Lightning test in lab. Symmetrical fault and protection. Olof Samuelsson

Transcription

1 Lightning test in lab Symmetrical fault and protection Olof Samuelsson

2 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 2

3 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 3

4 Series Fault

5 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 5

6 Lightning most common Statistically 80 % of faults on overhead lines are due to lightning 6

7 Power lines and trees 400 kv 50 kv 400 kv lines unaffected by Gudrun 10 kv Distribution lines most affected 7

8 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) 8

9 Heating Resistive losses RI 2 heat energy = RI 2 dt = RI 2 t Temperature rises with stored heat energy (if no cooling) Same I 2 t gives equal heating Time=1/I 2 Safe = no time limit Overload I 2 t=constant Short-circuit fault Current 9

10 Symmetrical 3-phase short-circuit Z TH Z TH + I SC I F V TH VF=VTH + I SC =V TH /Z TH I F =V F /Z TH =V TH /Z TH Thévenin gives only I F and not the prefault load current Prefault voltage V F often assumed same at all buses 10

11 3-phase short-circuit: Currents I I I Prefault I F + + I F System System V F System = sources V F + at V F = + V F System sources at 0 + V F Current during fault I = I prefault + I F Prefault current often << I F and neglected 11

12 Network during fault Standard simplifications to find fault current Transformers: Only X eq, no phase shift Transmission lines: Only series reactance Generators: E g behind X d, no saliency, R a or saturation Large motors: Like generators Small motors: Neglected Non-rotating loads: Neglected 12

13 Series impedances limit S-C currents All transformer x to same base: 400/130 kv, x= MVA MVA base 130/20 kv, x= MVA MVA base x p.u. 20/0.4 kv, x= MVA MVA base 50 x 0.25 p.u. and x p.u. The last transformer dominates Z TH Z TH400 j0.013 j0.25 j12.5 V TH ~ 400 kv 130 kv 20 kv 0.4 kv

14 S-C currents at different voltage levels Try 0Ω fault at 0.4 kv. Assume ΣZ=20 p.u. I SC =0.05 p.u. I base400 =100MVA/( 3 400kV)=144 A A=7.2 A I base130 =100MVA/( 3 130kV)=444 A A= 22 A I base30 =100MVA/( 3 20kV)=2.9 ka ka= 145 A I base0.4 =100MVA/( 3 0.4kV)=144 ka ka= 7200 A Z TH400 j0.013 j0.25 j12.5 V TH ~ 400 kv 130 kv 20 kv 0.4 kv

15 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 15

16 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 Open Closed Source: Nicklasson 16

17 Circuit breaker Sweden USA Source:ABB Open Closed Interrupts large current Perhaps 63 ka in 20 ms Short-circuit current Hidden contacts Control Protection systems Manual remote Design challenge Speed and current 17

18 Disconnecting circuit breaker Combines breaker and disconnector More reliable and compact Disconnector Breaker Line Combined breaker Line Lindome 400 kv station, also in Lund 18

19 More compact switchgear SF 6 Air 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 19

20 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) 20

21 Basic fault clearing system (Sw. felbortkopplingssystem) CT CB PT CB - Circuit Breaker CT - Current Transformer PT - Potential Transformer Relay Batteries for CB operation Possible communication 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 GPS 21

22 Current transformer (Sw. strömtransf.) Reduces current Typically 1000/2 A Current monitored Control center Protection equipment P, Q transducers Source:ABB 22

23 Voltage/potential transformer (Sw. spänningstransformator) Source:ABB Reduces voltage Typically x kv/110 V Voltage monitored Control center Protection equipment P, Q transducers Also C voltage divider 23

24 Capacitive Voltage Transformer 24

25 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: ABB 25

26 Layout Disconnector Current transformer Circuit breaker Disconnector Busbar Voltage transformer Surge arrester 26

27 Protection system tasks Detect fault Is there a fault? Short-circuit or only high load? I SC =5-7 p.u. of synchronous generators simplifies this! I SC 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 27

28 Different protection for different objects Differential protection where I in -I out >0 means fault Lines Transformers Busbars Generators Overcurrent protection I in I out I in I out I in I out1 I out2 Lines in radial (distribution) systems Overcurrent relay with directional sensitivity Lines in meshed (transmission) systems Generators 28

29 Current differential protection Compare i in and i out i in i out 0 no internal fault i in i out >>0 internal fault: Trip CB Applicable to generators, transformers, lines, busbars Generators i in and i out of each winding Communication needed for lines G M 29

30 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 G M 30

31 Time-delay overcurrent relay for line Detect overcurrent Wait delay time T Trip CB Time T Trip Constant delay characteristic 1 Relative overcurrent Multiple overcurrent relays can be co-ordinated Different delays decide tripping order Few fixed delay times practical, e.g. 0, 0,25, 1 s 31

32 Time-delay overcurrent relay for line Detect overcurrent Wait delay time T(I) Trip CB Time Trip Inverse 1/t characteristic 1 Relative overcurrent Fuses also have 1/t characteristic Easy to co-ordinate inverse time relays with fuses 32

33 Example: Co-ordination radial system I SC increases when approaching source R1 has higher current setting than R2 R1 CB1 Time R2 CB2 Load1 R1 R2 Load2 Relative overcurrent 33

34 Example: Fault in radial system R1 R2 CB1 CB2 Load2 L1» 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 Time Extra delay of R1 before it operates CB1 Isolates fault + Load 2 but also Load 1 Fault clearing non-selective 34 R1 R2 Current

35 Fault in radial system: At home F2 F1 Me F3 Neighbor» 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 Time Isolates fault + Me but also Neighbor Non-selective fault clearing F1 F2 Current 35

36 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 36

37 Line fed from both ends G R1 R2 R3 R4 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 Direction is obtained from phase difference of V and I measured by relay 37

38 Impedance relay Let relay measure V/I=Z=R+jX Normally load makes Z > Z line (Think Thévenin!) Fault on line makes Z < Z line TRIP! X Trip Radius= Z line R 38

39 Impedance relay types Directional X Z line Admittance or MHO Trip limit a certain admittance X Trip Trip R R 39

40 Distance protection (Sw. distansskydd) Series impedance ~ distance along line Z <0.8 Z line equivalent to» Zero Ω fault within 80% of line length» The reach of the relay is 80% 40

41 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: %, longer delay G Time A B C D Zone 3 Zone 2 Zone 1 G Distance 41

42 Distance protection coordination G A B C D Time Time Distance Distance Shown fault Primary protection from Zone 1 at C and D Backup protection from Zone 3 at A 42

43 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 43

44 When lights go out in radial system (Youtube blackout sandy jersey city) 1. A fault occurs. Voltages go low or zero. 2a. An upstream fuse/relay detects fault 2b. Fuse or breaker isolates downstream system. Voltage of unfaulted parts recover. 3. Fault is removed 4. Automatic reclosing after delay (successful if fault not permanent) or manual replacement of fuse Voltage of faulted parts recover. 44

45 Short-circuit transients Usually transients are steps and sinusoids are stationary AC power system (equivalent) R L i(t) E(t) = 2E sin(ωt +α) SW SW closes at t=0 Determine i(t) 45

46 R-L transients Math L di(t) + Ri(t) = 2E sin(ωt +α) with i(0) = 0 dt i(t) = i stationary (t) + i transient (t) = i AC (t) + i DC (t) i AC (t) = 2E sin(ωt +α θ) Z i DC (t) = 2E sin(α θ)e Z t T Z = R 2 + ( ωl) 2 ; θ = tan 1 ( ωl / R); T = L / R 46

47 R-L transients Power eng. Avoid α dependence Use worst case: α=(θ-π/2) Avoid instantaneous values i DC (t) = 2E Z t e T Use rms: I AC =E/Z Treat I DC as constant I RMS (t) = 2 I AC + [ I DC (t)] 2 2 = I AC where τ is time in cycles t /T + [ 2I AC e ] 2 2t = I AC 1+ 2e 2t /T T = 2 τ f R L = 4πτ X / R K(τ) 47

48 Exponential component Depends on initial condition Different α in the three phases Asymmetrical current Slow decay for high L/R (low losses) Increases peak current I RMS up to 3 I AC 48

49 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 =

50 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 50

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