This gives idea about necessity of protection of transmission line and protection based on time grading as well as on current grading. Also includes three step distance protection of transmission line

1. Protection of Transmission
lines (Distance Protection)
By,
Rohini Haridas
Assistant Professor,
Dept of Electrical Engineering,
SSGM College of Engineering,Shegaon

2. As the length of electrical transmission line is
generally long enough and it runs through open
atmosphere, the probability of occurring fault
in electrical power transmission line is much higher
than that of transformers and alternators .
That is why a transmission line requires much more
protective schemes than a transformer and an
alternator.

3. Features of protection of transmission line
1. During fault, the only circuit breaker closest to the fault point
should be tripped.
1. If the circuit breaker closest the faulty point, fails to trip1. If the circuit breaker closest the faulty point, fails to trip
the circuit breaker just next to this breaker will trip as back up.
2. The operating time of relay associated with protection of line
should be as minimum as possible in order to prevent
unnecessary tripping of circuit breakers associated with other
healthy parts of power system.

4. The main methods of transmission line
protection
1. Time graded
over current protection 4.Differential protection
2. Current graded over
current protection.
3. Distance protection.
4.Differential protection
5.Carrier current protection

5. Protection of Radial Feeder
In radial feeder, the power flows in one direction
only, that is from source to load.
This type of feeders can easily protected by using
either definite time relays or inverse time relays.

6. Line Protection by Definite Time Relay

7. ADVANTAGE
simplicity
DISADVANTAGE
If the number of sections in
the line is quite large, the time
setting of relay nearest to theduring fault, only
nearest CB towards the
source from fault point
will operate to isolate
the specific position of
the line.
setting of relay nearest to the
source, would be very long. So
during any fault nearer to the
source will take much time to
be isolated. This may cause
severe destructive effect on the
system.

8. Over Current Line Protection by Inverse
Relay

9. Over Current Protection of Parallel
Feeders

11. Protection of Ring main system

12. The two lines leaving the generating stations should be equipped
with non-directional over current relays ( in this case relay 1 and 8)

13. At each bus directional relay should be placed in both
incoming and outgoing line (2,3,4,5,6,7)
Direction of tripping should be away from the bus.
**If the direction of flow of power is same as that of the direction of relay
then only relay trips

14. There should be relative time setting of the relay. Going
round the loop E-A-B-C-D-A-E, the outgoing relays
are set with decreasing time limits (relays 1,3,5,7)

15. Similarly Going round the loop in opposite direction E-
A-D-C-B-A-E, the outgoing relays are set with
decreasing time limits (relays 8,6,4,2)
Direction of tripping should be away from the bus

20. Protection of Ring main system

21. Current graded protection
**The short ckt current along the length of protected ckt decreases with
increase in distance between supply end and fault point

22. Difficulties in current graded protection
1. The relay can not discriminate between the fault in the next section and
the end of first section.
**Hence for discrimination the relays are set to protect only part of the line,
usually 80%
2. fault currents are different for different types of fault so difficulty2. fault currents are different for different types of fault so difficulty
experienced in relay setting
3. For the ring mains, parallel feeders ,where power can flow to fault from
either direction , a system without directional control is not suited.
For this reason current grading alone can not
be employed

23. Distance protection of transmission line

24. Three step Distance protection
(FIRST STEP)

25. Three step Distance protection
(FIRST STEP)

26. Three step Distance protection

27. Three step Distance protection

28. Three step Distance protection

29. Step Reach Operating Time Remarks
First
Step
80 to 90% of line
Section
Instantaneous i.e. No
intentional time delay
(T)
1. Provides primary protection to line
upto 80 to 90%
Second
Step
100 % of line section
under consideration
+50% of adjoining line
T + Selective time
interval =T1
1. Provides primary protection to the
part of line left out of first step (20
to 10%.
2. Full Backup protection to line
section under consideration
3. Backup protection to next line upto
50%
Third
Step
100 % of line section
under consideration
+100% of adjoining line
+ 10 to 20% Extra
T1+ Selective time
interval =T2
1. Full Backup protection to line
section under consideration
1. Full Backup protection to next line

30. Three step Distance protection

32. Three step Distance protection using
mho relay

33. Comparison between Distance Relay
Factors Simple
impedance relay
Reactance relay Mho relay
Operating
quantity Current Current
Directional
element
Restraining
voltage
Directional VoltageRestraining
quantity
voltage
Directional
element
Voltage
Directional
property
No No Yes
Effect of fault
resistance
Under reaches Reach
unaffected
Under reaches
Area occupied
on R-X diagram
Moderate Very large Smallest

34. Overload or overcurrent protection

35. Combine overcurrent and earth fault
protection