POWER SYSTEM PROTECTION

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