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Symmetrical fault and protection
Olof Samuelsson
Lightning test in lab
Outline
• Three-phase short-circuit fault current• Network representation• Circuit breakers and disconnectors• Measurement transformers• Fuses and protection relays• Relay coordination
• Short-circuit fault current transient
2EIEN15 Electric Power Systems L3
Open-circuit faults (Sw. avbrott)Also series fault• A fault for which the impedances of each of the three
phases are not equal, usually caused by the interruption of one or two phases. (IEC definition)
Examples• One phase of circuit breaker stuck open• Conductor falling down
Short-circuit faults more common
3EIEN15 Electric Power Systems L3
Short-circuit faults (Sw. kortslutningar)Also shunt fault• A fault that is characterized by the flow of current
between two or more phases or between phase(s) and earth… (IEC definition)
Examples• Lightning• Dirt/salt on insulators• Flashover (Sw. överslag) line-line (wind) or line to tree• Tower/pole or conductor falls• Objects fall on conductors• Cable insulation failure
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Lightning most common
Statistically80 % of faults onoverhead lines are due to lightning
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Power lines and trees400 kV 50 kV
10 kV
Distribution lines most affected6EIEN15 Electric Power Systems L3
400 kV lines unaffected by Gudrun
Effects of short-circuit current
• Arc (Sw. ljusbåge)– Compare with welding
• Heating– Fire and explosion (movie transformer blast)
• Vibration due to magnetic forces – Parallel conductors are attracted (F=B·i·l)
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Heating
• Resistive losses RI2 heat energy = RI2dt = RI2t • Temperature rises with stored heat energy (if no cooling)• Same I2t gives equal heating
8EIEN15 Electric Power Systems L3
Time=1/I2
Current
Overload
Short-circuit fault
I2t=constantSafe = no time lim
it
Symmetrical 3-phase short-circuit
9EIEN15 Electric Power Systems L3
VTH
ZTH
ISC
VF=VTH
ZTH
IF+
–
–
+
ISC=VTH/ZTH IF =VF/ZTH=VTH/ZTH
•Thévenin gives only IF and not the prefault load current•Prefault voltage VF often assumed same at all buses
3-phase short-circuit: Currents
10EIEN15 Electric Power Systems L3
System
I
System
I
+
–VF
+
–VF
= Systemsources
at VF
IPrefault
+
–VF
= +
Current during fault I = Iprefault + IF
Prefault current often << IF and neglected
System
IF
+
–VF
sourcesat 0
IF
Network during fault
• Standard simplifications to find fault current– Transformers: Only Xeq, no phase shift– Transmission lines: Only series reactance– Generators: Eg behind X”d, no saliency, Ra or saturation
– Large motors: Like generators– Small motors: Neglected– Non-rotating loads: Neglected
11EIEN15 Electric Power Systems L3
Series impedances limit S-C currentsAll transformer x to same base:• 400/130 kV, x=0.1 p.u. @ 750 MVA
– 0.013 p.u. @ 100 MVA base
• 130/20 kV, x=0.1 p.u. @ 40 MVA– 0.25 p.u. @ 100 MVA base– 18.75 x 0.013 p.u.
• 20/0.4 kV, x=0.1 p.u. @ 0.8 MVA– 12.5 p.u. @ 100 MVA base– 50 x 0.25 p.u. and 937.5 x 0.013 p.u.
• The last transformer dominates ZTH
VTH
ZTH400
~ 400 kV 130 kV 20 kV 0.4 kV
j0.013 j0.25 j12.5
S-C currents at different voltage levelsTry 0 fault at 0.4 kV. Assume Z=20 p.u. ISC=0.05 p.u.• Ibase400=100MVA/(3400kV)=144 A
– 0.05144A=7.2 A
• Ibase130=100MVA/(3130kV)=444 A– 0.05444A= 22 A
• Ibase30=100MVA/(320kV)=2.9 kA– 0.052.9 kA= 145 A
• Ibase0.4=100MVA/(30.4kV)=144 kA– 0.05144 kA= 7200 A
VTH
ZTH400
~ 400 kV 130 kV 20 kV 0.4 kV
j0.013 j0.25 j12.5
Interrupting large currents
• Fuses (Sw. säkringar)– Use the melting effect of the fault current (arc)
• Circuit breakers (Sw. effektbrytare) interrupt kA in ms– Extinguish arc using pressurized air (arc energy),
vacuum, oil etc.
• Circuit breaker operation– Automatic by relay protection– Manual remote control from control center
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Disconnector/Isolator not for short-circuits(Sw. frånskiljare)
• Visible interruption• Motorized or manual (rural MV)• Interrupts < Load current
– OK: Youtube 110 kV disconnector closes and opens– Too large current: Youtube 138 kV Elkford
• Design challenge: Weather
Sweden USA
Open Closed
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Open Closed
Source: Nicklasson
Circuit breaker
• Interrupts large current– Perhaps 63 kA in 20 ms– Short-circuit current– Hidden contacts
• Control– Protection systems– Manual remote
• Design challenge– Speed and current
Sweden USA
Open Closed
Sour
ce:A
BB
16EIEN15 Electric Power Systems L3
Disconnecting circuit breaker• Combines breaker and disconnector• More reliable and compact
17EIEN15 Electric Power Systems L3
Lindome 400 kV station, also in Lund
Breaker
Disconnector
Line
Line Combinedbreaker
More compact switchgear
AirSF6
Source: Lakervi
• Disconnecting circuit breaker in air insulated station• Gas Insulated Switchgear (GIS) uses SF6 gas
– Isolates much better than air - reduces size– SF6 a greenhouse gas – alternatives are sought for
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Protection for automatic fault clearing
Need– Detect fault– Isolate faulted component– Restore service for unfaulted components
Aims– Continued supply for rest of system– Protect faulted part from damage
(A fuse does this, but needs manual replacement)
19EIEN15 Electric Power Systems L3
Basic fault clearing system(Sw. felbortkopplingssystem)
• The protection relay takes CB trip decision based on inputs• Many relays can operate same breaker• Initially relays were electro-mechanical, then electronic
and now use a microprocessor/DSP and GPS20EIEN15 Electric Power Systems L3
Relay
PT
CT CB
CB - Circuit BreakerCT - Current TransformerPT - Potential Transformer
Batteries for CB operationPossible communication
Current transformer (Sw. strömtransf.)
• Reduces current– Typically 1000/2 A
• Current monitored– Control center– Protection equipment– P, Q transducers
Sour
ce:A
BB
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Voltage/potential transformer(Sw. spänningstransformator)
• Reduces voltage– Typically x kV/110 V
• Voltage monitored– Control center– Protection equipment– P, Q transducers
• Also C voltage divider
Sour
ce:A
BB
22EIEN15 Electric Power Systems L3
Surge arrester/lightning arrester(Sw. ventilavledare)
• Passive overvoltage protection• Alternative to air gap• Nonlinear resistance gives
short-circuits at high voltages• Sends lightning to ground
Source: ABB23EIEN15 Electric Power Systems L3
24EIEN15 Electric Power Systems L3
Layout DisconnectorCurrent transformerCircuit breakerDisconnectorBusbar
Voltage transformer
Surge arrester
Protection system tasksDetect fault – Is there a fault?
– Short-circuit or only high load?– ISC=5-7 p.u. of synchronous generators simplifies this!– ISC of power electronic “generators” only about 1.2 p.u.!
Isolate fault – Open (“trip”) circuit breaker(s) (CB)Many alternatives Coordination required
– Which protection unit should react and open which CB?– Isolate as small area as possible– Isolation must happen also if one component fails
25EIEN15 Electric Power Systems L3
• Differential protection where |Iin-Iout |>0 means fault– Lines– Transformers– Busbars– Generators
• Overcurrent protection– Lines in radial (distribution) systems
• Overcurrent relay with directional sensitivity– Lines in meshed (transmission) systems– Generators
Different protection for different objects
26EIEN15 Electric Power Systems L3
Iin
Iin
Iin
Iout
Iout
Iout1
Iout2
Current differential protection
• Compare iin and iout
• |iin– iout|≈0 no internal fault• |iin– iout|>>0 internal fault: Trip CB
• Applicable to generators, transformers, lines, busbars• Generators
• iin and iout of each winding• Communication needed for lines
27EIEN15 Electric Power Systems L3
MG
Protection zones• Defined for protected objects
– Dedicated protection for each zone• Zone border where current is measured• Zones overlap with CB in overlap zones• Isolated at fault anywhere inside• Perfect for differential protection
MG
28EIEN15 Electric Power Systems L3
Time-delay overcurrent relay for line
• Detect overcurrent – Wait delay time T – Trip CB
• Multiple overcurrent relays can be co-ordinated
– Different delays decide tripping order– Few fixed delay times practical, e.g. 0, 0,25, 1 s
29EIEN15 Electric Power Systems L3
Time
Relative overcurrent
1
T
Trip Constant delaycharacteristic
Time-delay overcurrent relay for line
• Detect overcurrent – Wait delay time T(I) – Trip CB
• Fuses also have 1/t characteristic– Easy to co-ordinate inverse time relays with fuses
30EIEN15 Electric Power Systems L3
Time
Relative overcurrent
1
Trip Inverse 1/t characteristic
Example: Co-ordination radial system
CB1
CB2Load1
Load2
R2
•ISC increases when approaching source•R1 has higher current setting than R2
Time
Relative overcurrent
R1R2
31EIEN15 Electric Power Systems L3
R1
» R1 and R2 detect overcurrent
– Delay of R2 smallest
» R2 operates CB2 first
– Isolates fault + Load 2
– No overcurrent R1 reset
– Fault clearing selective» If R2 or CB2 fails
– R1 not reset
– Extra delay of R1 before it operates CB1
– Isolates fault + Load 2 but also Load 1
– Fault clearing non-selective
Example: Fault in radial system
CB1
CB2L1
Load2
R2
Time
Current
R1R2
32EIEN15 Electric Power Systems L3
R1
» Both F1 and F2 detect overcurrent
» Delay of F2
» Fuse F2 blows first
– Isolates fault and Me
– Selective fault clearing
» If fuse F2 fails
– Extra delay of F1
– F1 blows
– Isolates fault + Me but also Neighbor
– Non-selective fault clearing
Fault in radial system: At home
Me
F1
F2
Time
Current
F1F2
Neighbor
F3
33EIEN15 Electric Power Systems L3
Co-ordination (Sw. selektivplanering)
Relays 1 and 2 coordinated in example:For the line,• Relay 2 is primary protection and provides selective fault
clearing (Sw: selektiv felbortkoppling)• Relay 1 is backup protection and provides non-selective
fault clearing (Sw: reservbortkoppling)Always true (regardless of I) since t(I) curves do not cross
Rule: Longer delay close to source
34EIEN15 Electric Power Systems L3
Line fed from both ends
– 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
– Direction is obtained from phase difference of V and I measured by relay
GGR1 R2 R3 R4
35EIEN15 Electric Power Systems L3
Impedance relay
Let relay measure V/I=Z=R+jXNormally load makes Z > Zline (Think Thévenin!)Fault on line makes Z < Zline TRIP!
36EIEN15 Electric Power Systems L3
Trip
R
X
Radius=|Zline |
Impedance relay types
37EIEN15 Electric Power Systems L3
Trip
R
X
Trip
R
X
Directional Admittance or “MHO”Trip limit a certain admittance
Zline
Distance protection (Sw. distansskydd)
– Series impedance ~ distance along line– |Z|<0.8|Zline| equivalent to
» Zero fault within 80% of line length
» The reach of the relay is 80%
38EIEN15 Electric Power Systems L3
Distance protection zones
– Zone 1 relay at A, Primary: 80%, no delay– Zone 2 relay at A, Backup 1: 120%, delay– Zone 3 relay at A, Backup 2: 120+100%, longer delay
39EIEN15 Electric Power Systems L3
GGA B C D
Distance
Time
Zone 1
Zone 2
Zone 3
Time
Distance
Distance protection coordination
• Shown faultPrimary protection from Zone 1 at C and D Backup protection from Zone 3 at A
40EIEN15 Electric Power Systems L3
GA B C D
Distance
Time
Protection system performance
High “dependability”– Always isolate targeted fault– High sensitivity good
High “security”– Only react to targeted faults– High sensitivity bad
Fast– Good for (transient) stability– Safety
41EIEN15 Electric Power Systems L3
Compromise
When lights go out in radial system
2a. An upstream fuse/relay detects fault
42EIEN15 Electric Power Systems L3
1. A fault occurs. Voltages go low or zero.
2b. Fuse or breaker isolates downstream system.Voltage of unfaulted parts recover.
4. Automatic reclosing after delay (successful if fault not permanent) or manual replacement of fuseVoltage of faulted parts recover.
3. Fault is removed
(Youtube blackout sandy jersey city)
Short-circuit transients
43EIEN15 Electric Power Systems L3
R L
SW
i(t)
E(t) 2E sin(t )
Usually transients are steps and sinusoids are stationary…
AC power system (equivalent)
SW closes at t=0Determine i(t)
R-L transients – Math
44EIEN15 Electric Power Systems L3
L di(t)dt
Ri(t) 2E sin(t ) with i(0) 0
iAC(t) 2EZ
sin(t )
i(t) istationary(t) itransient (t) iAC(t) iDC(t)
iDC(t) 2EZ
sin( )e
tT
RLTRLLRZ / );/(tan ;)( 122
R-L transients – Power eng.
Avoid dependence– Use worst case: =(-/2)
Avoid instantaneous values– Use rms: IAC=E/Z– Treat IDC as constant
– where is time in cycles
45EIEN15 Electric Power Systems L3
IRMS(t) IAC2 IDC (t) 2 IAC
2 2IACe t /T 2
IAC 1 2e2 t /T 2tT 2
f RL 4
X / R
iDC(t) 2EZ
e
tT
K()
Exponential component
• Depends on initial condition• Different in the three phases
– Asymmetrical current• Slow decay for high L/R (low losses)• Increases peak current
– IRMS up to IAC
46EIEN15 Electric Power Systems L3
3
Summary
• Compute short-circuit fault current with…• Circuit breaker is used for…• Disconnector is used for…• Fault clearing system includes protective relay with
– Sensors:…– Actuator:…
• 3 protection types:…, …, …• Backup protection acts after delay if … protection fails• Deenergizing minimum area = … fault clearing• S-C current transient = ….. + …..
47EIEN15 Electric Power Systems L3
Summary
• Compute short-circuit fault current with Thévenin equivalent• Circuit breaker is used for interrupting fault current• Disconnector is used for interrupting load current or less• Fault clearing system includes protective relay with
– Sensors: Current and voltage/potential transformers– Actuator: Circuit breaker
• 3 protection types: differential, overcurrent, distance protection• Backup protection acts after delay if primary protection fails• Deenergizing minimum area = selective fault clearing• S-C current transient = sinusoidal AC + expontial DC
48EIEN15 Electric Power Systems L3