line protection basic

1. ©ABBGroup-1-
19-Mar-08
Line
Protection

2. ©ABBGroup-2-
19-Mar-08
Line Protection
Electrical faults in the power system
Transmission lines 85%
Busbar 12%
Transformer/ Generator 3%
100%

3. ©ABBGroup-3-
19-Mar-08
Line Protection
Fault types
Transient faults
are common on transmission lines, approximately 80-85%
lightning are the most common reason
can also be caused by birds, falling trees, swinging lines etc.
will disappear after a short dead interval
Persistent faults
can be caused by a broken conductor fallen down
can be a tree falling on a line
must be located and repaired before normal service

4. ©ABBGroup-4-
19-Mar-08
Line Protection
Fault types on double circuit lines
Simultaneous and Interline faults
On parallel line applications a problem
can occur with simultaneous faults.
A full scheme relay is superior when the
protection is measuring two different fault
types at the same time.
L1
L3
L3
L1
L2
L2
~~
Z
<
L2-
N
L1-
N

5. ©ABBGroup-5-
19-Mar-08
Line Protection
Main requirements on line protection are:
• SPEED
• SENSITIVITY
• SELECTIVITY
• DEPENDABILITY
• SECURITY

6. ©ABBGroup-6-
19-Mar-08
Line Protection
Measuring principles
Over current protection
Differential protection
Phase comparison
Distance protection
Directional- wave protection

7. ©ABBGroup-7-
19-Mar-08
Line Protection
Over current protection
Are normally used in radial networks with system
voltage below 70 kV where relatively long
operating time is acceptable.
On transmission lines directional or non-directional
over current relays are used as back-up
protections.
I > I > I >
I >
block

8. ©ABBGroup-8-
19-Mar-08
Line Protection
Dependent Time Overcurrent Relays
CHARACTERISTICS OF
DEPENDENT TIME OVERCURRENT RELAYS
0.1
1.0
10.0
100.0
1 10 100
Current (multiple of setting)
OperateTime[s]
Long Time Inverse
Extremely Inverse
Normal Inverse
Very Inverse

9. ©ABBGroup-9-
19-Mar-08
Line Protection
Two or Three Phase Over current Relays
Two phase over current relays and one residual over
current relay give complete protection on power lines
and cables
A third phase relay provides back-up protection
In case of a D/Y-connected transformer, the fault current
in one phase may be twice that in the other two phases
and it may be necessary to provide three phase over
current relays

10. ©ABBGroup-10-
19-Mar-08
Line Protection
Directional Over current Relays
Relays on radial lines do not need directional element
Directional elements are useful on parallel lines, on
looped lines, and in meshed networks

11. ©ABBGroup-11-
19-Mar-08
Line Protection
Directional Residual Overcurrent Relays
Reverse
operation
Forward
operation
Upol
-3U03I0D
0.6 3I0Dx
3I0 >
φφφφ = the characteristic angle of zero
sequence source impedance
φ=65φ=65φ=65φ=65

12. ©ABBGroup-12-
19-Mar-08
Line Protection
Directional Residual Overcurrent Relays
Residual voltage polarization requires a sensitive
directional element
Third harmonics in voltage must not cause incorrect
operation of the directional element

13. ©ABBGroup-13-
19-Mar-08
Line Protection
Pilot wire differential protection
Pilot wires can be in soil or on towers.
The resistance in the wires will limit the use on longer
lines. The use is mostly restricted to distances up to 10
km.
High sensitivity
Can be used on short lines
Very useful on series compensated lines
Insensitive to power swing
Weak source no problem
Why differential protection?

14. ©ABBGroup-14-
19-Mar-08
Line Protection
Idiff = Delta Current = 0
Differential protection - operating principle
Idiff = Delta Current > 0

15. ©ABBGroup-15-
19-Mar-08
Line Protection
Digital differential communication
Digital communication with optical
fibers or by multiplexed channels
L1
L2
L3
DL1
DL2
DL3
DL1
DL2
DL3

16. ©ABBGroup-16-
19-Mar-08
Line Protection
Phase comparison
Phase comparison relays
compare the angle difference
between the two currents at
both ends of the line.
The measured time for zero
crossing is transmitted to the
other end.
Normally a start criteria is
added to the phase angle
requirement.
I1 I2
e
1
e
2
e
2
e1
-
φφφφ
>
φφφφ
>
I1 I2
load
I2
I2
I1func-
tion
αααα
αααα
φφφφ
φφφφ

17. ©ABBGroup-17-
19-Mar-08
Line Protection
Directional wave protection
The basic principle of directional wave protection is to observe the
polarities of the instantaneous change in voltage and current. Here by
one can determine the direction of a fault with respect to the location
of the measurement.
Tripping is achieved when both protections detects a fault in forward
direction.
~ ~
A B
F
I U
+ +
- -
- +
+ -
Trip
0
0
1
1

18. ©ABBGroup-18-
19-Mar-08
Line Protection
Why:
Local current and voltage: No need for communication
Fault on protected line: Reach independent of fault
current level
Impedance characteristics can be chosen with different
reach for different impedance phase angles.
Enables remote back-up protection
Application of distance protection

19. ©ABBGroup-19-
19-Mar-08
Line Protection
The principle of distance protection
ZK=Uk/ Ik
Uk=0Uk
IkZ<
A B
metallic faultZk
The impedance is proportional to the distance!

20. ©ABBGroup-20-
19-Mar-08
Line Protection
The principle of distance protection
• Power lines have impedances of 0,3- 0,4 ohm/ km
and normal angles of 80 - 85 degrees in a 50Hz
systems.
• The line impedance may have to be converted to
secondary values with the formula:
A
Z<
B
Z<
ZL=R+jX
Zsec=
VTsec
VTprim CTsec
CTprim
Zprimx x

21. ©ABBGroup-21-
19-Mar-08
Line Protection
Fault resistance
multi-phase faults
consists only of arc resistance
earth faults
consists of arc and tower
footing resistance
L1
L3
L3
L1
L2
L2
Footing resistance
Rarc =
28707 x L
1.4
I
Warrington´s
formula
L= length of arc in
meters
I= the actual fault current in
A

22. ©ABBGroup-22-
19-Mar-08
Line Protection
Application of distance protection
A
Z<
B
Z<
C
t1
t2
t3
Distance protection has different functional zones with different
impedance reaches
With a combination of distance reach setting and functional delay
for each zone selectivity is relatively easy to achieve.

23. ©ABBGroup-23-
19-Mar-08
Line Protection
A B C
t1
t2
t3
t1
t2
t3
Z< Z< Z< Z<
Application of distance protection

24. ©ABBGroup-24-
19-Mar-08
Line Protection
A B C
f1 f2
t1
t2
t3
t1
t2
t3
Z< Z<
Application of distance protection

25. ©ABBGroup-25-
19-Mar-08
Line Protection
A B C
f3
t1
t2
t3
t1
t2
t3
Z< Z< Z< Z<
Application of distance protection

26. ©ABBGroup-26-
19-Mar-08
Line Protection
Design of distance protection
Switched scheme
consists of a start relay which detects the type of fault and select
(switch) the measuring loop to the single measuring relay. The
relevant loop voltages and currents are switched to the
measuring unit.
Full scheme
has a measuring element for each measuring loop and for each
zone
~~
Z<
L2-N
L1-N

27. ©ABBGroup-27-
19-Mar-08
Line Protection
Requirements on Distance relay Zones
Zone-1
Must not overreach
Zone-2
Must overreach
Must co-ordinate with next section
Provides back-up for the next busbar
Provides back-up for the first part of next line
Zone-3
Can provide back-up for next line
Can provide back-up for next busbar
In feed of fault current at the remote busbar affects the effective
reach of the overreaching zones

28. ©ABBGroup-28-
19-Mar-08
Line Protection
Measuring loop for earth faults
The distance protection relays are always set based on
the phase impedance to the fault
Zs RL XL
RN
XN
The measured Impedance is a function of
positive and zero sequence impedance
IL1
UL1
IN

29. ©ABBGroup-29-
19-Mar-08
Line Protection
Measuring loop for two- phase faults
The distance protection relays are always set based on
the phase impedance to the fault
Zs RL XL
UL1-L2
IL1
IL2
The measured impedance is equal to the
positive sequence impedance up to the fault
location

30. ©ABBGroup-30-
19-Mar-08
Line Protection
Measuring loop for three- phase faults
• The distance protection relays are always set
based on the phase impedance to the fault
Zs RL XL
UL1
IL1
IL2
The measured impedance is equal to the
positive sequence impedance up to the fault
location
IL3UL2
UL3

31. ©ABBGroup-31-
19-Mar-08
Line Protection
The earth fault measurement
U= I1Z1+I0Z0+I2Z2
Z1=Z2
U= Z1( I1+I2+I0 ) +I0Z0 -I0Z1 I= I1+I2+I0
U=I Z1+I0 ( Z0 - Z1 ) 3I0=IN
U=IZ1+IN(
Z0 - Z1
3
)U=I Z1+
IN
3
( Z0 - Z1 )

32. ©ABBGroup-32-
19-Mar-08
Line Protection
The earth fault measurement
The current used is thus the phase current plus the residual
current times a factor KN = (Z0-Z1) / 3Z1, the zero sequence
compensation factor.
The factor KN is a transmission line constant and Z0/ Z1 is
presumed to be identical throughout the whole line length.
(1+KN) Z1 gives the total loop impedance for the earth fault
loop for single end infeed.

33. ©ABBGroup-33-
19-Mar-08
Line Protection
Measurement Loops
Fault Voltage Current
R-Earth VR IR +Kn⋅3I0
S-Earth VS IS +Kn⋅3I0
T-Earth VT IT +Kn⋅3I0
R- S VR - VS IR - IS
S- T VS - VT IS - IT
T- R VT - VR IT - IR
R- S- T Anyphase-earthvoltage
anyphase-phasevoltage
Correspondingphasecurrent
Correspondingphase-phasecurrent
R- S- T- Earth Anyphase-earthvoltage
anyphase-phasevoltage
Correspondingphasecurrent
Correspondingphase-phasecurrent

34. ©ABBGroup-34-
19-Mar-08
Line Protection
Directional measurement
When a fault occurs close to the relay location the
voltage can drop to a value where the directional
measurement can not be performed.
Modern distance protection relays will instead use the
healthy voltage e.g. for L1- fault the voltage UL2-L3,
shifted 90 degrees compared to UL1. This cross
polarisation is used in different proportions between
healthy and faulty phases in different products.
At three- phase fault close to the station all phase
voltages are low and cross polarisation is not of any
use. Instead a memory voltage is used to secure correct
measurement.

35. ©ABBGroup-35-
19-Mar-08
Line Protection
Distance protection on short lines
Distance protection with mho
characteristic can not see an
average fault resistance
RF
XF
jX
R

36. ©ABBGroup-36-
19-Mar-08
Line Protection
Distance protection on short lines
Quadrilateral characteristic
improves sensitivity for higher RF/XF
ratio
It still has some limitations:
the value of set RF/XF ratio is
limited to 5
jX
RXF
RF

37. ©ABBGroup-37-
19-Mar-08
Line Protection
Distance protection on short lines
Overreaching permissive
schemes increase the
sensitivity
Weak infeed logic for very
high fault resistance
Independent underreaching
zone 1 gives additional
advantage
jX
R
RF
XF

38. ©ABBGroup-38-
19-Mar-08
Line Protection
Distance protection on long lines
Load impedance limits the reach
in resistive direction
High value of RF/XF ratio is
generally not necessary
Circular (mho) characteristic
Has no strictly defined reach
in resistive direction
Needs limitations in resistive
direction (blinder)
R
jX

39. ©ABBGroup-39-
19-Mar-08
Line Protection
Double end infeed
I1 I2
UF RF
UF = RF ( I1 + I2 )
RF ( I1 + I2 )
RF1=
I1
U1 U2
I Load

40. ©ABBGroup-40-
19-Mar-08
Line Protection
Resistive fault, double end fed
ZSCA ZSCBk ZL (1-k) ZL
Rf
+
EA
-
+
EB
-
IA IB
VA
f
A
BA
L
A
A
A R
I
II
Zk
I
V
Z ⋅
+
+⋅==( ) fBAALA RIIIZkV ⋅++⋅⋅=
The fault has more or less fault resistance.
If the fault is an arcing fault the fault resistance is normally very small.
The influence of the fault resistance depends on the fault current
infeed from the remote line end.

41. ©ABBGroup-41-
19-Mar-08
Line Protection
Resistive fault, double end fed
f
A
BA
LA R
I
II
ZkZ ⋅
+
+⋅=
The fault resistance seen by the distance protection can be
increased compared to its real value.
fR f
A
BA
R
I
II
⋅
+
LZk⋅
UNDERREACH!

42. ©ABBGroup-42-
19-Mar-08
Line Protection
Resistive fault, double end fed
f
A
BA
LA R
I
II
ZkZ ⋅
+
+⋅=
There is a risk that zone 1 will trip for faults outside its border.
fR
f
A
BA
R
I
II
⋅
+
LZk⋅
OVERREACH!
The apparent fault resistance
can also get a phase shift,
depending on the load
conditions before the load.

43. ©ABBGroup-43-
19-Mar-08
Line Protection
Compensation of overreach in Zone1 due to load
ph - E
R
X
Fault resistance reach influence
Zone 1 of the REL 5XX/REL 670 terminal has a compensation of
the characteristic due to the overreach caused by the load current.
In case of active power out from the station the characteristic is
automatically tilted. This is valid only for Ph-E loops.

44. ©ABBGroup-44-
19-Mar-08
Line Protection
Remote faults
Due to current contribution If2 and If3 in substation B, the
distance protection in station A will measure a higher
impedance than the "true" impedance to the fault.
The relay will thus underreach and this means in practice it
can be diffcult to get a remote back-up.
Z<
If
1
If
2
If
3
If=If1+If2+If3
ZL
ZF
A B
Um
Um= If1 x ZL+ (If1+If2+If3) x ZF

45. ©ABBGroup-45-
19-Mar-08
Line Protection
Zero- sequence mutual coupling on parallel lines
ZA<
overreaching
ZB< underreaching
~ ZOM
ZL
ZL
~
ZA< ZB<
~
~

46. ©ABBGroup-46-
19-Mar-08
Line Protection
Parallel line out of service and earthed at both ends
∆∆∼ ∼
∆Z = - ZL
KOM • ZOM / ZOL
1 + KO
•
= - 0.23 ZL

47. ©ABBGroup-47-
19-Mar-08
Line Protection
Parallel line in Service
∆Z =
∼ ∆
D
KOM
1 + KO
• ZL
= 0.38 ZL

48. ©ABBGroup-48-
19-Mar-08
Line Protection
Distance relay settings for parallel lines
The influence of zero sequence coupling can be
compensated in two different ways
Different K factor for different Zones within same
group setting parameters
Different groups of setting parameters for different
operating conditions

49. ©ABBGroup-49-
19-Mar-08
Line Protection
Communication equipment
Power line carrier (PLC) equipment is based on a
capacitive connection of signals with frequency in
range 50- 500 kHz on the power line.
Radio link is a good and reliable communication
equiment, but is rarely used due to the high cost.
Optical fibres have the advantage to be insensitive
to noise and can transmit a huge amount of
information.

50. ©ABBGroup-50-
19-Mar-08
Line Protection
Permissive schemes.
PermissivePermissive
UnderreachUnderreach
OverreachOverreach
permission to trip instantaneously
to an overreaching zone.
The permission is sent by
an Under reaching zone
The permission is sent by
an Overreaching zone

51. ©ABBGroup-51-
19-Mar-08
Line Protection
Permissive underreaching scheme
CS = ZM1
Trip = ZM1 + ZM2 *(T2 + CR) +ZM3 * T3
ZM2, T2
ZM1, T1
A B
ZM2, T2
ZM1, T1
Permission is sent by
an Underreaching zone (ZM1)
Permission to trip instantaneously
to an overreaching zone (ZM2).
If B has a weak source,
it could not see the
fault and fail to send
the carrier to A.

52. ©ABBGroup-52-
19-Mar-08
Line Protection
Permissive communication schemes
Communication signal carrier send (CS) is sent to remote end when
the fault is detected in forward direction. Tripping is achieved when
the commmunication signal carrier receive (CR) is received and the
local relay has detected a forward fault.
In a permissive underreaching scheme the communication signal is
sent from a zone that underreaches the remote end.
In a permissive overreaching scheme the communication signal is
sent from a zone that overreaches the remote end.
A
Z< Z<
B
Carrier send CS = Z< forward, under or
overreach
Trip = ZM1 + ZM2 (t2 + CR) + ZM3 x t3

53. ©ABBGroup-53-
19-Mar-08
Line Protection
Permissive Underreach Distance Protection

54. ©ABBGroup-54-
19-Mar-08
Line Protection
Permissive overreaching scheme
CS = ZM2
Trip = ZM1 + ZM2 *(T2 + CR) +ZM3 * T3
A B
Permission is sent by
an Overreaching zone (ZM 2)
Permission to trip instantaneously
to an overreaching zone (ZM2).
The carrier is sent by
both relays for faults
on the whole line.
ZM2, T2
ZM1, T3
ZM3, T3
ZM2, T2
ZM1, T1
ZM3, T3
Good for weak-end infeed.
Echo carrier signal is sent back
from B if a carrier has been
received but no fault detected
in ZM1, ZM2 and ZM3.

55. ©ABBGroup-55-
19-Mar-08
Line Protection
Permissive Overreach Distance Protection

56. ©ABBGroup-56-
19-Mar-08
Line Protection
Permissive overreaching schemes are adopted for short
lines( Also called directional comparison schemes)
Advantages are
• Better performance for high resistance faults.
• Superior to pilot wire as digital decisions are
exchanged and not analogue
• Superior to phase comparison which requires
faithful transmission of phase information.
Permissive Overreach Distance Protection

57. ©ABBGroup-57-
19-Mar-08
Line Protection
Blocking communication schemes
Communication signal (CS) is sent to remote end when the fault
is detected in the reverse direction. Tripping is achieved when
this blocking signal is not received within a time T0 (20-40 ms)
and the local relay has detected a fault in the forward direction.
A
Z< Z<
B
Carrier send CS = Z< reverse zone
Trip = ZM1 + ZM2 (t2 + CR x T0) + ZM3 x t3

58. ©ABBGroup-58-
19-Mar-08
Line Protection
Blocking overreaching scheme
ZM2, T2
ZM1, T1
A B
Block signal is sent by
the reverse zone (Zone 3)
Overreaching inst. zone to be
Blocked by a block signal).
• Carrier is sent when
the line is healthy
• Good for short lines,
where it is impossible to
set 80-90% of the line
length.
• Series compensated
lines
ZM3, T3
CS = ZM3
Trip = ZM1 + ZM2 * TCR* CR+ (ZM3 * T3 + ZM2 * T2)
ZM2, T2
ZM1, T1
ZM3, T3
Waiting time for the
block signal (tCoord)
Block signal.

59. ©ABBGroup-59-
19-Mar-08
Line Protection
Blocking Overreach Distance Protection

60. ©ABBGroup-60-
19-Mar-08
Line Protection
This function is based on condition
3Uo > 20 % of Un / √√√√ 3 and 3Io < 20 % of In
It can be selected to block protection and give alarm
or just to give alarm.
Fuse fail supervision is blocked for 200ms following
Line energisation in order not to operate for unequal
pole closing and also during auto-reclosing.
MCB can also be used.
FUSE FAIL SUPERVISION

61. ©ABBGroup-61-
19-Mar-08
Line Protection
Switch On To Fault (SOTF)
When energizing a power line onto a forgotten earthing, no
measuring voltage will be available and the directional
measuring can thus not operate correctly.
A special SOTF function is thus provided. Different principles
can be used, from one phase current to non-directional
impedance measuring.
Z<
U=0
V
SOTF condition can either be
taken from the manual closing
signal activating the (BC) input
or it can be detected internally
by a logic.

62. ©ABBGroup-62-
19-Mar-08
Line Protection
A power swing can start by sudden load change or due
to a fault somewhere in the network.
Close to the centre of the power swing, low voltage and
thus low impedance will occur.
A distance protection relay must then be blocked during
the power swing.
This can be done by measuring the transit time of the
impedance locus passing two dedicated impedance
zones.
Normally the time used is 35-40 ms.
Power Swing Blocking function

63. ©ABBGroup-63-
19-Mar-08
Line Protection
Power Swing Blocking function
∆∆∆∆t
∆∆∆∆t = 40 ms
X
R
Power swing
locus

64. ©ABBGroup-64-
19-Mar-08
Line Protection
• When power swing detection unit operates any impedence
zone can be selected to be blocked or not as required.
• Operation of power swing detection unit is inhibited when zero
sequence current is detected. This feature is included to ensure
tripping of high resistance earth faults where fault resistance
can decrease slowly.
• The residual current inhibit condition ensure PSD will not
block due to unbalanced load or residual current experienced
with un-transposed transmission lines.
Power Swing Blocking function

65. ©ABBGroup-65-
19-Mar-08
Line Protection
Stub protection function It is not possible for the
distance protection relay to
measure impedance when the
line disconnector is open. Not
to risk incorrect operation the
distance protection must be
blocked and a Stub protection
is released.
The Stub protection is a simple
current relay.
line disc
open
I STUB >
& trip
25ms
Bus A
Bus B
> Z+

66. ©ABBGroup-66-
19-Mar-08
Line Protection
Current reversal logic
~~
A:
1
B:
1
A:
2
B:
2
~~
A:
1
B:
1
A:
2
B:
2
Permissive overreaching
schemes can trip healthy line
without C.R.L
1 Fault occurs on line 1
Fault detection by protection A:1 B:1 and A:2
2 Relay B:1 trips CB and sends carrier to A:1
Relay A:2 sees fault in forward direction and
sends carrier to B:2
3 Fault cleared at B:1, current direction changed
on line 2
4 Carrier from A:2 and forward looking measuring
element in relay A:2 does not reset before relay
B:2 detects the fault in forward direction and
trips, also relay A:1 will trip when receiving carrier
from B:1
C.R.L allows slowly resetting
communication equipment without
risking to tripping the healthy line.

67. ©ABBGroup-67-
19-Mar-08
Line Protection
Simultaneous faults

68. ©ABBGroup-68-
19-Mar-08
Line Protection
On parallel line applications a problem
can occur with simultaneous faults.
A full scheme relay is superior when
the protection is measuring two
different fault types at the same time.
Simultaneous faults

69. ©ABBGroup-69-
19-Mar-08
Line Protection
Weak end infeed
Weak end infeed is a condition which can occur on a transmission line,
either when the circuit breaker is open, so there is no current infeed
from that line end, or when the current infeed is low due to weak
generation behind the protection.
lt1
t2
t3
CS = ZM2
TRIP = ZM1 + ZM2(CR + t2)
CS (echo)=CR x low voltage x no start forward or
reverse
Z< Z<CS
CS
(echo)
CR
CR

70. ©ABBGroup-70-
19-Mar-08
Line Protection
∼∼∼∼∼∼∼∼
∼∼∼∼
-
+
L
F
A B
IA
I
F
I
B
ZA ZB
RF
pZL ( I - p )ZL
pZL ( 1- p ) ZL
ZA ZB
Fault Locator Measuring Principle
UA=IA X P ZL + IFA X RF
DA
DA =
(I-P) ZL +ZB
ZA+ZL +ZB

71. ©ABBGroup-71-
19-Mar-08
Line Protection
Series compensated system
• Correct direction
discrimination at voltage
reversal (negative fault
reactance)
• variation in resulted line
impedance
Consideration for line
distance protections
BA
F1
X =70%C X =100%l
R
jX
A
B
B´
70%
100%
gap not flashed
gap flashed

72. ©ABBGroup-72-
19-Mar-08
Line Protection

73. ©ABBGroup-73-
19-Mar-08
Line Protection
(i) Zone-I: to be set to cover 80-85% of protected line length
(ii) Zone II: to be set to cover minimum 120% of length of principle
line section. However, in case of D/C lines 150% coverage must be
provided to take care of, under reaching due to mutual coupling effect
but, care is to be taken that it does not reach into next lower voltage
level.
3.0 SETTING CRITERIA3.0 SETTING CRITERIA
3.1 Reach settings of distance protection3.1 Reach settings of distance protection

74. ©ABBGroup-74-
19-Mar-08
Line Protection
(iii) Zone-III:
For 400kV lines Zone-III to be set to cover 120% of principle section
plus adjacent longest section subject to a reach restriction so that it
does not reach into next lower voltage level.
For 220 kV lines, Zone-III reach may be provided to cover adjacent
longest section if there is no provision of LBB or all protection are
connected to single DC source at remote end substation.
(iv) Resistive reach should be set to give maximum coverage subject to
check of possibility against load point encroachment considering
minimum expected voltage and maximum load. Attention has to be
given to any limitations indicated by manufacturer in respect of
resistive setting vis-a-vis reactance setting.

75. ©ABBGroup-75-
19-Mar-08
Line Protection
• A Zone-II timing of 0.3 second is recommended. If a long line is follow-
ed by a short line, then a higher setting may be adopted on long line to
avoid indiscriminate tripping through Zone-II operation on both lines.
• Zone-III timer should be set so as to provide discrimination with the
operating time of relays provided in subsequent sections with which Zone-
III reach of relay being set overlaps.
3.2 Time setting of distance protection3.2 Time setting of distance protection

76. ©ABBGroup-76-
19-Mar-08
Line Protection
3.3.1
• Low set voltage may be set at 110% with a typical time delay of 5
seconds.
• A time grading of 1 second may be provided between relays of
different lines at a station.
• Longest time delay should be checked with expected operating time
of overfluxing relay of the transformer to ensure disconnection of
line before tripping of transformer.
3.3.2
• High set stage may be set at 150% with a time delay of 100 m second.
3.3 O / V Protection3.3 O / V Protection

77. ©ABBGroup-77-
19-Mar-08
Line Protection
• Decisions pertaining to allowing which Zone to trip and which to block
should be taken based on system studies on case to case basis.
3.4 Power Swing Blocking Function Associated with Distance Relays3.4 Power Swing Blocking Function Associated with Distance Relays

78. ©ABBGroup-78-
19-Mar-08
Setting of Protections
(distance relays)

79. ©ABBGroup-79-
19-Mar-08
Line Protection
Zone-1
Offers instantaneous circuit-local back-up
protection for nearby faults, but not for the entire
transmission circuit from both terminals.
Set to under reach protected circuit to ensure
external security
In case of parallel circuit may be necessary to
increase degree of under reach
For multi circuit lines reach reduces further due to
in feeds

80. ©ABBGroup-80-
19-Mar-08
Line Protection
Zone-2 Reach Setting Criteria
Should overreach all terminals of the protected circuit
by an acceptable margin (typically 20% of highest
impedance seen) for all fault conditions and for all
intended modes of system operation.
As far as possible, should be less than Zone-1
coverage of all adjacent lines, to minimize the
required Zone-2 time delay setting.

81. ©ABBGroup-81-
19-Mar-08
Line Protection
Zone-2 reach setting

82. ©ABBGroup-82-
19-Mar-08
Line Protection
Zone-2
Zone-2 time Setting Criteria
Must be set to coordinate with clearance of adjacent circuit faults,
within reach, by the intended main protection or by breaker fail
protection
tZ2 = Required Zone-2 time delay
tMA = Operating time of slowest adjacent circuit main
protection or Circuit Local back-up for faults within Zone-2 reach
tCB = Associated adjacent circuit breaker clearance time
tZ2reset= Resetting time of Zone-2 impedance element with load
current Present
tS = Safety margin for tolerance (e.g. 100ms)
sresetzCBMAz ttttt +++> 22

83. ©ABBGroup-83-
19-Mar-08
Line Protection
Zone-2
Effect of parallel lines
Where common impedance settings exist for
phase and ground fault impedance elements, or
where independent residual compensation settings
are not available for each zone of protection the
phase fault Zone-2 reach will unavoidably be
extended in order to satisfy ground fault reach
requirements.
This can create Zone-2 back-up co-ordination
difficulties, particularly where adjacent sections or
transmission circuit are of unequal length

84. ©ABBGroup-84-
19-Mar-08
Line Protection
Zone-2
Multi terminal circuits
The primary Zone-2 setting criterion must be met with
allowance for the highest apparent impedance seen for a
fault at any remote circuit terminal.
The Zone-2 reach setting may reach very high
percentage of the circuit impedance between the closest
terminals. The reach may need to be further enhanced to
address under reach for ground faults when protecting
parallel multi-terminal circuits.

85. ©ABBGroup-85-
19-Mar-08
Line Protection
Zone-2 in multi-terminal lines

86. ©ABBGroup-86-
19-Mar-08
Line Protection
Zone-2
Load encroachment
One problem with mho impedance elements is that the
fault resistance coverage varies with the forward reach
setting. When applying Zone-1 elements to short lines,
fault resistance coverage may be insufficient. It can also
be disadvantage for Zone–2 elements to be set with
unusually high forward reach setting in relation to the
minimum load impedance. It may be possible for the
minimum load impedance to encroach upon the Zone-2
operating region.

87. ©ABBGroup-87-
19-Mar-08
Line Protection
Zone-2 load encroachment

88. ©ABBGroup-88-
19-Mar-08
Line Protection
Zone-3 Remote back-up
General
Usually set to provide remote back-up protection for
adjacent sections of a transmission circuit.
May have independently adjustable forward and
reverse reach setting
Usually forward reach provides remote back-up
protection.
With duplicate main protection, there may be a
case for not applying Zone-3 remote back-up
protection at all.
In case of long 400kV lines it may be desirable
either to reduce the reach or to block 3rd zone of
distance relay for reasons of security.

89. ©ABBGroup-89-
19-Mar-08
Line Protection
Zone-3 remote back-up
Reach setting
Zone –3 should overreach the remote terminal of the
longest adjacent line by an acceptable margin (typically
20% of highest impedance seen) for all fault conditions
and in feed conditions associated with all intended modes
of system operation
Zone-3 reach should be less than the Zone-2 protection
coverage of the shortest adjacent transmission circuit and
it should not see through power transformers into
distribution systems, in order to minimize the required
zone-3 time delay setting.

90. ©ABBGroup-90-
19-Mar-08
Line Protection
Zone-3 remote back-up
Time setting
Must be set to co-ordinate with clearance of faults by adjacent
circuit-local back-up protection. Zone-2 distance protection or time
delayed over current protection
The following formula would be the basis for determining the
minimum acceptable Zone-3 time setting:
Where:
tZ3 = Required Zone-3 time delay
tMA = Operating time of slowest adjacent circuit local back-up
protection
tCB = Associated adjacent circuit breaker clearance time
tZ3reset = Resetting time of Zone-3 impedance element with load
current present
tS = Safety margin for tolerance (e.g. 100ms)
sresetzCBMAz ttttt +++> 33

91. ©ABBGroup-91-
19-Mar-08
Line Protection
Zone-3 remote back-up
Consideration of mutual coupling
As for Zone-2 protection the under reaching effect
of zero sequence mutual coupling for remote
ground faults must also be considered.
Such consideration is quite complex, since there
may well be differences in the levels of mutual
coupling for the protected circuit and any number of
adjacent circuits.
In addition, some circuit sections may be multi-
circuit while other sections may not be.

92. ©ABBGroup-92-
19-Mar-08
Line Protection
Zone-3 remote back-up
Considerations for intervening fault currents
The under reaching effects are encountered in relation to
adjacent circuit impedance, when applying Zone-3 remote
back-up protection.
These under-reaching effects are particularly difficult to
address, since they are variable according to the type of
fault.
Ground faults can invoke additional zero sequence
current in feed from transformers with grounded star-
connected primary windings and other delta-connected
windings.

93. ©ABBGroup-93-
19-Mar-08
Line Protection
Zone-3 remote back-up
Zone-3 load impedance encroachment
Encroachment due to the minimum load impedance under expected
modes of system operation and the minimum impedance that might
be sustained for seconds or minutes during abnormal or emergency
system situations.
Use of blinders in case of Mho type of elements or by use of polygon
type impedance elements.
In case of long 400kV transmission lines it may be desirable to limit
the reach of Zone-3 for reasons of security. In such cases, if the
adjacent station has bus bar protection and breaker failure protection,
Zone-3 can be dispensed with.
ESKOM (South Africa) have a practice of not using third zone
impedance protection in long line applications . They use enhanced
local breaker back-up .

94. ©ABBGroup-94-
19-Mar-08
Line Protection
Effect of in feeds on Zone-3

95. ©ABBGroup-95-
19-Mar-08
Line Protection
Zone-3 load encroachment

96. ©ABBGroup-96-
19-Mar-08
Line Protection
Zone-3 coordination

97. ©ABBGroup-97-
19-Mar-08
Line Protection
Zone- 4 substation local back-up
An additional zone of reverse-looking protection (e.g.
Zone-4) to offer substation-local back-up protection.
The Zone-4 reverse reach must adequately cover
expected levels of apparent bus bar fault resistance,
when allowing for multiple in feeds from other
circuits.
Sometimes when separate reverse looking element is
not available the above is achieved by offset reach of
Zone-3 of distance relay.

98. ©ABBGroup-98-
19-Mar-08
Line Protection