The document discusses power system protection and protective relaying. It covers the need for protection systems to maintain reliable power supply and prevent damage during faults. The key elements of a protection system include protective relays, circuit breakers, transducers, and communication channels. Relays detect faults using principles like overcurrent, directional overcurrent, distance and differential protection. Coordination between primary and backup protection is also discussed. Digital relays offer advantages like multifunctionality, adaptability and reduced maintenance over electromechanical relays. The document emphasizes the importance of properly designing protection schemes for reliable power system operation.

1. Agenda
 Why protection is needed
 Principles and elements of the
protection system
 Basic protection schemes
 Digital relay advantages and
enhancements

2. Disturbances: Light or Severe
 The power system must maintain acceptable
operation 24 hours a day
 Voltage and frequency must stay within certain
limits
 Small disturbances
 The control system can handle these
 Example: variation in transformer or generator
load
 Severe disturbances require a protection system
 They can jeopardize the entire power system
 They cannot be overcome by a control system

3. Power System Protection
Operation during severe disturbances:
 System element protection
 System protection
 Automatic reclosing
 Automatic transfer to alternate power
supplies
 Automatic synchronization

4. Electric Power System Exposure to
External Agents

5. Damage to Main Equipment

6. Protection System
A series of devices whose main purpose
is to protect persons and primary electric
power equipment from the effects of faults
The “Sentinels”

7. Blackouts
 Loss of service in
a large area or
population region
 Hazard to human
life
 May result in
enormous
economic losses
 Overreaction of
the protection
system
 Bad design of the
protection system
Characteristics Main Causes

8. Short Circuits Produce High
Currents
Fault
Substation
a
b
c
I
I
Wire
Three-Phase Line
Thousands of Amps

9. FAULTS ON POWER SYSTEMS RISK :
Severe damage to the faulted equipment :
 Excessive current may flow;
 Causes burning of conductors or equipment
windings;
 Arcing - energy dissipation;
 Risk of explosions for oil - filled switchgear, or
when in hazardous environments.
Damage to adjacent plant :
 As the fault evolves, if not cleared quickly;
 Due to the voltage depression / loss of supply.

10. Mechanical Damage During
Short Circuits
 Very destructive in busbars, isolators, supports,
transformers, and machines
 Damage is instantaneous
i1
i2
f1 f2
Rigid Conductors f1(t) = k i1(t) i2(t)
Mechanical
Forces

11. The Fuse
Fuse
Transformer

12. Essential qualities of
protection:
 Reliability
 Selectivity-
Absolute or relative
 Fastness
 Discrimination

13. Protection System Elements
 Protective relays
 Circuit breakers
 Current and voltage transducers
 Communications channels
 DC supply system
 Control cables

14. Protective relays:
 A device which detect intolerable or
unwanted conditions within the
assigned area.
 * A watchman or watchdog for the
equipment/area
 * Silent sentinels to power system.

15. How relays are differentiated?
 Can be differentiated based on:
 * Functional categories
 * Input quantities
 *Operating Principles
 * Performance Characteristics.

16. What are various design
criteria?
 * Dependability/Reliability
 * Security
 * Selectivity
 *Speed
 * Simplicity/flexibility
 *Stability
 *Performance Vs. Economy

17. What are various technique
used?
 * Electromechanical
 *Solid state/Static
 * Microprocessor/Numerical

18. Non-Unit, or Unrestricted
Protection :
No specific point downstream up to which
protection will protect
 Will operate for faults on the protected
equipment;
 May also operate for faults on downstream
equipment, which has its own protection;
 Need for discrimination with downstream
protection, usually by means of time
grading.

19. Unit, or Restricted Protection :
Has an accurately defined zone of
protection
An item of power system plant
is protected as a unit;
Will not operate for out of zone
faults, thus no back-up
protection for downstream
faults.

20. Types of relays
As per function:
 Main
 Auxiliary
 Signal
As per actuating quantity
 Overrelays
 Underrelays

21. Types…
As per connection
 Primary
 Secondary(common)
As per action on CB
 Direct acting
 Indirect acting
As per construction
 Electromagnetic

22. Types..
 Static
 Numerical
As per comparator types
 Single input comparator
 Two input comparator
 Multiple input comparator

23. Methods of disciminations:
 To locate fault
by time
by current grading
by time and direction
by distance
by time, current and distance
by current balance
by power direction comparison
 Type of fault

24. Three-Phase Diagram of the Protection
Team
CTs
VTs
Relay
CB
Control
Protected
Equipment

25. DC Tripping Circuit
SI
52
TC
DC Station
Battery
SI
Relay
Contact
Relay
Circuit
Breaker
52a
+
–
Red
Lamp

26. Circuit Breakers

27. Current Transformers
Very High Voltage CT
Medium-Voltage CT

28. Voltage Transformers
Medium Voltage
High Voltage
Note: Voltage transformers
are also known as potential
transformers

29. Protective Relays

30. Examples of Relay Panels
Old Electromechanical
Microprocessor-
Based Relay

31. How Do Relays Detect Faults?
 When a fault takes place, the current, voltage,
frequency, and other electrical variables behave in a
peculiar way. For example:
 Current suddenly increases
 Voltage suddenly decreases
 Relays can measure the currents and the voltages
and detect that there is an overcurrent, or an under
voltage, or a combination of both
 Many other detection principles determine the design
of protective relays

32. Primary Protection

33. Primary Protection Zone
Overlapping
Protection
Zone B
Protection
Zone A
To Zone B
Relays
To Zone A
Relays
52 Protection
Zone B
Protection
Zone A
To Zone B
Relays
To Zone A
Relays
52

34. Backup Protection
A
C D
E
Breaker 5
Fails
1 2 5 6 11 12
T
3 4 7 8 9 10
B F

35. Typical Short-Circuit Type
Distribution
Single-Phase-Ground: 70–80%
Phase-Phase-Ground: 17–10%
Phase-Phase: 10–8%
Three-Phase: 3–2%

36. Balanced vs.
Unbalanced Conditions
Balanced System Unbalanced System
c
I
a
I
b
I
a
I
c
I
b
I

37. Decomposition of an Unbalanced
System
Positive-Sequence
Balanced Balanced
Negative-Sequence
1
b
I
1
c
I
1
a
I
2
b
I
2
a
I
2
c
I
0
a
I
0
b
I
0
c
I
a
I
c
I
b
I
Zero-Sequence
Single-Phase

38. Power Line Protection Principles
 Overcurrent (50, 51, 50N, 51N)
 Directional Overcurrent (67, 67N)
 Distance (21, 21N)
 Differential (87)

39. Characteristics of overcurrent
relays:
 Definite time
 IDMT- inverse definite minimum time
 Very inverse
 Extremely inverse

40. Application of Inverse-Type
Relays
t
Relay
Operation
Time
I
Fault Load
Radial Line

41. Distance
Distance
t
I
  
T

Inverse-Time Relay
Coordination
T
 T


42. 50/51 Relay Coordination
Distance
Distance
t
I
  
T
 T
 T


43. Directional Overcurrent Protection
Basic Applications
K
L

44. Distance Relay Principle
Three-Phase
Solid Fault
d
L
Radial
Line
21
Suppose Relay Is Designed to Operate
When:
|
||
|
)
8
.
0
(
|
| 1 a
L
a I
Z
V 
c
b
a I
I
I ,
,
c
b
a V
V
V ,
,

45. The Impedance Relay
Characteristic
2
1
2
2
r
Z
X
R 

R
X Plain Impedance Relay
Operation Zone
Zr1
Radius Zr1
1
r
Z
Z 

46. Need for Directionality
1 2 3 4 5 6
F1
F2
R
X
RELAY 3
Operation Zone
F1
F2
Nonselective
Relay Operation

47. Three-Zone Distance Protection
1 2 3 4 5 6
Zone 1
Zone 2
Zone 3
Time
Time
Zone 1 Is Instantaneous

48. Circular Distance Relay Characteristics
MHO
OFFSET
MHO (1)
PLAIN
IMPEDANCE
R
X
R
X
R
X
OFFSET
MHO (2)
R
X
LENS
(RESTRICTED MHO 1)
TOMATO
(RESTRICTED MHO 2)
R
X
R
X

49. Differential Protection Principle
No Relay Operation if CTs Are Considered Ideal
External
Fault
IDIF = 0
CT CT
50
Balanced CT Ratio
Protected
Equipment

50. Differential Protection Principle
Internal
Fault
IDIF > ISETTING
CTR CTR
50
Relay Operates
Protected
Equipment

51. Problem of Unequal CT
Performance
 False differential current can occur if a CT saturates
during a through-fault
 Use some measure of through-current to desensitize
the relay when high currents are present
External
Fault
Protected
Equipment
IDIF  0
CT CT
50

52. Possible Scheme – Percentage
Differential Protection Principle
Protected
Equipment
ĪR
ĪS
CTR CTR
Compares:
Relay
(87)
OP S R
I I I
 
| | | |
2
S R
RT
I I
k I k

  
ĪRP
ĪSP

53. Differential Protection
Applications
 Bus protection
 Transformer protection
 Generator protection
 Line protection
 Large motor protection
 Reactor protection
 Capacitor bank protection
 Compound equipment protection

54. Differential Protection
Summary
 The overcurrent differential scheme is simple
and economical, but it does not respond well to
unequal current transformer performance
 The percentage differential scheme responds
better to CT saturation
 Percentage differential protection can be
analyzed in the relay and the alpha plane
 Differential protection is the best alternative
selectivity/speed with present technology

55. Advantages of Digital Relays
Multifunctional
Compatibility with
digital integrated
systems
Low maintenance
(self-supervision)
Highly sensitive,
secure, and
selective
Adaptive
Highly reliable
(self-supervision)
Reduced burden
on
CTs and VTs
Programmable
Versatile
Low Cost

56. Why study this protection
scheme??
 Protection scheme plays a vital & important
role for the normal operation or the steady
state operation of different components of
power system network, which must be reliable,
fast and efficient.
 In order to achieve all these features, it is
essential that these should be proper care in
designing and choosing an appropriate and
efficient protection scheme.
 The protective relays functions as the
brain behind the whole schemes…

57. THANK YOU