symmetrical fault and protection olof samuelsson - iea€“ transmission lines: ... transformers,...

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

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

Lightning most common

Statistically80 % of faults onoverhead lines are due to lightning

Power lines and trees400 kV 50 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)

Heating

• Resistive losses RI2 heat energy = RI2dt = RI2t • Temperature rises with stored heat energy (if no cooling)• Same I2t gives equal heating

Time=1/I2

Current

Overload

Short-circuit fault

I2t=constantSafe = no time lim

Symmetrical 3-phase short-circuit

VF=VTH

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

System

= Systemsources

IPrefault

Current during fault I = Iprefault + IF

Prefault current often << IF and neglected

System

sourcesat 0

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

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

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

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

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

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

Disconnecting circuit breaker• Combines breaker and disconnector• More reliable and compact

Lindome 400 kV station, also in Lund

Breaker

Disconnector

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

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)

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

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

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

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

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

• 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

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

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

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

Relative overcurrent

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

Trip Inverse 1/t characteristic

Example: Co-ordination radial system

CB2Load1

•ISC increases when approaching source•R1 has higher current setting than R2

» 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

Current

» 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

Current

Neighbor

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

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

Impedance relay

Let relay measure V/I=Z=R+jXNormally load makes Z > Zline (Think Thévenin!)Fault on line makes Z < Zline TRIP!

Radius=|Zline |

Impedance relay types

Directional Admittance or “MHO”Trip limit a certain admittance

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%

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

GGA B C D

Distance

Zone 1

Zone 2

Zone 3

Distance

Distance protection coordination

• Shown faultPrimary protection from Zone 1 at C and D Backup protection from Zone 3 at A

GA B C D

Distance

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

Compromise

When lights go out in radial system

2a. An upstream fuse/relay detects fault

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

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

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

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

IRMS(t) IAC2 IDC (t) 2 IAC

2 2IACe t /T 2

IAC 1 2e2 t /T 2tT 2

f RL 4

iDC(t) 2EZ

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

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 = ….. + …..

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

symmetrical fault and protection olof samuelsson - iea€“ transmission lines: ... transformers,...

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