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Knowledge Is PowerSM
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Introduction to Distance Protection
Jay GosaliaVice President of Marketing
Doble Engineering Company
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Objective of Relay Protection
Protect persons and equipment in the surrounding of the power system
Protect apparatus in the power system
Separate faulty parts from the rest of the power system to facilitate the operation of the healthy part of the system
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Electrical Fault in Power System
Transmission lines 85%Busbar 12%Transformer/ Generator 03%
85%
12% 3%
Transmission lines Busbar TFR/Gen
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Fault Statistics
Single phase to earth 80%Two phases to earth 10%Phase to phase faults 5%Three phase faults 5%
80%
10%5% 5%
Ph-G Flt Ph-Ph-G Flt Ph-Ph Flt 3 Ph Flt
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Fault Type in Transmission Line
Transient faultsCommon on transmission lines, approximately 80-85%Lightnings are the most common reasonCaused by birds, falling trees, forest growth,Swinging lines, High velocity winds etc.
• Disappear after a short dead interval
Persistent faultsCaused by a broken conductor fallen downTree falling on a line
• Must be located and repaired before normal service
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Protection Types
Unit ProtectionNon-Unit protection
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Protection Types
Unit ProtectionDifferential protection
• Transformer differential protectionBus differential protection
• Generator differential protection• Line differential protection
Pilot protection• Transfer trip schemes• Under/over reaching pilot protection with distance
protectionNon-Unit protection
Over Current protection• Time over current or instantaneous protection
3 Zones of distance protection
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Unit Protection
TFR
Protection
Unit protection provides the protection if the fault is inside the Zone of protectionIt does not provide the protection if the fault is outside the Zone of protection
Unit protection provides the protection if the fault is inside the Zone of protectionIt does not provide the protection if the fault is outside the Zone of protection
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Non-Unit ProtectionA B DC
Distance
Time
Zone 1Zone 2 Zone 3
Zone 1 provides the instantaneous protection for the line AB Zone 2 and Zone 3 are the back up protection
Zone 1 provides the instantaneous protection for the line AB Zone 2 and Zone 3 are the back up protection
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It’s a Non Unit Protectionbut can be modified to Unit Protection when combined with signaling channel
Non-Unit Protection
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DistanceProtection
Theory of operationTheory of operation
DesignDesign
Application ExamplesApplication Examples
Basics of Distance Protection
Basics of Distance Protection
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Distance Protection: Principle
Protection works on impedance seen by the protectionImpedance is directly proportional to distance so it’s a distance protectionRequires inputs for voltage and current
Using CTs and PTsCurrent is an operating force and voltage is a restraining force
Normal load condition restraining force is higher then the operating force
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Distance Protection: Principle
Current Voltage
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Fault condition
Normal conditionNormal voltage & load current69 Volts and 1 A load current
Fault conditionDepressed voltage & high current20 Volts & 10 A fault current
A B DC
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Fault condition
Quick isolation of the faulted sectionReduces damage caused by the faultLess stress on the electrical apparatusMaintains the flow of electricity to healthy section
Quick and fast detection of the fault condition Fast operation of the distance protection
A B DC
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Distance Protection: Principle
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Distance Protection: Ph-Ph Fault
The measured impedance is equal to thePositive & Negative sequence impedance up to the fault location
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Distance Protection: 3 Ph Faults
The measured impedance is equal to thePositive Sequence impedance up to the fault location
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Distance Protection: Ph-G Faults
The measured impedance is equal to thePositive, Negative & Zero Sequence impedance up to the fault location
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Distance Protection: Ph-G Faults
The current is Phase current + the Residual current Residual current = Iph * (Z0-Z1) / 3Z1,
KN = Zero Sequence compensation factor.The factor KN is a transmission line constant
Identical throughout the whole line length.Total loop impedance = (1+KN) Z1
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Distance Protection: Principle
A B DC
Resistance : R
Reactance : XZ
RF
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Distance Protection: Principle
IR
IX IZ
V
ZI
V
IZ
+ OutputAngleComparator
>= 900
V-IZ
Angle = 900
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Distance Protection: Principle
ZI
V
IZ
+ OutputAngleComparator
>= 900
IR
IX IZ
V
V-IZ
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Distance Protection: Principle
Angle between the two cords drawn from the diameterOf a circle is always 90 degrees.
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Distance Protection
IR
IX IZ
V
ZI
V
IZ
+ OutputAngleComparator
>= 900
V-IZ Internal faultAngle >=900Internal faultAngle >=900
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Distance Protection
IR
IX IZ
V
ZI
V
IZ
+ OutputAngleComparator
>= 900
V-IZ
External faultAngle < 900External faultAngle < 900
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Distance Protection : Design
Set a replica impedanceReplica of line impedanceMagnitude and angle : ZRCalled reach of the Relay
Convert current I in to vector IZRDerive voltage of the system : V
Reference voltage V , Polarizing Voltage : Vpol
Calculate voltage vector Vpol - IZRMeasure the angle between Vpol and Vpol – IZROutput if the angle is 900 or greaterThis produces “MHO” Characteristic
IR
IX IZR
Vpol
Vpol - IZR
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Distance Protection : Ph-G Fault
For A-G faultIZR is IA*ZR
Vpol is VA
Vpol – IZR is VA – IAZR
When polarizing voltage = Fault voltageSelf polarized RelayFor B-G fault
• Polarized voltage = Fault voltage = VB
Earlier designs were self polarized distance relays
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Self Polarized Protection
LimitationsWhat happens if the fault is at the terminal of the breaker?
• Fault voltage is 0• Polarized voltage is 0• No reference voltage Vpol to compare with
V-IZ• Self polarized distance protection no good for
0 voltage phase to ground faultSolution
Use memory voltage instead of faulted phase voltage
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Memory Polarized Protection
ZI
V
IZ
+ Output
AngleComparator
>= 900
Memory
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Memory Polarized Protection
What memory voltage does to the “MHO”characteristic?
ZI
V
IZ
+ Output
AngleComparator
>= 900
Memory
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Memory Polarization
Pre-Fault voltage at the protection before the fault is E (neglecting load current drop in Zs)
A B
G Zs
E21
E
Load Current
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Memory Polarization
Pre-Fault voltage at the protection before the fault is E (neglecting load current drop in Zs)Just after the fault the E = VF + IFZs
A B
G Zs
EVF 21
Fault Current IF
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Memory Polarization
IR
IX IZR
VF
V-IZ
Angle > 900Angle > 900
IZs
Vpol = V pre fault
E = VF + IFZs
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Mho Ch. : Memory Polarization
IR
IX IZR
VF
V-IZ
IZs
Vpol = V pre fault
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Memory Polarization Effect
IR
IX IZR
VF
V-IZ
IZsVpol = V pre fault
More fault resistanceCoverage due to memory Polarization
More fault resistanceCoverage due to memory Polarization
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Memory Polarization: Summary
Provides reference voltage (Vpol) under all phase and ground faultsExpand the self polarized characteristic to cover more fault resistance
No overreaching at reach pointCircle with a diameter = Source impedance + Reach impedance
• Higher the source impedance (weak source) larger the diameter means more fault resistance coverage
Numerical protections uses memory, self, healthy phase voltages for polarizations and or different combinations of the same
Partially or fully cross polarized protections are very common now a days
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Cross Polarization
It is very common to use healthy phase voltage for phase to ground fault
Provides polarizing voltage for a zero voltage faultFor A-G fault: Polarizing voltage is -(VB+VC)
• Called Cross polarizing• Same effect as Memory Polarization
VA
VC
VB
-(VB+VC)
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Memory Polarization : Questions
How protection works when there is a 3 phase zero voltage faults?How the protection works when there is a permanent 3 phase zero voltage faults during reclosing?Looks like that protection can trip for a reverse faults. True?
IR
IX IZR
IZs
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Distance Protection: Architecture
ZRI
V
IZ
+Output
Timer = 0.25Cycles
Memory
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Distance Protection: Architecture
ZRI
V
IZ
+Output
MemoryPhaseShift- 900
PhaseDetector
V-IZ in phaseOr lagVpol
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Distance Protection: Architecture
ZRI
V
IZ
+Output
MemoryPhaseShift- 900
PhaseDetector
V-IZ in phaseOr lagVpol
ZR
V-IZ
ZsVpol
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Distance Protection: Architecture
ZRI
V
IZ
+Output
MemoryPhaseShift- 900
PhaseDetector
V-IZ in phaseOr lagVpol
ZR
V-IZ
ZsVpol
ZR
V-IZ
ZsVpol
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Review Question - 1ZR
V-IZ
Zs
Vpol
How protection works for a 3 phase zero voltage faults?Typically the protection memorizes 16-20 cycles of pre fault voltages which is used when there is Zero Voltage Fault.
How protection works for a 3 phase zero voltage faults?Typically the protection memorizes 16-20 cycles of pre fault voltages which is used when there is Zero Voltage Fault.
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Review Question - 2
How the protection works for a permanent 3 phase zero voltage faults during closing of the breaker?If the grounding chains were left on the breaker & breaker is closed, protection has no voltage in the memory as well as the fault voltage is zero. Protection sees only fault current.Switch On To Fault - SOTF feature is employed which trips the breaker if protection sees the current but no voltage following breaker close.
How the protection works for a permanent 3 phase zero voltage faults during closing of the breaker?If the grounding chains were left on the breaker & breaker is closed, protection has no voltage in the memory as well as the fault voltage is zero. Protection sees only fault current.Switch On To Fault - SOTF feature is employed which trips the breaker if protection sees the current but no voltage following breaker close.
A B
G Zs
21
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Review Question - 3
Looks like that protection can trip for a reverse faults. True? Characteristic is true only for forward faults for reverse fault protection will not operate
Looks like that protection can trip for a reverse faults. True? Characteristic is true only for forward faults for reverse fault protection will not operate
ZR
V-IZ
Zs
Vpol
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Memory Polarization : FactsA B
G
21
ZsZL
Characteristic is circle whose diameter is Zs - ZRFor faults behind the protection ZS = Zs + ZRSo characteristic should be a circle whose diameter is Zs - ZRSubstitute the value for ZsCharacteristic should be circle with a diameter equal to Zs
ZR
V-IZ
Zs
Vpol
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Memory Polarization : FactsA B
G
21
ZsZL
Characteristic is circle whose diameter is Zs - ZRFor faults behind the protection ZS = Zs + ZRSo characteristic should be a circle whose diameter is Zs - ZRSubstitute the value for ZsCharacteristic should be circle with a diameter equal to Zs
ZR
Zs
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Memory Polarization : ConclusionA B
G
21
ZsZL
ZR
Zs
Memory polarized MHO characteristic is very secure and not prone to operate for reverse faults
Memory polarized MHO characteristic is very secure and not prone to operate for reverse faults
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Fault Res.: Memory Vs. Self Polarized A B
G
21
ZsZLZs
RF
ZR
Zs
ZR
Zs
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Fault Res.: Strong Vs. Weak SourceA B
G
21
ZsZLZs
RF
ZR
Zs
ZR
ZsStrong SourceWeak Source
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Fault Res.: Short Vs. Long Line
G
21
ZsZLZs
RF
ZR
Zs
ZR
ZsStrong Source/Short line Weak Source/long line
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Fault Resistance CoverageStrong source & short line reduces fault resistance coverage
Use quadrilateral characteristic to improve fault resistance coverage
Weak Source & long line increases fault resistance coverageMemory polarized protection is more secure under all fault conditionsSelf polarized protection does not provide good fault resistance coverageMemory polarization increases fault resistance coverage compare to self polarized protectionHealthy phase polarization provides the same effect as memory polarized protection
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Quadrilateral Characteristics
Four comparators are used to detect fault conditionsIf all four comparators produces output, protection tripsQuad characteristic provide good fault resistance coverage
Four comparators are used to detect fault conditionsIf all four comparators produces output, protection tripsQuad characteristic provide good fault resistance coverage
ZR
ZL
R
X
Load Area
Dir
ReachLoad Blinder
KR
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Quadrilateral Characteristics
ZR
ZL
R
X
Dir
ReachLoad Blinder
KR
How directional line works?How directional line works?
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Directional line : Int. Fault
IR
IZ = Signal B
VF
VF <-900 = Signal A
IX
If A lags BBy 00 – 1800
True
Dir line
Operate
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Directional line : Ext. Fault
IR
IZ = Signal B
VF
VF <-900 = Signal A
IX
If A lags BBy 00 – 1800
True
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Reach line : Ext. Fault
IR
IZ VF
I*Kr = Signal B
IX
If A lags BBy 00 – 1800
VF - IZ = Signal A
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Reach line : Int. Fault
IR
IZ VF
I*Kr = Signal B
IX
If A lags BBy 00 – 1800
VF - IZ = Signal A
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Reach line
IR
IZ VF
I*Kr = Signal B
IX
If A lags BBy 00 – 1800
VF - IZ = Signal A
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Load Blinder: Internal Fault
IR
IZ
VF
I*KR
IX
If A lags BBy 00 – 1800VF - IZ = Signal B
KR Setting for the load blinder
VF-IKR=Signal A
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Load Blinder: External Fault
IR
IZ
VF
I*Kr
IX
If A lags BBy 00 – 1800
VF - IZ = Signal B
KR Setting for the load blinder
VF-IKR=Signal A
Operate
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Quadrilateral Characteristic : Facts
Quad characteristic is better for short line and/or strong source as it provides better fault resistance coverageVery good for ground fault protection
Fault resistance can be high • Tower footing resistance, Tree touching the
line etc.Reactance and resistance reach can be set independent of each other for optimum fault coverage
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Ph-G Faults: High Resistance Fault
High Resistance Faults•can be caused by growing trees, bushfire or objects touching a conductor• this type of high resistive faults can not be detected by impedance protection
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Quad Characteristic : Reactance Line
Reactance line is not a straight line parallel to R axis.Top line has a tilt of 30
The tilt enables protection to not operate for an external faultIf the tilt is not there then protection can operate for an external fault due to effect of load on the lineHow?
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Effect of Load : Quad Ch.
G G
21
ZsZLZs
Vx <00
X Y
VY <-300
Ix Iy
Ix+Iy <-Ө
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Impedance Seen at Protection X
G G
21
ZsZLZs
Vx <00 VY <-300
Ix Iy
Ix+Iy <-Ө
(Ix+Iy)*R
(Ix)*ZL
Tilt of the reactance linePrevents tripping for the External faults
Tilt of the reactance linePrevents tripping for the External faults
Ix
IyIx + Iy
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Effect of Load : Quad Ch.
G G
21
ZsZLZs
Vx <00
X Y
VY <+300
Ix Iy
Ix+Iy <+Ө
Ix
IyIx + Iy
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Impedance Seen at Protection X
G G
21
ZsZLZs
Vx <00 VY <-300
Ix Iy
Ix+Iy <-Ө
(Ix+Iy)*R
(Ix)*ZLTilt of the reactance linePrevents operation for the Internal faults
Tilt of the reactance linePrevents operation for the Internal faults
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Quad Characteristics : Ext. Fault
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Quad Characteristics : Int. Fault
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Quad Characteristics : Int. Fault
By polarizing the top line with –Ve or Zero sequence current, it will adapt to load ConditionBy polarizing the top line with –Ve or Zero sequence current, it will adapt to load Condition
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MHO Characteristics : Zone 3
Zone 3 provides the back up protection in case Zone 1 and Zone 2 fails to operateZone 3 is typically time delayed zoneIt is used in blocking scheme to determine direction of the faultZone 3 is mostly offset characteristic that it includes the Origin in the characteristic means it can trip for a reverse faultTo avoid load encroachment due to large setting of Zone 3 it’s shape can lens
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Zones of Protection
Load profile
Zone 3 : Offset MHO
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OFFSET MHOIZ
-IZ’
V-IZ
V-IZ’
Z = Forward ReachZ’ = Reverse Reach
Z = Forward ReachZ’ = Reverse Reach
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OFFSET MHOIZ
-IZ’
Z = Forward ReachZ’ = Reverse Reach
Z = Forward ReachZ’ = Reverse Reach
Load profile
Two Comparators are phase shifted by same angle in opposite direction Two Comparators are phase shifted by same angle in opposite direction
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OFFSET MHOIZ
-IZ’
Z = Forward ReachZ’ = Reverse Reach
Z = Forward ReachZ’ = Reverse Reach
Load profile
Major axis to minor axis ratio is equal to tan (180 - Ө)/2Each comparator is shifted by an angle Ө
Major axis to minor axis ratio is equal to tan (180 - Ө)/2Each comparator is shifted by an angle Ө
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MHO or Quad : Pros and ConsSimple and directionalLees sensitive to power swings
Reach does not extend as far along R-Axis
Limited fault resistance coverage for short lines
Simple and directionalLees sensitive to power swings
Reach does not extend as far along R-Axis
Limited fault resistance coverage for short lines
Good fault resistance coverage as char. can be set along R- Axis
Good for short line and strong source
Sensitive to power swing
Characteristic extends on R-Axis
Good fault resistance coverage as char. can be set along R- Axis
Good for short line and strong source
Sensitive to power swing
Characteristic extends on R-Axis
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Knowledge Is PowerSM
Apparatus Maintenance and Power Management for Energy Delivery