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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 protection system 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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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 economic losses
Overreaction of the protection system
Bad design of the protection system
Characteristics Main Causes
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Short Circuits Produce High Currents
FaultSubstation
abc
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 DuringShort 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
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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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Voltage Transformers
Medium Voltage
High Voltage
Note: Voltage transformers are also known as potential transformers
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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
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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 Zone Overlapping
ProtectionZone B
ProtectionZone A
To Zone BRelays
To Zone ARelays
52 ProtectionZone B
ProtectionZone A
To Zone BRelays
To Zone ARelays
52
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Typical Short-Circuit Type Distribution
Single-Phase-Ground: 70–80%
Phase-Phase-Ground: 17–10%
Phase-Phase: 10–8%
Three-Phase: 3–2%
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Balanced vs. Unbalanced Conditions
Balanced System Unbalanced System
cI
aI
bI
aI
cI
bI
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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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Addition of Instantaneous OC Element
tRelay Operation
Time
I
Fault Load
Radial Line
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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
LIMITFAULT ZZE
I+′
=
Relay operates when the following condition holds:
SETTINGaFAULT III >=
As changes, the relay’s “reach” will change, since setting is fixed
1sZ
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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 ≤
cba III ,,
cba VVV ,,
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The Impedance Relay Characteristic
21
22rZXR ≤+
R
X Plain Impedance RelayOperation Zone
Zr1
Radius Zr11rZZ ≤
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Need for Directionality
1 2 3 4 5 6
F1F2
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)
( )MTMZZ ϕϕ −≤ cos
( )MTMZIV ϕϕ −≤ cosOperates when:
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Three-Zone Distance Protection
1 2 3 4 5 6
Zone 1
Zone 2Zone 3
Time
TimeZone 1 Is Instantaneous
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Circular Distance Relay Characteristics
MHO
OFFSETMHO (1)
PLAIN IMPEDANCE
R
X
R
X
R
X
OFFSETMHO (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
RESTRICTEDDIRECTIONAL
R
X
RESTRICTEDREACTANCE
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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Basic DCB Logic
Zone 2
RCVRTrip
CC
0
Carrier Coordination
Time Delay
Key XMTRZone 3
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Differential Protection Principle
No Relay Operation if CTs Are Considered Ideal
ExternalFault
IDIF = 0
CT CT
50
Balanced CT Ratio
ProtectedEquipment
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Differential Protection Principle
InternalFault
IDIF > ISETTING
CTR CTR
50
Relay Operates
ProtectedEquipment
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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
ExternalFault
ProtectedEquipment
IDIF ≠ 0
CT CT
50
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Possible Scheme – Percentage Differential Protection Principle
ProtectedEquipment
ĪRĪS
CTR CTR
Compares:
Relay(87)
OP S RI I I= +
| | | |
2S R
RT
I Ik I k
+× = ×
ĪRPĪSP
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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
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Differential ProtectionSummary
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
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Multiple Input Differential SchemesExamples
Differential Protection Zone
Bus Differential: Several Inputs
ĪRPĪSP
OP
ĪT
I1 I2 I3 I4
Three-Winding TransformerDifferential: Three Inputs
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Advantages of Digital Relays
MultifunctionalCompatibility withdigital integrated
systems
Low maintenance(self-supervision)
Highly sensitive,secure, and
selectiveAdaptive
Highly reliable(self-supervision)
Reduced burden on
CTs and VTs
ProgrammableVersatile
Low Cost