Transmission Line Protection
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Transcript of Transmission Line Protection
![Page 1: Transmission Line Protection](https://reader033.fdocuments.in/reader033/viewer/2022061106/54473b86b1af9fe4108b4a7f/html5/thumbnails/1.jpg)
Fundamentals of Distance Protection
GE Multilin
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2 /GE /
April 7, 2023
Outline• Transmission line introduction• What is distance protection?• Non-pilot and pilot schemes• Redundancy considerations• Security for dual-breaker
terminals• Out-of-step relaying• Single-pole tripping• Series-compensated lines
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April 7, 2023
Transmission Lines
A Vital Part of the Power System: • Provide path to transfer power between generation and load• Operate at voltage levels from 69kV to 765kV• Deregulated markets, economic, environmental requirements have pushed utilities to operate transmission lines close to their limits.
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April 7, 2023
Transmission Lines
Classification of line length depends on: Source-to-line Impedance Ratio (SIR),
and Nominal voltage
Length considerations: Short Lines: SIR > 4 Medium Lines: 0.5 < SIR < 4 Long Lines: SIR < 0.5
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April 7, 2023
Typical Protection SchemesShort Lines
• Current differential• Phase comparison• Permissive Overreach Transfer Trip (POTT)• Directional Comparison Blocking (DCB)
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April 7, 2023
Typical Protection SchemesMedium Lines
• Phase comparison• Directional Comparison Blocking (DCB)• Permissive Underreach Transfer Trip (PUTT)• Permissive Overreach Transfer Trip (POTT) • Unblocking• Step Distance• Step or coordinated overcurrent• Inverse time overcurrent• Current Differential
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April 7, 2023
Typical Protection SchemesLong Lines
• Phase comparison• Directional Comparison Blocking (DCB)• Permissive Underreach Transfer Trip (PUTT)• Permissive Overreach Transfer Trip (POTT) • Unblocking• Step Distance• Step or coordinated overcurrent• Current Differential
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April 7, 2023
What is distance protection?
For internal faults:> IZ – V and V approximately
in phase (mho)> IZ – V and IZ approximately
in phase (reactance)
RELAY (V,I)
IntendedREACH point
Z
F1
I*Z
V=I*ZF
I*Z - V
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April 7, 2023
What is distance protection?
For external faults:> IZ – V and V approximately
out of phase (mho)> IZ – V and IZ approximately
out of phase (reactance)
RELAY (V,I)
IntendedREACH point
Z I*Z
V=I*ZF
I*Z - V
F2
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April 7, 2023
What is distance protection?
RELAY
IntendedREACH point
Z
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April 7, 2023
Source Impedance Ratio, Accuracy & Speed
LineSystem
Relay
Voltage at the relay:SIRf
fVV
PULOC
PULOCNR
][
][
Consider SIR = 0.1
Fault location
Voltage (%)
Voltage change (%)
75% 88.24 2.76
90% 90.00 0.91
100% 90.91 N/A
110% 91.67 0.76
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April 7, 2023
Source Impedance Ratio, Accuracy & Speed
Line
SystemRelay
Voltage at the relay:SIRf
fVV
PULOC
PULOCNR
][
][
Consider SIR = 30
Fault location
Voltage (%)
Voltage change (%)
75% 2.4390 0.7868
90% 2.9126 0.3132
100% 3.2258 N/A
110% 3.5370 0.3112
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April 7, 2023
Challenges in relay design>Transients:
– High frequency– DC offset in currents– CVT transients in voltages
CVT output
0 1 2 3 4
steady-stateoutput
power cycles
-30
-20
-10
0
10
20
30
volt
ag
e,
V
C1
C22
3 5
6
1
4
7
High Voltage Line
Seco
ndar
y Vo
ltage
Out
put
8
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April 7, 2023
Challenges in relay design>Transients:
– High frequency– DC offset in currents– CVT transients in voltages
C1
C22
3 5
6
1
4
7
High Voltage Line
Seco
ndar
y Vo
ltage
Out
put
8
CVToutput
0 1 2 3 4
steady-stateoutput
-60
-40
-20
0
20
40
power cyclesvolt
ag
e,
V
60
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April 7, 2023
Challenges in relay design
-0.5 0 0.5 1 1.5-100
-80
-60
-40
-20
0
20
40
60
80
100
Volta
ge [V
]
-0.5 0 0.5 1 1.5-3
-2
-1
0
1
2
3
4
5
Curr
ent [
A]
vA vB vC
iA
iB, iC
-0.5 0 0.5 1 1.5-100
-50
0
50
100
Reacta
nce c
om
para
tor
[V]
power cycles
SPOL
SOP
Sorry… Future (unknown)
>In-phase = internal fault>Out-of-phase = external fault
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April 7, 2023
Transient Overreach
• Fault current generally contains dc offset in addition to ac power frequency component• Ratio of dc to ac component of current depends on instant in the cycle at which fault occurred• Rate of decay of dc offset depends on system X/R
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April 7, 2023
Zone 1 and CVT Transients
Capacitive Voltage Transformers (CVTs) create certain problems for fast distance relays applied to systems with high Source Impedance Ratios (SIRs):>CVT-induced transient voltage components
may assume large magnitudes (up to 30-40%) and last for a comparatively long time (up to about 2 cycles)
>60Hz voltage for faults at the relay reach point may be as low as 3% for a SIR of 30
>the signal may be buried under noise
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April 7, 2023
CVT transients can cause distance relays to overreach. Generally, transient overreach may be caused by: >overestimation of the current (the magnitude of
the current as measured is larger than its actual value, and consequently, the fault appears closer than it is actually located),
>underestimation of the voltage (the magnitude of the voltage as measured is lower than its actual value)
>combination of the above
Zone 1 and CVT Transients
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Distance Element Fundamentals
XL
XC
R
Z1 End Zone
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April 7, 2023
-10 -5 0 5 10-5
0
5
10
15R
eact
ance
[ohm
]
Resistance [ohm]
18
22
26
30
3442
44 Actual FaultLocation
LineImpedance
Trajectory(msec)
dynamic mhozone extendedfor high SIRs
-10 -5 0 5 10-5
0
5
10
15R
eact
ance
[ohm
]
Resistance [ohm]
18
22
26
30
3442
44 Actual FaultLocation
LineImpedance
Trajectory(msec)
dynamic mhozone extendedfor high SIRs
Impedance locus may pass below the origin of the Z-plane - this would call for a time delayto obtain stability
Impedance locus may pass below the origin of the Z-plane - this would call for a time delayto obtain stability
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April 7, 2023
>apply delay (fixed or adaptable)>reduce the reach>adaptive techniques and better filtering
algorithms
CVT Transient Overreach Solutions
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April 7, 2023
>Optimize signal filtering:– currents - max 3% error due to the dc
component– voltages - max 0.6% error due to CVT transients
>Adaptive double-reach approach– filtering alone ensures maximum transient
overreach at the level of 1% (for SIRs up to 5) and 20% (for SIRs up to 30)
– to reduce the transient overreach even further an adaptive double-reach zone 1 has been implemented
CVT Transients – Adaptive Solution
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April 7, 2023
The outer zone 1:The outer zone 1:
> is fixed at the actual reach> applies certain security delay to cope with CVT
transients
DelayedTrip
InstantaneousTrip
R
XThe inner zone 1:The inner zone 1:
> has its reach dynamically controlled by the voltage magnitude
> is instantaneous
CVT Transients – Adaptive Solution
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April 7, 2023
Desirable Distance Relay AttributesFilters:
>Prefiltering of currents to remove dc decaying transients– Limit maximum transient overshoot (below 2%)
>Prefiltering of voltages to remove low frequency transients caused by CVTs– Limit transient overreach to less than 5% for an
SIR of 30>Accurate and fast frequency tracking algorithm>Adaptive reach control for faults at reach points
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April 7, 2023
Distance Relay Operating Times
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April 7, 2023
Distance Relay Operating Times
20ms
15ms
25ms 30ms
35ms
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Distance Relay Operating Times
SLG faults LL faults
3P faults
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April 7, 2023
0 5 10 15 20 25 300
10
20
30
40
50
60
70
80
90
100
Max
imum
Rac
h [%
]
SIR
0 5 10 15 20 25 300
10
20
30
40
50
60
70
80
90
100
Max
imum
Rac
h [%
]
SIR
Actual maximum reach curvesActual maximum reach curves
Relay 1
Relay 3
Relay 2
Relay 4
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Maximum Torque Angle
• Angle at which mho element has maximum reach• Characteristics with smaller MTA will accommodate larger amount of arc resistance
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April 7, 2023
Traditional
Directional angle lowered and “slammed”
Directional angle “slammed”
Both MHO and directional angles “slammed” (lens)
Mho Characteristics
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Typical load characteristic impedance
+R
Operate
area
No Operate area
+XL
+ = LOOKING INTO LINE normally considered forward
Load Trajectory
Rea
ch
Load Swings
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Load swingLoad swing
“Lenticular” Characterist
ic
Load Swings
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Load Encroachment Characteristic
The load encroachment element responds to The load encroachment element responds to positive sequence voltage and current and can positive sequence voltage and current and can
be used to block phase distance and phase be used to block phase distance and phase overcurrent elements.overcurrent elements.
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April 7, 2023
Blinders
• Blinders limit the operation of distance relays (quad or mho) to a narrow region that parallels and encompasses the protected line• Applied to long transmission lines, where mho settings are large enough to pick up on maximum load or minor system swings
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Quadrilateral Characteristics
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Ground Resistance (Conductor falls on ground)
XL
R Resultant impedance outside of the mho operating region
Quadrilateral Characteristics
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April 7, 2023
Mho Quadrilateral
Better coverage for ground faults
due to resistance
added to return path
Lenticular
Used for phase elements with long heavily loaded lines
heavily loaded
Standard for phase elements
JX
R
Distance Characteristics - Summary
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Distance Element PolarizationThe following polarization quantities are commonly used in distance relays for determining directionality:• Self-polarized• Memory voltage• Positive sequence voltage• Quadrature voltage• Leading phase voltage
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Memory Polarization
>Positive-sequence memorized voltage is used for polarizing:– Mho comparator (dynamic, expanding Mho)– Negative-sequence directional comparator
(Ground Distance Mho and Quad)– Zero-sequence directional comparator (Ground
Distance MHO and QUAD)– Directional comparator (Phase Distance MHO and
QUAD)>Memory duration is a common distance settings (all
zones, phase and ground, MHO and QUAD)
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Memory PolarizationjX
R
Dynamic MHO characteristic for a reverse fault
Dynamic MHO characteristic for a forward fault
Impedance During Close-up FaultsImpedance During Close-up Faults
Static MHO characteristic (memory not established or expired)
ZL
ZS
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Memory Polarization
Memory Polarization…Improved Resistive Coverage
Dynamic MHO characteristic for a forward fault
Static MHO characteristic (memory not established or expired)
jX
R
ZL
ZS
RL
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Choice of Polarization
•In order to provide flexibility modern distance relays offer a choice with respect to polarization of ground overcurrent direction functions:– Voltage polarization– Current polarization– Dual polarization
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Ground Directional Elements>Pilot-aided schemes using ground mho distance relays
have inherently limited fault resistance coverage>Ground directional over current protection using either
negative or zero sequence can be a useful supplement to give more coverage for high resistance faults
>Directional discrimination based on the ground quantities is fast:
– Accurate angular relations between the zero and negative sequence quantities establish very quickly because:
During faults zero and negative-sequence currents and voltages build up from very low values (practically from zero)
The pre-fault values do not bias the developing fault components in any direction
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Distance Schemes
Pilot Aided Schemes
No Communication between Distance
Relays
Communication between Distance
relays
Non-Pilot Aided Schemes
(Step Distance)
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Step Distance Schemes• Zone 1:
– Trips with no intentional time delay– Underreaches to avoid unnecessary operation for faults beyond
remote terminal– Typical reach setting range 80-90% of ZL
• Zone 2:– Set to protect remainder of line– Overreaches into adjacent line/equipment– Minimum reach setting 120% of ZL
– Typically time delayed by 15-30 cycles• Zone 3:
– Remote backup for relay/station failures at remote terminal– Reaches beyond Z2, load encroachment a consideration
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BU
S BU
S
Z1Z1
Z1Z1
LocalLocal
RemotRemotee
Step Distance Schemes
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BU
S BU
S
Z1Z1
Z1Z1
End End ZoneZone
End End ZoneZone
LocalLocal
RemotRemotee
Step Distance Schemes
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April 7, 2023
BU
S
Z1Z1
Z1Z1BreakeBreaker r TrippeTrippedd
BU
S
BreakeBreaker r ClosedClosed
LocalLocal
RemotRemotee
Step Distance Schemes
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April 7, 2023
BU
S
Z1Z1
Z1Z1B
US
Z2 (time Z2 (time delayed)delayed)
RemotRemotee
LocalLocal
Step Distance Schemes
Z2 (time Z2 (time delayed)delayed)
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BU
S
Z1Z1
BU
S
Z2 (time Z2 (time delayed)delayed)
Step Distance SchemesZ3 (remote Z3 (remote
backup)backup) …
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Step Distance Protection
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Local Relay – Z2Local Relay – Z2
Zone 2 PKPZone 2 PKP
Local RelayLocal Relay Remote RelayRemote Relay
Remote Relay – Z4Remote Relay – Z4
Zone 4 PKPZone 4 PKP
Over LapOver Lap
Distance Relay Coordination
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BU
SBU
S
Communication Communication ChannelChannel
Local Local RelayRelay
Remote Remote RelayRelay
Need For Pilot Aided Schemes
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April 7, 2023
Pilot Communications Channels• Distance-based pilot schemes traditionally utilize simple on/off communications between relays, but can also utilize peer-to-peer communications and GOOSE messaging over digital channels• Typical communications media include:
– Pilot-wire (50Hz, 60Hz, AT)– Power line carrier– Microwave– Radio– Optic fiber (directly connected or multiplexed
channels)
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Distance-based Pilot Protection
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Pilot-Aided Distance-Based Schemes
DUTT – Direct Under-reaching Transfer Trip
PUTT – Permissive Under-reaching Transfer Trip
POTT – Permissive Over-reaching Transfer Trip
Hybrid POTT – Hybrid Permissive Over-reaching Transfer Trip
DCB – Directional Comparison Blocking Scheme
DCUB – Directional Comparison Unblocking Scheme
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April 7, 2023
Direct Underreaching Transfer Trip (DUTT)• Requires only underreaching (RU) functions which overlap in reach (Zone 1).•Applied with FSK channel
– GUARD frequency transmitted during normal conditions
– TRIP frequency when one RU function operates• Scheme does not provide tripping for faults beyond RU reach if remote breaker is open or channel is inoperative.• Dual pilot channels improve security
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Bus
Line
Bus
Zone 1
Zone 1
DUTT Scheme
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Permissive Underreaching Transfer Trip (PUTT)
• Requires both under (RU) and overreaching (RO) functions • Identical to DUTT, with pilot tripping signal supervised by RO (Zone 2)
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Bus
Line
Bus
Zone 1
Zone 2
Zone 2
Zone 1
To protect end ofline
& Local Trip Zone 2
Rx PKP
OR Zone 1
PUTT Scheme
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Permissive Overreaching Transfer Trip (POTT)• Requires overreaching (RO) functions (Zone 2).• Applied with FSK channel:
– GUARD frequency sent in stand-by– TRIP frequency when one RO function
operates• No trip for external faults if pilot channel is inoperative• Time-delayed tripping can be provided
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Bus
Line
Bus
Zone 1
Zone 2
TripLine
Breakers
OR
t
Rx
Tx
AND
(Z1)
(Z1)
o
Zone 1
Zone 2
Zone 2
Zone 1
POTT Scheme
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POTT Scheme
POTT – Permissive Over-reaching POTT – Permissive Over-reaching Transfer TripTransfer Trip
BU
S BU
S
End End ZoneZone
Communication Communication ChannelChannel
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Local Local RelayRelay
Remote Remote RelayRelay
Remote Remote Relay Relay FWD IFWD IGNDGND
Ground Dir OC FwdGround Dir OC Fwd
OROR
Local Relay – Z2Local Relay – Z2
ZONE 2 PKPZONE 2 PKP
Local Local Relay FWD Relay FWD I IGNDGND
Ground Dir OC Ground Dir OC FwdFwd
OROR
TRIPTRIP
Remote Relay – Z2Remote Relay – Z2
POTT TX
ZONE 2 ZONE 2 PKPPKP
POTT RX
CommunicatCommunication Channelion Channel
POTT Scheme
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POTT TX 4
POTT TX 3
POTT TX 2
POTT TX 1 A to GA to G
B to GB to G
C to GC to G
Multi PhaseMulti Phase
Local RelayLocal Relay Remote RelayRemote Relay
POTT RX 4
POTT RX 3
POTT RX 2
POTT RX 1
Com
munications
Channel(s)
POTT Scheme
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Local RelayLocal Relay Remote RelayRemote Relay
POTT TX ZONE 2 ORZONE 2 OR
GND DIR OC FWDGND DIR OC FWD
Communication Communication ChannelChannel
TRIPTRIP
GND DIR OC REVGND DIR OC REVGND DIR OC REVGND DIR OC REV POTT RX
Start Start TimerTimerTimer Timer ExpireExpire
GND DIR OC FWDGND DIR OC FWD
POTT SchemeCurrent reversal example
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Local RelayLocal Relay
OpenOpen
Remote RelayRemote Relay
Remote FWD Remote FWD IIGNDGND
POTT TX
Remote – Z2Remote – Z2
Communication Communication ChannelChannel
POTT RX
OPENOPEN
POTT TX
Communication Communication ChannelChannel
POTT RX
TRIPTRIP
POTT SchemeEcho example
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Hybrid POTT
• Intended for three-terminal lines and weak infeed conditions• Echo feature adds security during weak infeed conditions • Reverse-looking distance and oc elements used to identify external faults
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Bus
Line
Bus
Zone 1
Zone 2
Zone 2
Zone 1 Zone 4
LocalRemoteWeak
system
Hybrid POTT
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Directional Comparison Blocking (DCB)• Requires overreaching (RO) tripping and blocking (B) functions• ON/OFF pilot channel typically used (i.e., PLC)
– Transmitter is keyed to ON state when blocking function(s) operate
– Receipt of signal from remote end blocks tripping relays
• Tripping function set with Zone 2 reach or greater• Blocking functions include Zone 3 reverse and low-set ground overcurrent elements
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Bus
Line
Bus
Zone 1
Zone 2
Zone 2
Zone 1
LocalRemote
DCB Scheme
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BU
S
BU
S
End ZoneEnd Zone
Communication ChannelCommunication Channel
Directional Comparison Blocking (DCB)
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Directional Comparison Blocking (DCB)Internal Faults
Local RelayLocal Relay Remote RelayRemote Relay
Local Relay – Z2Local Relay – Z2
Zone 2 PKPZone 2 PKP
TRIP Timer TRIP Timer StartStart
FWD IFWD IGNDGND
GND DIR OC FwdGND DIR OC Fwd
ORORDir Block RXNONO
TRIPTRIP
ExpiredExpired
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Local RelayLocal Relay Remote RelayRemote Relay
Remote Relay – Z4Remote Relay – Z4
Zone 4 PKPZone 4 PKP
REV IREV IGNDGND
GND DIR OC RevGND DIR OC Rev
OROR
DIR BLOCK TX
Local Relay – Z2Local Relay – Z2
Zone 2 PKPZone 2 PKP
Dir Block RX
Communication Communication ChannelChannel
FWD IFWD IGNDGND
GND DIR OC FwdGND DIR OC Fwd
OROR
TRIP Timer TRIP Timer StartStart No TRIPNo TRIP
Directional Comparison Blocking (DCB)External Faults
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Directional Comparison Unblocking (DCUB)• Applied to Permissive Overreaching (POR) schemes to overcome the possibility of carrier signal attenuation or loss as a result of the fault• Unblocking provided in the receiver when signal is lost:
– If signal is lost due to fault, at least one permissive RO functions will be picked up
– Unblocking logic produces short-duration TRIP signal (150-300 ms). If RO function not picked up, channel lockout occurs until GUARD signal returns
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Bus
Line
Bus
TripLine
Breakers
Tx1(Un-Block)
Forward
Forward
Tx2(Block)
Forward
Rx2
Rx1
to
AND to
AND
AND
AND
Lockout
(Block)
(Un-Block)
DCUB Scheme
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BU
S
BU
S
End ZoneEnd Zone
Communication ChannelCommunication Channel
Directional Comparison Unblocking (DCUB)
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Directional Comparison Unblocking (DCUB)Normal conditions
Local RelayLocal Relay Remote RelayRemote Relay
GUARD1 TXGUARD1 RX
Communication Communication ChannelChannel
GUARD2 TX GUARD2 RXNO Loss of GuardNO Loss of Guard
FSK CarrierFSK Carrier FSK CarrierFSK Carrier
NO PermissionNO Permission
NO Loss of GuardNO Loss of Guard
NO PermissionNO Permission
Load CurrentLoad Current
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Directional Comparison Unblocking (DCUB)Normal conditions, channel failure
Local RelayLocal Relay Remote RelayRemote Relay
GUARD1 TXGUARD1 RX
Communication Communication ChannelChannel
GUARD2 TX GUARD2 RX
FSK CarrierFSK Carrier FSK CarrierFSK Carrier
Loss of GuardLoss of Guard
Block Timer StartedBlock Timer Started
Loss of GuardLoss of Guard
Block Timer StartedBlock Timer Started
Load CurrentLoad Current
NO RX
NO RX
Block DCUB Block DCUB until Guard OKuntil Guard OK
ExpiredExpired
Block DCUB Block DCUB until Guard OKuntil Guard OK
ExpiredExpired
Loss of ChannelLoss of Channel
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Directional Comparison Unblocking (DCUB)Internal fault, healthy channel
Local RelayLocal Relay Remote RelayRemote RelayGUARD1 TXGUARD1 RX
Communication Communication ChannelChannel
GUARD2 TX GUARD2 RX
FSK CarrierFSK Carrier FSK CarrierFSK Carrier
Loss of GuardLoss of Guard
PermissionPermission
TRIP1 TX
Local Relay – Z2Local Relay – Z2
Zone 2 PKPZone 2 PKP
TRIP1 RX
TRIP2 TX
TRIPTRIP
Remote Relay – Z2Remote Relay – Z2
ZONE 2 PKPZONE 2 PKP
TRIP Z1TRIP Z1
TRIP2 RX
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Directional Comparison Unblocking (DCUB)Internal fault, channel failure
Local RelayLocal Relay Remote RelayRemote RelayGUARD1 TXGUARD1 RX
Communication Communication ChannelChannel
GUARD2 TX GUARD2 RX
FSK CarrierFSK Carrier FSK CarrierFSK Carrier
TRIP1 TX
Local Relay – Z2Local Relay – Z2
Zone 2 PKPZone 2 PKP
NO RX
TRIP2 TX
TRIPTRIP
Remote Relay – Z2Remote Relay – Z2
ZONE 2 PKPZONE 2 PKP
TRIP Z1TRIP Z1
NO RX
Loss of GuardLoss of Guard
Loss of ChannelLoss of Channel
Loss of GuardLoss of GuardBlock Timer StartedBlock Timer StartedDuration Timer StartedDuration Timer StartedExpiredExpired
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Redundancy Considerations
• Redundant protection systems increase dependability of the system:Multiple sets of protection using same protection
principle and multiple pilot channels overcome individual element failure, or
Multiple sets of protection using different protection principles and multiple channels protects against failure of one of the protection methods.
• Security can be improved using “voting” schemes (i.e., 2-out-of-3), potentially at expense of dependability.• Redundancy of instrument transformers, battery systems, trip coil circuits, etc. also need to be considered.
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BU
S
BU
S
End ZoneEnd Zone
Communication Channel 1Communication Channel 1
Communication Channel 2Communication Channel 2
Loss of Channel 2Loss of Channel 2
AND Channels:AND Channels:
POTT Less ReliablePOTT Less Reliable
DCB Less SecureDCB Less Secure
OR Channels:OR Channels:
POTT More ReliablePOTT More Reliable
DCB More SecureDCB More Secure
More Channel SecurityMore Channel Security More Channel DependabilityMore Channel Dependability
Redundant Communications
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Redundant Pilot Schemes
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• Integrated functions:weak infeedecholine pick-up (SOTF)
• Basic protection elements used to key the communication:distance elementsfast and sensitive ground (zero and
negative sequence) directional IOCs with current, voltage, and/or dual polarization
Pilot Relay Desirable Attributes
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Pre-programmed distance-based pilot schemes:Direct Under-reaching Transfer Trip (DUTT)Permissive Under-reaching Transfer Trip (PUTT)Permissive Overreaching Transfer Trip (POTT)Hybrid Permissive Overreaching Transfer Trip (HYB
POTT)Blocking scheme (DCB)Unblocking scheme (DCUB)
Pilot Relay Desirable Attributes
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Security for dual-breaker terminals• Breaker-and-a-half and ring bus terminals are common designs for transmission lines.• Standard practice has been to:
– sum currents from each circuit breaker externally by paralleling the CTs
– use external sum as the line current for protective relays
• For some close-in external fault events, poor CT performance may lead to improper operation of line relays.
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Security for dual-breaker terminals
Accurate CTs preserve the reverse current direction under weak remote infeed
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Security for dual-breaker terminals
Saturation of CT1 may invert the line current as measured from externally summated CTs
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Security for dual-breaker terminals • Direct measurement of
currents from both circuit breakers allows the use of supervisory logic to prevent distance and directional overcurrent elements from operating incorrectly due to CT errors during reverse faults.• Additional benefits of direct measurement of currents:
independent BF protection for each circuit breaker independent autoreclosing for each breaker
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Security for dual-breaker terminalsSupervisory logic should:
– not affect speed or sensitivity of protection elements– correctly allow tripping during evolving external-to-
internal fault conditions– determine direction of current flow through each
breaker independently:• Both currents in FWD direction internal fault• One current FWD, one current REV external
fault– allow tripping during all forward/internal faults– block tripping during all reverse/external faults– initially block tripping during evolving external-to-
internal faults until second fault appears in forward direction. Block is then lifted to permit tripping.
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Single-pole Tripping
• Distance relay must correctly identify a SLG fault and trip only the circuit breaker pole for the faulted phase.• Autoreclosing and breaker failure functions must be initiated correctly on the fault event• Security must be maintained on the healthy phases during the open pole condition and any reclosing attempt.
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Out-of-Step Condition
• For certain operating conditions, a severe system disturbance can cause system instability and result in loss of synchronism between different generating units on an interconnected system.
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Out-of-Step RelayingOut-of-step blocking relays
– Operate in conjunction with mho tripping relays to prevent a terminal from tripping during severe system swings & out-of-step conditions.
– Prevent system from separating in an indiscriminate manner.
Out-of-step tripping relays– Operate independently of other devices to detect
out-of-step condition during the first pole slip.– Initiate tripping of breakers that separate system in
order to balance load with available generation on any isolated part of the system.
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Out-of-Step TrippingThe locus must stay for some time between the outer and middle characteristics
Must move and stay between the middle and inner characteristics
When the inner characteristic is entered the element is ready to trip
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Power Swing BlockingApplications:>Establish a blocking signal for stable power swings
(Power Swing Blocking)>Establish a tripping signal for unstable power swings
(Out-of-Step Tripping) Responds to:>Positive-sequence voltage and current
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Series-compensated lines
EXs SC XL Infinte
Bus
Benefits of series capacitors:• Reduction of overall XL of long lines • Improvement of stability margins • Ability to adjust line load levels• Loss reduction• Reduction of voltage drop during severe disturbances• Normally economical for line lengths > 200 miles
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Series-compensated lines
EXs SC XL Infinte
Bus
SCs create unfavorable conditions for protective relays and fault locators:• Overreaching of distance elements• Failure of distance element to pick up on low-current faults• Phase selection problems in single-pole tripping applications• Large fault location errors
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Series-compensated linesSeries Capacitor with MOV
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Series-compensated lines
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Series-compensated linesDynamic Reach Control
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Series-compensated linesDynamic Reach Control for External Faults
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Series-compensated linesDynamic Reach Control for External Faults
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Series-compensated linesDynamic Reach Control for Internal Faults
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Distance Protection Looking Through a Transformer• Phase distance elements can be set to see beyond any 3-phase power transformer• CTs & VTs may be located independently on different sides of the transformer• Given distance zone is defined by VT location (not CTs)• Reach setting is in sec, and must take into account location & ratios of VTs, CTs and voltage ratio of the involved power transformer
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Transformer Group Compensation
Depending on location of VTs and CTs, distance relays Depending on location of VTs and CTs, distance relays need to compensate for the phase shift and magnitude need to compensate for the phase shift and magnitude
change caused by the power transformerchange caused by the power transformer
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Setting Rules
• Transformer positive sequence impedance must be included in reach setting only if transformer lies between VTs and intended reach point• Currents require compensation only if transformer located between CTs and intended reach point• Voltages require compensation only if transformer located between VTs and intended reach point• Compensation set based on transformer connection & vector group as seen from CTs/VTs toward reach point
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>Multiple reversible distance zones> Individual per-zone, per-element characteristic:
– Dynamic voltage memory polarization– Various characteristics, including mho, quad,
lenticular> Individual per-zone, per-element current supervision (FD)>Multi-input phase comparator:
– additional ground directional supervision– dynamic reactance supervision
>Transient overreach filtering/control>Phase shift & magnitude compensation for distance
applications with power transformers
Distance Relay Desirable Attributes
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>For improved flexibility, it is desirable to have the following parameters settable on a per zone basis:– Zero-sequence compensation– Mutual zero-sequence compensation– Maximum torque angle– Blinders– Directional angle– Comparator limit angles (for lenticular
characteristic)– Overcurrent supervision
Distance Relay Desirable Attributes
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>Additional functions– Overcurrent elements (phase, neutral, ground,
directional, negative sequence, etc.)– Breaker failure– Automatic reclosing (single & three-pole)– Sync check– Under/over voltage elements
>Special functions– Power swing detection– Load encroachment– Pilot schemes
Distance Relay Desirable Attributes
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