Welcome T APR SE AIONON O E NT TINSULATION- CO-ORDINATION BY A.SAI PRASAD SARMA
INSULATION CO-ORDINATION• It is selection of suitable insulation levels of various components in any electrical system and their rational arrangement.• It is required to ensure3) Insulation shall withstand all normal stresses and majority of abnormal ones4) Efficient discharge of over voltages due to internal /external causes5) B/D shall be only due to external causes6) B/D shall be at such places where least damage is caused
Determination of Insulation coordination – contd. Steps in the determination of Insulation coordination• Determination of live Insulation• Selection of BIL and Insulation levels of other equipment• Selection of Lightning Arrestors.
Definition:- Flash over voltages• Dry flash over voltage (Dry for) Power frequency voltage. Which will cause flashover of the Insulation.• Wet flash over voltage:- Power frequency voltage. Which will cause flash- over when sprayed with water of a resistance 9000-11000 ohm-cms drawn from a source of supply at a temp within 10°c of the ambient temperature in the neighbour- hood of insulation under testing and directed at an angle of 45° the volume of water being equivalent to precipitation of 0.305 cm /min
Definition:- Flash over voltages Impulse flash over voltage:-• The voltage which will cause flash over of an Insulation When subjected to a 1.2x50µs impulse• (British standards1x50µ sec)• (American standards 1.5 x 40µsec)
Definition:- Flash over voltages• Basic Insulation level :- The crest voltage of standard wave that will not cause flashover of the insulation is referred to as “Basic insulation level” (Basic impulse insulation voltages are levels expressed in impulse crest voltage with a standard wave not longer than 1.2x50 µs (Indian standards) Equipment insulation as tested shall be equal or above the BIL
Impulse spark over volt- time characteristic• This characteristic is obtained by plotting --Time which elapses between the moment the voltage wave is applied and the moment of spark over -- on abscissa -Voltage at the movement of spark over (i) Occurring on the wave front (ii) Occurring on the wave peaks (iii) Crest of the voltage for spark over occurring on the wave tail
Impulse spark over volt- time characteristic -contd.• This characteristic is established by means of a 1/50 impulse wave• A line drawn meeting the three B/D values is the characteristic• Proper insulation co-ordination will ensure that the voltage time Curve of any equipment will lie above the volt -time curve of the protective equipment, say, Lightning arrestor.
LINE INSULATION• Extra high voltage line can be made lightning proof by2 Efficient shielding3 Low tower footing resistance equal to or less than 10 ohms shielding angleTransmission lines up to 220kV 30°400 kV at and above 20°
Line insulation -contd.• Line insulation shall be sufficient to prevent a flashover from the power- frequency over voltages and Switching Surges.• It shall take into consideration the local un favourable circumstances which decrease the flash over voltage (rain, dirt, Insulation pollution etc.,)
OVER VOLTAGE FACTORSLine Switching Power frequency flashVoltages Surge flash over (Dry & Wet) over220kV 6.5 V pn 0.3400kV 5.0 V pn 3.3Vpn = Phase to Neutral Voltage (rms)Add one or two more Insulators for each string.
OVER VOLTAGE FACTORS— Contd.-To take care of one disc in the string becoming defective. -Facilitate hot line maintenance Up to 220 kV Line – 1 disc for each string 400 kV Line – 2 discs for each string
FLASH OVER VOLATAGE(FOV) OF DISCS 254 X 145 mmNO DRY FOV WET FOV ImpulseOF ( kV rms) FOVDISCS (Standard full wave)9 540 375 86010 590 415 94514 785 565 126515 830 600 134525 1280 900 2145
RECOMMENDED INSULATION LEVEL OF LINENormal Vpn Switching over No ofsystem In kV volt. (Wet) kV * discsVoltage (Vph/√3) required132kV 76 76 x6.5=495 5220kV 127 127x6.5=825 9400kV 231 231x5=1755 13* Compared with Impulse FOV (Value)
RECOMMENDED INSULATION LEVEL OF LINE—contd.Normal Vpn Power freq. No. No. of As persystem In kV over volt of discs practiceVoltage (wet) discs recom. (kVrms) req.132kV 76 76x3=228 6 7 9/10220kV 127 127x3=381 10 11 13/14400kV 231 231x3=762 20 22 23/24
• Tower forting resistance 10ohms• severest lightning discharge 50kA (rms)• Impulse strength of Insulation=√2x50x10³x10=700kV• As per the table for 7 discs, the impulse FOV ( kVp =695kVp)• For better performance tower forting resistance shall be brought down.• For 132kV best is 7 ohms
Co-ordination of line Insulation and Sub-Station Insulation• Line Insulation is not directly related to the Insulation of equipment within the Sub-Station.• Impulse flash over voltage of line Insulation determine the highest surge voltage that can travel into the sub-station.• Current through lighting arrestor can be calculated from4 Surge impudence of line5 Surge voltage arriving over the line
Co-ordination of line Insulation and Sub-Station Insulation• Discharge voltage of the LA on that current is the basic protective level of the substation equipment.• Discharge voltage across LA varies with surge current.
BASIC INSULATION LEVEL AS PER IS (2165 – 1962)Nominal Highest Impulse withstand One minute powersystem system volt kVp for test frequent volt kV (rms)volt kV volt kV Full Reduced Full Reduced(rms) (rms) insulation insulation insulation insulation132 kV 145 650 550 275 230220 kV 245 1050 900 460 395400 kV 420 1550 680 1425 630Reduced insulation is used where system is effectively earthed.
INSULATION LEVELS OF EQUIPMENT• Transformers, Isolators, Instrument Transformers are manufactured for the standard Insulation level.• Some times transformers, are manufactured for one step lower insulation level for the sake of economy. (LAs will be designed for a still lower level)• Where LAs are provided right on the top of the transformer, some of the equipment may lie well out side the protective zone of the LA.
INSULATION LEVELS OF EQUIPMENT• Protective zone is determined based onA With stand level of equipmentB Discharge volt of LAC Distance between LA and equipment.• Such equipment shall be designed for one step higher Bill.• Generally BILL of substation equipment other than transformer are designed for10% higher BIL than that of Transformer .
INSULATION LEVELS OF EQUIPMENT• BIL of Open poles of a disconnect switch shall be 10 to 15% higher than that provided between poles and earth.
• EHV system must be designed to operate under stresses associated not only with normal operating power frequency voltage but also those caused by transient over voltage.• These transient over voltage rise principally from lightning over voltage and switching operations• The former is predominant in system at 100 kV and below.• Switching over voltage are of concern in system at 220 kV and above
INSULATION CO-ORDINATION Over Voltage• Let Un = line to line normal RMS voltage• Let Um = Rated highest system voltage rms line to line• √2 Un / √ 3 = Peak of rms voltage phase to ground for nominal system voltage• √2 Um / √ 3 = Peak of rms voltage phase to ground voltage for highest system voltage• Any voltage higher than √ 2/ √ 3 Um is called over voltage
Over voltages• In addition, temporary over voltages also occur at power and harmonic frequencies at times for considerable time under certain conditions.• The insulation strength and characteristics of various components of a system (including those of voltage limiting devices) must be selected relating to those stresses. i. To reduce frequency of supply interruptions ii. To reduce component failures• The selected level of voltage shall be low enough to be operationally and economically acceptable
• IEC 71 covers “ Insulation Co-ordination”• IEC -71- Part-I definition, principles• IEC 71- Part – II Guidance for selection of rules (i) electric strength of the plant, (ii) electric strength of LAs or protective spark gapsIEC 71-3 • Phase to phase insulation co-ordination • Complimentary to part I & II • Standard phase to phase insulation level for voltages up to and above 300 kV • Voltage stresses In service and clearances in air
Data required:2. Field data on lightning induced and switching surges appearing on the system3. Establishing insulation strength of various insulating components of the system through lab tests
Causes of over voltage:• Phase to earth faults ( it is assumed that resulting temporary voltages will not exceed –1.4 Pu for solidly earthed networks –1.7 Pu for resistance earthed networks –2.0 Pu for reactance earthed networks• Load rejection (supplying capacitive current through a large inductive reactance ex. A smaller generator connected to a long cable or over head line)• Ferro resonance ( inter change of stored energy for series or parallel combination of inductive and capacitive reactance)
Causes of over voltage: contd.• Ferranti effect: (receiving end voltage greater than sending end voltage under no load or light load conditions)• By care full design and natural earthing sustained over voltages involving resonance and arcing ground faults are eliminated• Below 145 kV method of earthing will normally determine the level temporary over voltages.
Switching surges• They are of short duration and irregular form• Typical switching impulse standard form is the 250/2500 sec. ( time to crest/ time to half value way)• The magnitude of internally operated switching surges is related to the system operating voltage• In a system where CBS are not subjected to multi re striking the switching surges will rarely exceed 3 pu• 2.5 pu would be typical maximum based on which the discharge duty of LA is assessed• However in systems above 300 kV, it may be necessary to suppress maximum switching surges to 2 pu or less by the installation of a shunt reactor and/or closing resistors on the circuit breakers
Resonance effects• For voltage level below 300 kV. Resonance effects occur i. When switching transformer ii. When switching cable and overhead line combination iii. Between lumped capacitive and reactive elements and over head lines iv. Charging long lines without shunt reactor compensation
Resonance effects-- contd• Ferro resonance encountered on a transformer feeder greater than 5 to 10 Km in length• When one feeder/transformer on a double circuit is switched out but parallel feeder remains energized, the dead circuit draws energy by captive coupling from the parallel line circuit which resonates with transformer impedance at a sub harmonic frequency• (operation procedure such as opening the line isolator at the transformer end on the disconnected circuit will eliminate the problem)
Mode of action of flash over on a line• A lightning flash can impress over voltage on a over head line by a) Induction when it discharges to earth close to line b) By direct contact on the line either to the earthed structure or to the phase conductor
Induced Voltage Surge– A close flash to ground up to about 14 m away can induce a voltage rise on phase conductors– The highest amplitude normally associated is in the region of 200 kV– Significant in case of low voltage lines– At 11 kV estimated that it accounts for some 90% of all faults– Little significance on lines of 275 kV and above
Direct stroke• A direct stroke can be to the earthed tower top or on phase conductor• Stroke on earthed lower top, for transmission of shielded design, is innocuous• Raise in potential caused by passage of current through tower impedance to earth will be less than with stand strength of line
Direct stroke—contd.• However the rise in potential can be severe and exceed with stand capability, if – Tower footing resistance is high – Rate of rise of current exceeds a certain level• Flashover may occur• Through the system voltage, losses is the frequency of flash over
Direct stroke—contd.• Direct stroke on phase conductor• May occur if there is a shielding failure i.e. stroke avoids earth wire and lands on line conductor.• Discharge current flows equally in both directions.• Impedance to earth is half the surge impedance (Z0) of the conductor. IN a 400 kV line Z0 = 175 ohms• Voltage rise is sufficient to cause failure of line insulation• Minimum critical current for flash over Ic = 2 V I0 Z0 VI0 = minimum flash over voltage for 1/50 Wave• At flash over the impedance through which the discharge current flows drops abruptly from Z0/2 to impedance of tower, x -arm, tower footing
Surge propagation:• Surge waves are propagated at the velocity of light along the conductor• On arrival at substation, equipment there in get stressed.• Rod gaps and surge arrestors provide necessary protection• Waves are subjected to considerable attenuations due to losses both in the conductor (ohmic losses) and corona losses
Lightning discharges• Clarification of lightning discharges stroke (A) stroke (B) Stroke (A) : produced by the charged cloud which induces a charge on the stationery objects such as high buildings etc.• Charge distribution causes concentration of potential at the top most point• Electro static stress being great at that point ionization of surrounding atmosphere takes place• Dielectric strength of surrounding air decreases giving an easy path to lightning stroke.• Decrease in dielectric strength of surrounding air takes considerable time
Lightning dischargesStroke B:• A, B & C are three clouds with A and C positively charged and B negatively charged• When there is a stroke between (A) and (B) the charge on (C) becomes free and immediately and indiscriminately strikes on any object on the ground• For stroke (B) there is no time lag• Stroke (B) may completely ignore highest building and strike bare ground.• No protection can be arranged against stroke `B`• Stroke `A` can be made safe by channelising the charge through a lightning conductor placed on the top of the building
Static induced charges• An over head conductor accumulates statically induced charge when a charged cloud comes above • When the cloud is swept away charge on the conductor is released • The charge travels on either side giving rise to two travelling waves • The earth wire does not prevent such surges
Lightning strokes• Over voltage due to lightning strokes surge impedance of the line = Zs Discharge current = Id Over voltage due to direct stroke = Vd = Id x Zs However current travels in both directions over voltage = Vd = Id x Zs 2 when lightning strikes over earth wire or a towerOver voltage = Id x Ze + Lc di dt Ze = impedance of earth wire Lc is the inductance of the line conductor
Protection against lightning1. Protection of transmission lines from direct strokes2. Protection of power stations and substations from direct strokes3. Protection of electrical equipment from traveling waves
Protection of transmission linesAgainst the direct strokes :• Most harmful• Effective protection required shielding to prevent lightning from striking the electrical conductors.• There shall be adequate drain facilities so that the charge can be grounded without affecting Insulators or line conductors.
Design of transmission line against lightning• Design shall consists of (a) General wire of adequate mechanical strength to provide shielding for line conductor. They shall also be non –corrosive Resistance of ground wire shall be low for better protection against direct stroke.(b) Adequate clearance between 1. Line conductor and tower 2. Line conductor and earth 3. Clearance between line conductor and ground wire all through the span including mid Span or point of lowest sag.(c) Tower footing resistance shall be low (d) Angle of protection (shielding angle) angle between the normal passing through the ground wire and line joining the supported center points of outer conductor and ground wire. It shall be 30° for 132 & 220 kV lines 20 ° for 400 kV lines
Effect of number of earth wires• In the absence of a ground wire:• When there is a charge cloud over a transmission line without any ground wire• There will be two capacitances (1) Between cloud and conductor C2 (2) Between conductor and earth C1 Induced voltage on the line V L1 = C1 x Ec C1+C2• When ground wire is present it increases capacitance between conductor and earth i.e. C1 Decreases induced voltage on the line.• It is observed that presence of a ground wire reduces induced voltage on line to half.• For two ground wires the induced voltage comes down to one third• Presence of two ground wires also provides better shielding
Earth wires• Disadvantages with ground wire: (a) higher line cost (b) Probable direct shorting between line conductor and ground wire when the later gets cut In 400kV system transmission line towers will have twu earth wires.
Alternative method of line protection• Even after providing ground and reducing the likely induced voltages, harmful voltages can still develop• Lightning arrestors act as additional protective devisees by by-passing the surges to ground• Protector tube is a fiber tube with electrode at earth end.• Fitted directly below the conductor• The arc type electrode on the top of the tube forms a series gap with conductor
Alternative method of line protection• The lower electrode is solidly grounded• In case of surge on the conductor, an arc develops between conductor and top electrode of the tube.• Arc shifts within the tube and vaporises some of the fiber of tube wall to emit gases which will quench the arc• This tube successfully prevents re-striking• The break down voltage of tube shall be less than flash over voltage of the insulation.
Protection against traveling wavesThe traveling waves cause the following damages: i. High peak voltage of surge may cause flash over in the internal winding or external flashover between the terminals of the equipment. ii. steep wave front may cause internal flash over between turns of the transformer iii. Steep wave front resulting into resonance and high voltage may cause internal or external flash over causing building up of oscillations in the equipment• Protective equipment : LAs and Surge diverters• They are connected between line and earth
Action of the Surge diverter• A traveling wave reaches surge diverter and attains a prefixed voltage• A spark is formed across the gap• The diversion provides a low impedance path to earth• The surge impedance of the line limits the amplitude of the current flowing to earth to prevent break down of insulation• Important aspect is that the surge diverter shall provide low impedance path to earth only when traveling surge reaches the surge diverters
Action of the Surge diverter• It shall absorb any current during normal operation for over voltage surges.• It means that it shall not function at power frequencies but function only when abnormal frequencies are applied• When there is a discharge through them they shall be capable of carrying the discharge current for some time interval.• After the over voltage discharge it must be capable interrupting normal frequency current from flowing to earth as soon as the voltage reaches below the break down value
Switching over voltage protection in a substation• Operation of breakers causes transient over voltages• Over voltage value varying between 1.1 Pu to 6 Pu based on switching duty and the type of circuit breaker• Over voltage occurs mainly due to exchange of energy between system inductance ½ LI2 and system capacitance ½ CV2• Over voltage occurs during the opening of circuits and closing of long EHV lines• Most severe over voltages occurs during the closing unloaded transmission line• Preventive measure – Provision of Pre insertion resistors ( 400 to 800 ohms per phase)• Simultaneous closing of lines at both ends• Using shunt reactors, surge arresters etc.
Switching Over voltages in SubstationsSwitching duty of Applications and PhenomenaC.B. Remedial ActionsOpening of capacitor Switching of shunt Re strike in circuitbank currents, cable capacitor banks used for breakers giving overcharging circuits, filter p.f. correction. voltage.banks - Use of re strike free C.B. for capacitor switching duty.EHV lines * Long EHV transmission. Traveling waves* Closing unloaded - Use of pre-closing travel to and frolines resistors with circuit giving rise to a breakers. Use of lightning switching surge.* Closing chargedlines arresters. Use of shunt reactors in transmission* Auto re closing ofC.B. lines.
Methods of Reducing Switching Over VoltagesSwitching operation Method to reducecausing over voltage switching over voltageEnergising an uncharged High voltage shunt reactorsline are connected to line to reduce power frequency over voltages.Elimination of trapped Line shunting after openingcharged on the line by means of earthing switchReduction of current Opening resistorschopping ( Resistance switching with CB) used only with ABCB
Methods of Reducing Switching Over VoltagesSwitching operation Method to reducecausing over voltage switching over voltageReducing the switching over Single stage pre closing resistorvoltages due to closing insertion with CB. Two stage pre closing resistor insertion with CB. Closing resistors in between circuit breaker and shunt reactorReducing switching over voltages Synchronous switching of threeby improved switching sequence poles. Simultaneous operation of circuit breakers at both ends of line,Use of surge arrestors While closing of line While disconnecting reactor
Rod gaps or coordinating gaps• They are used on insulators, equipment and bushings• Conducting rods are provided between line terminal and earth terminal with an adjustable gap ( Air insulation)• Rods are of 12mm dia approx.• The gap is adjusted to break down at about 20% below the flash over voltage of the insulation.• Spark over causes dead Short circuit• Voltage of phase with respect to ground falls very low• The rod gaps are no more used consequent to development of surge arrestors.
Over-voltage in Network and RemediesPhenomena Causes Effect RemediesSurges Lightning strokes on Line insulation flash -Use of Ground overhead lines or over or puncture. wire substation The traveling wave - Surge Diverters reaches substations. -Earthing of The insulation of towers equipment is -Lightning Masts stressed by impulse surgeSwitching Breaking inductive circuit, Wave travels from -Use of openingsurges the energy stored C.B. to both sides resistors with C.B. inductance gives rise a Transmission line - Use of restrike voltage rise across insulator, stressed. free C.B. capacitor. Terminal apparatus -Use pre-insertion Switching of capacitive, insulation stressed resistors with C.B. line charging currents give rise to a over voltage due to restrike. Closing of EHV lines
Over-voltage in Network and RemediesPhenomena Causes Effect RemediesResonance The fault causing Very high, voltage Filters to resonance between surges occur. eliminate inductance and Insulation failure harmonics capacitance in a part of likely to occur. the circuitTraveling High voltage waves get Reflected waves -Properwaves reflected – on reaching gets superimposed switching a junction or end. for initial wave. sequence. Voltage may rise to several time the normal voltage.Sustained Poor voltage control Failure of -Proper VoltagePower transformers and controlfrequency Rotating Machinesovervoltage
Protective Devices Against Lightning Over voltages Device Where applied RemarksRod gaps Across insulator string, -Difficult to coordinate bushing insulators -Create dead short circuit -CheapOverhead Ground -Above overhead lines -Provide effectiveWires (earthed) -Above the substation protection against area direct strokes on line conductors towers sub station equipmentVertical Masts in -- in sub stations -instead of providingsubstations overhead shielding wiresLightning Masts/Rods - Above tall buildings Protect buildings(earthed) against direct strokes. Angle of Protection œ = 300
Protective Devices Against Lightning Over voltages Device Where applied RemarksSurge Arresters -- on incoming lines in -- Diverts over voltage to each substation earth without causing -Near terminals of short circuit Transformers and -Used at every voltage generators level in every sub- -Near motor and station and for each line. generators terminalsSurge Absorbers -- near rotating machines -Resistance connected between phase Capacitance and ground Combination absorbs the over voltage surge and reduces steepness of wave
Lightning arrester selection• 1. To determine the magnitude of the power frequency phase to ground voltage expected at the proposed arrester location during phase to ground fault, or other abnormal conditions which cause higher voltages to ground than normal.• 2. To make a tentative selection of the power frequency voltage rating of the arrester. This selection may have to be reconsidered after step (6) is completed.• 3. To select the impulse current likely to be discharged through the arrester.• 4. To determine the maximum arrester discharge voltage for the impulse current and type of arrester selected.• 5. To establish the full-wave impulse voltage withstand level of the equipment to be protected.• 6. To make certain that the maximum arrester discharge voltage is below the full wave impulse, withstand level of the equipment insulation to be protected, by adequate margin.• 7. To establish the separation limit between the arrester and the equipment to be protected.
Types of Earthing• For purpose of selection of voltage rating of a LA three types of earthing are considered(I) Effective earthed system: a system is effectively earthed if under any fault condition the line to earth voltages of healthy phases do not exceed 80 % of the system line to line voltage• If in a system all transformers have star connected winding with neutrally solidly earthed then the system is effectively earthed• However if only few transformers are earthed like that, it is not effectively earthed system
Types of Earthing - conted.(II) Non effectively earthed system: a) if the line to earth voltage in healthy phases in case of a fault exceed 80% of the line to line voltage but does not exceed 100% of it, the system is called non effectively earthed system b) System with few solidly earthed neutrals c) Systems with neutral Earthed through resistors or reactors of low ohmic value or arc suppression coil(III) Isolated or un earthed neutral systems :- system neutrals are not earthed. Line to earth voltage of healthy phases exceed 100% of the line to line voltage.
Selection of lightening arrestors• Tentative selection of arrestor Voltage:• Arrestor Voltage rating shall not be less than product of system highest voltage x co-efficient of earthing• Co-efficient of earthing : – Effectively earthed system – 80% – Non effectively earthed system - 100 % and isolated earth system
Selection of lightening arrestors• In a 220 kV effectively earthed system – Highest system voltage = 245 kV – Co-efficient of earthing = 80% – Arrestor voltage rating >= 245x0.8 = 196 kV – As per IS 3070 (part –I) 1965 the rating is 198 kV• By going for a higher voltage rating for a surge arrestor, the degree of protection for equipment gets reduced.
Selection of arrestor discharge current• This can be calculated from (a) Spark over voltage of transmission line insulation (b) Surge impedance of the line (c) Residual discharge voltage of LA Ia = 2E- Ea Z Ia = Arrestor discharge current E = Magnitude of incoming surge voltage Ea = Residual discharge voltage of an arrestor Z = Surge impedance of the line
Selection of arrestor discharge current• In a 220 kV system using 11 insulators Transmission line will not permit a traveling wave of a value more than 1025 kVp• As per IS 3010 (Part 1) -1965 the residual voltages of LA at a discharge current of 10kA is 649 kV.• Considering the surge impedance as 450 ohms• Maximum value of discharge current of LA = 2(1025000)-649000 = 3100 Amps 450• The LAs normally in 200 kV system have a discharge current rating of 10 kA.
Selection of arrestor discharge Voltage• Most important characteristic of LA determining the protection level being offered• The arrestor discharge voltage shall be less than BIL of equipment for effective protection• Discharge voltage depends on (I) discharge current (II) rate of rise of current applied (III) Wave shape of current applied• Discharge voltage of LA increases with discharge current. But increase is much restricted due to non –linear resistance property.• Increase in discharge from 5 kA to 20 kA produces only 25% rise in discharge voltage.• Increase in rate of current from 1000 to 5000 Amps per micro second increases discharge voltage by only 35%.
Protective margin of LA• Protective margin of LA = BIL of the equipment--- maximum discharge voltage of LA• While determining protection level offered by a LA 10% allowances towards drop in lead length and manufacturing tolerance shall be allowed.• Protective margin shall be 20% of the BIL of the equipment when closely located• In a 220 kV systemDischarge voltage of LA = 649 kVAllowing 10 % margin protection level = 713 kVBIL of equipment = 900 kVpProtection margin = 900-713 = 187 kVp There is more than 20 % of the BIL of 180 kV
Protective margin of LA-Continue.• In American system Average discharge voltage x 1.25 +40 kV = BIL protected When adequate margin is not available LAs with lower rating shall be chosen taking risk.
Insulation Co-ordination Scheme• For 220 KV system.• L.A. Voltage rating=system highest voltage x co-efficient of earthing =245x.8=196Kv.• Selecting standard rating from Table 12.1 column 1,L.A. voltage rating=198 Kv• Discharge current rating= 10KA (assumed)• Residual voltage, from column 3 of table 12.1,=649Kv (peak)• Protection level of the L.A. =649x1.1=714Kv• For a margin of 20% between the B.I.L. and the protection level of L.A., the B.I.L. should be =714x1.2=857Kv.• Choose standard B.I.L. Table 14.3 (b) Col. 4=900 Kv,• The corresponding power freq. I minute test voltage =395kv• Switching surge flashover voltage =220 x6.5=825kv• √3• Check it is less than B.I.L. of 900kv.• Power frequency over voltage=220x3=228kv rms √3• This is less than 395kv.• B.I.L. of CBs, instrument transformer, disconnect switches etc,.=900x1.1=990kv.• Choose standard B.I.L.=1175kv.
The L.A. voltage ratingRated system Highest system Arrester ratingvoltage KV voltage KV in KV132 145 120/132220 245 198/216400 420 336
Establishment of Separation Limit• When arrestor are to be located away from equipment.• A traveling wave coming into the station to location to the discharge voltage of the arrestor.• Proximity to transformer or breakers. - Transformer is most expensive price. - Repair to transformer is costly and with higher revenue loss. - Transformers are always at the end of a circuit where voltage regulation.. For circuit breakers and disconnecting switches flash over distance between terminals when in open position in grater than between terminals and ground.. Surge in excess to insulation strength will flash over to ground with out damaging the equipment.. At best there can be only outage .. By reducing BIL of transformer savings in the cost of insulation can be obtained.. Not possible incase of CB or disconnections switches.. Hence a set of LAS shall be closer to transformers.
Location of Lightning Arresters:• The electrical circuit length between L.A. and the transformer bushing terminal (inclusive of lead length in metes for effectively earthed) should not exceed the limits given below: Rated syst. BIL KV Max. voltage KV Peak distance 132kV 550 35.0 650 45.0 220kV 900/1050 Closer 400kV 1425/1550 to Trans.