Transmission Line Protection
Transmission Line Theory
Transmission Line Fundamentals:
Characteristics of Lines:
• A “line” is a circuit used to transmit power
• Lines are supported by insulators, which are connected to
structures
• structures, which can be made of wood, metal or
concrete keep the lines above the ground.
• Electrical properties of the line are affected by material of
the structure, the physical arrangement of the components,
and the geography plus location of the conductors and
towers.
Transmission Line Theory
Transmission Line Fundamentals –
Line Classification:
Terminal:
• physical location of a line boundary
• switch, breaker, bus or transformer defines the end of a
transmission line
Transmission Line Theory
Transmission Line Fundamentals –
Line Classification:
INTER-CONNECTOR
Transmission Line Classifications to describe the
configuration, function and location in the system:
• Parallel Lines
• Inter-connector Lines
• Generator Export Lines
• Process Plant Load Lines
Transmission Line Theory
Transmission Line Fundamentals –
Line Classification:
SINGLE-FEED
(Radial Line)
DOUBLE-ENDED
MULTI-ENDED
Transmission Line Theory
Transmission Line Fundamentals –
Line Voltage Classification:
Distribution:
~ 2kV - 50 kV
Sub-transmission: ~ 30 - 150 kV
Transmission:
~ >100kV
Most Line Distance protection applications: > 50 kV
Transmission Line Theory
Transmission Line Fundamentals –
Line Classification by Length:
< 80 km (SIR > 4)
SHORT LINES
80 km – 240 km ( 0.5 > SIR < 4 )
MEDIUM LINES
> 240 km ( SIR < 0.5 )
LONG LINES
> 240 km
Transmission Line Theory
Transmission Line Fundamentals –
Shunt Capacitance:
500kV
550kV
90 deg
Voltage
0 deg
Typical 3-Phase MVAR values:
• 500kV lines
1.3 to 1.5 MVAR per km
• 230kV lines
0.15 to 0.4 MVAR per km
Capacitance adds
leading MVARs on the
line to bring current
and voltage back to
unity power factor.
90 deg
Current
0 deg
Voltage
Current
Negative MVARs
Leading MVARs
Transmission Line Theory
Transmission Line Fundamentals –
Fault Protection:
Transmission Line Theory
Transmission Line Fundamentals –
Fault Protection:
Source to Line Impedance Ratio (SIR):
1 Ohm
5 Ohms
SIR = 1 Ohm / 5 Ohms
= 0.2
Strong
Source G
15 Ohms
Weak
Source G
SIR = Source Impedance
Line Impedance
5 Ohms
SIR = 15 Ohms / 5 Ohms
=3
Transmission Line Theory
Transmission Line Fundamentals –
Fault Protection:
Computer-aided Software:
Equivalent Line Model:
Sequence Components:
e.g. π-model for Medium Lines
R-X
Diagram
VS
Y/2
Y/2
Z0
X
Z1
VR
R
Transmission Line Categories:
• Short Lines
• Medium Lines
• Long Lines
Positive and Zero
Sequence Impedances
Transmission Line Protection Elements
Distance Protection:
Impedance of Power System:
V = 230 kV
I = 1000 A
3 Ohms
20 Ohms
207 Ohms
Source
Impedance
Line
Impedance
Load
Impedance
Z Total = 3 Ohms + 20 Ohms + 207 Ohms = 230 Ohms
Transmission Line Protection Elements
Distance Protection:
Impedance of Power System with Fault:
V = 230 kV
I = 10000 A
3 Ohms
20 Ohms
207 Ohms
Source
Impedance
Line
Impedance
Load
Impedance
Z Total = 3 Ohms + 20 Ohms = 23 Ohms
Transmission Line Protection Elements
Distance Protection:
Fault Distance calculated based on the Ratio of
Impedance of Fault to Transmission Line
V = 177 kV
I = 17700 A
Z Fault
Z Fault
Z TL
V
=
I
=
X 100% =
177 kV
17.7 kA
10 Ohms
20 Ohms
=
10 Ohms
X 100% = 50%
Transmission Line Protection Elements
Distance Protection:
Impedance of Transmission Line on R-X Diagram:
20 Ohms
Inductive
20 Ohms
XL
Impedance of line is mostly
inductive during fault.
10 Ohms
R
XC
Capacitive
Resistive
Transmission Line Protection Elements
Distance Protection:
Impedance of Transmission Line on R-X Diagram:
3 Ohms
20 Ohms
XL
207 Ohms
Impedance of line is mostly
resistive with load.
R
XC
Transmission Line Protection Elements
Distance Protection:
Impedance of Transmission Line on R-X Diagram:
3 Ohms
20 Ohms
XL
207 Ohms
Fault on line causes the loss of load
and becomes highly inductive.
R
XC
Transmission Line Protection Elements
Distance Zones Characteristics:
XL
Admittance
/ Mho
Reactive
R
Quadrilateral
Impedance
Resistance
XC
Transmission Line Protection Elements
Zones of Protection – Impedance Characteristics
:
XL
Impedance
Characteristics:
Reach = protection coverage
• non-directional
R
• generator
backup protection
Fault is in reverse to the
protected transmission line
XC
Transmission Line Protection Elements
Zones of Protection – Reactance Characteristics :
Reactance
Characteristics:
XL
• fault is highly
resistive
R
• supervise
another
distance zone of
protection
XC
Transmission Line Protection Elements
Zones of Protection – Resistance Characteristics:
Resistance
Characteristics:
XL
• fault is highly
inductive
R
• supervise
another
distance zone of
protection
XC
Transmission Line Protection Elements
Zones of Protection – Admittance / Mho Circle:
XL
Reach = protection coverage
Admittance / Mho
Characteristics:
• directional
• most common
transmission line
distance protection
R
Fault out of
the zone.
XC
Transmission Line Protection Elements
Zones of Protection – Admittance / Mho Circle:
Example:
V = 230 kV
I = 1000 A
XL
Z TL = V
I
= 230 kV
1 kA
= 230 Ohms
18 Ohms
R
XC
Transmission Line Protection Elements
Zones of Protection – Admittance / Mho Circle :
Example:
V = 230 kV
I = 20000 A
XL
Additional
Resistive Load
Z TL + Load = V
I
= 230 kV
20 kA
= 11.5 Ohms
18 Ohms
R
XC
Transmission Line Protection Elements
Zones of Protection – Lenticular Zone:
XL
Reach
Lenticular Zone:
• altered mho zone
• highly loaded
systems
Encroached Impedance in
Mho Zone close to origin due
to Highly Resistive Load
R
XC
Transmission Line Protection Elements
Zones of Protection – Tomato Characteristics:
Tomato
Characteristics:
XL
Reach
• altered mho zone
• highly resistive
faults
R
Transmission Line Protection Elements
Zones of Protection – Quadrilateral Characteristics:
XL
Reaches: inductive & resistive
Quadrilateral
Characteristics:
• custom zone
R
• set reactance and
resistive boundaries
Quadrilateral Characteristics
XC
Transmission Line Protection Elements
Zones of Protection – Enhanced Mho Element:
Mho element with
memory voltage
polarization:
XL
• Static zone
• Dynamic zone for
forward fault
•Dynamic zone for
reverse fault
R
Zone expanded dynamically for Forward Fault
VM
ES
ES’
ZL
ZS
IM
ZS’
ZREACH
VM=IM*ZM
jX
VOP=IM*ZREACH – VM= IM*(ZREACH – ZM)
ZREACH - ZM
VPOL= ES = VM+ IM*ZS = IM*(ZM + ZS)
Where VM, IM and ZM are the
measured voltage, current and
apparent impedance at the relay
location
ZM
Static Zone
R
ZS
Dynamic Zone
ZS+ZM
Zone shifted up dynamically for Reverse Fault
VM
ES
ZL
ZS
IM
ZS’
ZREACH
VM = IM*ZM
ZM – (ZL+ZS’)
jX
VOP = IM*ZREACH – VM = IM*(ZREACH – ZM)
ZS’
VPOL = ES’ = VM – IM*(ZL + ZS’)
ZL
Dynamic Zone
= IM*(ZM – (ZL + ZS’))
Where VM, IM and ZM are the
measured voltage, current and
apparent impedance at the relay
location
ES’
ZREACH - ZM
ZM
Static Zone
R
Transmission Line Protection Elements
Modern Distance Protection:
• Phase Distance element - detects phase-to-phase faults
• Ground Distance element - detects phase-to-ground faults
Reverse Direction
XL
Forward Direction
R
Transmission Line Protection Elements
Modern Distance Protection:
Max Torque Angle
XL
Blinders
Directional Angle
Comparator Limit Angle
R
Transmission Line Protection Elements
Life is not really that simple!
Relays are simple comparators – An actual distance relay
compares the angle of two voltages
Transmission Line Protection Elements
Distance Zones Protection:
Z Measured = V
I
Z Measured
ZActual = 20 Ohms
Z Fault = 20.5 Ohms
Z Measured = 19.5 Ohms
If Z Measured < Z Actual ,
then Line is Shorter indicating a Fault
• due to inaccuracies of CTs, VTs
and transient effects.
Transmission Line Protection Elements
Distance Zones Protection:
Under-Reaching
• covers part of
transmission
line
Over-Reaching
• covers
transmission
line and part of
next adjacent
line
Transmission Line Protection Elements
Distance Zones Protection:
Out Of the Line
Into the Line
• protects in the
reverse direction
• protects in the
forward direction
Transmission Line Protection Elements
Distance Schemes:
Pilot Aided Schemes
• Communication
Non-Pilot Aided Schemes
• No Communication
• Delays and Coordination
Transmission Line Protection Elements
Non-Pilot Aided Distance Schemes:
• Stepped Distance
• Zone 1 Extension
Transmission Line Protection Elements
Non-Pilot Aided - Stepped Distance Schemes:
Example:
Transmission Line Protection Elements
Non-Pilot Aided - Stepped Distance Schemes:
Zone 1 - Under-reaching:
Zone 1 = 80-90%
End Zone
Transmission Line Protection Elements
Non-Pilot Aided - Stepped Distance Schemes:
Zone 2 - Over-reaching:
Zone 2 = 120%
Time Delay 0.25 - 0.4 Sec
Zone 1 = 80-90%
Transmission Line Protection Elements
Non-Pilot Aided - Stepped Distance Schemes:
Example:
Zone 1 - Pickup – OFF
- Operate - OFF
Zone 2 - Pickup – OFF
- Operate - OFF
Zone 2
Zone 1
Transmission Line Protection Elements
Non-Pilot Aided - Stepped Distance Schemes:
Example 1: Fault on Transmission Line 1 within Zone 1
Zone 1 - Pickup – ON
- Operate - ON
Zone 2 - Pickup – ON
- Operate - OFF
Zone 2
Zone 1
• When Fault Cleared:
Zone 1 - Pickup – OFF
- Operate - OFF
Zone 2 - Pickup – OFF
- Operate - OFF
Transmission Line Protection Elements
Non-Pilot Aided - Stepped Distance Schemes:
Example 2: Fault on Transmission Line 1 outside Zone 1
Zone 1 - Pickup – OFF
- Operate - OFF
Zone 2 - Pickup – ON
- Operate - ON
Zone 2
Zone 1
• When Fault Cleared:
Zone 1 - Pickup – OFF
- Operate - OFF
Zone 2 - Pickup – OFF
- Operate - OFF
Transmission Line Protection Elements
Non-Pilot Aided - Stepped Distance Schemes:
Example 3: Fault on Transmission Line 2 within
Zone 2 of Relay 1 and Zone 1 of Relay 2
Distance Relay 1
Zone 2 - Pickup – ON
- Operate - OFF
Distance Relay 2
Zone 1 - Pickup – ON
- Operate - ON
Zone 2
Zone 1
Relay 1
Zone 1
Relay 2
Transmission Line Protection Elements
Non-Pilot Aided - Stepped Distance Schemes:
Zone 3:
Zone 3 = 220%
Time Delay of 1.0 Sec
Zone 2 = 120%
Zone 1 = 80-90%
Zone 3
Adjacent Zone 2
Transmission Line Protection Elements
Non-Pilot Aided - Stepped Distance Schemes:
Zone 4 & 5:
Zone 3 = 220%
Zone 2 = 120%
Zone 1 = 80-90%
Zone 4 = 20-50%
Time Delay 0.75 – 1.0 Sec
Time Delay 1.0 Sec
Transmission Line Protection Elements
Non-Pilot Aided - Stepped Distance Schemes:
Z3
Z2
Z1
Z4
Z1
Z2
Z3
Z4
Transmission Line Protection Elements
Non-Pilot Aided - Zone 1 Extension Schemes:
• based on Stepped Distance Scheme
• based on principle that transmission line faults
are typically transient
Transmission Line Protection Elements
Non-Pilot Aided - Zone 1 Extension Schemes:
Example: transient fault cleared by tripping circuit breaker
Transmission Line Protection Elements
Non-Pilot Aided - Zone 1 Extension Schemes:
Z3
Z2
120% with Time Delay 0.25 - 0.4 Sec
Z1 X
120% with No Time Delay
Z1
80-90% with No Time Delay after 1st Reclosure
Z4
Transmission Line Protection Elements
Non-Pilot Aided - Zone 1 Extension Schemes:
Examples:
Z3
Z2
120% with Time Delay 0.25 - 0.4 Sec
Z1 X
120% with No Time Delay
Z1
80-90% with No Time Delay after 1st Reclosure
Z4
Z1
80-90%
Transmission Line Protection Elements
Pilot Aided Schemes:
Z3
Z2
Z1
Z4
Z1
Z2
Z3
Z4
Transmission Line Protection Elements
Pilot Aided Schemes:
• Fault in Zone 1 of both relays cleared instantly by
local and remote relays
Z1
Z1
Transmission Line Protection Elements
Pilot Aided Schemes:
• Fault in end zone only cleared instantly by the Remote
relay without pilot scheme
Local
Z2
End Zone
Z1
Breaker
Closed
Breaker
Tripped
End Zone
Z1
Remote
Transmission Line Protection Elements
Pilot Aided Schemes:
• Communication Channel speeds up clearing faults on
transmission lines and in the end zones
End Zone
Communication Channel
• power line carriers, microwave
radio channels, SONET channels, etc.
Transmission Line Protection Elements
Pilot Aided Schemes:
• Communication Channels make all the difference
• Fiber Optic tends to be very high speed and have
high availability and high bandwidth. NOT AFFECTED
DURING FAULT!
• Microwave channels tend to be fairly high speed, but
can be affected by weather conditions, vegation
growth, etc. Fairly high bandwidth
• Power line carrier is low bandwidth and can be
severely affected during faults.
Transmission Line Protection Elements
Pilot Aided Schemes:
• Power Line Carrier
• On-Off Keying –
• Carrier signal is either on or off
• You don’t know if you have a good comm channel
• Frequency Shift Keying (FSK) –
• Two Frequencies called Guard and Trip
• Loss of channel can be detected
Transmission Line Protection Elements
Pilot Aided 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
Transmission Line Protection Elements
Pilot Aided Schemes – DUTT:
DUTT – Direct Under-reaching Transfer Trip
Local Relay - Z1
TRIP
TRIP
Remote Relay - Z1
TRIP
Local Relay
DUTT KEY
Remote Relay
Zone 1 - OP
Transmission Line Protection Elements
Pilot Aided Schemes – PUTT:
PUTT – Permissive Under-reaching Transfer Trip
Local Relay – Z2
Remote Relay - Z1
PUTT Key
TRIP
Local Relay
AND
Zone 2 PKP
PUTT KEY
Remote Relay
Transmission Line Protection Elements
Pilot Aided Schemes – POTT:
POTT – Permissive Over-reaching Transfer Trip
Local Relay
FWD IGND
Remote Relay
FWD IGND
Local Relay – Z2
Remote Relay – Z2
TRIP
Local
Communication
Channel
POTT RX
Relay
Remote
POTT TX
Relay
ZONE 2 PKP
ZONE 2 PKP
OR
OR
Ground Dir OC Fwd
Ground Dir OC Fwd
Transmission Line Protection Elements
Pilot Aided Schemes – POTT:
• Conditions to cause relays to trip under the POTT scheme
Diagram 1:
Local Relay – Z2
Remote Relay – Z2
Diagram 3:
Local Relay – Z2
Remote Relay FWD GND
Diagram 2:
Local Relay FWD GND
Remote Relay – Z2
Diagram 4:
Local Relay FWD GND
Remote Relay FWD GND
Transmission Line Protection Elements
Pilot Aided Schemes – POTT:
• distance elements can also determine which phases of
the transmission line are faulted
POTT RX 2
POTT RX 3
POTT RX 4
Local Relay
Communications
Channel(s)
POTT RX 1
POTT TX 1
A to G
POTT TX 2
B to G
POTT TX 3
C to G
POTT TX 4 Multi Phase
Remote Relay
Transmission Line Protection Elements
Pilot Aided Schemes – POTT:
Example: Current Reversal
Local Relay
Remote Relay
Communication
Channel
GND DIR OC REV
POTT RX
POTT TX
GND DIR OC FWD
Transmission Line Protection Elements
Pilot Aided Schemes – POTT:
Example (cont’d): Current Reversal
TRIP
TRIP
Local Relay
GND DIR OC FWD
Timer
Expire
Remote Relay
Communication
Channel
POTT RX
POTT TX
GND DIR OC REV
Transmission Line Protection Elements
Pilot Aided Schemes – POTT:
Example: Echo
Remote Relay
FWD IGND
Open
Remote Relay – Z2
OPEN
Communication
Channel
POTT RX
Local Relay Echo
POTT TX
TRIP
POTT TX
POTT RX
Communication
Channel
Echo
Remote Relay
Transmission Line Protection Elements
Pilot Aided Schemes – Hybrid POTT:
Local Relay
BLOCK
REV IGND
Remote Relay
FWD IGND
Remote Relay – Z2
Local Relay
Zone 4 PKP - ON
BLOCK
Local Relay
BLOCK
Hybrid POTT Key
Remote Relay
Transmission Line Protection Elements
Pilot Aided Schemes – Hybrid POTT:
High Source Impedance
V = LOW
I = LOW
IGND = LOW
Local Relay
Remote Relay
Transmission Line Protection Elements
Pilot Aided Schemes – Hybrid POTT:
• D60 Hybrid POTT scheme and Weak Infeed feature
High Source Impedance
Voltage = LOW
Neutral/Neg Seq Dir OC FWD = OFF
TRIP
Local Relay
Neutral/Neg Seq Dir OC REV = OFF
ZONE 2 PKP = OFF
ZONE 4 PKP = OFF
Transmission Line Protection Elements
Pilot Aided Schemes – DCB:
(Internal Faults)
DCB - Directional Comparison Blocking Scheme
Local Relay – Z2
FWD IGND
TRIP
TRIP Timer
Expired
ZONE 2 PKP
OR
Local Relay
NO
GND DIR OC Fwd
Dir Block RX
Remote Relay
Transmission Line Protection Elements
Pilot Aided Schemes – DCB:
(External Faults)
Local Relay – Z2
FWD IGND
Remote Relay – Z4
No
TRIP Timer TRIP
REV IGND
Start
DIR Block RX
Local Relay
DIR Block TX
Communication
ZONE 2 PKP
ZONE 4 PKP
Channel
OR
OR
GND DIR OC Fwd
GND DIR OC Fwd
Remote Relay
Transmission Line Protection Elements
Pilot Aided Schemes – DCUB:
(Normal Conditions)
DCUB - Directional Comparison UnBlocking
Scheme
Load Current
FSK Carrier
FSK Carrier
GUARD1 RX
GUARD1 TX
Local Relay
NO Loss of Guard
AND
NO Permission
Remote Relay
GUARD2 TX
GUARD2 RX
Communication
Channel
NO Loss of Guard
AND
NO Permission
Transmission Line Protection Elements
Pilot Aided Schemes – DCUB:
(Normal Conditions, Channel Fails)
Load Current
Loss of Channel
FSK Carrier
Local Relay
Loss of Guard
AND
Block Timer Expired
Block DCUB
until Guard OK
No RX
FSK Carrier
GUARD1 TX
Remote Relay
GUARD2 TX
No RX
Communication
Channel
Loss of Guard
AND
Block Timer Expired
Block DCUB
until Guard OK
Transmission Line Protection Elements
Pilot Aided Schemes – DCUB (POTT Mode):
(Internal Fault, Normal Channel)
Local Relay – Z2
Remote Relay – Z2
TRIP
FSK Carrier
Local Relay
Zone 2 PKP
AND
Loss of Guard
AND
Permission
TRIP1 RX
TRIP Z1
FSK Carrier
TRIP1 TX
Remote Relay
Zone 2 PKP
TRIP2 TX
TRIP2 RX
Communication
Channel
Transmission Line Protection Elements
Pilot Aided Schemes – DCUB (DCB Mode):
(Internal Fault, Channel Fail)
Local Relay – Z2
Remote Relay – Z2
TRIP
FSK Carrier
Local Relay
Loss of Channel
GUARD1
RX
No RX
Zone 2 PKP
AND
TRIP2 TX
GUARD2
TX
Loss of Guard
AND
Communication
Block Timer Started
Channel
Duration Timer Expired
TRIP Z1
FSK Carrier
GUARD1
TRIP1TXTX
No RXRX
GUARD2
Remote Relay
Zone 2 PKP
AND
Loss of Guard
Transmission Line Protection Elements
Current Differential Principle
End A
End B
IA
IF
IB
Relay A
Relay B
Communication Link
IA + IB = 0 Healthy
IA + IB ≠ 0 (= IF) Fault
Transmission Line Protection Elements
Three Ended Line Protection
IA
End A
IF
IB
IC
Relay B
End C
IA + IB + IC = 0 Healthy
IA + IB + IC ≠ 0 (= IF faulty)
Transmission Line Protection Elements
Current Differential - Advantages
No voltage transformers needed
Good for multi-ended lines
Detect very high resistance faults
Immune to power swings
Uniform trip time
No problems with series compensation
Simple to set with no coordination problems
References
ANSI/IEEE Device Numbers, C37.2
ANSI Device Numbers, Publication No. GER-3977
IEEE CT Burdens (5 Amps), C57.13
Required information to apply protection
IEEE Protective Relaying Standards
“The Art of Protective Relaying,” Publication No. GET7201
• “Protective Relaying Principles and Applications” by J.
Lewis Blackburn and Thomas J. Domin
•
•
•
•
•
•
References
ANSI / IEEE C37.2
Device Numbers:
References
ANSI / IEEE C37.2
Device Numbers:
References
ANSI Device Numbers:
Publication No. GER-3977
C37.2 Partial Listing
References
IEEE C57.13 CT Burdens (5 Amps):
Application
Burden
Designation
Impedance
(Ohms)
VA @
5 amps
Power
Factor
Metering
B0.1
B0.2
B0.5
B0.9
B1.8
0.1
0.2
0.5
0.9
1.8
2.5
5
12.5
22.5
45
0.9
0.9
0.9
0.9
0.9
Relaying
B1
B2
B4
B8
1
2
4
8
25
50
100
200
0.5
0.5
0.5
0.5
References
Required Information 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 Expected Expansion
10. Any special considerations for application
References
IEEE Protective Relaying Standards:
IEEE Standard
Description
C37-91
IEEE guide for protective relay applications to power transformers
C37.96
IEEE Guide for AC Motor Protection
C37.97
IEEE guide for protective relay applications to power system buses
C37.99
IEEE guide for the protection of shunt capacitor banks
C37.101
IEEE guide for generator ground protection
C37.102
IEEE Guide for AC Generator Protection
C37.110
IEEE Guide for the Application of Current Transformers Used for Protective
Relaying Purposes
C37.113
IEEE guide for protective relay applications to
C37.119
IEEE Guide for Breaker Failure Protection of Power Circuit Breakers
PC37.230
IEEE (draft) Guide for Protective Relay Applications to Distribution Lines
C37-94
IEEE standard for N times 64 kilobit per second optical fiber interfaces
between teleprotection and multiplexer equipment