This document discusses protection methods for transmission lines. It describes: 1. Transmission lines require more protective schemes than other equipment due to their long lengths and exposure, making faults more common. 2. Key methods of transmission line protection include time-graded overcurrent protection, differential protection, current-graded overcurrent protection, and distance protection. 3. Distance protection uses impedance relays that can discriminate between faults along the line and those near the end, providing more selective operation than overcurrent protection alone. It describes implementations using simple impedance, reactance, and mho relays.

1. Protection of Transmission
lines
ByBy
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
Non- unit type Protection
1. Time graded
over current protection
Unit type protection
1. Differential protection
2. Current graded over
current protection.
3. Distance protection.
2. 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 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 outgoing relays are set with
decreasing time 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 fault point

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%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
43
2
2
2
1 )cos( kVIkVkIkT +−++= τθ
The universal torque equation is given as :
Thus trip law for simple impedance relay is as follows :
ZsetZseen pIf 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
vφ
Reference quantity
v
1φ
φ
θ
τ

26. Characteristics of directional impedance relay

27. Effect of Arc resistance 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 as under-reach

28. Trip Law for Simple Reactance Relay Using
Universal Torque Equation
The universal torque equation is given as :
43
2
2
2
1 )cos( kVIkVkIkT +−++= τθ
Thus ,trip law for simple impedance relay is as follows:
XsetXseen pIf Then trip, else restrain

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

33. Distance protection of transmission line

34. Three step 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. Combine overcurrent and earth fault
protection