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06-DistanceProtection.ppt
- 1. LARSEN & TOUBRO LIMITED ECC DIVISION – EDRC, HQ CHENNAI Transmission Line Protection (Distance Protection)
- 2. 2 Contents Introduction Distance protection characteristics Operating principle Distance protection schemes
- 4. 4 Introduction to distance protection Why distance relays ? Coordination of overcurrent and directional overcurrent relays is achieved using time delays The time delays would become exceptionally long when applied to large power systems Not possible to protect complex systems using overcurrent & directional overcurrent relays Distance protection was introduced to overcome these problems Most systems use one or another form of distance relay for protecting transmission and sub-transmission lines
- 5. 5 Limitation of instantaneous overcurrent protection Introduction to distance protection IF1 = 115kV/3(5+4) = 7380A Is > 7380A F1 115kV 50 IF1 ZS = 10 ZS = 10 ZL = 4 F2 115kV 50 IF2 ZS = 10 ZL = 4 IF2 = 115kV/3(10) = 6640A 7380A> Is < 6640A Impossible to set
- 6. 6 Introduction to distance protection Distance relay is very popular for protecting transmission lines. Offers fast operating times (1 cycle) Can work independently or in co-ordination with other relays in the system Easy to design and proven to be the best solution for wide range of system configuration Communication aided schemes can give 100% instantaneous protection. Additional time delayed back-up available.
- 7. 7 Introduction to distance protection The line can include the following which falls between these circuit breakers Sections of the bus Overhead conductor Underground cable Other apparatus including line traps, series capacitors, shunt reactors, and autotransformers
- 8. 8 Introduction to distance protection EA ZSA CB1 CB2 EB ZSB ZL Transmission Line RA RB RA
- 10. 10 Distance protection characteristics A distance relay response characteristic is usually represented in R-X impedance diagram – reach setting of the relay Z-T diagram – operating time of each zone
- 11. 11 MHO R Zs jX IMPEDANCE R Zs jX OFFSETMHO R Zs jX Zs’ CROSS/MEMORY POLARIZED MHO R Zs jX LENTICULAR jX Zs R Zs’ Distance protection characteristics QUADRILATERAL Zs R jX POLYGON Zs R jX
- 12. 12 Simple impedance characteristics Operating characteristics is a circle with its centre as origin in the R-X plane. Good resistive reach coverage Non-directional, thus requires additional directional unit. Higher chances of load encroachment Susceptible to power swings R Zs jX Operate Restrain
- 13. 13 Self polarized mho characteristics Operating characteristics is a circle passing through the origin in the R-X plane Inherently directional Less susceptible to power swings Characteristics defined by the reach setting (ZS) and relay characteristics angle () Less fault resistance coverage R Zs jX Operate Restrain
- 14. 14 Offset mho characteristics Has a reverse reach. Usually used for time delayed zones to provide back-up for forward and reverse faults Ensures secure operation for close-up faults. R Zs jX Operate Restrain ZR
- 15. 15 Cross polarized mho characteristics To ensure proper operation for close-up forward and reverse faults with zero (very close to zero!!) fault voltage “Cross polarization” technique is used Using healthy phase(s) voltage to polarize the mho characteristics 100% cross polarized - When full magnitude of healthy phase voltage is used Partially cross polarized - When a percentage of healthy phase voltage is used In addition to providing secure operation, it also offers higher coverage for resistive faults For example for a “B-C” phase fault use “A” phase voltage as the polarizing signal
- 16. 16 Three phase close-up faults During three phase close-up faults the fault voltage can go down to a very low value Since this happens in all three phases, cross polarization will not help. This can again lead to wrong decision from the relay. This is solved by using memory voltage for polarizing. R X ZL Uncertain area Due to low fault voltage
- 19. 19 Distance protection operating principle Consider a simple single phase system LO A D L R R R Z Z V Z IR ZL ZS VR VS ZLOAD Measured impedance :-
- 20. 20 Distance protection operating principle R jX ZL LOAD Lag PF Lead PF Load and protected line IR ZL ZS VR VS ZLOAD
- 21. 21 Distance protection operating principle When a fault occurs….. Since IF>>IL we can ignore load and write L F F R L F R R R R mZ Z Ι V I Ι V Ι V Z IR (1-m)ZL ZS VR VS ZLOAD mZL IF IL RF=0
- 22. 22 Distance protection operating principle R jX LOAD ZL ZF=mZL Fault on protected line IR (1-m)ZL ZS VR VS ZLOAD mZL IF IL RF=0
- 23. 23 Distance protection operating principle With external fault resistance….. Still assuming IF>>IL we can write F L F F R L F R R R R R mZ Z Ι V I Ι V Ι V Z IR (1-m)ZL ZS VR VS ZLOAD mZL IF IL RF0
- 24. 24 Distance protection operating principle R jX LOAD ZL ZF=mZL+RF Fault with fault resistance mZL RF IR (1-m)ZL ZS VR VS ZLOAD mZL IF IL RF0
- 25. 25 Distance protection operating principle jX R LOAD ZL Long line - Load Encroachment
- 26. 26 Distance protection operating principle ZF=mZL+RF Fault resistance coverage – Short line R jX LOAD ZL
- 28. 28 Basic distance scheme Z1 A B Z2 Z3 Z1 Z2 Z3 OR Trip Z1 Z2 Z3 t2 t3
- 29. 29 Zone 1 set to 80% leaves 20% end zones Faults in end zone results in instantaneous tripping at one end and time delayed at the other Time delay may lead to system stability problems Sequential clearance leaves no dead time for high speed A/R cycle (transient fault becomes permanent) Longer clearance times - More damage Basic distance scheme disadvantages
- 30. 30 Auto Reclose Majority of the faults on the line are transient faults caused by lightning & temporary contact with external objects such as tree branches. Transient faults such as insulator flashover can be cleared by immediately tripping the breaker and will not recur when the line is energised. Auto reclose schemes are employed to carry out this duty automatically thereby minimizing interruption of power supply
- 31. 31 Zone 1 extension scheme Z1 A B Z2 Z3 Z1 Z2 Z3 Z1X Z1X OR Trip Z1 Z2 Z3 & Z1X A/R OR t2 t3
- 32. 32 No signalling channel required (may be used as temporary replacement for carrier aided scheme when communication channel goes out of service) Provides fast fault clearance at both ends for a transient fault anywhere along the line length Allows the use of high speed A/R cycle Zone 1 extension advantages
- 33. 33 Tripping can occur for external faults (but will be followed by an autoreclose) Basic distance scheme logic applies following reclose (i.e. potential for time delayed clearance for permanent faults) Only suitable to systems where autoreclose is used (for example can not be used on cable circuits) Zone 1 extension scheme disadvantages
- 34. 34 Direct transfer tripping Z1 A B Z2 Z3 Z1 Z2 Z3 OR Trip Z1 Z2 Z3 t2 t3 Trip Z1 Z2 Z3 t2 t3 Tx Rx Tx Rx OR End A End B
- 35. 35 Advantages All faults anywhere along the protected line can be cleared instantaneously at both line ends Scheme can be advantageous for protecting 3 terminal lines due to ease of application Disadvantages A very secure signalling channel is required :- incorrect operation leads to false tripping Circuit breakers at both line ends must be closed and contribute fault current to obtain high speed fault clearance If the channel fails only the basic scheme logic will be provided Direct transfer trip
- 36. 36 Permissive underreach scheme Z1 A B Z2 Z3 Z1 Z2 Z3 & OR Trip Z1 Z2 Z3 t2 t3 Trip Z1 Z2 Z3 t2 t3 Tx Rx Tx Rx End A End B & 100 100 OR
- 37. 37 Advantage Only a simplex signalling channel required Scheme is very secure as signalling channel only keyed for internal fault (Zone 1 initiation) Disadvantage If one terminal of the line is open then only Basic scheme logic will apply If there is a weak in-feed at one terminal then only Basic scheme logic will apply If signalling channel fails then only Basic scheme logic will apply Resistive coverage is governed by Zone 1 setting (may be limited on short lines) Permissive underreach scheme
- 38. 38 Permissive overreach scheme Z1 A B Z2 Z3 Z1 Z2 Z3 & OR Trip Z1 Z2 Z3 t2 t3 Trip Z1 Z2 Z3 t2 t3 Tx Rx Tx Rx End A End B & OR
- 39. 39 Permissive overreach scheme with circuit breaker echo logic Z1 A B Z2 Z1 Z2 Fault current in feed at one line end is insufficient to operate any distance Zone element or if one Circuit breaker is left open the fault clearance is delayed. To avoid this slow tripping the weak infeed relay will echo back the signal received immediately.
- 40. 40 Advantages Provides better resistive coverage, especially on short lines, where MHO measuring elements are used For cases where one line terminal is open, open breaker echo logic can be used For cases of weak or zero in-feed at one line terminal weak in- feed logic can be used (reverse looking zone required) Disadvantages Duplex signalling channel required Scheme is theoretically less secure then PUR as signalling channel is keyed for external faults If signalling channel fails then only Basic scheme logic will apply Permissive overreach scheme
- 41. 41 Practical blocking scheme Z1 A B Z2 Z3 Z1 Z2 Z3 Trip Z1 Z2 Z3 t2 t3 Tx Rx & Td & Tw Channel in service OR Trip Z1 Z2 Z3 t2 t3 Tx Rx & Td & Tw Channel in service OR
- 42. 42 Blocking Scheme Advantages Provides better resistive coverage than PUR on short lines where MHO elements are used Fast tripping will still be possible at closed end of line for all fault positions with remote breaker open or conductor snapping Fast tripping will still be possible at strong in-feed terminal for all fault positions where remote terminal has no or weak in-feed Disadvantages One zone has to be dedicated for blocking signal sending If signalling channel fails then only Basic scheme logic will apply Current sensitivity is lower as tripping elements (Z2) are controlled by high set current level detectors (to ensure blocking elements Z3 is designed more sensitive than tripping elements)
- 43. 43 Permissive less reliable - require a signal from remote relay plus local operation to trip Blocking less secure - require a signal from remote relay to prevent a trip Permissive schemes are marginally faster and more sensitive (timer plus high set current elements on Blocking scheme) Permissive schemes Vs Blocking schemes
- 44. 44 Switch On to Fault Provided for high speed Clearance of any fault detected immediately following manual closure of the Circuit breaker Enabled for 500ms after CB closure Provided only for manual closure Over current element used to trip the CB instantaneously for 500ms,where maintenace Earthswitch are left closed
- 45. 45 Thank you