L6
Content
How do blackouts start? Faults and protection
Joint of disconnector
Outline
• • • • • • • Faults in power systems Power system protection Zones of protection Time-delay overcurrent relay Coordination Directional relays Reclosers and sectionalizers
Electric Power Systems - Olof Samuelsson 2
Disconnector initiates Swedish blackout 23 september
Electric Power Systems - Olof Samuelsson 1
Open-circuit faults
• One phase of circuit breaker stuck open • Conductor falling down Short-circuit faults more common • • • • • • •
Short-circuit faults
Lightning Dirt/salt on insulators Flashover line-line (wind) Flashover to tree Tower/pole or conductor falls Objects fall on conductors Cable insulation failure
Electric Power Systems - Olof Samuelsson 4
Electric Power Systems - Olof Samuelsson
3
Lightning most common
Power lines and trees
400 kV 50 kV
Statistically 80 % of faults on overhead lines are due to lightning
10 kV
www.dmi.dk
Electric Power Systems - Olof Samuelsson 5
Distribution lines most affected
Electric Power Systems - Olof Samuelsson
6
Galopping spectacular
Effects of short-circuit current
• Arc
– Compare with welding – http://205.243.100.155/frames/longarc.htm
• Heating
– Fire and explosion
• Vibration due to magnetic forces
– Parallel conductors are attracted (F=B·i·l)
Electric Power Systems - Olof Samuelsson
7
Electric Power Systems - Olof Samuelsson
8
Heating
• Resistive losses RI2 • Temperature rise with stored heat energy I2t (no cooling assumed) • Same I2t gives equal heating (see graph)
Time Overload
Interrupting large currents
• Fuses
– Use the melting effect of the arc
• Circuit breakers interrupt kA in ms
– Extinguish arc
Short-circuit fault Current
Electric Power Systems - Olof Samuelsson 9
• Breaker operation – Automatic by relay protection
Electric Power Systems - Olof Samuelsson 10
Protection including fuses
Need
– Detect fault – Isolate faulted component – Restore faulted component
When lights go out…
1. An upstream fuse/relay has detected a fault 2. Downstream system isolated by fuse or breaker 3. Automatic reclosing after delay successful if fault not permanent
Electric Power Systems - Olof Samuelsson 12
Aims
– Continued supply for rest of system – Protect faulted part from damage
Electric Power Systems - Olof Samuelsson
11
Protection system performance
High reliability
– Always isolate targeted fault – High sensitivity good
Protection system tasks
Is there a fault?
– Short-circuit or only high load? – All situations must be known!
High selectivity
– Only react to targeted faults – High sensitivity bad
Compromise
Coordination
– Which protection unit should react? – Isolate as small area as possible – Must work also if component fails
Electric Power Systems - Olof Samuelsson 14
Fast
– Good for (transient) stability – Safety
Electric Power Systems - Olof Samuelsson 13
Zones of protection
• Defined for protected objects
– Dedicated protection for each zone
Protection types
• Overcurrent protection
– Lines (distribution)
• Zones overlap • CB in overlap zones • Isolated at fault anywhere inside
G M
• Directional overcurrent relay
– Lines (transmission), generators
• Differential protection
– Lines – Transformers – Busbars – Generators
Ex
Electric Power Systems - Olof Samuelsson 16
Electric Power Systems - Olof Samuelsson
15
Line protection components
CT CB
Time-delay overcurrent relay
Detect overcurrent – Wait delay time T – Trip CB Time
PT Relay
CB - Circuit Breaker CT - Current Transformer PT - Potential Transformer T 1
Trip
Constant delay characteristic
Large number of relays needed: One for each phase and fault type
Electric Power Systems - Olof Samuelsson 17
Relative overcurrent
18
Electric Power Systems - Olof Samuelsson
Time-delay overcurrent relay
Detect overcurrent – Wait delay time T(I) – Trip CB Time
R1 CB1
Radial system
•ISC increases when approaching source •R1 has higher current setting than R2 Time
Load1 CB2 R1 R2
Trip
Inverse 1/t characteristic Similar for fuses
R2
1
Relative overcurrent
19
Load2
Relative overcurrent
Electric Power Systems - Olof Samuelsson 20
Electric Power Systems - Olof Samuelsson
Fault in radial system
• R1 and R2 detect overcurrent • Delay of R2 smallest Time • R2 operates CB2 first
R1 R2 CB2 CB1 L1
Fault in radial system: At home
• Both F1 and F2 detect overcurrent • Delay of F2 Time • Fuse F2 blows first
F1 F2 F3
– Isolates fault + Load 2 – R1 reset
– Isolates fault and Me
• If fuse F2 fails
R1 R2
• If R2 or CB2 fails
– – – –
Load2
R1 not reset Current Extra delay of R1 R1 operates CB1 Isolates fault + Load 2 but also Load 1
– Extra delay of F1 – F1 blows – Isolates fault + Me but also Neighbor
F1 F2 Current
Me Neighbor •
Fault clearing is selective
– Coordination works
Electric Power Systems - Olof Samuelsson 22
Electric Power Systems - Olof Samuelsson
21
Coordination
Relays 1 and 2 coordinated in example: For the line, • Relay 2 provides Primary protection • Relay 1 provides Backup protection Always true since t(I) curves do not cross Rule: Longer delay close to source
Electric Power Systems - Olof Samuelsson 23
Line fed from both ends
R1 R2 R3 R4
G
G
– Rule not applicable due to many sources – Use directional relays:
• R1 and R3 only trip for fault to their right • R2 and R4 only trip for fault to their left
– V and I phase difference gives direction
Electric Power Systems - Olof Samuelsson
24
Impedance relay
Let relay measure V/I=Z=R+jX Normally load makes Z > Zline Fault on line makes Z < Zline - TRIP!
X Trip R Radius=|Zline |
Electric Power Systems - Olof Samuelsson 25
Impedance relay types
Directional X Trip R Zline Admittance or MHO X Trip R
Electric Power Systems - Olof Samuelsson
26
Distance protection
– Series impedance ~ distance along line – |Z|<0.8|Zline| equivalent to
• Zero Ω fault within 80% of line length • The reach of the relay is 80%
Distance protection zones
– Zone 1, Primary: 80%, no delay – Zone 2, Backup 1: 120%, delay – Zone 3, Backup 2: 120+100%, longer delay
A B C D
G
Time Zone 3 Zone 2 Zone 1
G
Distance
Electric Power Systems - Olof Samuelsson 27 Electric Power Systems - Olof Samuelsson 28
Distance protection coordination
A B C D
Current differential protection
• Compare iin and iout
• |iin– iout|≈0 no internal fault • |iin– iout|>>0 internal fault: Trip CB
G
Time
Distance Time
• Generators
• iin and iout of each winding
• Communication needed for lines
G
Distance
Electric Power Systems - Olof Samuelsson 29
M
Ex
Electric Power Systems - Olof Samuelsson
30
Example: Permanent fault Automatic sectionalizer
– Cost-effective restoration of service
• Sectionalizer with transducers and logic
R S Load3 S Load2 S Load1 R
S1 Load1
R S
R S
R S
R S
R S
– Operates when recloser R is open
• Need not interrupt fault current • Simpler than circuit breaker • Operation not electrically powered
S
S2 Load2 Load3
S
S
S
S
S
S
– Counts periods of fault current
• Opens after preset number • Maximum area restored • Radio message to repairman
Electric Power Systems - Olof Samuelsson 31
Normal S1 preset to 3 S2 preset to 2
Fault occurs S1 count 1 S2 count 1
R opens
R recloses S1 count 2 S2 count 2 Fault not temporary!
R opens S2 opens
32
R closes
Electric Power Systems - Olof Samuelsson
System protection
• ”Unit protection”=Fault protection
– Protects generator, line, transformer… – Weakens system when tripping CB
Calculating fault current
• Short-circuit protection input data
– Minimum short-circuit current – Maximum load current
• System protection=Blackout protection
– Acts to avoid system blackout – E.g. Sacrifice some load to save the rest
• Circuit breaker selection input data
– Maximum short-circuit current
• All protection is based on knowledge
– Normal and abnormal operation – Coordinating protection in nuclear plant!
Electric Power Systems - Olof Samuelsson 33
• Short-circuit current calculation
– Based on network data – Detailed like load flow or Thévenin-based
Electric Power Systems - Olof Samuelsson 34
Summary
• Short-circuit most common fault (lightning) • Fuses and protection
– Dependable = not miss a fault – Selective = not overreact – Many relay types – One zone for each relay
• Limiting blackout-area
– Coordination, autoreclosing, autosectionalizing
Electric Power Systems - Olof Samuelsson 35
Joint of disconnector
Outline
• • • • • • • Faults in power systems Power system protection Zones of protection Time-delay overcurrent relay Coordination Directional relays Reclosers and sectionalizers
Electric Power Systems - Olof Samuelsson 2
Disconnector initiates Swedish blackout 23 september
Electric Power Systems - Olof Samuelsson 1
Open-circuit faults
• One phase of circuit breaker stuck open • Conductor falling down Short-circuit faults more common • • • • • • •
Short-circuit faults
Lightning Dirt/salt on insulators Flashover line-line (wind) Flashover to tree Tower/pole or conductor falls Objects fall on conductors Cable insulation failure
Electric Power Systems - Olof Samuelsson 4
Electric Power Systems - Olof Samuelsson
3
Lightning most common
Power lines and trees
400 kV 50 kV
Statistically 80 % of faults on overhead lines are due to lightning
10 kV
www.dmi.dk
Electric Power Systems - Olof Samuelsson 5
Distribution lines most affected
Electric Power Systems - Olof Samuelsson
6
Galopping spectacular
Effects of short-circuit current
• Arc
– Compare with welding – http://205.243.100.155/frames/longarc.htm
• Heating
– Fire and explosion
• Vibration due to magnetic forces
– Parallel conductors are attracted (F=B·i·l)
Electric Power Systems - Olof Samuelsson
7
Electric Power Systems - Olof Samuelsson
8
Heating
• Resistive losses RI2 • Temperature rise with stored heat energy I2t (no cooling assumed) • Same I2t gives equal heating (see graph)
Time Overload
Interrupting large currents
• Fuses
– Use the melting effect of the arc
• Circuit breakers interrupt kA in ms
– Extinguish arc
Short-circuit fault Current
Electric Power Systems - Olof Samuelsson 9
• Breaker operation – Automatic by relay protection
Electric Power Systems - Olof Samuelsson 10
Protection including fuses
Need
– Detect fault – Isolate faulted component – Restore faulted component
When lights go out…
1. An upstream fuse/relay has detected a fault 2. Downstream system isolated by fuse or breaker 3. Automatic reclosing after delay successful if fault not permanent
Electric Power Systems - Olof Samuelsson 12
Aims
– Continued supply for rest of system – Protect faulted part from damage
Electric Power Systems - Olof Samuelsson
11
Protection system performance
High reliability
– Always isolate targeted fault – High sensitivity good
Protection system tasks
Is there a fault?
– Short-circuit or only high load? – All situations must be known!
High selectivity
– Only react to targeted faults – High sensitivity bad
Compromise
Coordination
– Which protection unit should react? – Isolate as small area as possible – Must work also if component fails
Electric Power Systems - Olof Samuelsson 14
Fast
– Good for (transient) stability – Safety
Electric Power Systems - Olof Samuelsson 13
Zones of protection
• Defined for protected objects
– Dedicated protection for each zone
Protection types
• Overcurrent protection
– Lines (distribution)
• Zones overlap • CB in overlap zones • Isolated at fault anywhere inside
G M
• Directional overcurrent relay
– Lines (transmission), generators
• Differential protection
– Lines – Transformers – Busbars – Generators
Ex
Electric Power Systems - Olof Samuelsson 16
Electric Power Systems - Olof Samuelsson
15
Line protection components
CT CB
Time-delay overcurrent relay
Detect overcurrent – Wait delay time T – Trip CB Time
PT Relay
CB - Circuit Breaker CT - Current Transformer PT - Potential Transformer T 1
Trip
Constant delay characteristic
Large number of relays needed: One for each phase and fault type
Electric Power Systems - Olof Samuelsson 17
Relative overcurrent
18
Electric Power Systems - Olof Samuelsson
Time-delay overcurrent relay
Detect overcurrent – Wait delay time T(I) – Trip CB Time
R1 CB1
Radial system
•ISC increases when approaching source •R1 has higher current setting than R2 Time
Load1 CB2 R1 R2
Trip
Inverse 1/t characteristic Similar for fuses
R2
1
Relative overcurrent
19
Load2
Relative overcurrent
Electric Power Systems - Olof Samuelsson 20
Electric Power Systems - Olof Samuelsson
Fault in radial system
• R1 and R2 detect overcurrent • Delay of R2 smallest Time • R2 operates CB2 first
R1 R2 CB2 CB1 L1
Fault in radial system: At home
• Both F1 and F2 detect overcurrent • Delay of F2 Time • Fuse F2 blows first
F1 F2 F3
– Isolates fault + Load 2 – R1 reset
– Isolates fault and Me
• If fuse F2 fails
R1 R2
• If R2 or CB2 fails
– – – –
Load2
R1 not reset Current Extra delay of R1 R1 operates CB1 Isolates fault + Load 2 but also Load 1
– Extra delay of F1 – F1 blows – Isolates fault + Me but also Neighbor
F1 F2 Current
Me Neighbor •
Fault clearing is selective
– Coordination works
Electric Power Systems - Olof Samuelsson 22
Electric Power Systems - Olof Samuelsson
21
Coordination
Relays 1 and 2 coordinated in example: For the line, • Relay 2 provides Primary protection • Relay 1 provides Backup protection Always true since t(I) curves do not cross Rule: Longer delay close to source
Electric Power Systems - Olof Samuelsson 23
Line fed from both ends
R1 R2 R3 R4
G
G
– Rule not applicable due to many sources – Use directional relays:
• R1 and R3 only trip for fault to their right • R2 and R4 only trip for fault to their left
– V and I phase difference gives direction
Electric Power Systems - Olof Samuelsson
24
Impedance relay
Let relay measure V/I=Z=R+jX Normally load makes Z > Zline Fault on line makes Z < Zline - TRIP!
X Trip R Radius=|Zline |
Electric Power Systems - Olof Samuelsson 25
Impedance relay types
Directional X Trip R Zline Admittance or MHO X Trip R
Electric Power Systems - Olof Samuelsson
26
Distance protection
– Series impedance ~ distance along line – |Z|<0.8|Zline| equivalent to
• Zero Ω fault within 80% of line length • The reach of the relay is 80%
Distance protection zones
– Zone 1, Primary: 80%, no delay – Zone 2, Backup 1: 120%, delay – Zone 3, Backup 2: 120+100%, longer delay
A B C D
G
Time Zone 3 Zone 2 Zone 1
G
Distance
Electric Power Systems - Olof Samuelsson 27 Electric Power Systems - Olof Samuelsson 28
Distance protection coordination
A B C D
Current differential protection
• Compare iin and iout
• |iin– iout|≈0 no internal fault • |iin– iout|>>0 internal fault: Trip CB
G
Time
Distance Time
• Generators
• iin and iout of each winding
• Communication needed for lines
G
Distance
Electric Power Systems - Olof Samuelsson 29
M
Ex
Electric Power Systems - Olof Samuelsson
30
Example: Permanent fault Automatic sectionalizer
– Cost-effective restoration of service
• Sectionalizer with transducers and logic
R S Load3 S Load2 S Load1 R
S1 Load1
R S
R S
R S
R S
R S
– Operates when recloser R is open
• Need not interrupt fault current • Simpler than circuit breaker • Operation not electrically powered
S
S2 Load2 Load3
S
S
S
S
S
S
– Counts periods of fault current
• Opens after preset number • Maximum area restored • Radio message to repairman
Electric Power Systems - Olof Samuelsson 31
Normal S1 preset to 3 S2 preset to 2
Fault occurs S1 count 1 S2 count 1
R opens
R recloses S1 count 2 S2 count 2 Fault not temporary!
R opens S2 opens
32
R closes
Electric Power Systems - Olof Samuelsson
System protection
• ”Unit protection”=Fault protection
– Protects generator, line, transformer… – Weakens system when tripping CB
Calculating fault current
• Short-circuit protection input data
– Minimum short-circuit current – Maximum load current
• System protection=Blackout protection
– Acts to avoid system blackout – E.g. Sacrifice some load to save the rest
• Circuit breaker selection input data
– Maximum short-circuit current
• All protection is based on knowledge
– Normal and abnormal operation – Coordinating protection in nuclear plant!
Electric Power Systems - Olof Samuelsson 33
• Short-circuit current calculation
– Based on network data – Detailed like load flow or Thévenin-based
Electric Power Systems - Olof Samuelsson 34
Summary
• Short-circuit most common fault (lightning) • Fuses and protection
– Dependable = not miss a fault – Selective = not overreact – Many relay types – One zone for each relay
• Limiting blackout-area
– Coordination, autoreclosing, autosectionalizing
Electric Power Systems - Olof Samuelsson 35
