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TO A PRESENTATION ON

INSULATION- CO-ORDINATIONBYA.SAI PRASAD SARMA

INSULATION CO-ORDINATIONIt is selection of suitable insulation levels of various components in any electrical system and their rational arrangement.It is required to ensureInsulation shall withstand all normal stresses and majority of abnormal onesEfficient discharge of over voltages due to internal /external causes B/D shall be only due to external causesB/D shall be at such places where least damage is causedSafety of operating personnel and public.

Determination of Insulation coordination contd. Steps in the determination of Insulation coordinationDetermination of live InsulationSelection of BIL and Insulation levels of other equipmentSelection 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 10c 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 1.2x50s impulse (British standards1x50 sec)(American standards 1.5 x 40sec)

Definition:- Flash over voltagesBasic 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 s (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 waveA line drawn meeting the three B/D values is the characteristicProper 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 INSULATIONExtra high voltage line can be made lightning proof byEfficient shielding Low tower footing resistance equal to or less than 10 ohmsshielding angleTransmission lines up to 220kV 30400 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 FACTORS

OVER VOLTAGE FACTORSContd. -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 mm

RECOMMENDED INSULATION LEVEL OF LINE

RECOMMENDED INSULATION LEVEL OF LINEcontd.

Tower forting resistance 10ohms severest lightning discharge 50kA (rms)Impulse strength of Insulation=2x50x10x10=700kVAs 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 InsulationLine 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 from Surge impudence of lineSurge voltage arriving over the line

Co-ordination of line Insulation and Sub-Station InsulationDischarge 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)

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 EQUIPMENTProtective zone is determined based onAWith stand level of equipmentBDischarge volt of LACDistance 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 operationsThe 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-ORDINATIONOver VoltageLet Un = line to line normal RMS voltageLet Um = Rated highest system voltage rms line to line2 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 voltageAny voltage higher than 2/ 3 Um is called over voltage

Over voltagesIn 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.To reduce frequency of supply interruptionsTo reduce component failures The selected level of voltage shall be low enough to be operationally and economically acceptable

IEC 71 covers Insulation Co-ordinationIEC -71- Part-I definition, principlesIEC 71- Part II Guidance for selection of rules (i) electric strength of the plant, (ii) electric strength of LAs or protective spark gapsIEC 71-3Phase to phase insulation co-ordinationComplimentary to part I & IIStandard phase to phase insulation level for voltages up to and above 300 kVVoltage stresses In service and clearances in air

Data required:Field data on lightning induced and switching surges appearing on the systemEstablishing 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 exceed1.4 Pu for solidly earthed networks1.7 Pu for resistance earthed networks2.0 Pu for reactance earthed networksLoad 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 eliminatedBelow 145 kV method of earthing will normally determine the level temporary over voltages.

Switching surgesThey are of short duration and irregular formTypical 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 voltageIn 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 assessedHowever 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 effectsFor voltage level below 300 kV. Resonance effects occur When switching transformer When switching cable and overhead line combinationBetween lumped capacitive and reactive elements and over head linesCharging long lines without shunt reactor compensation

Resonance effects-- contdFerro resonance encountered on a transformer feeder greater than 5 to 10 Km in lengthWhen 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 Induction when it discharges to earth close to lineBy 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 conductorsThe highest amplitude normally associated is in the region of 200 kVSignificant in case of low voltage linesAt 11 kV estimated that it accounts for some 90% of all faultsLittle significance on lines of 275 kV and above

Direct stroke A direct stroke can be to the earthed tower top or on phase conductorStroke on earthed lower top, for transmission of shielded design, is innocuousRaise in potential caused by passage of current through tower impedance to earth will be less than with stand strength of line

Direct strokecontd.

However the rise in potential can be severe and exceed with stand capability, ifTower footing resistance is highRate of rise of current exceeds a certain levelFlashover may occurThrough the system voltage, losses is the frequency of flash over

Direct strokecontd.Direct stroke on phase conductorMay 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 ohmsVoltage rise is sufficient to cause failure of line insulationMinimum critical current for flash over Ic = 2 V I0 Z0VI0 = 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 conductorOn arrival at substation, equipment there in get stressed.Rod gaps and surge arrestors provide necessary protectionWaves 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 pointElectro static stress being great at that point ionization of surrounding atmosphere takes placeDielectric 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 chargedWhen there is a stroke between (A) and (B) the charge on (C) becomes free and immediately and indiscriminately strikes on any object on the groundFor stroke (B) there is no time lagStroke (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 chargesAn over head conductor accumulates statically induced charge when a charged cloud comes aboveWhen the cloud is swept away charge on the conductor is releasedThe charge travels on either side giving rise to two travelling wavesThe earth wire does not prevent such surges

Lightning strokesOver voltage due to lightning strokessurge impedance of the line = ZsDischarge current = IdOver voltage due to direct stroke = Vd = Id x ZsHowever current travels in both directions over voltage = Vd = Id x Zs 2when lightning strikes over earth wire or a towerOver voltage = Id x Ze + Lc di dtZe = impedance of earth wireLc is the inductance of the line conductor

Protection against lightningProtection of transmission lines from direct strokesProtection of power stations and substations from direct strokesProtection 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 betweenLine conductor and towerLine conductor and earthClearance 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 wiresIn the absence of a ground wire:When there is a charge cloud over a transmission line without any ground wireThere will be two capacitances(1) Between cloud and conductor C2(2) Between conductor and earth C1Induced voltage on the lineV L1 = C1 x Ec C1+C2When 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 thirdPresence of two ground wires also provides better shielding

Earth wiresDisadvantages 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 protectionEven after providing ground and reducing the likely induced voltages, harmful voltages can still developLightning arrestors act as additional protective devisees by by-passing the surges to groundProtector tube is a fiber tube with electrode at earth end.Fitted directly below the conductorThe arc type electrode on the top of the tube forms a series gap with conductor

Alternative method of line protectionThe lower electrode is solidly groundedIn 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 arcThis tube successfully prevents re-strikingThe 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:High peak voltage of surge may cause flash over in the internal winding or external flashover between the terminals of the equipment.steep wave front may cause internal flash over between turns of the transformerSteep wave front resulting into resonance and high voltage may cause internal or external flash over causing building up of oscillations in the equipmentProtective equipment : LAs and Surge divertersThey are connected between line and earth

Action of the Surge diverterA traveling wave reaches surge diverter and attains a prefixed voltageA spark is formed across the gapThe diversion provides a low impedance path to earthThe surge impedance of the line limits the amplitude of the current flowing to earth to prevent break down of insulationImportant 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 diverterIt 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 appliedWhen 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 substationOperation of breakers causes transient over voltagesOver voltage value varying between 1.1 Pu to 6 Pu based on switching duty and the type of circuit breakerOver voltage occurs mainly due to exchange of energy between system inductance LI2 and system capacitance CV2 Over voltage occurs during the opening of circuits and closing of long EHV linesMost severe over voltages occurs during the closing unloaded transmission linePreventive measureProvision of Pre insertion resistors ( 400 to 800 ohms per phase)Simultaneous closing of lines at both endsUsing shunt reactors, surge arresters etc.

Switching Over voltages in Substations

Methods of Reducing Switching Over Voltages

Methods of Reducing Switching Over Voltages

Rod gaps or coordinating gapsThey are used on insulators, equipment and bushingsConducting 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 circuitVoltage of phase with respect to ground falls very lowThe rod gaps are no more used consequent to development of surge arrestors.

Over-voltage in Network and Remedies

Over-voltage in Network and Remedies

Protective Devices Against Lightning Over voltages

Protective Devices Against Lightning Over voltages

Lightning arrester selection1. 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 EarthingFor 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 voltageIf in a system all transformers have star connected winding with neutrally solidly earthed then the system is effectively earthedHowever 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 systemb) System with few solidly earthed neutralsc) 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 earthingCo-efficient of earthing :Effectively earthed system 80%Non effectively earthed system - 100 % and isolated earth system

Selection of lightening arrestorsIn a 220 kV effectively earthed systemHighest system voltage =245 kVCo-efficient of earthing= 80%Arrestor voltage rating >= 245x0.8 = 196 kVAs per IS 3070 (part I) 1965 the rating is 198 kVBy 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 fromSpark over voltage of transmission line insulation Surge impedance of the lineResidual discharge voltage of LA Ia = 2E- Ea ZIa = Arrestor discharge currentE = Magnitude of incoming surge voltageEa = Residual discharge voltage of an arrestorZ = Surge impedance of the line

Selection of arrestor discharge currentIn a 220 kV system using 11 insulators Transmission line will not permit a traveling wave of a value more than 1025 kVpAs 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 ohmsMaximum value of discharge current of LA =2(1025000)-649000 = 3100 Amps 450The LAs normally in 200 kV system have a discharge current rating of 10 kA.

Selection of arrestor discharge VoltageMost important characteristic of LA determining the protection level being offeredThe arrestor discharge voltage shall be less than BIL of equipment for effective protectionDischarge voltage depends on(I) discharge current(II) rate of rise of current applied(III) Wave shape of current appliedDischarge 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 LAProtective margin of LA = BIL of the equipment--- maximum discharge voltage of LAWhile 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 locatedIn 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 systemAverage 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 SchemeFor 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 KvDischarge current rating= 10KA (assumed) Residual voltage, from column 3 of table 12.1,=649Kv (peak) Protection level of the L.A. =649x1.1=714KvFor 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 =395kvSwitching surge flashover voltage =220 x6.5=825kv 3Check it is less than B.I.L. of 900kv.Power frequency over voltage=220x3=228kv rms 3This 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 rating

Establishment of Separation LimitWhen 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. voltage KVBIL KV PeakMax. distance132kV55065035.045.0220kV400kV900/10501425/1550Closer to Trans.