The document outlines the fundamentals of power system protection, emphasizing the necessity of protection against severe disturbances that can jeopardize power system operations. It details elements like protective relays, circuit breakers, and current/voltage transformers, as well as various protection schemes and guidelines for their implementation. Additionally, it discusses the advantages of digital relays and the importance of reliable communication systems in enhancing protection mechanisms.

1. Power System Protection
Fundamentals
Dr. Youssef A. Mobarak
y.a.Mobarak@Gmail.com
2014
1

2. AGENDA
Why protection is needed
Principles and elements of the protection system
Basic protection schemes
Digital relay advantages and enhancements
2
Topic_1

3. 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

4. POWER SYSTEM PROTECTION
Operation during severe disturbances:
 System element protection
 System protection
 Automatic reclosing
 Automatic transfer to alternate power supplies
 Automatic synchronization
4

5. Generation-typically at 4-35kV
Transmission-typically at 230-765kV
Subtransmission-typically at 69-161kV
Receives power from transmission system and transforms
into subtransmission level
Receives power from subtransmission system and
transforms into primary feeder voltage
Distribution network-typically 2.4-69kV
Low voltage (service)-typically 120-600V
TYPICAL BULK POWER SYSTEM
5

6. PROTECTION ZONES
1. Generator or Generator-Transformer Units
2. Transformers
3. Buses
4. Lines (transmission and distribution)
5. Utilization equipment (motors, static loads, etc.)
6. Capacitor or reactor (when separately protected)
Unit Generator-Tx zone
Bus zone
Line zone
Bus zone
Transformer zone
Transformer zone
Bus zone
Generator
~
XFMR Bus Line Bus XFMR Bus Motor
Motor zone
6

7. WHAT INFO IS REQUIRED TO APPLY PROTECTION
1. One-line diagram of the system or area involved
2. Impedances and connections of power equipment, system frequency,
voltage level and phase sequence
3. Existing schemes
4. Operating procedures and practices affecting protection
5. Importance of protection required and maximum allowed clearance
times
6. System fault studies
7. Maximum load and system swing limits
8. CTs and VTs locations, connections and ratios
9. Future expansion expectance
10. Any special considerations for application. 11

8. C37.2: DEVICE
NUMBERS
Partial listing

9. ONE LINE DIAGRAM
Non-dimensioned diagram showing how pieces of electrical
equipment are connected
Simplification of actual system
Equipment is shown as boxes, circles and other simple graphic
symbols
Symbols should follow ANSI or IEC conventions
13

10. LINE SYMBOLS [1]
14

11. LINE SYMBOLS [2]
15

12. LINE SYMBOLS [3]
16

13. LINE SYMBOLS [4]
17

14. 1-LINE [1]
18

15. PROTECTION SYSTEM
A series of devices whose main purpose is to
protect persons and primary electric power
equipment from the effects of faults
19
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

16. SHORT CIRCUITS PRODUCE HIGH CURRENTS
FaultSubstation
a
b
c
I
I
Wire
Three-Phase Line
Thousands of Amps
20

17. ELECTRICAL EQUIPMENT THERMAL DAMAGE
I
t
In Imd
Damage Curve
Short-Circuit
Current
Damage
Time
Rated Value
21

18. 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
22

19. ELECTRIC POWER SYSTEM EXPOSURE TO EXTERNAL
AGENTS
23

20. DAMAGE TO MAIN EQUIPMENT
24

21. THE FUSE
Fuse
Transformer
25

22. PROTECTION SYSTEM ELEMENTS
Protective relays
Circuit breakers
Current and voltage transducers
Communications channels
DC supply system
Control cables
26

23. THREE-PHASE DIAGRAM OF THE PROTECTION TEAM
CTs
VTs
Relay
CB
Control
Protected
Equipment
27

24. DC TRIPPING CIRCUIT
SI
52
TC
DC Station
Battery
SI
Relay
Contact
Relay
Circuit
Breaker
52a
+
–
Red
Lamp
28

25. CIRCUIT BREAKERS
29

26. CURRENT TRANSFORMERS
Very High Voltage CT
Medium-Voltage CT
30

27. VOLTAGE TRANSFORMERS
Medium Voltage
High Voltage
Note: Voltage transformers
are also known as potential
transformers
31

28. TYPICAL CT/VT CIRCUITS
Courtesy of Blackburn, Protective Relay: Principles and Applications
32

29. CT/VT CIRCUIT VS. CASING GROUND
Case ground made at IT location
Secondary circuit ground made at first point of use
Case
Secondary Circuit
Prevents shock exposure of personnel
Provides current carrying capability for the ground-fault
current
Grounding includes design and construction of substation
ground mat and CT and VT safety grounding

30. SUBSTATION TYPES
• Single Supply
• Multiple Supply
• Mobile Substations for emergencies
• Types are defined by number of transformers, buses,
breakers to provide adequate service for application
34

31. SWITCHGEAR DEFINED
Assemblies containing electrical switching, protection,
metering and management devices
Used in three-phase, high-power industrial, commercial
and utility applications
Covers a variety of actual uses, including motor control,
distribution panels and outdoor switchyards
The term "switchgear" is plural, even when referring to a
single switchgear assembly (never say, "switchgears")
May be a described in terms of use:
 "the generator switchgear"
 "the stamping line switchgear"
35

32. PROTECTIVE RELAYS
38

33. EXAMPLES OF RELAY PANELS
Old Electromechanical
Microprocessor-
Based Relay
39

34. 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 undervoltage, or
a combination of both
Many other detection principles determine the design of
protective relays
40

35. MAIN PROTECTION REQUIREMENTS
Reliability
 Dependability
 Security
Selectivity
Speed
 System stability
 Equipment damage
 Power quality
Sensitivity
 High-impedance faults
 Dispersed generation
41

36. PRIMARY PROTECTION
42

37. 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
43

38. BACKUP PROTECTION
A
C D
E
Breaker 5
Fails
1 2 5 6 11 12
T
3 4 7 8 9 10
B F
44

39. BALANCED VS. UNBALANCED CONDITIONS
Balanced System Unbalanced System
cI
aI
bI
aI
cI
bI
45
Typical Short-Circuit Type Distribution
Single-Phase-Ground: 70–80%
Phase-Phase-Ground: 17–10%
Phase-Phase: 10–8%
Three-Phase: 3–2%

40. DECOMPOSITION OF AN UNBALANCED SYSTEM
Positive-Sequence
Balanced Balanced
Negative-Sequence
1bI
1cI
1aI
2bI
2aI
2cI
0aI
0bI
0cI
aI
cI
bI
Zero-Sequence
Single-Phase
46

41. CONTRIBUTION TO FAULTS
47

42. FAULT TYPES (SHUNT)
48
X
X

Z
Z
Z
G
BC
A
Short Circuit Calculation
Fault Types – Single Phase to Ground
X
X

Z
Z
Z
G
BC
A
Short Circuit Calculations
Fault Types – Line to Line
Z
Z
Z
G
BC
A
X
X

X

Short Circuit Calculations
Fault Types – Three Phase

43. AC & DC CURRENT COMPONENTS OF FAULT CURRENT
49VARIATION OF CURRENT WITH TIME DURING A FAULT
VARIATION OF GENERATOR REACTANCE
DURING A FAULT

44. USEFUL CONVERSIONS
50

45. PER UNIT SYSTEM
Establish two base quantities:
Standard practice is to define
 Base power – 3 phase
 Base voltage – line to line
Other quantities derived
with basic power equations
51

46. SHORT CIRCUIT CALCULATIONS
PER UNIT SYSTEM
Per Unit Value = Actual Quantity
Base Quantity
Vpu = Vactual
Vbase
Ipu = Iactual
Ibase
Zpu = Zactual
Zbase
52
3 x kV L-L base
I base =
x 1000MVAbase
Z base =
kV2
L-L base
MVAbase
 Zpu2 =Zpu1 x kV 2
base1 x MVAbase2
kV 2
base2 MVAbase1

47. FAULT INTERRUPTION AND ARCING
57

48. POWER LINE PROTECTION PRINCIPLES
Overcurrent (50, 51, 50N, 51N)
Directional Overcurrent (67, 67N)
Distance (21, 21N)
Differential (87)
58
t
Relay
Operation
Time
I
Fault Load
Radial Line
APPLICATION OF INVERSE-TYPE RELAYS

49. INVERSE-TIME RELAY COORDINATION
59
tRelay
Operation
Time
I
Fault Load
Radial Line
Distance
Distance
t
I
  T T T

50. DIRECTIONAL OVERCURRENT PROTECTION
BASIC PRINCIPLE
F2
Relay
F
1
Forward Fault (F1)Reverse Fault (F2)
V
I
V
I
IV
60
11 )8.0( LS
SETTING
ZZ
E
I


11
)(
)8.0( LS
LIMITFAULT
ZZ
E
I


 Relay operates when the following condition holds:
SETTINGaFAULT III 
 As changes, the relay’s “reach” will change, since
setting is fixed

51. DISTANCE RELAY PRINCIPLE
L
Three-Phase
Solid Fault
d
Radial
Line2
1
Suppose Relay Is Designed to Operate When:
||||)8.0(|| 1 aLa IZV 
cba III ,,
cba VVV ,,
61
2
1
22
rZXR 
R
X Plain Impedance Relay
Operation Zone
Zr1
Radius Zr1
1rZZ 

52. NEED FOR DIRECTIONALITY F1
1 2 3 4 5 6
F2
R
XRELAY 3
Operation Zone
F1
F2
Nonselective Relay
Operation
62
1 2 3 4 5 6
F1F2
R
XRELAY 3
Operation Zone
F1
F2
The Relay Will
Not Operate for
This Fault
Directional Impedance
Relay Characteristic
 MTMZZ   cos
ZM
Z
R
X
MT

 MTMZIV   cosOperates when:

53. THREE-ZONE DISTANCE PROTECTION
1 2 3 4 5 6
Zone 1
Zone 2
Zone 3
Time
Time
Zone 1 Is Instantaneous
63
X
R
A
B
C
D

54. DISTANCE PROTECTION SUMMARY
Current and voltage information
Phase elements: more sensitive than 67 elements
Ground elements: less sensitive than 67N elements
Application: looped and parallel lines
64
Communications
Channel
Exchange of logic information
on relay status
RL
Relays Relays
T
R
R
T
LI RI

55. PERMISSIVE OVERREACHING TRANSFER TRIP
1 2 3 4 5 6
FWD
FWD
Bus A Bus B
65
1 2 3 4 5 6
FWD
FWD
RVS
RVS
Bus A Bus B

56. DIFFERENTIAL PROTECTION PRINCIPLE
No Relay Operation if CTs Are Considered Ideal
External
Fault
IDIF = 0
CT CT
50
Balanced CT Ratio
Protected
Equipment
66
Internal
Fault
IDIF > ISETTING
CTR CTR
50
Relay Operates
Protected
Equipment

57. 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
67

58. POSSIBLE SCHEME – PERCENTAGE DIFFERENTIAL
PROTECTION PRINCIPLE
Protected
Equipment
ĪRĪS
CTR CTR
Compares:
Relay
(87)
OP S RI I I 
| | | |
2
S R
RT
I I
k I k

  
ĪRPĪSP
68

59. DIFFERENTIAL PROTECTION APPLICATIONS
Bus protection
Transformer protection
Generator protection
Line protection
Large motor protection
Reactor protection
Capacitor bank protection
Compound equipment protection
69

60. 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
70

61. MULTIPLE INPUT DIFFERENTIAL SCHEMES
EXAMPLES
Differential Protection Zone
Bus Differential: Several Inputs
ĪRPĪSP
OP
ĪT
I1 I2 I3 I4
Three-Winding Transformer
Differential: Three Inputs
71

62. 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
72

63. A GOOD DAY IN SYSTEM PROTECTION……
 CTs and VTs bring electrical info to relays
 Relays sense current and voltage and declare fault
 Relays send signals through control circuits to circuit breakers
 Circuit breaker(s) correctly trip
73
A BAD DAY IN SYSTEM PROTECTION……
 CTs or VTs are shorted, opened, or their wiring is
 Relays do not declare fault due to setting errors, faulty relay, CT
saturation
 Control wires cut or batteries dead so no signal is sent from relay to
circuit breaker
 Circuit breakers do not have power, burnt trip coil or otherwise fail
to trip

64. PROTECTION PERFORMANCE STATISTICS
Correct and desired: 92.2%
Correct but undesired: 5.3%
Incorrect: 2.1%
Fail to trip: 0.4%
74
THE FUTURE
Improvements in computer-based protection
Highly reliable and viable communication systems (satellite, optical
fiber, etc.)
Integration of control, command, protection, and communication
Improvements to human-machine interface
Much more