Statistical Approach to Insulation Coordination o INS COORDINATION

Statistical Approach to Insulation Coordination o INS. COORDINATION Outcomes: 1 -selection or specification of the Elec. Strength 2 -ph. to Gr and Ph. to ph. Clearances & leakage or creepage distance 3 -selection of surge arresters

Suggested Steps (air insulated Substations) 1 -rating of surge arrester 2 -creepage length of porcelain insulator (contamination & Equivalent. BIL & BSL) 3 -Arrester close to transformer & its BIL, BSL 4 -BIL of other Equipments & ph. to Gr clearance, lightning O. V. s (if BIL & clearance excessive, New Arr. & the BIL again. ) 5 -BSL of other equipment, ph. to Gr. & ph. to ph. Clearances (required by switching OV. s) 6 -Protection of opened CCT. Breaker (gap type or arrester)

Insulation coordination approach o Ins. Design of substation equipments ◊Transformer as an Example: 1 -highest surges enter substation/generate within it 2 -stress on coil-to-coil & turn-to-turn INS. 3 -INS. Sys. need capability of 20% above calculated 4 -Surge arrester rating coordinated with Transformer INS. Safety factors employed: to cover lack of knowledge

Compensating the Over Design ◊ However many INS. Designs are overdesigned ◊ certain risk of failure is acceptable o In Self Healing Insulation Systems: This compensate the over design

Probabilistic Nature of voltage stress & Insulation strength o Insulation of O/H line : 1 -reducing No. of insulators: save money; cost risk; reduce cross arm length, width of right of way, weight of tower; affect foundation 2 -to justify this change, risk should be quantified 3 -INS. Strength Determined by R. O. F.

Parameters for Risk Assessment ◊stress has a f(Va) of Normal Dis. ◊ F(va) prob. of Voltage less than Va ◊ Q(Va) prob. of Voltage exceed Va Breakdown Strength of Ins. Also Statistic

Risk of Insulation Failure o o o Matching probabilities of Insulation stress & strength, risk of failure determined Prob. of flashover g(Vw) & its cum. G(Vw) then prob. of V 1 occur : f(V 1) prob. Flashover at less than V 1, G(V 1) ◊ prob. of V 1 & flash Over: f(V 1)x. G(V 1)

Risk Assessment o Normal Dis. For stress: f(Va)=1/[σs√ 2Π] exp{-1/2[(Va-S)/σs]} where: σs=standard dev. Surge with crest of S 2=S+2σs, S: mean for sw. surge INS. Design o Possibility of a surge exceed S 2 ≈ 2% S 2 is referred to as : ”statistical switching overvoltage” Eq (1)

Probability of Failure ◊ Surge known by: f(Va) applied an Ins. Of o CFO & σ The prob. of flashover can calculated : from o Eq (1) & Eq of Normal Dis. As follows:

Risk Assessment continued o o resultant probability: Where: y=[S-CFO]/√(σ�+σs�) ◊ Transmission Lines N elements Prob. At least one flash over: P(N, V)=1 - [1 - P(V)]^N

Example of Risk Assessment A line with 500 towers which CFO is 2. 3 pu, & σ=4. 5% and stress characterized by: s=1. 6 and σs=10% o σ= 2. 3 x 0. 045=0. 1035, σs=0. 16 pu Y=[1. 6 -2. 3]/√(0. 1035�+0. 16�)=-3. 67 o P(V)=1. 22 x 10^-4 ignoring neg. surges P(V)=6. 1 x 10^-5 o This is for 1 Tower, for the whole line: o P(N, V)={1 -[1 -0. 000061]^500}=3. 0 x 10^-2 o means 30 switch op. s per 1000 applications o

Protection of Systems and Equipment against Transient OVs Surges in PWR SYS can be Very destructive o Important to protect against them 1 - stop Lightning surge generated outside to enter 2 - minimize effects of those entered o 3 - sw. surges & surges gen. by faults are inevitable, however reduced by proper design o o How to protect PWR SYS Equip. s and control CCTs or relays The surge protection devices

Surge Protection Devices & their Application o Ground Wires 1 - 1 or 2 are bounded to tower above ph. Conductors 2 - Ground wires are at ground potential under normal 3 - lightning strokes stopped to be terminated on ph. o Strike Distance: S (Love’s Formula) 1 - as a leader stroke approaches earth’s surface, it is attracted towards tall objects 2 - s: strike distance ; stroke tip reaches within distance “S” of Gr object prob. of terminating on that > prob. striking another object in a distance more than s o S=10 x I ^ 0. 65 (I in k. A, S in m) EQ. 1

Strike Distance Application If I=10 k. A, S=44. 7 m if I=50 k. A, S=127 m o Anderson approach α: shielding angle βs: h of horizontal line strokes within βs of Earth terminate on it, rather than on Gr or ph o β=0. 8 for EHV o β=0. 67 for UHV o This line & 2 arcs Define 3 regions, where: ◊ PQ; unshielded section

Discussion of unshielded section o o PQ reduces for greater S related to higher I at some I, Imax points P & Q will coincide or no shielding failure at I>Imax As S reduces, arc PQ & X increases There is a Imin below it stroke to ph. , generate insufficient voltage to cause flashover Imin=2 VCFO/Z 0 EQ(2) VCFO=Ins. Critical flashover voltage Z 0=surge Impedance of Line

Shielding Failure Rate o o o Xs: is X corresponding to Imin Shield. Fail. : Imin<I<Imax & strokes within Xs P (Imin<I<Imax)=Pmin - Pmax No. of strokes to earth/sq. km/yr N=k. T T: keraunic level K: a constant 1<k<0. 19 NSF=k. T/10 Xs/2 (Pmin-Pmax)/100 km/yr Procedure should be performed for each ph cond. w. r. t. its most protective Gr wire to obtain overall Shielding Failure Rate

Shielding Failure …continued o o Combining (1) & (2) Smin=10(2 VCFO/Z 0)^0. 65 Q is located; being Smin from most exposed phase conductor and βSmin above the ground Where : Smin from most exposed ph. Cond. and βsmin, above GR o o Shield wire now a distance Smin from Q a circular arc centered on Q of radius Smin This assures P & Q coincide and leaving no unshielded region

Lightning Shielding of Substation o o Gr wires above Substation Frequently equipped by 1 -lightning rods above structural steel work 2 -O/H ground cage solidly bounded to Gr mat to provide a low resistance ground economically.

Surge Suppresors & Lightning Arreters o To protect against possible S. F. s: 1 -family of devices developed known: (a) Surge diverter, (b) Surge suppressor, (c) Lightning arrester 2 -these placed in parallel & close 3 -permanently connected or sw. in by Spark over of a series gap

Performance of Protective Devices o Normal operation; open gap represents a high impedance o When gap flash over; switch over to low impedance mode o o Arc voltage few hundred or a few thousands volts for long gap Surge voltage divided between sys imp. & the protector Imp.

Surge Voltage divided at protector terminal o If: Z 1 surge Imp. of sys generated surge Zp surge Imp. of protector o o Z 2 surge Imp. Of load Current in S when closes: I=V/{Zp+[Z 1 Z 2/(Z 1+Z 2)]}= V[Z 1+Z 2]/[Zp. Z 1+Zp. Z 2+Z 1 Z 2] V 1= V Zp(Z 1+Z 2)/ [Zp. Z 1+Zp. Z 2+Z 1 Z 2] Z 2>>Z 1 surge doubled Dissipating energy pot.

Nonlinear Resistor Protectors o Rod gap disadvantage: 1 -flashover through a fault on CCT & need o o o CCT interruption 2 - do not protect fast rising surges A device limit voltage without creating a fault more attractive Nonlinear Resistor is Such a device These resistors resistance diminish as voltage increase

Nonlinear Surge Diverters and The Analysis of their surge reponse Characteristic : I=k. V^α o Si. C type 2<α<6 Fig 16. 4 (V-I) o Si. C versus Zn. O(20<α<50) Fig 16. 13 o Employed at all voltage levels 1 -small elements to protect relays, 2 -in large junks, under oil, across windings of pwr Transformer o o Determination of over voltage protection

Examples of characteristic Si. C material o o Fig gives instantaneous VI characteristic of Si. C nonlinear resistors

Operation of Nonlinear Protector o o o CCT Diagram: Thevenin eq CCT Zs parallel of Z 1 & Z 2 V/I characteristic of Zp shown in Fig (d). o Fig© shows variation of surge without arrester Lines on Fig (d) with a slope of tan−�Zs intersects Zp I 2 flowing in Zp o I 2 Zs=V 2 -V’ 2 o o

Surge Arrester Response…continued o o As surge reaches V 1, V 2, V 3, … voltage on protected object passes through V’ 1, V’ 2, V’ 3, …with a considerable reduction It found following the surge variation with combination of Eq CCT & Arrester V/I Characteristic

Traditional Lightning Arresters o o Trad. L. Arr. Use nonlinear resistance They have gap or gaps in series It is possible to design the resistor element to satisfy the energy dissipation and voltage-limiting under surge conditions Preferred material Zn. O & traditionally Si. C

Si. C Arresters o o o When suppressor operates an arc in gap This arc must quenched as surge pass, or resistor will be destroyed In other cases the gap is not required Arresters vary depending on their voltage class & duty however has: Gaps, coils, valve elements (nonlinear res. ) They are stacked in series & hermetically sealed in a porcelain housing(6 k. V element) A station-type arrester for 96 k. V shown in next slide

station-type arrester o In which similar elements are connected in series

Operation of Gapped Arresters little different from plain nonl. Resis. o Initially behave like a gap with a volt/time curve turn-up relatively slight, less than that of a rod gap o Once sparkover occur(in front, peak, or on tail)nonlinear resistor inserted o Sequence shown in Fig. 16. 8 o

Metal Oxide Arresters Metal Oxide Varistors 1 - introduced for O/V protection(1960) 2 -larger α than Si. C 3 - like Si. C is crystalline 4 -90% Zn. O & metal oxides 5 -material is ground, mixed, pressed, & sintered and shape o disk blocks 6 -the nonlinear property depond on boundary layers between crystals 7 -Fig 16 -12, VI characteristic, Dyna Var 209 k. V

Traditional Lightning Arresters o o Traditional lightning arresters uses nonlinear resistance elements as before however have a gap or gaps series with them So resistor is isolated from cct under normal conditions & is introduced when a surge appears by sparkover of gap It is possible to design resistor element from energy dissipation & voltage-limiting under surge conditions

Preferred Type of Arrester o o Preferred material for application is Zinc Oxide (Zn. O) however traditionally Si. C used traditional type still in a vast number are in service A different approach relates to a type of surge suppressor, in which when suppressor operates and an arc is established in gap this arc must be quenched when surge passed or resistor will be destroyed by current that flow

Arresters Assembly o o Arresters vary in sophistication upon the voltage class & duty generally comprise: gap units, coil units, valve elements of nonlinear resistance material These are stacked in series & hermetically sealed in porcelain housing Principle is shown in next fig.

Valve type Arrester path of : a- surge current b-follow current o Components Shown comprise Requirement for a 6 k. V arrester o

Operation of Valve type Arresters o o o Magnetic field created by coil follow current in coil reacts upon this current in arcs of gap assemblies causing them to be driven into arc quenching chambers arc extinction occur at first current zero by elongating & cooling arc Operation of a gapped arrester is little different from plain nonlinear resistor at least up to point of gap spark-over The sequence illustrated in fig.

Operation of gapped type surge Arrester o o Surge impinges on arrester Voltage follows surge voltage to point of sparkover P Upon voltage drop to Q, defined by Q’ determined by load line P’Q’ Then surge climbs to R but protected object sees only S

Examination of Efficient Performance of this type(application of repeated 1. 5/40 μs impulses) o o o Done according to IEC and USA standards For a 12 k. V maximum system voltage arrester average time to spark-over (a)0. 4μs (b)1. 8μs

Spark-over Curves o o Volt/time spark-over curves for arresters Surges of: 1. 2/50 μs (turn up is evident)

Performance of a 36 k. V arrester of this type o Discharging a 5000 A surge current of 8 x 20 μs Note: peaks (voltage & current) do not coincide

Metal Oxide Arresters o o o Metal Oxide Varistors 1 st for O/V protection in 1960 (safe guard electronic components) Years passed until technology advanced to where large disks of consistent quality & stability made & applied in PWR SYS impact on PWR INDUS. Since then is profound metal oxide material different from silicon carbide in exponent α which typically 20 rather than 4 for Si. C It is about 90% Zn. O & rest of other metal oxides

Metal Oxide Arrester … o o o Material is ground, mixed, pressed, & sintered to form disk-shaped blocks with a dense, fine structure Property of Si. C derives from bulk material itself, while in Zn. O it resides in boundary layers between crystals Grain size & number of boundaries is dependent on sintering process , so VI controlled by sintering as well as composition as shown in next fig.

Metal Oxide Arrester …. o o Influence of Zn. O grain size upon Varistor Voltage However VI characteristic of a real sample named “Dyna Var” 209 k. V metal Oxide arrester shown in fig of next slide

Comparison of Si. C & Zn. O o Fig 16. 13: (for application in 345 k. V) 1 - a Zn. O Arrester 2 - a Si. C Arrester 3 - a Linear Resistor 1&2 a protective level of 2 pu in 10 k. A o The 296 k. V intersect MOV in < 1 m. A o line intersect the Si. C’s in 200 -500 A o for MOV : 1 - could be operated without a gap 2 - If gap employed protected level can be reduced o

Comparison of a Zn. O & Si. C o o fig shows comparison of these two & a linear resistor for application 345 k. V

Gapless Arresters o Must support Normal Voltage continuously o Therefore the L. H. S. of characteristic o Shown in Fig 16 -14 is sensitive to Temp. (in o o given Voltage increase with Temp. ) Working at elevated Temp. increase dissipation & increase Temp. further same situation for MOV operation continuously at too high a voltage Each device a Max. Con. Op. Volt: MCOV Normally close to max. Line to N rms voltage

Temperature Effect o On V/I curve of metal Oxide Varistor

MOV Parameters & gap type o o o Dashed line in Fig shows cap. &res. Current components At this voltage level act as a capacitor with mild loss Its dielectric constant about 1000 During quiescent voltage Dis. Between gap and MOV based on capacitance: C 1: across Gap ; C 3 : MOV capacitance C 2 disturb balance between gap 1 & 2, when fast rising surge applied 1 st , No. one spark over then controlled by MOV 2 & then voltage

Schematic of Gapped Metal Oxide arrester o Under quiescent condition, voltage distribution between gap section & MOV determined by capacitance: C 1 across gaps & C 3, inherent capacitance of MOV’s share assures thermal stability even though it typically has 10% fewer disks than a gapless type C 2 becomes influential with advent of a fast rising surge disturb balance between gap 1 &2 first 1 spark over and then 2 follow quickly as voltage appear across it

TIME TO SPARK OVER for MOV with gap o o voltage is controlled by MOV characteristics rather than the initial spark over voltage of gaps which determines max. voltage experienced by protected object Spark-over occurs at a lower voltage for a somewhat slower rising surges Is Different from a Si. C type For extra HV, H. E. applications 2 columns of Zn. O disks used

Surge Arresters Characteristics o o o MCOV : Max Con operating Voltage Rating is 15 -30% more than that & is the highest voltage at which duty cycle test can be performed Test ANSI/IEEE C 62. 11 -1987; should be subjected to 20, 8 x 20 μs current surges at special intervals followed by a test to show thermal stability Energy capability: kilojoules/k. V

Maximum Energy Capability for MOV arresrter o TABLE

Parameters Continued o o energy limited to 85% of table & repeatable a minute after some cooling Table 16. 3 Si. C 1 st column rating in k. V, 2 nd “front of wave” spark over voltage with very fast surges, 3 rd spark over voltage with standard 1. 5 x 40 μs wave, 4 th Max. switching spark over, 5 th Max 60 Hz spark over voltage o Similar Data on Zn. O some gapped(VS, VX) and ungapped

TYPICAL PROTECTIVE CHARACTERISATICS of Silicon Carbide station type Arresters

Typical Protective Characteristics of MOV station type Arresters

Application of Surge Arresters o Objective: 1 - protect insulation of other equipment 2 - without putting itself at risk ◊ Highest protective margin or protection ratio desirable; as margin increase energy demand increase ◊proper application need: a compromise ◊contingencies: T. O. V. , Lightning , Sw. Surge ◊min protective ratios: 1 -chop. wave withstand/Front-of-wave prot. level≥ 1. 20 2 -Full wave withstand (BIL)/Imp. Prot. Level ≥ 1. 20 3 -Sw. surge withstand/Sw. surge prot. Level ≥ 1. 15

Ratios Requirements and Protection against Switching surge Ratios met, if possible exceeded (since insulation deteriorate with time) Energy loadings not exceeded o Protection against Sw. surge An example of protection of a Transformer switched in through a transmission line, where the line energized from the other end o

Example of SW. Surge Protection o Arrangement: where; L=13 m. H, Z 0=350Ω, l=200 miles ◊345 KV sys, 362 k. V max design voltage o Arrester, with: MCOV=362/√ 3=209 k. V

Example continued o Surge traveling down the line: V(t)=(1 -e^-αt)V , α = Z 0/L V=voltage across the switch at closing o o 1/α=37 μs short compared to: travel time ≈ 1. 075 ms When reach far end approach 2 V after 3 to 4 time constants

Discussion on Arrester Response o Closing at peak; Voltage at transformer : 591 k. V Transformer BIL 900 k. V & SIL=0. 83 x 900=747 k. V However, Arrester conduct, What is the energy absorbed?

Example continued…. . o Neglecting corona, & other dampings 1 -surge at Arr. rise to 591 KV remain constant until reflection return from source (after 2. 15 ms) 2 -arrester restrict voltage at transformer by: its characteristic & load line

Example Continued…Normal case ◊ Q 1, dissipated power OP 1 Q 1 R 1: 423 x 480=203040 k. W ◊ energy in 2. 15 ms =436. 5 k. J ◊ In term of arrester: 436. 5/209=2. 09 k. J/k. V

Example continued …with Trapped charge Breaker recloses at pos peak voltage when neg peak voltage trapped on line o voltage at arrester attempt swing–vp to 3 vp o o Reach 887 k. V which exceeds the transformer SIL o Shifting load line to right at Q 2, 530 k. V & 1020 A o Voltage within SIL limit of transformer Energy dissipation: 530 x 1020 x 2. 15 ms=1162 k. J o In term of arrester 1162/209=5. 56 k. J/k. V o

Protection Against Temporary Over Voltage o o Temporary power frequency over voltage (due to single to ground fault, on unfaulted phases, Ferranti rise due to load rejection and ferro-resonance) TOVs caused problem for gapped Si. C arresters if a surge cause spark over while TOV present Zn. O arresters have some tolerances for TOVs however is limited Power dissipated and temperature increase Rapidly with increase in voltage

Protection Against Temporary Over Voltage o o o o A 209 k. V MCOV arrester can withstand a TOV of 304 k. V for one second & maintain stability Arresters can withstand lower TOVs for longer periods and vice versa Ambient temperature and any dissipation immediately prior to TOV will affect its tolerance i. e. voltage as per unit of TOV capability versus duration Liklihood of TOVs should be studied where applying arresters ANSI IEEE C 62. 2 (12) can be consulted w. r. t. magnitude Duration should be taken into account considering the operating time of back-up breakers

voltage as per unit of TOV capability o Figure

Arrester/ Equipment Insulation o o Information on shielding against: lightning, surge arresters and application of surge arresters for a 230 k. V system summarized in one figure Equipment at this voltage normally has a BIL of 750 k. V, however in this figure reduced to 600 k. V, recognizing its aging

Metal Oxide arrester insulation Coordination for 140 k. V MCOV arrester suitable for 230 k. V system

Arrester for Line Protection

Assignment N 0. 4 (Solution) o o Question 1 13. 8 KV, 3 ph Bus L=0. 4/314=1. 3 m. H Xc=13. 8�/5. 4=35. 27 Ω, C=90. 2μF o Z 0=10√ 1. 3/9. 02=3. 796Ω o Vc(0)=11. 27 KV o Ipeak=18000/3. 796= 4. 74 KA

Question 1 o o o 1 - Vp=2 x 18 -11. 27=24. 73 KV Trap 2 - Assuming no damping, reaches Again the same neg. peak and 11. 27 KV trap 3 - 1/2 cycle later –(18 -11. 27)=-6. 73 Vp 2=-(24. 73+2 x 6. 73)=-38. 19 KV

Question 2 C. B. reignites during opening&1 st o Peak voltage on L 2=352, L 1=15 m. H, o C=3. 2 n. F So reigniting at Vp, 2 comp. : o Ramp: Vs(0). t/[L 1+L 2]= 138√ 2 x 10�/[√ 3(352+15)x 10�]=0. 307 x 10^6 t o Oscill. of : f 01=1/2Π x {√[L 1+L 2]/L 1 L 2 C} Z 0=√{L 1 L 2/[c(L 1+L 2)]} o component 2: as Sw closes Ic=[Vs(0)-Vc(0)] /√{L 1 L 2/[c(L 1+L 2)]} o ≈2 Vp√C/L 1=104. 1 A

Question 2 continued Eq of Reignition current I’ t + Im sinω0 t which at current zero: sinω0 t=-I’t/Im , ω0=1/√LC 1=1. 443 x 10^5 o Sin 1. 443 x 10^5 t=-0. 307 x 10^6 t/104. 1=2. 949 x 10^3 t o Sin 1. 443 x 10^5 t =-2. 949 x 10^3 t o t(μs): 70 68 -0. 6259 -0. 3780 -0. 2064 0. 2005 67 -0. 2409 -0. 1376 66. 7 -0. 1987 -0. 1967 66. 8 -0. 1959 -0. 1966

Question 2 o o t=66. 68μs I 1=0. 307 x 66. 68=20. 47 A Vp=I 1√L 2/C=20. 47 x 10. 488=214. 7 KV

Question 3 69 KV, 3 ph Cap. N isolated, poles interrupt N. Seq. o 160◦ 1 st reignite o Xc=69�/30=158. 7 o C=20μF, CN=0. 02μF Vs-at-reig=69√ 2/3 cos 160 =-52. 94 KV o o Trap Vol. : V’A(0)=56. 34 KV V’B(0)=20. 62 KV, V’C(0)= -76. 96 KV, VCN(0)=28. 17 KV o Vrest=56. 34+28. 17+52. 94=137. 45 KV

Question 3 continued o o o o Z 0=√L/CN=√ 5. 3 x 0. 2 x 100=514Ω Ip-restrike=137. 45/514=0. 267 KA=267 A F 0=1/[2Π√LCN]=10^6/{2Π√ 53 x 2}=15. 45 KHz Voltage swing N=2 x 137. 45=274. 9 KV VN=28. 7 -274. 9=-246. 73 KV VB’=-246. 73+20. 6=-226. 13 KV VC’=-246. 73+-76. 96=-323. 69 KV