This document discusses power system protection schemes, including: - Zones of protection with protective relays coordinated between zones - Attributes of reliable, selective, and fast relaying - Fault clearing times of relays and circuit breakers - Protection of system components like feeders, transmission lines, transformers, generators It provides examples of overcurrent protection design using time-graded and current-graded discrimination. Directional relays, differential protection, and power line carrier communication are also summarized.

1. PROTECTION SCHEMES
PROTECTIVE RELAYING

2. ZONE OF PROTECTION
• Power system is protected in zones, each
containing an alternator, a transformer, a
bus bar section or a transmission line.
Each zone has one or more protective
schemes, which are coordinated with
the overall protection with the following
characteristics

3. ATTRIBUTES OF RELAYING
• RELIABILITY: security (avoid false operation) &
Dependability ( correct operation)
• SELECTIVITY: minimum disconnection & Maximum
continuity of service
• SPEED :
– Improved power system stability
– Decreased amount of damage incurred
– Less annoyance to electric power consumers
– Decreased likelihood of development of one type of fault into
other more severe type
– Rapid re-closure of circuit breakers to restore service to
customer.

4. Maximum load
Permissible over load
Minimum Fault current
Maximum Fault current
over load

5. Fault Clearing Time
• Relay operating Time
– High speed or instantaneous Relay: 1.0 to
2.0 cycles i.e., 0.02 to 0.04 sec.
• Circuit Breaker clearing Time:
– High speed C.Bs: 2.5 to 3.0 cycles i.e. 0.05
to 0.06 sec.
• Total clearing time = relay operating time+
circuit breaker clearing time
– Total clearing time may be between 0.07 to
0.1 sec.

6. PROTECTION OF VARIOUS
COMPONENTS OF POIWER SYSTEM
• Feeder protection
– Radial system
– Mesh Network
• HV TXN Line protection
• Transformer Protection
• Alternator Protection
• Motor Protection

7. Feeder protection

8. Protection of Distribution Network
And Feeders
• Time graded
Discrimination
• Current graded
Discrimination

9. CURRENT GRADED
DISCRIMINATION

10. Criteria for over current
protection of radial feeder
• The relay at the far end is operated in the shortest time as it does
not have to give back up to any other relay. Upstream relays
(moving back to source side) are time graded with about 0.3 second
delays. Definite time relays can be used where source impedance is
large as compared to the line impedance i.e., small variation of
current for near and far end faults.
• Inverse time (IDMT) relays can be used if lines are long and fault
level is much smaller at the far end fault than it is for source end
fault.
• Very or extremely inverse time can be selected where the line
impedance is high as compared to source impedance or in case
where coordination with fusses or re-closure is necessary.

11. Feeder Protection by IDMT relay
• Both current graded & time graded
discrimination can be implemented
• Requires adjustment of PS and TMS
• Coordination delay time (C.D.T)= 0.3 or
0.5 seconds

12. Example
• Use 2.2 IDMT relay to design a well coordinated over current
protection scheme for a radial distribution system with the help
of following information available:
substatio
n
C.T ratio Fault
current
A 400:5 6000
B 200:5 5000
C 200:5 4000
Bus A Bus B Bus C
Load current
IDMT
relay
IDMT
relay
Fault
current
Fault Location
A B
If,A
If,B
If,C

13. Bus A Bus B Bus C
Load current
IDMT
relay
IDMT
relay
Fault
current
Fault Location
A B
If,A
If,B
If,C

14. • Start relay settings for the substation C farthest from the source
• set Relay B for back up protection for Rc
• Use CDT=0.5 seconds
• Check operating time of RB for the plug setting done in the preceding
step for a fault at substation B.
• Set PS and TMS of RA for backup of RB; using the CDT=0.5 seconds
• Check RA operating time for a fault at sub station A
Fault location RA
PS=125%
TMs=37.9%
RB
PS=125%
TMs=29%%
RC
PS=100%
TMs=10%
C - 0.72s 0.22s
B 1.138s 0.638s -
A 0.985s - -

15. Use of Directional over current relay
page 366

16. Directional relay
For DC current
For ac : induction cup
relay

18. EARTH FAULT PROTECTION
• Restricted Earth
fault scheme
• Unrestricted Earth
fault scheme

19. Use of residual current & core
balance C.T

21. TXN LINE PROTECTION

22. Problems with transmission
lines protection
• System configuration changes continuously
• More load added time to time
• Outages of T.L and/or generating units are
frequent.
• Due to complex meshed network and
interconnections, various loops exist in the
system. Hence selectivity can not be achieved
through simple over current relays

23. Reach of over current relay
• Reach of over current
relay depends on:
– type of fault
– Generation level or source
impedance value
23

24. Distance relaying
Distance relying scheme is
independent of source impedance
variations
24

25. Impedance relay
Balanced beam relay
25

26. 26

27. 27

28. schematic
28

29. Relay characteristics-RX
diagram
29

30. 30

31. Effect of Power swings and arc resistance on the
performance of simple impedance relay

32. Effect of Arc resistance on
Relay operation
32

33. Effect of Load swing on Relay
operation
33

34. Reactance Relay
• Current operated directional restraint relay
34

35. 35

36. Performance during Normal Load and
effect of arc resistance
36

37. Directional Property
37

38. Use of directional features with
simple impedance relay
38

39. Mho Relay
• It is directional relay with voltage Restraint
39

40. 40

41. Effect of Arc resistance and
performance under Normal Load
41

42. 42

43. distance protection
43

44. 44

45. 45

46. 46

47. 47

48. 48

49. Power line carrier communication(PLCC)
• For 80% first zone protection setting, only 60% of the line covers
fast tripping zone of both relay located at each end of a double fed
Tx. Line
• Opening of circuit breakers located on both ends is necessary to
maintain system stability
• Therefore, a communication link is to be established between these
two relays to ensure opening of both Circuit breakers
“simultaneously”

50. Possible communication
Channels
• The information to be transmitted is only about
state of the Circuit breaker i.e., either closed or
Tripped. Therefore, no requirement of large
bandwidth sets the carrier frequency just above
audible frequency range ( 50kHz to 200kHz)
• Possible channels are
– Telephone lines
– Microwave
– Satellite communication
– Power line conductor itself

51. Unit Protection
• Graded over current schemes drawbacks
– Satisfactory grading can not always be
arranged for complex network
– Setting may cause greater tripping times at
point in the system making protection
insufficient for excessive disturbance
• Concept of unit protection
– Where sections of the power system are
protected as a complete unit without
reference to other parts

52. Example : Differential protection
• Works on difference between incoming and
outgoing currents
Pilot wires

53. Healthy condition
Faulty condition

55. Requirements for differential scheme
• Current magnitude seen by the relay on both sides should be equal
and in phase
• Relay must not trip for external or through faults
– Because of CT mismatch of characteristics, relay can mal-operate for external
fault as well.
– Therefore a restraint coil is used in addition to operating coil.
– Relay characteristics are changed to percent differential characteristics.

57. Use of Percent differential relay

60. Difficulties with transformer protection
• No load transformer
Switching transients
(inrush current) may be
differentiated from fault current by
measuring harmonics contents,(2nd
harmonic). Relay false operation is
blocked by harmonic restraint
• C.T ratio error is prominent for
through faults because of CTs
saturation at different level. Percent
bias coil is to be used
• Phase shift in YD
transformers is addressed by
proper connections of CTs on both
sides

61. Protection of YD
Transformer

62. Requirements:
• C.Ts secondary line currents ( or current in
pilot wires) as seen by the relay should be
in phase
• C.Ts ratio must be adjusted so that current
seen by the relay should be equal in
magnitude under normal condition

63. CT connections-30° phase shift offset

65. Ex 14.7
• A 3-phase transformer rated
for 33kV/6.6kV is connected
star-delta and the protecting
current transformer on the LV
side have a ratio of 400:5.
determine the ratio of the CT
on HV side
• A 3 phase , 200 kVA 110.4 kV,
DY transformer has CTs on
0.4 kV side a turns ratio of
500:5. what should be the CT
ratio on HV side of the
transformer? Also determine
the out of balance current
when a fault of 750A of the
following type occurs on LV
side: a) Earth fault within
protection zone and b) earth
fault outside the zone