Protection of Transmission lines

1. Protection
of
Transmission
lines

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 .
T h a t 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 breake closest the faulty point fail t 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
Non- unit type Protection Unit type protection
1. Time graded 1. Differential protection
over current protection
2. Current graded over
current protection.
3. Distance protection.
2. Carrier current protection

5. Protection of
Radial Feeder
I n radial feeder, the power flows in one direction
only, that is from source to load.
T h i s type of feeders can easily protected byusing
either definite time relays or inverse time relays.

6. Line Protection by Definite Time Relay

7. ADVANTAGE
 simplicity
during fault, only
nearest CB towards the
source from fault point
will operate to isolate
the specific position of
the line.
DISADVANTAGE
If the number of sections in the
line is quite large, the time
setting of elay arest 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
lines lines (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
decreasing time
outgoing relays are set with
limits (relays 8,6,4,2)
Direction of tripping should be away from the bus

16. Protection of Ring
main system

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

18. 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. For the ring mains, parallel feeders ,where power can flow to
fault from either direction , a system without directional
control is not suited.

19. Trip Law for Simple Impedance Relay Using Universal
Torque Equation
The universal torque equation is given as :
43
T =kkVI2
k VI 2
1 2
cos(θk+ τ ) +−+
Thus trip law for simple impedance relay is as follows :
Zseenp ZsetIf Then trip, else restrain

20. Introduction to
Distance Protection

21. Principle of R-X
Diagram

22. Characteristics of
impedance relay
** It is non directional. It responds to the faults on both sides of CT,VT location
i.e. forward (I-quadrant )and reverse direction(III-quadrant)

23. Characteristics of impedance
relay on R-X
diagram

24. Implementation of simple impedance
relay using balance beam
structure

25. Phasor diagram for
directional element
I
Max. Torque line
Zero torque line
v
Reference quantity
1

26. Characteristics of directional
impedance relay

27. on Reach of SIR
(Under-reach)
**The tendency of distance to restrain (not to operate ) at the preset value of
the impedance or impedances less than preset value is known asunder-reach

28. Trip Law for Simple Reactance Relay
Using Universal Torque
Equation
The universal torque equation is given as :
43
T =kkVI2
k VI 2
1 2
cos(θk+ τ ) +−+
Thus ,trip law for simple impedance relay is as follows:
Xseenp XsetIf Then trip, else restrain

29. Characteristics of
Reactance Relay
X
Restrain
Xset or Xn
Line fault characteristics
R
TripTrip

30. Effect of Arc resistance
on Reach of
Reactance relay

31. Directional property exhibited by
reactance relay

32. Comparison between
Distance Relay
Factors Simple
impedance relay
Reactance relay Mho relay
Operating
quantity Current Current
Directional
element
Restrainin Directional
quantity
o tage
element
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

33. Distance protection of
transmission line

34. Distance
protection

35. Three step Distance
protection

36. Three step Distance
protection using
mho relay

37. Three step Distance
protection

38. Overload or
overcurrent protection

39. and earth fault
protection