3. Types of faults in the transmission system
0Short circuit faults Frequency
0Phase – Ground faults 85%
0Phase- Phase faults 8%
0Phase – Phase –Ground faults 5%
03 Ph faults 2%
0Open circuit faults
0Broken conductor
0Open jumper
4. Protection Scheme
0Protection 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.
0Types of main Protections:
• Over Current Protection.
• Differential or Phase Comparison or Unit Protection.
• Distance Protection.
5. 0Requirements of distance protection:
• 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.
• Max. operating time under given conditions shall be as follows
1. SIR: 4, Setting: 10Ohms, Fault location : 60% Trip contacts: 4, Fault resistance: 0
Max.Operating time including trip relays: 45ms
2. SIR: 15, Setting: 2 Ohms, Fault location : 60% Trip contacts: 4, Fault resistance: 0;
Max.Operating time including trip relays: 45 ms (3ph fault), 60ms (others)
6. 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
X
X
X
X
X
Over Current Protection
7. Phase Comparison Protection
0 Current Phase comparison type
0 Suitable for operation with PLCC
0 High sensitivity and selectivity for all types of faults
0 Capable of single and three pole tripping.
0 Un effected By:
0 Heavy load transfer
0 Power swings
0 CT saturation
0 CT Phase errors
0
8. Distance Protection
0Type of distance relays
0Reactance
0 Suitable for short lines
0 Not effected by fault resistance
0 Effected by power swings
0 Non directional
0Impedance
0 Suitable for medium lines
0 Non directional
0 Effected by fault resistance
0Mho
0 Directional
0 Least effected by power swings
0 Less effected by fault resistance
10. 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
11. OFF set MHOcharacteristic
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
12. 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
13. 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
14. 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
15. Terms associatedwithdistance protection
Reach:
Reach is the impedance of the tr. line up to which the
distance relay protects the line from the faults.
over reach
Relay measured impedance less than the actual fault
Impedance. i.e effective reach of the relay increases
Under reach
Relay measured impedance is more than actual impedance.
i.e. Effective reach of the relay decreases
16.
17. Parallel Compensation
0Necessity of parallel compensation:
For the fault on the parallel line, fault current also fed from healthy line and
0 this current pass through ground. This current changes the mutual
0 inductance and in turn causes relay measuring impedance to increase and
0 is more than actual fault impedance.
This effect will be compensated by connecting neutral current of the line to
0 parallel line.
This compensation will not work, if the parallel line neutral current is more
0 than line neutral current.
18. ZONE - I CHARACTERISTICS
Should isolate the faulted section instantaneously
Should cover protected circuit &fault resistance with some
margin to take care of errors in measurement
Selective phase tripping for 1ph to ground faults for
auto reclose of the breaker.
Fast operation with secured directional discrimination.
Should be stable in the presence of:
1. Fault resistance, arc resistance and tower footing resistance
2. Power swings
3. Heavily loaded condition
4. In feed condition
5. Errors in CT, CVT and CT saturation
6. Transients in CT & CVT
19. 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
20. 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
21. CARRIER SCHEMES - P U R
CHANNEL
Z1A
Z2A
Z1B
Z2B
A
B
CARRIER
RELAY
RELAY
Fault
Trip = Z1 + Z2.CR+Z3.T3+Z2.T2
CS = Z1
Under reaching zone sends carrier signal
22. CARRIER SCHEMES - P O R
RELAY
Z1A
Z2A
Z1B
Z2B
A
B
CARRIER RELAY
Fault
Trip = Z1+Z2.CR+ Z2.T2+Z3.T3
CS = Z2
Over reaching zone sends carrier signal
23. WEAK END FEED
It is a condition which occurs on a line when no current infeed
from the line terminal or when the current infeed is low due
to weak Generation behind protection.
If the fault current infeed is too low to operate the protection
at the week end, the following might occur, depending on
The selected communication scheme.
In permissive schemes or blocking schemes CB at week infeed
end may fail to trip instantaneously, due to no relay operation
in the weak end.
In permissive over reach scheme, if the fault is in Zone 2 from
stronger end, both CBs may fail to trip instantaneously due to
no relay operation in the weak end.
24. WEAK END FEED LOGIC IN P.O.R
Fault in Z2A+UV relay + Receipt
Of echo signal etc
= Trip breaker A + Send echo signal
Fault in Z1B= Trip CB B
Fault in Z2B=Sends Echo
signal
Z1A
Z2A
Z1B
Z2B
A
B
CARRIER RELAY
RELAY
Fault
Weak
End
25. 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
26. 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
27. SEMI PERMANENT FAULTS
This type of faults requires more than one De energised
interval before it disappears. Such faults are prevalent on
EHV lines traversing forest.
An example is a tree falling on the line and getting burnt
up by the arc when the line is re energised.
10% of the reclosures are successful with second shot.
However this can cause unnecessary wear on EHV CBs.
Therefore second shot is not recommended for EHV Systems.
28. 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.
29. 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
30. Power Swing
0Power Swings are disturbances in system due to various reasons
0such as sudden load throw, bad synchronization etc
0Power swings are characterized by slow power flow oscillations,
0resulting in swinging of voltages and currents, resulting in
0operating point movement into distance relay characteristics,
0in turn can cause tripping of distance relays.
0Tripping during power swings is undesirable since no actual fault
0is present and moreover a line outage during power swing may
0cause further deterioration to system stability.
0Detection of power swing will block the distance protection
0Zones 2,3,4. Normally tripping in Zone-I is not blocked even after
detection of power swing.
32. Fuse Failure Function
0Asymmetrical measuring voltage failure:
Substantial asymmetry of measured voltage, while the measured
0 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
0 protection switched to emergency over current function automatically.
0 If the asymmetry in measured current is detected during blocking by FF
0 function, then FF block will released.
33. Switch on to fault
0This feature provide protection against energisation of the tr.
line with fault or dead short.
0Distance protection will not provide protection in
0this case as voltage is not available for distance measurement.
0It can be activated by TNC switch or CB aux. binary input or
internal detection of current rise.
0It provides instantaneous 3Ph trip and blocks auto reclose.
34. One and Half Breaker Scheme
Ckt-1 Ckt-2
Bus-1
Bus-2
21
Stub Protection
35. DEF Protection
0It provides back up protection for tr. line.
0It provides reliable protection for high resistance
earth faults.
0It uses cross polarized voltage for directional
discrimination.
36. Local breaker back up protection
0It is the secondary protection
0To provide back up isolation during failure of breaker
to open.
0It opens source to that breaker (i.e other end breakers,
bus bar, etc.)
0It will be triggered by operation of any primary
protection (like distance, DEF, bus bar, etc..)
0It sends direct trip command to other end.
37. Direct trip Scheme
0 It is required to trip other end breakers without any
0 checking the status at other end during following
0 conditions:
• Operation of over voltage protection.
• Operation of bus bar with tie breaker open.
• Manual tripping of both the breakers (main&Tie)
• Operation of LBB
On receipt of command through PLCC at other end
breakers will trip directly.
38. Over Voltage Protection
0It will have 2 stages
0Stage-I:
0Setting: 110%
0Time delay: 5 Sec.
0Stage –II
0Setting: 140%
0Time delay: Instantaneous.
