Types of faults in the transmission system0Short circuit faults Frequency0Phase – Ground faults 85%0Phase- Phase faults 8%0Phase – Phase –Ground faults 5%03 Ph faults 2%0Open circuit faults0Broken conductor0Open jumper
Protection Scheme0Protection Scheme for Transmission lines as per CBIP guidelines• Should have two independent high speed main protectionschemes• Two stage over voltage protection• Sensitive IDMT directional E/F relays• Auto reclose relay suitable for 1 ph/3ph (with deadlinecharging and synchro check) reclosure.0Types of main Protections:• Over Current Protection.• Differential or Phase Comparison or Unit Protection.• Distance Protection.
0Requirements of distance protection:• Shall have min. of three independent zones with directional characteristics.• Shall be non switched type with separate measurement for both earth faults and phase faults• Capable of 1phase and 3 phase tripping.• Capable of operation for close up faults and switch on to faults• Accuracy of better than 5% of reach setting for Zone 1, 10% for Zone-2 &3.• Shall have variable residual compensation.• Shall include power swing detection feature for selectively blocking.• Shall include fuse failure feature to monitor all types of fuse failures and block distance protection.• Max. operating time under given conditions shall be as follows 1. SIR: 4, Setting: 10Ohms, Fault location : 60% Trip contacts: 4, Fault resistance: 0 Max.Operating time including trip relays: 45ms 2. SIR: 15, Setting: 2 Ohms, Fault location : 60% Trip contacts: 4, Fault resistance: 0; Max.Operating time including trip relays: 45 ms (3ph fault), 60ms (others)
Over Current Protection E ZS ZL X X X X X If If = E/(ZS+ZL)The reach of over current relay is function of SourceImpedance which varies considerably, making it difficultto get fast and Selective tripping .
Phase Comparison Protection0 Current Phase comparison type0 Suitable for operation with PLCC0 High sensitivity and selectivity for all types of faults0 Capable of single and three pole tripping.0 Un effected By:0 Heavy load transfer0 Power swings0 CT saturation0 CT Phase errors0
Distance Protection0Type of distance relays0Reactance0 Suitable for short lines0 Not effected by fault resistance0 Effected by power swings0 Non directional0Impedance0 Suitable for medium lines0 Non directional0 Effected by fault resistance0Mho0 Directional0 Least effected by power swings0 Less effected by fault resistance
Impedance Relay Characteristics X Z3 Z2 Z1 Load R Area
MHO relay characteristicThe characteristic of a mho impedance element , when plottedon a R/X diagram, is a circle whose circumference pass throughthe origin . X Ψ R Ψ = relay characteristic angle
OFF set MHO characteristicUnder close up faults, when the voltage is near to zero thenMHO will not operate. The mho characteristic can be shiftedtowards origin for operation of close up faults. This is knowas OFF set MHO. X Ψ R Ψ = relay characteristic angle
Lenticular characteristicsThe characteristic of lenticular mho will be useful toprovide maximum load transfer condition withmaximum fault resistance coverage. Z-3 Z-2 Z-1 Ψ Load R Z-3 Ψ = relay characteristic angle
Quadrilateral characteristic It is a basically a reactance relay superseded with controlled resistive reach. Z-3 Z-2 Z-1 Ψ Z-1&2Z-3 Ψ = relay characteristic angle
Zones of Distance Protection: Z3 Z2 Z1 X X X X X X BASIC SETTING PHILOSOPHYZONE –1 : 80 % of protected lineZONE –2 : 100 % of protected line + 20 % of shortest adj. line section or 100% + 50% of transformer impedanceZONE –3 : 100% of protected line + 100 % of longest adj. line or 100 % + 100% of transformer impedance.ZONE -4 : To cover close up back-up non-directional faults generally reverse reach will be provided in relays (10%).
erms associated with distance protectio each:each is the impedance of the tr. line up to which thestance relay protects the line from the faults.ver reachelay measured impedance less than the actual faultmpedance. i.e effective reach of the relay increases nder reachelay measured impedance is more than actual impedance.e. Effective reach of the relay decreases
Parallel Compensation0Necessity of parallel compensation:For the fault on the parallel line, fault current also fed from healthy line and0 this current pass through ground. This current changes the mutual0 inductance and in turn causes relay measuring impedance to increase and0 is more than actual fault impedance.This effect will be compensated by connecting neutral current of the line to0 parallel line.This compensation will not work, if the parallel line neutral current is more0 than line neutral current.
ZONE - I CHARACTERISTICSShould isolate the faulted section instantaneouslyShould cover protected circuit &fault resistance with somemargin to take care of errors in measurementSelective phase tripping for 1ph to ground faults forauto reclose of the breaker.Fast operation with secured directional discrimination.Should be stable in the presence of:13. Fault resistance, arc resistance and tower footing resistance14. Power swings15. Heavily loaded condition16. In feed condition17. Errors in CT, CVT and CT saturation18. Transients in CT & CVT
ZONE-II CHARACTERISTICSDelayed tripping and non selective phase tripping.Provide back up protection for part of adjacent line.Trip the faulty line instantaneously using carrier aided tripping.Time delay is normally 500ms ZONE-III CHARACTERISTICSThis provides back up protection for the adjacent lines ortransformerTime delay is normally 1500ms ZONE –IV CHARACTERISTICSThis provides back up protection for the station faults It is normally in the reverse directionTime delay is normally 1500ms
Distance Schemes:1 . P . U . R -- Permissive under reach scheme3. P . O . R -- Permissive Over Reach scheme3. BLOCKING SCHEME4. WEAK END FEED
CARRIER SCHEMES - P U R Z2A Z1A Fault A Z1B Z2B B CARRIERRELAY RELAY CHANNEL Trip = Z1 + Z2.CR+Z3.T3+Z2.T2 CS = Z1 Under reaching zone sends carrier signal
CARRIER SCHEMES - P O R Z2A Z1A Fault A Z1B Z2B BRELAY CARRIER RELAYTrip = Z1+Z2.CR+ Z2.T2+Z3.T3CS = Z2Over reaching zone sends carrier signal
WEAK END FEEDIt is a condition which occurs on a line when no current infeedfrom the line terminal or when the current infeed is low dueto weak Generation behind protection.If the fault current infeed is too low to operate the protectionat the week end, the following might occur, depending onThe selected communication scheme.In permissive schemes or blocking schemes CB at week infeedend may fail to trip instantaneously, due to no relay operationin the weak end.In permissive over reach scheme, if the fault is in Zone 2 fromstronger end, both CBs may fail to trip instantaneously due tono relay operation in the weak end.
WEAK END FEED LOGIC IN P.O.R Z2A Weak Z1A Fault End A Z1B Z2B B RELAY CARRIER RELAYFault in Z2A+UV relay + Receipt Fault in Z1B= Trip CB BOf echo signal etc Fault in Z2B=Sends Echo= Trip breaker A + Send echo signal signal
AUTORECLOSE – PHILOSOPHYNEED FOR AUTO RECLOSE3. REDUCING OUTAGE TIME4. IMPROVED RELIABILITY5. RESTORATION OF NETWORK STABILITY AND SYNCHRONISMTYPES OF FAULTS10. TRANSIENT FAULTS11. SEMI PERMANENT FAULTS12. PERMANENT FAULTS
TRANSIENT FAULTS -CHARACTERISTICChracterised by disappearnance after Short dead time and aredisapper without any action being taken.TYPES OF TRANSIENT FAULTS8. Lightning strokes resulting in fashovers9. Conductor swinging due to high winds10. Bird fault11. Temporary contact with foreign objects like tree etc.About 85 % of faults on transmission lines are transient in nature
SEMI PERMANENT FAULTSThis type of faults requires more than one De energisedinterval before it disappears. Such faults are prevalent onEHV lines traversing forest.An example is a tree falling on the line and getting burntup by the arc when the line is re energised.10% of the reclosures are successful with second shot.However this can cause unnecessary wear on EHV CBs.Therefore second shot is not recommended for EHV Systems.
DEAD TIME :The time between the autoreclose scheme being energisedand the operation of the contacts which energise the CB closingCircuit.RECLAIM TIME :The time following a successful closing operation measuredfrom the instant the A/R relay closing contacts make, whichmust elapse before the autoreclose relay will initiate reclosingsequence in the event of a further fault.
CHOICE OF RECLAIM TIMEThe reclaim time must not be set to such a low value thatthe intended operating cycle of the breaker is exceededwhen two fault incidents occurs close together.for example the reclaim time for a air blast circuit breakersmust allow time for air pressure to recover to its normalvalue.CHOICE OF DEAD TIMEDead time for EHV system lower limit is decided byde-ionising time, upper limit is decided by transient stabilityand synchronism
Power Swing0Power Swings are disturbances in system due to various reasons0such as sudden load throw, bad synchronization etc0Power swings are characterized by slow power flow oscillations,0resulting in swinging of voltages and currents, resulting in0operating point movement into distance relay characteristics,0in turn can cause tripping of distance relays.0Tripping during power swings is undesirable since no actual fault0is present and moreover a line outage during power swing may0cause further deterioration to system stability.0Detection of power swing will block the distance protection0Zones 2,3,4. Normally tripping in Zone-I is not blocked even afterdetection of power swing.
Power Swing detection X Z3 Z2 Power swing detection Z1 zones RTime taken by fault locus to cross the power swingdetection zones is more than 40-50ms, then it is calledpower swing.
Fuse Failure Function0Asymmetrical measuring voltage failure:Substantial asymmetry of measured voltage, while the measured0 currents are in symmetry indicates fuse failAsymmetry of voltage detected by 3Uo or U2 > thresholdSymmetry in current detected by 3Io or I2 < thresholdDuring blocking of distance protection by fuse fail, the distance0 protection switched to emergency over current function automatically.0 If the asymmetry in measured current is detected during blocking by FF0 function, then FF block will released.
Switch on to fault0This feature provide protection against energisation of the tr.line with fault or dead short.0Distance protection will not provide protection in0this case as voltage is not available for distance measurement.0It can be activated by TNC switch or CB aux. binary input orinternal detection of current rise.0It provides instantaneous 3Ph trip and blocks auto reclose.
Stub Protection Bus-2 Bus-121 Ckt-1 Ckt-2 One and Half Breaker Scheme
DEF Protection0It provides back up protection for tr. line.0It provides reliable protection for high resistance earthfaults.0It uses cross polarized voltage for directionaldiscrimination.
Local breaker back up protection0It is the secondary protection0To provide back up isolation during failure of breakerto open.0It opens source to that breaker (i.e other end breakers,bus bar, etc.)0It will be triggered by operation of any primaryprotection (like distance, DEF, bus bar, etc..)0It sends direct trip command to other end.
Direct trip Scheme0 It is required to trip other end breakers without any0 checking the status at other end during following0 conditions:• Operation of over voltage protection.• Operation of bus bar with tie breaker open.• Manual tripping of both the breakers (main&Tie)• Operation of LBBOn receipt of command through PLCC at other endbreakers will trip directly.
Over Voltage Protection0It will have 2 stages0Stage-I:0Setting: 110%0Time delay: 5 Sec.0Stage –II0Setting: 140%0Time delay: Instantaneous.