Surviving a strike. ABB helps distribution transformers survive rapid voltage transients

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Surviving a srike ABB helps disribuion ransformers survive rapid volage ransiens WOJCIECH PIASECKI, MAREK FLORKOWSKI, MAREK FULCZYK, PAWEŁ KŁYS, EGIL STRYKEN, PIOTR GAWA D A lighning srike is no a

1 Surviving a srike ABB helps disribuion ransformers survive rapid volage ransiens WOJCIECH PIASECKI, MAREK FLORKOWSKI, MAREK FULCZYK, PAWEŁ KŁYS, EGIL STRYKEN, PIOTR GAWA D A lighning srike is no a pleasan experience under any circumsances. For elecrical equipmen i is no jus he volage peak ha can cause damage, bu also he sudden and rapid volage rise. In some cases he volage ransiens are far seeper han hose ha occur in ypical siuaions covered by common sandards hey can surge by megavols in he space of microseconds. The insulaion covering he windings of ransformers and moors is no normally designed for such ransiens and can suffer permanen damage if no addiional proecion is used. Sudies show ha despie each piece of equipmen being designed o wihsand ypical surges, as many as 35 percen of oal dielecric failures in power equipmen may be caused by such surges [1]. One soluion is o oally redesign equipmen o beer cope wih such ransiens. A simpler approach involves adding a componen ha proecs he equipmen from surges wihou affecing normal everyday operaion. ABB has developed precisely such a componen. 68 ABB review 1 11

2 1 Transiens in power neworks Transiens in power neworks are consequences of: Exernal evens (eg, lighnings) Evens wihin power neworks (swiching, fauls) 50 Hz + = (ms) Fas and very fas ransiens affec he equipmen by: Overvolages High d/d High frequency oscillaions overvolage oscillaions d/d Transiens in power neworks resul in overvolages and oscillaions superimposed on he phase volages and currens. T he high-frequency componens in he specrum of a volage surge 1 resul in a highly nonuniform volage disribuion. This causes local sresses on he insulaion sysem, going far beyond hose encounered during normal operaing con diions. Furhermore, he complicaed inernal srucures of elecric apparaus can ac as muliresonan circuis. High frequencies can herefore addiionally cause local amplificaion. The resuling sresses on he insulaion sysem can significanly compromise he equipmen s lifeime, and ofen leads o inernal shor circuis. The volage impulse he insulaion maerial can wihsand srongly depends on he impulse rise ime. This is normally aken ino accoun when choosing insulaion for elecric moors. Of special concern is dry ype insulaion on roaing elecric machines when hese machines are fed by drives ha use solid-sae swiching devices a high frequency. Manufacurers of such machines ofen provide guidelines regarding he limis of surge volage ampliudes and corresponding fron imes. Of special concern are surges whose rise ime is ypically below 1μs [2], resuling in a nonlinear iniial volage disribuion along he winding. This iniial uneven volage disribuion produces high-volage insulaion sress and can lead o breakdowns, corona and parial discharges 2. The increasing demand for a higher wihsand level is also observed in disribuion ransformers. This is being me by nonconvenional designs of windings ha are increasing boh design and manufacuring coss. Of special concern are ransformers exposed o frequen amospheric discharges. Operaors of such neworks ofen reques compliance wih a highly demanding norm, requiring he esing of ransformers wih a seepfroned impulse. The Finnish sandard SFS 2646 prescribes a volage rise (d/d) of 2 M/μs [3]. This norm requires ha he ransformer should be proeced agains overvolages by a spark gap in is proximiy. As a spark gap is relaively slow o reac, he volage a he ransformer erminals may (under es condiions) rise o levels largely in excess of he basic insulaion level (BIL) deermined by he operaing volage of he equipmen. In addiion, he high d/d of he waveform resuls in a highly nonlinear iniial volage disribuion in a convenional winding, leading o local oversressing of he insulaion sysem 3. Experimenal resuls confirm his and indicae ha a spark gap may no provide sufficien proecion for disribuion ransformers [4] of sandard design. The common soluion is o apply a special winding design comprising addiional elemens (elecrosaic screens) equalizing he iniial poenial disribuion. Such a soluion helps avoid local oversressing of he insulaion by high d/d values, bu i adds complexiy o boh he design and he manufacuring process of he ransformer. Furhermore, he peak overvolage neverheless reaches he ransformer winding and hus he insulaion mus be dimensioned o wihsand overvolages in excess of he sandard BIL. SmarChoke series-impedancebased proecion ABB has developed an alernaive o such a special ransformer design by providing a series-conneced filering elemen (referred o as a choke) upsream of he proeced equipmen. The basic principle of he series elemen is o provide an appropriae impedance-frequency characerisic 4. This makes he Of special concern are surges, whose rise ime is ypically below 1μs, resuling in he iniial volage disribuion along he winding being nonlinear. device pracically ransparen a 50 or 60 Hz while suppressing very high frequency componens. Surviving a srike 69

3 2 Iniial, nonlinear disribuions of elecrical poenial along he ransformer winding 4 Idealized represenaion of he frequency characerisic of he impedance of he series choke Winding lengh = 500 ns olage (%) impedance Z(f) 0 Z(f) R = 100 ns frequency 3 Impac of a fron-chopped lighning surge on a ransformer peak 1.2 μs 2M/μs = 2M Lighning impulse chopped a fron by a spark gap peak = olage a ransformer Proeced wih spark gap = reference BIL level for a sandard ransformer The sandard BIL waveform (green line) is a reference. Experimens have demonsraed he effeciveness of his mehod in reducing he d/d resuling from ransiens associaed wih reigniions and presrikes in a circui breaker. The exernally insalled series choke elemen combined wih a small shun capacior reduced d/d o a safe level and also eliminaed he highfrequency oscillaions ha would oherwise have followed such a ransien [5]. The successful miigaion of swichinginduced d/d ransiens raised he quesion of wheher a similar approach could be used o miigae he lighninginduced chopped d/d ransiens o which pole-mouned disribuion ransformers are frequenly exposed. Furhermore, ABB sough o inegrae he device wihin he ransformer iself. The series choke elemen can be regarded as an alernaive o specially designed winding described above ha uses elecrosaic screens for he equalizaion of poenial disribuion a high frequencies. The series choke elemen forms a lowpass filer when combined wih he self capaciance of a ransformer (a high frequency he ransformer winding s characerisic can be represened by is surge capaciance). This capaciance varies depending on he ype and size of he device. For oil-filled ransformers i can be beween a single and a few nanofarads per phase. The frequency response of such a filer can hen be opimized by appropriae selecion of he R and L parameers. I mus furhermore be ensured ha he choke behaves correcly under normal operaing condiions and is able o wihsand he shor-circui es. Depending on he relaion beween he R, L, and C values, he circui response can be eiher aperiodic, or i can addiionally conain oscillaory componens. If he resisance value is lower han he criical value Rc, he periodic erms are equal o zero and he oupu volage is a combinaion of exponenial funcions. Therefore, selecing an appropriae value for he damping resisance is exremely imporan in order o achieve he highes possible reducion in d/d, while sill prevening overshoos and oscillaions of he volage downsream of he choke elemen 5. The choke is series-conneced upsream of he proeced equipmen. Is basic principle is o provide an appropriae impedance-frequency characerisic In his approach, he capaciance C is he equivalen phase-o-ground capaciance of he ransformer. Since, as menioned above, he limis of his capaciance are known for a given class of ransformer, he opimizaion of he R 70 ABB review 1 11

5 Concep of series device proecing he disribuion ransformer agains high d/d U peak 2M/μS Z(ω) <1M/μS 6 Transformer bushing wih inegraed SmarChoke filer and L values can be achieved o cover all ypical

Considering he idealized siuaion of 3, in which a 2 M lighning impulse is chopped wih a spark gap, he unproeced equipmen (eg, pole-mouned ransformer) would experience he phase-o-ground volage a a

4 5 Concep of series device proecing he disribuion ransformer agains high d/d U peak 2M/μS Z(ω) <1M/μS 6 Transformer bushing wih inegraed SmarChoke filer and L values can be achieved o cover all ypical ransformers of a cerain caegory. Considering he idealized siuaion of 3, in which a 2 M lighning impulse is chopped wih a spark gap, he unproeced equipmen (eg, pole-mouned ransformer) would experience he phase-o-ground volage a a peak above 270 k (characerized by a d/d of 2 M/μs). Wih he series choke elemen insalled upsream of he proeced device, he d/d is no only lower, bu he peak value of he volage surge reaching he ransformer is also reduced. 7 Experimenally measured reducion in he overvolage and d/d 8 SmarChoke-proeced, high d/d resisan ransformer by ABB 1 wihou SmarChoke 2 wih SmarChoke bushing SmarChoke-proeced ransformer The described series choke elemen concep has been implemened in new ABB disribuion ransformers, giving hem a superior proecion agains high Selecing an appropriae value for he damping resisance is exremely imporan in order o achieve he highes possible reducion in d/d. d/d ransiens. The SmarChoke elemen is inegraed wihin he ransformer bushing 6 and hus he filering of he high d/d ransien akes place before i reaches he winding. The parameers of he choke elemen embedded wihin he epoxy-cas ransformer bushing are seleced so ha he same bushing can be used o proec all ypical sizes of disribuion ransformers working in pole-mouned subsaions in disribuion neworks. Experimens performed have shown ha a reducion in d/d exceeds a facor of wo wih respec o he sandard ransformer bushing. A significan reducion in peak overvolage was also observed 7. A ransformer feauring he SmarChoke bushing 8 has been cerified according o he SFS 2646 sandard a he H Surviving a srike 71

5 9 Comparison of differen proecion scenarios Spark gap d/d of incoming surge d/d unchanged >> 1. Spark gap and convenional bushing ery high d/d and exceeding sandard BIL Non-sandard winding design needed Spark gap d/d of incoming surge d/d reduced 2. Spark gap and SmarChoke bushing d/d reduced and wihin sandard BIL sandard winding design MO d/d of incoming surge d/d unchanged < 3. MO and convenional bushing ery high d/d and below sandard BIL Non-sandard winding design may be needed Wojciech Piasecki Marek Florkowski Marek Fulczyk MO d/d of incoming surge d/d reduced < 4. MO and SmarChoke bushing d/d reduced and below sandard BIL sandard winding design ABB Corporae Research Cracow, Poland Experimens have shown ha reducion in d/d exceeds a facor of wo wih respec o he sandard ransformer bushing. laboraory of he Helsinki Universiy of Technology in Espoo, Finland. I was demonsraed ha he ransformer using a sandard winding design and ha has proecive chokes embedded in he bushings as well as a 2 40 mm spark gap could be safely exposed o a 2 M/μs lighning impulse. Successful proecion The use of a choke upsream of he equipmen o be proeced has been shown o represen an aracive alernaive o complex redesigning of he equipmen iself. Alhough chokes reduce d/d and consequenially also he peak value of he fron-chopped wave, i should be poined ou ha he primary funcion of he proecive device is he reducion in d/d of a ransien caused by a lighning surge. I is herefore complemenary o he sandard overvolage proecion provided, for example, by proecive spark gaps or meal oxide arresers 9. Paweł Kłys ABB Power Producs, Disribuion Transformers Lodz, Poland pawel.klys@pl.abb.com Egil Sryken ABB Power Producs, Disribuion Transformers Drammen, Norway egil.sryken@no.abb.com Pior Gawa d ABB Power Producs, Apparaus Przasnysz, Poland pior.gawad@pl.abb.com References [1] Agrawal, K. C. (2001). Indusrial Power Engineering and Applicaions Handbook. Newnes. [2] IEEE Working Group. (1981, Augus). Impulse volage srengh of AC roaing machines. In IEEE Transacions on Power Apparaus and Sysems: ol. PAS-100(8), ( ). [3] Finnish Sandards Associaion. (1987, June 29). SFS 2646,, Pole mouned subsaion. [4] Burrage, L. M., Shaw, J. H., McConnell, B. W. (1990, April). Disribuion ransformer performance when subjeced o seep fron impulses. In IEEE Transacions on Power Delivery, ol. 5(2). [5] Glinkowski, M., Piasecki, W., Florkowski, M., Fulczyk, M., Arauzo, F. (2008, May). Smar- Choke-proecing power equipmen from fas ransiens. Paper presened a IEEE PES Transmission and Disribuion Conference, Chicago, IL, Unied Saes. Tile picure Lighning srikes are a cause of seep volage ransiens ha can damage elecrical equipmen. 72 ABB review 1 11

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,