power swing.

29. Fuse Failure Function
Asymmetrical measuring voltage failure:
Substantial asymmetry of measured voltage, while the measured
 currents are in symmetry indicates fuse fail
Asymmetry of voltage detected by 3Uo or U2 > threshold
Symmetry in current detected by 3Io or I2 < threshold
During blocking of distance protection by fuse fail, the distance
 protection switched to emergency over current function automatically.
 If the asymmetry in measured current is detected during blocking by FF
 function, then FF block will released.

30. Switch on to fault
This feature provide protection against energisation of the tr.
line with fault or dead short.
Distance protection will not provide protection in
this case as voltage is not available for distance measurement.
It can be activated by TNC switch or CB aux. binary input or
internal detection of current rise.
It provides instantaneous 3Ph trip and blocks auto reclose.

31. One and Half Breaker Scheme
Ckt-1 Ckt-2
Bus-1
Bus-2
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Stub Protection

32. DEF Protection
It provides back up protection for tr. line.
It provides reliable protection for high resistance earth
faults.
It uses cross polarized voltage for directional
discrimination.

33.  Carrier current protection scheme is mainly used for the
protection of the long transmission line.
 In the carrier, current protection schemes, the phase angle of
the current at the two phases of the line are compared instead
of the actual current.
 Phase angle of the line decides whether the fault is internal
and external.
 The main elements of the carrier channel are a transmitter,
receiver, coupling equipment, and line trap
Carrier Current Protection of
Transmission Lines

34.  In Absence Of BUSBAR Protection,
Fault Clearance takes place
in Zone II of Distance Relay by
Remote End Tripping
 This Means Slow & Unselective Tripping
and wide spread blackout
NEED FOR BUS BAR PROTECTION

35.  Minimizing damage at fault location
 Maintaining system stability
 Localizing isolation to avoid wide
spread disruption
 Delayed clearance create shock to inter
connected equipment like Generator shaft
and windings of Transformer
NEED FOR BUS BAR PROTECTION

36.  High speed operation
 Selectivity – shall isolate the faulty bus bar only
 Stability – stable for through faults upto 40 KA
fault level
 Reliability – Check feature
 Applicable for any type of bus bar protection
 Shall provide zone indication
REQUIREMENT OF BUS BAR
PROTECTION

37.  Continuous supervision for CT Secondaries
against any possible Open Ckt.
 In case of detection of any Open Circuiting of CT
secondary, after a time delay, the affected zone
of protection shall be rendered(provide) in-
operative and an alarm initiated.
REQUIREMENT OF BUS BAR PROTECTION

38.  Bus bar protection must be provided in all new 400kV and
220kV Substations as well as Generation Switch Yards
 For existing substations, provision of bus bar protection is
considered a must at 400kV level and for 220kV level it is
essential at substations having multiple feed.
 In case of radially fed 220kv substations, having more
than one bus it is desirable to have bus bar protection but
is not a must
RECOMMENDATIONS FOR PROVIDING BUS BAR
PROTECTION

39. NEED FOR BUSBAR PROTECTION
In Its Absence Fault Clearing Takes Place In Zone-2 Of Distance Relay By Remote
End Tripping. this means slow and unselective tripping and wide spread blackout.
EFFECT OF DELAYED CLEARENCE
 Greater damage at fault point.
 Indirect shock to connected equipment like shafts of generator and windings of
transformer.
BASIC THEORY
Kirchoff’s current law states that the sum of the currents entering a given node must
be equal to the currents leaving at that node.
BASICS OF BUSBAR PROTECTION

40. TYPES OF BUS BAR PROTECTION
 Low Impedance
 High Impedance
Both use Circulating Current Differential
Principle - To isolate the entire bus bar section
by disconnecting all the feeders connected to
the bus.

41. HIGH SPEED DIFFERENTIAL PROTECTION
Simple Current Differential Circuit:

42. Simple Current Differential Circuit with Bus Fault

43. Simple Current Differential Circuit with Close in Line Fault

44. AUTO-RECLOSING
 It is placed on over head transmission line and nearer to
the CB .
 It is also one of the protective relay.
44

45. What is
Auto – Reclosing ?
45

46. Introduction to auto-reclosing
scheme
46

47. Types Of Faults
• Transient Faults
• Semi Permanent faults
• Permanent Faults
Utility Substation Customers
47

48. 48
STEPS TO BE FOLLOWED IN AUTORECLOSING
Visual Check
Check of the
Power Supply
Test of the
Auto-re-closure settings
Functional Testing
(involving operation of
Protective Relays
& Circuit Breaker(s)
INSTALLATION
OPERATION

49. Scheme Of Operation
• Live bus Dead Line Charging
• Other end closing under synchronism check relay
• Live bus Live line charging
49

50. Types of Auto Reclosing
1. Medium voltage auto-reclose
• Aim of continuity of supply
• the obvious advantages are continuous supply except for short
duration when tripping and re-closure operations are being performed.
• high speed protection decreases the amount of damage incurred and
thus increases the probability of auto reclosure.
2. High voltage auto-reclose
• main considerations are of stability and synchronizing.
• the fault levels associated are extremely high.
• High speed re-closure in high voltage circuits improves the stability
to a considerable extent on single-circuit ties.

51. FACTORS TO BE CONSIDERED FOR
AUTO RECLOSING SYSTEM
• Protection Characteristics
• De- Ionization of Fault Arc
• Circuit Breaker Characteristics
• Reclaim Time
• Number of Shots
51

52. SINGLE PHASE AUTO – RECLOSING
ADVANTAGES:-
•Maintenance of System Integrity
• Reduced switching overvoltage due to reclosing
• Reduced shock to generators.
• Sudden changes in mechanical output are less
• Longer De – ionization time
DISADVANTAGE:-
52

53. 1. Densely interconnected systems.

Where the Loss of a single line is unlikely to
cause loss of synchronism between two ends
2. Dead Time is allowed to be long enough for any
power swings due to fault and tripping allowed
to decay before reclosing

a) Less shock to system than with high speed
A/R
b) Problems of arc de – ionization times, and
circuit breaker operating characterstics are
eliminated.
Delayed Auto-Reclose Application
53

54. Operating feature of Auto reclosing
schemes
• Dead Timer
• Anti Pumping Devices
• Reclaim timer
• CB Lockout
• Manual Closing
• Multi Shot Scheme
54

55. Auto - Reclose Blocking
• Receipt of transfer Trip
• Manual Trip
• Breaker failure
• Three phase Fault
• Faults On Buses
• Under frequency/ Under voltage load shedding trips
55

56. Benefits Of Auto - Reclosing
• Minimizing the interruption of supply.
• Maintenance of system stability and Synchronism.
• Minimum Outage and least expenditure of manpower.
• Ability to run substation unattended.
56