Power System Protection Fundamental
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Copyright SEL 2008
Power System Protection
Fundamentals
What should we teach students
about power system protection?
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Agenda
Why protection is needed
Principles and elements of the protection
system
Basic protection schemes
Digital relay advantages and enhancements
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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 protectionsystem
They can jeopardize the entire power system
They cannot be overcome by a control system
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Power System Protection
Operation during severe disturbances:
System element protection
System protection
Automatic reclosing
Automatic transfer to alternate power
supplies
Automatic synchronization
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Electric Power System Exposure to
External Agents
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Damage to Main Equipment
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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
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Blackouts
Loss of service in a
large area or
population region Hazard to human life
May result in
enormous economiclosses
Overreaction of the
protection system
Bad design of theprotection system
Characteristics Main Causes
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Short Circuits Produce High
Currents
FaultSubstation
a
bc
I
IWire
Three-Phase Line
Thousands of Amps
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Electrical Equipment Thermal Damage
I
t
In Imd
Damage Curve
Short-Circuit
Current
Damage
Time
Rated Value
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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)= ki1(t) i2(t)
Mechanical
Forces
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The Fuse
Fuse
Transformer
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Protection System Elements
Protective relays
Circuit breakers
Current and voltage transducers
Communications channels
DC supply system
Control cables
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Three-Phase Diagram of the Protection Team
CTs
VTs
Relay
CB
Control
Protected
Equipment
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DC Tripping Circuit
SI
52
TC
DC Station
Battery SI
Relay
Contact
Relay
CircuitBreaker
52a
+
Red
Lamp
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Circuit Breakers
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Current Transformers
Very High Voltage CTMedium-Voltage CT
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Voltage Transformers
Medium Voltage
High Voltage
Note: Voltage transformers
are also known as potential
transformers
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Protective Relays
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Examples of Relay Panels
Old Electromechanical
Microprocessor-
Based Relay
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How Do Relays Detect Faults?
When a fault takes place, the current, voltage,
frequency, and other electrical variablesbehave 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 acombination of both
Many other detection principles determine the
design of protective relays
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Main Protection Requirements
Reliability
Dependability
Security
Selectivity
Speed
System stability
Equipment damage
Power quality
Sensitivity
High-impedance faults
Dispersed generation
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Primary Protection
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Primary Protection Zone Overlapping
Protection
Zone B
Protection
Zone A
To Zone BRelays
To Zone A
Relays
52 Protection
Zone B
Protection
Zone A
To Zone B
Relays
To Zone A
Relays
52
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Backup Protection
A
C D
E
Breaker 5Fails
1 2 5 6 11 12
T
3 4 7 8 9 10
B F
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Typical Short-Circuit Type
Distribution
Single-Phase-Ground: 7080%
Phase-Phase-Ground: 1710%
Phase-Phase: 108%
Three-Phase: 32%
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Balanced vs.
Unbalanced Conditions
Balanced System Unbalanced System
cI
aI
bI
aI
cI
bI
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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
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Power Line Protection Principles
Overcurrent (50, 51, 50N, 51N)
Directional Overcurrent (67, 67N)
Distance (21, 21N)
Differential (87)
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Application of Inverse-Type
Relays
tRelay
Operation
Time
I
Fault Load
Radial Line
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Distance
Distance
t
I
T
Inverse-Time Relay Coordination
T T
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Addition of Instantaneous OC
Element
tRelay
Operation
Time
I
Fault Load
Radial Line
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50/51 Relay Coordination
Distance
Distance
t
I
T T T
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Directional Overcurrent ProtectionBasic Applications
K
L
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Directional Overcurrent ProtectionBasic Principle
F2
Relay
F1
Forward Fault (F1)Reverse Fault (F2)
V
IV
I
IV
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Overcurrent Relay Problem
11 )8.0( LSSETTING
ZZ
EI
11
)()8.0(
LS
LIMITFAULTZZ
EI
Relay operates when the following condition
holds:
SETTINGaFAULTIII
As changes, the relays reach will change,since setting is fixed1s
Z
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Distance Relay Principle
Three-Phase
Solid Fault
d
L
RadialLine21
Suppose Relay Is Designed to Operate
When:||||)8.0(|| 1 aLa IZV
cbaIII ,,
cbaVVV ,,
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The Impedance Relay Characteristic
21
22rZXR
R
X Plain Impedance Relay
Operation Zone
Zr1
Radius Zr11rZZ
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Need for Directionality
1 2 3 4 5 6F1F2
R
XRELAY 3Operation Zone
F1
F2Nonselective
Relay Operation
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Directionality Improvement
1 2 3 4 5 6
F1F2
R
XRELAY 3Operation Zone
F1
F2The Relay Will
Not Operate for
This Fault
Directional Impedance
Relay Characteristic
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Mho Element Characteristic
(Directional Impedance Relay)
MTM
ZZ cos
ZM
Z
R
X
MT
MTM
ZIV cosOperates when:
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Three-Zone Distance Protection
1 2 3 4 5 6
Zone 1
Zone 2
Zone 3
Time
Time
Zone 1 Is Instantaneous
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Line Protection With Mho Elements
E
X
RA
B
C
D
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Circular Distance Relay Characteristics
MHO
OFFSETMHO (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
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Semi-Plane Type Characteristics
REACTANCE
OHM
DIRECTIONAL
R
X
R
X
R
X
RESTRICTED
DIRECTIONAL
R
X
RESTRICTED
REACTANCE
QUADRILATERAL
R
X
R
X
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Distance ProtectionSummary
Current and voltage information
Phase elements: more sensitive than 67
elements
Ground elements: less sensitive than 67N
elements
Application: looped and parallel lines
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Directional Comparison
Pilot Protection Systems
Communications
Channel
Exchange of logic information
on relay status
RL
Relays Relays
T
R
R
T
LI RI
O
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Permissive Overreaching
Transfer Trip
1 2 3 4 5 6
FWD
FWD
Bus A Bus B
B i POTT L i
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Basic POTT Logic
Zone 2 Elements
RCVR
Key XMTR
TripAND
Di ti l C i
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Directional Comparison
Blocking Scheme
1 2 3 4 5 6
FWD
FWD
RVS
RVS
Bus A Bus B
B i DCB L i
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Basic DCB Logic
Zone 2
RCVRTrip
CC
0
Carrier CoordinationTime Delay
Key XMTRZone 3
Diff ti l P t ti P i i l
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Differential Protection Principle
No Relay Operation if CTs Are Considered Ideal
ExternalFault
IDIF = 0
CT CT
50
Balanced CT Ratio
Protected
Equipment
Diff ti l P t ti P i i l
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Differential Protection Principle
InternalFault
IDIF > ISETTING
CTR CTR
50
Relay Operates
Protected
Equipment
P bl f U l CT P f
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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
ProtectedEquipment
IDIF 0
CT CT
50
P ibl S h P t
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Possible Scheme Percentage
Differential Protection Principle
Protected
Equipment
RS
CTR CTR
Compares:
Relay
(87)
OP S RI I I
| | | |
2
S R
RT
I Ik I k
RPSP
Diff ti l P t ti A li ti
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Differential Protection Applications
Bus protection
Transformer protection
Generator protection
Line protection
Large motor protection
Reactor protection Capacitor bank protection
Compound equipment protection
Diff ti l P t ti
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Differential ProtectionSummary
The overcurrent differential scheme is simpleand economical, but it does not respond well to
unequal current transformer performance
The percentage differential scheme respondsbetter 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
M ltiple Inp t Differential Schemes
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Multiple Input Differential SchemesExamples
Differential Protection Zone
Bus Differential: Several Inputs
RPSP
OP
T
I1 I2 I3 I4
Three-Winding Transformer
Differential: Three Inputs
Advantages of Digital Relays
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Advantages of Digital Relays
MultifunctionalCompatibility withdigital integrated
systems
Low maintenance
(self-supervision)
Highly sensitive,secure, and
selective
AdaptiveHighly reliable
(self-supervision)
Reduced burdenon
CTs and VTs
Programmable
VersatileLow Cost
Synchrophasors Provide a
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Synchrophasors Provide a
Snapshot of the Power System
The Future
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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-machineinterface
Much more