TESTING CREEPING DISCHARGE BEHAVIOUR OF INSULATORS FOR ISOLATED LIGHTNING PROTECTION SYSTEMS

IX Inernaional Symposium on Lighning Proecion 26 h -3 h November 27 Foz do Iguaçu, Brazil TESTING CREEPING DISCHARGE BEHAVIOUR OF INSULATORS FOR ISOLATED LIGHTNING PROTECTION SYSTEMS Beierl, Omar Brocke, Ralph; Wechsler, Andre Universiy of Applied Sciences Nuremberg. Germany DEHN+SÖHNE, Neumark, Germany omar.beierl@fh-nuernberg.de ralph.brocke@echnik.dehn.de, andre.wechsler@echnik.dehn.de Kesslerplaz 12, D-9489 Nuremberg, Germany Absrac - The deerminaion of separaion disances a lighning proecion sysems was specified in he curren IEC 6235-3 sandard based on previously known ess on creepage sparkover and elecric srengh of insulaed lighning proecion componens". This aricle shows ha possible creeping discharges a he insulaing clearance have o be observed for he correc dimensioning of insulaors used. Creeping discharges presen a paricular problem for seep pulses. In his aricle, ess on insulaed lighning proecion componens are performed exemplarily and new deails on he performance of pulses wih shor rise ime are presened. The objecive of hese ess is o compare he resuls of impulse volage ess in he laboraory wih he requiremens of he sandard. For his purpose, he breakdown behaviour of a comparison se-up is compared wih he sparkover behaviour of lighning proecion componens o be esed for pulses in he sub-µs range. Conformiy wih requiremens specified in he sandard is ensured via he evaluaion of he consan-area-crierion. Comparing he U/ characerisic of insulaed lighning proecion componens wih he comparison se-ups shows ha he lighning proecion sysem is safe in cerain areas of he impulse volage characerisic and ha a sparkover or breakdown may occur in oher areas. The resuls prove he urgen need of es mehods for insulaed lighing proecion componens according o he sandard and are a rough guideline on how o srucure hese mehods. 3 [1]. Several ess have shown ha lighning curren componens of he maximum possible rae of curren rise have o be considered when defining separaion disances. Volages a he poin of proximiy caused by inducion in insallaion loops can easily reach values of several 1 kv. Minimum disances beween lighning proecion sysem and meal insallaion ensuring safe isolaion a he poin of proximiy are wihin he accepable proximiy range. These minimum disances are influenced by he induced volage u ind and he elecric srengh of he insulaion a he poin of proximiy. The elecric srengh depends on he ime characerisic of he volage (see Fig. 1). Insallaion Proximiy EBB 1 MV Volage u induced by lighning impulse curren i a he poin of proximiy u 25 ns i 25 ka 1 INTRODUCTION When designing and dimensioning a lighning proecion sysem, i has o be ensured ha lighning impulse currens are safely discharged and separaion disances are mainained in order o preven uninenional sparkovers o he meal insallaion wihin a srucure. The quesion wheher a lighning proecion sysem is designed as insulaed sysem or wheher all pars of he meal insallaion in he building o be proeced are inegraed ino he equipoenial bonding srucure mainly depends on he proecion objecive. Basic principles for he correc deerminaion of he separaion disances required are specified in IEC 6235- Fig. 1: Proximiy beween lighning proecion sysem and elecrical insallaion in he building Proximiies in buildings can be conrolled by differen measures, e.g. insallaion of down conducors of he exernal lighning proecion sysem on insulaing spacers or use of insulaed down conducors. 2 SEPARATION DISTANCE If a lighning proecion sysem is insalled in a building as insulaed sysem, he separaion disance beween meal insallaion inside he building and lighning proecion sysem has o be mainained according o IEC 6235-3. In order o preven dangerous sparkovers

caused by induced volages, he separaion disance s required by he sandard has o be mainained. ki kc s = L km (1) where: s Separaion disance k i Facor depending on he seleced class of LPS k c Facor depending on he division of lighning curren k m Facor depending on he elecrical insulaion maerial L Verical disance from he poin a which he separaion disance has o be deermined o he neares equipoenial bonding poin I is obvious ha he value of he separaion disance depends on he lengh of he down conducor, he class of LPS seleced, he division of he lighning curren o differen down conducors and he insulaing maerial in he spark gap. 3 ELECTRIC STRENGTH OF INSULATING CLEARANCES The elecric srengh of he insulaion in he area of a proximiy beween lighning proecion sysem and meal insallaion depends on he dielecric srengh of he insulaing maerial used. This shows ha he elecric srengh depends on he ime characerisic of he volage. k m =.7 has proven is worh in many pracical applicaions [5] if insulaing maerials such as rods or pipes made of a maerial wih a high wihsand volage e.g. GRP are used. 3.2 Applicaion of he consan-area-crierion o differen pulses In order o compare heoreical consideraions of he impulse wihsand volage of an insulaing clearance deermined via he consan-area-crierion wih he resuls of measuremens, i is necessary o apply he consanarea-crierion o pulses wih differen rise imes and duraion (Fig. 2 o Fig. 4). u() U ( A + U ) = U ˆ (4) Fig. 2: Consan-area-crierion for square-wave pulses (induced volage drop a a down conducor as normal pulse for he deerminaion of separaion disances) u() 3.1 Impulse wihsand volage and consan-areacrierion The deerminaion of he impulse wihsand volage is sill based on he impulse breakdown volage of he air. The consan-area-crierion considers he differen ypes of volage pulses [2]: u( ) U d = (2) [ ] A The ime characerisic of he impulse volage is inegraed via he period <, in which he volage u() exceeds he saic breakdown volage U. The following parameers were deermined for a rod-rod spark gap wih a clearance d in air [2]: U 5 = 6 1 d[ m] V ; A =.6 d[ ] Vs (3) m Calculaion of he k i facor according o he sandard is based on hese parameers as described below [3]. This assumpion can only be pu ino pracice if insulaion is mainly esablished by an air gap. Tess carried ou in [4] prove ha a maerial facor k m =.5, which akes he reduced elecric srengh of consrucion maerials ino accoun, has o be used for solid consrucion maerials such as concree or brickwork. A maerial facor of U r C 2 U r A + 2 = (5) ( U ) Fig. 3 : Consan-area-crierion for ramp pulses u() U A = Uˆ 2 U (6) U + 2 2 Fig. 4 : Consan-area-crierion for riangular pulses

Comparing differen ypes of pulses via he consan-areacrierion by means of simple linear approximaion of he pulses is permied in a firs approximaion. However, i has o be considered ha he difference beween heoreical and acual volage-ime area increases if pulses are very shor. 3.3 Impulse volage characerisic curve of a rod-rod spark gap in air The calculaion of he separaion disance according o IEC 6235-3 is based on he applicaion of he consanarea-crierion o a rod-rod spark gap in air. The consans according o equaion 3 are used for a square-wave pulse (Fig. 2). This pulse is caused by an ideal induced volage drop of a lighning curren pulse, which is assumed o increase in a ramp form in order o simplify maers. A value of L' = 1.5 μh/m is used as inducance per uni lengh of a down conducor for all classes of LPS. The negaive subsequen srike, for which a rise ime of r =.25 μs is also used for all classes of LPS, is decisive for he separaion disance. Depending on he class of LPS, he peak value I max of he lighning curren varies. In he following, he required elecric srengh U' of a proximiy disance concerning one mere of an individual down conducor hrough which he oal lighning curren flows is shown in case LPS I is seleced: I 5 ka U = max L = 1.5μH/m 3 kv/m.25 μs = (7) r In Fig. 5, he impulse volage-ime curve of a rod-rod spark gap wih a clearance of d = 1m, which resuls from he evaluaion of he square curve shape according o Fig. 2, is compared wih he inducive volage drop a an individual down conducor of 1 m hrough which lighning curren ( r =.25 μs / I max = 5 ka) flows. he sandardised k i =.1 for class of LPS I. All furher k i values can be derived from his comparison. Calculaions are based on he same square volage pulse which resuls in all oher classes of LPS for lower lighning curren peak values and larger down conducors. I is appropriae o apply exacly hese heoreical consideraions o laboraory ess carried ou for insulaed lighning proecion componens in order o develop a es mehod according o he sandard. 4 TEST METHOD BASED ON THE CONSTANT- AREA-CRITERION Technology is no ye sufficienly advanced o creae es pulses in larger es volumes, which correspond o he square-wave pulse based on he sandard. Therefore i is eviden o use he seepes possible es pulses which can be generaed in he laboraory a reasonable effor. The consan-area-crierion is evaluaed according o he sandard. For measuremens presened in his aricle, a 12-sage Marx generaor originally designed for 1.2/5 waveform was rerofied by insalling low-impedance damping resisors which enabled a double exponenial es volage of.4/45 waveform. Furhermore, i is exremely difficul o use a rod-rod spark gap wih variable clearance and reproducible characerisics in he laboraory. Therefore in [5] a spark gap made of 2 "crossed" rods has been suggesed as comparison spark gap and i has been proven ha he performance of hese rods are o a large exen similar o a rod-rod spark gap in case of negaive es pulses. 4.1 Descripion of he experimenal se-up The following experimenal se-(fig. 6) up has been esablished aking hese consideraions ino accoun. Tes objec (e.g. spacer) Impulse generaor.4/45µs wih pulse volage measuremen Comparison se-up wih clearance s V Fig. 5: Comparison of an inducive volage drop of a down conducor L = 1 m wih he impulse volage-ime curve of rodrod spark gaps d = 1 m in case of differen ypes of pulses The comparison (see Fig. 5) shows ha he consan-areacrierion is compleely fulfilled for a square-wave pulse of a duraion of r =.25 μs and a volage U max = 3 kv. These calculaions direcly resuled in Fig. 6: Experimenal se-up (principle) The se-up consiss of he rerofied impulse generaor conneced in parallel, he comparison se-up and he es objec. The comparison se-up consiss of wo crossed rods of a lengh of 3 m each and a diameer of 8 mm. The earhed rod is posiioned a a heigh of 2 m.

This experimenal se-up enables o compare he sparkover performance of differen es objecs (e.g. GRP spacer) wih he breakdown performance of he comparison se-up. The comparison se-up serves a he same ime as reference spark gap and chopping gap. Fig. 7 shows he experimenal se-up which has been se up in he high volage laboraory. Tes generaor connecion Measuremens were carried ou wih impulse volages of.4/45 waveform. Peak values and momens of breakdown were indicaed as poins in he diagram in Fig. 8. The comparison according o Fig. 8 shows ha on he one hand he impulse volage can be simulaed in he rise and peak area by means of a ramp (he influence of he ail was deliberaely negleced). The parameers of he consan-area-crierion according o equaion 3 are confirmed in good approximaion. 4.3 Experimenal ess of GRP spacers For furher ess, he sparkover performance of wo differen GRP spacers of l GFK1 = 6 mm and l GFK2 = 4 mm was esed and compared wih measuremens a he comparison se-up. The lenghs seleced correspond o l V / l GFK = k m =.7. Comparison se-up Tes objec Fig. 7: Tes objec (spacer 6 mm) and comparison se-up (s V = 42 mm) 4.3.1 Tes resuls for dry GRP spacer (pulse form of.4/45) Tes resuls for dry, clean GRP spacers are addiionally lised in Fig. 9 as riangle symbols. 4.2 Sparkover performance of he comparison se-up In furher procedures, i is esed o wha exen he flashover performance of he comparison se-up corresponds o he crieria of he consan-area-crierion based on he sandard. For his purpose, he impulse volage-ime curve for clearances s V1 = 42 mm and s V2 = 28 mm were deermined using he parameers according o equaion 3. In order o simplify simulaion of he impulse volage wih a.4/45 waveform, calculaions were based on a curve in ramp form (see Fig. 3) wih a rise ime r =.4 μs (see curves in Fig. 8). 1 8 s V1 = 42 mm 6 4 2 s V2 = 28 mm [μs]..5 1. 1.5 2. Fig. 8: Comparison of measuremens.4/45 waveform wih calculaions ramp impulse volage r =.4 μs Fig. 9: Comparison of measuremens GRP spacers and comparison se-up (waveform.4/45) Tess were again carried ou wih impulse volages of.4/45 waveform. During hese ess, he GRP spacers showed significanly worse performance for high volage levels wih shor breakdown imes han he comparison se-up despie of considering he facor k m =.7. In he area of he inersecion, srenghs of GRP spacers and comparison se-up are very close as shows he discharge developmen in Fig. 1. The sparkover performance of he GRP rod of a lengh of l GFK1 = 6 mm is of paricular ineres for low volage loads in he long-ime limi (Fig. 11), which refer o

crossing U/ characerisic curves. In he long-ime limi, he srengh of he GRP spacer is significanly higher han he srengh of he comparison spark gap. This effec plays a minor for he shorer GRP rod of he lengh l GFK2 = 4 mm as he U/ characerisic curves cross afer laer or longer periods of ime. ap waer). Resuls for we GRP spacers have been added o he U/ diagram as square symbols (Fig. 12). The insulaion resisance is no noably deerioraed for shor volage loads compared o he resuls of dry GRP spacers. For long volage loads he impulse wihsand capabiliy is noably decreased. Fig. 1: Discharge developmen in he inersecion of he impulse characerisic curves (deailed picure) 8 7 6 5 Long-ime range GRP1 c [µs] 4 3 2 1 [μs]. 2. 4. 6. 8. Fig. 11: Comparison of measuremens a GRP spacers l GFK1 = 4 mm or l GFK1 = 6 mm in he long-ime range (.4/45 waveform) This means ha a GRP rod, which has a lower elecric srengh han he relevan comparison se-up in case of high volage loads or shor response imes, seems o have a higher elecric srengh han he comparison se-up in case of lower volage loads and longer response imes. This effec is even enhanced wih increasing lengh of he GRP rods esed. 4.3.2 Tes resuls for we GRP spacers (pulse form.4/45) The measuremens for dry GRP spacers described above have been repeaed for we GRP spacers (moisening wih Fig. 12: Comparison of measuremens GRP spacers and comparison se-up (.4/45 waveform) in addiion wih es resuls a we GRP spacers A possible reason for he relaively low impac of humidiy is he fac ha creeping discharges a he surface of insulaor se-ups spread a very high speed. If volage loads are high or pulses are very shor, he difference beween he sparkover a he GRP rod and he breakover a he comparison spark gap has lower effecs. 4.3.3 Ineracion beween es arrangemen and waveshape of he es impulse The measuremens show, ha an impulse generaor circuiry adjused o generae 1,2/5 µs pulses is more affeced by he influence of he es arrangemen han an circuiry wih a lower impedance (i.e. for,4/45 µs impulses). This ineracion will be caused by a beginning pre-discharge a he es sample which causes an addiional non-linear load. The increased curren causes an volage drop a he relaively high inner impedance of he impulse generaor and herefore an oupu volage deviaing form he open circui oupu volage (fig. 13). I can be shown ha wih lower generaor impedance he ineracion beween he impulse generaor and he es arrangemen become less.

25-25 -5-75 3 % 9 % 1 % T =,5 µs 1 Max. open circui volage -,2,2,4,6,8 /µs Fig. 13: Measured oupu volage in comparison wih he open circui volage level (1,2/5 µs) The measured volage impulses seems o be impulses wih shorer rises imes or impulse duraion. Wih respec o he U/-characerisics his deviaion appears as breakdowns wih lower peak volages or shorer breakdown imes due o he fac ha he consan-areacrierion is reached earlier (fig. 14). For comparison he calculaed u/-characerisics using equ. 5 wih he parameers of equ. 3 are also given in fig. 14. 9 8 7 6 5 4 3 2 1, 1, 2, 3, in µs Fig. 14: Measured oupu volage (do symbols 1,2/5 µs; riangle symbols,4/45 µs) in comparison wih he calculaed open circui volage level (solid line r = 1,2 µs; dashed line r =,4 µs) 5 SUMMARY AND OUTLOOK In he aricle a hand heoreical consideraions, on which he sandardised calculaions of separaion disances are based, are summed up on he basis of he consan-areacrierion and ransferred o he evaluaion of impulse volage ess carried ou in he laboraory. Technology is no ye sufficienly advanced o creae square-wave pulse volages used in he sandard. Therefore i is recommended o evaluae es volages in he laboraory wih he help of he crieria of he consan-area-crierion specified in he sandard. Thus, es mehods according o he sandard wih seep es volages, which can be creaed a a reasonable effor, can be derived and developed. This was demonsraed during impulse volage ess using a sandard Marx generaor, which was rerofied a minimal effor, in order o creae.4/45 waveforms. During he measures performed, he breakdown performance of a comparison se-up inroduced o he elecrical expers could be verified wih he parameers for he consan-area-crierion used in he sandard. Thus, a reference spark gap can also be used when developing es mehods according o he sandard. Exemplary ess of GRP spacers show ha shor and long volage loads have a clearly differen sparkover performance. While he srengh of he GRP spacer are below he elecric srengh of he comparison spark gap in he range of high volages or shor breakdown periods, he elecric srengh in he long-ime limi is higher han he elecric srengh of he reference spark gap. Tess revealed ha even componens designed for he conservaive value k m =.7 only have a dielecric srengh below he relevan comparison se-up. Such consideraions and resuls show ha es mehods in line wih he sandard are required for insulaed lighning proecion componens. Such es mehods have no been developed ye and are he objecive of furher research. Furher invesigaions should also ake ino accoun ha a significan volage impulse degradaion due o he ineracion beween he impulse generaor circuiry and he es arrangemen may occur. 6 REFERENCES [1] IEC 6235-3:26 Proecion agains lighning Par 3: Physical damage o srucures and life hazard [2] Thione, L: The Dielecric Srengh of Large Air Insulaion; in K. Ragaller: Surges in High-Volage Neworks. Plenum Press, New York, 198. [3] Beierl,.; Seinbigler, H.: Induziere Überspannungen im Bereich von Ableiungen bei maschenförmigen Fanganordnungen [Induced overvolages in he down conducor area for mesh-ype air-erminaion sysems], ICLP, München 1985, 4.1. [4] W. Zischank,. J. Wiesinger, P. Hasse, P. Zahlmann: "Teilsisoliere Blizschuz-Anlagen zum sicheren Einhalen von Näherungsabsänden". [Parly isolaed lighning proecion sysems o safely mainain proximiy disances] 2. VDE/ABB conference on lighning proecion, Ulm, 6.11.-7.11.1997, page 135 o 145 [5] W. Zischank,. J. Wiesinger, P. Hasse: "Insulaors for isolaed of parly isolaed Lighning Proecion Sysems o verify safey Disances" 23. ICLP, Florenz 1996, S. 513-518