System earthing. Industrial Electrical Engineering and Automation. Anna Guldbrand. Dept. of Industrial Electrical Engineering and Automation

Transcription

1 CODN:UTDX/(T-76)/-/(6) ndustrial lctrical nginring and Automation Systm arthing Anna Guldbrand Dpt. o ndustrial lctrical nginring and Automation und Univrsity

2 ntroduction... Solidly arthd systms... solatd nutral systms... sistanc arthd systms...4 sonant arthd systm...7 ctivly arthd systms... Othr kinds o arthing... Comparison... rncs...

3 ntroduction Th systm arthing, that is to say th connction btwn th transormr nutral points and arth, is o high importanc to th bhaviour o a powr systm during an unsymmtrical ault. Th arthing dsign is considrd th singl most important paramtr to dtrmin th arth ault bhaviour in a powr systm []. A powr systm can hav mor than on nutral point. All nutral points o on systm do not hav to b connctd to arth, using th sam arthing mthod. Two important unctions o nutral arthing ar to dtct arth aults and to control th ault currnt, sinc larg ault currnts can caus th potntial ris o xposd parts o th powr systm to rach dangrous lvls. n this chaptr solidly arthing as wll as th thr most common typs o non-solidly nutral arthing; isolatd nutral, rsistanc arthing and rsonant arthing, will b discussd. Solidly arthd systms n a solidly arthd systm a numbr o th transormr nutrals ar dirctly arthd []. Figur shows an arth ault in a systm with a solidly arthd nutral. Figur, arth ault in a ntwork with a solidly arthd nutral Th singl-phas arth ault currnt in a solidly arthd systm may xcd th thrphas ault currnt. Th magnitud o th currnt dpnds on th ault location and th ault rsistanc. On way to rduc th arth ault currnt is to lav som o th transormr nutrals unarthd. Th main advantag o solidly arthd systms is low ovr voltags, which maks th arthing dsign common at high voltag lvls (HV). solatd nutral systms A systm whr all transormr nutrals ar unarthd is calld an isolatd nutral systm. Th only intntional connction btwn an unarthd nutral and arth is via high impdanc quipmnt or protction or masurmnt purposs [] such as surg arrstrs or voltag transormrs. n a powr systm thr ar howvr always capacitiv connctions btwn th phass and arth. Th strngth o th capacitiv connction dpnds on typ and lngth o th powr systm circuit. Whn an arth

4 ault occurs in th systm, th capacitanc to arth o th aulty phas is bypassd. Figur shows an arth ault in a systm with on unarthd nutral. Figur, arth ault in a ntwork with an unarthd nutral Figur 3 shows th Thévnin quivalnt o th ntwork with an unarthd nutral. Figur 3, Thévnin quivalnt o a ntwork with an unarthd nutral n th cas o a solid arth ault, th rsistiv connctions btwn phas and arth ar small nough to b nglctd. Th arth ault currnt, as wll as th nutral point displacmnt voltag, dpnds only on th phas to arth voltag and capacitancs. quation givs th, thror solly capacitiv, arth ault currnt. c j3ωc quation Th maximum arth ault currnt o an isolatd systm is small providing th systm s capacitiv connction to arth is wak. Th prsnc o a ault rsistanc mans a rsistiv part is addd to th systms quivalnt impdanc. Th rducd ault currnt will thror consist o a rsistiv and a capacitiv part. quation givs th arth ault currnt in cas o a non-solid arth ault.

5 r c r + + j ( 3ωC ) ( 3ωC ) ( 3ωC ) c 3ωC + quation Th ault currnt givs ris to a zro squnc voltag across th capacitancs. This voltag is calld th nutral point displacmnt voltag. n cas o a solid arth ault this voltag quals th pr-ault phas to arth voltag o th aulty phas. th arth ault is non-solid, part o th phas to arth voltag will b a across th ault rsistanc. quation 3 givs th nutral point displacmnt voltag. U n 3ωC quation 3 Figur 4 shows th pr-ault phas voltags, th nutral point displacmnt voltag and th voltag o th halthy phass during a phas-to-arth ault in an isolatd systm. Th voltag btwn th nutral point and th halthy phass will rmain unchangd during th ault. A nutral point displacmnt voltag thror rmands a chang in th halthy phas to arth voltag lvl. Th maximum voltag o th halthy phass is 5 % o th pr-ault phas-to-phas voltag. Figur 4, Pr-ault voltags U A, U B, U C, nutral point displacmnt voltag U and voltag o halthy phass U B, U C during a phas-to-arth ault in an isolatd systm n isolatd nutral systms som phas-to-arth aults ar clard without involving any rlay opration. This is normally a good thing but can, in cas o intrmittnt aults and nutral point displacmnt voltag, lad to ovr voltags and additional aults in th powr systm []. Th nutral point displacmnt voltag and th arth ault currnt control th snsitivity o th rlay protction. in an isolatd systm th capacitiv connction to arth is too wak, th ovr currnt rlays will not b abl to dtct arth aults o high nough ault rsistancs. Th rason or th diicultis is that th dirnc btwn th currnt masurd during aults with high rsistancs and currnts du to 3

6 unsymmtrical conditions at normal opration is small. Unsymmtrical conditions at normal opration rsult in an unsymmtrical currnt (zro squnc currnt) much lik high ault impdanc, singl-phas arth ault currnt. Th ovr currnt rlays must b programmd not to oprat at this lvl. arth ault currnt calculations carrid out in Matlab conirm that th dirnc in arth ault currnt btwn solid arth aults and vry high impdanc arth aults is small or powr systms with vry wak capacitiv connction to arth, that is small c, s Figur 5. Th lattr th curv btwn dsirabl ault rsistanc dtction lvl and normal opration asymmtry, th hardr to dtct th ault. Figur 5, Fault currnt as unction o ault rsistanc, or dirnt capacitiv connctions to arth Advantags o isolatd systms: Small arth ault currnts, providing limitd capacitiv connction to arth. arg shar o th aults ar sl-claring. Disadvantags: Strong capacitiv connction to arth gnrats xtnsiv arth ault currnts. Too wak capacitiv connction to arth will rsult in diicultis dtcting th arth aults. isk o ovr voltags. Bcaus o th risk o ovr voltags th us o isolatd nutral is rstrictd to low and mdium voltag []. sistanc arthd systms To improv th arth ault dtction in a powr systm a rsistanc can b connctd btwn a transormr nutral point and th station arthing systm. A systm whr at last on o th nutral points is connctd to arth via a rsistor is calld a rsistanc arthd systm. Th purpos o th nutral point rsistor is to incras th 4

7 rsistiv part o th arth ault currnt and hnc improv th arth ault dtction. Figur 6 shows an arth ault in a systm with a rsistanc arthd nutral. Figur 7 shows th corrsponding Thévnin quivalnt. Figur 6, arth ault in a ntwork with a rsistanc arthd nutral Figur 7, Thévnin quivalnt o a ntwork with a rsistanc arthd nutral n a systm with vry wak capacitiv connction to arth th ractanc o th arth capacitanc will b larg compard to th nutral point rsistanc. Th nutral point rsistanc, instad as or th isolatd systms th capacitiv connction to arth, will thror dtrmin th maximum arth ault currnt. quation 4 givs th arth ault currnt in cas o a solid arth ault. + >> C C + j3ωc quation 4 Th prsnc o a ault rsistanc rducs th arth ault currnt. quation 5 givs th arth ault currnt in cas o a non-solid arth ault, th phas to arth capacitanc nglctd. 5

8 + quation 5 As mntion in prvious sction, systms with a vry wak capacitiv connction to arth ar normally rsistanc arthd. To mak th dirnc btwn ault currnts o isolatd and rsistanc arthd systms visibl, arth ault currnt calculations hav bn carrid out using Matlab. Th rsulting currnt as unctions o ault rsistanc is shown in Figur 8. t is an obvious dirnc in arth ault currnt or ault rsistancs around 5 k ohm, whil th dirnc in ault currnt or vry high impdanc aults, and hnc unsymmtrical condition during normal opration, is small. th systms capacitiv arth ault currnt instad is A thr will hardly b any dirnc in arth ault currnt o isolatd and rsistanc arthd systms or ault rsistancs abov a coupl o thousands ohm. Figur 8, Fault currnt as unction o ault rsistanc, or an isolatd systm c and rsistanc arthd systm r As in th cas o a ault in an isolatd systm, th ault currnt givs ris to a nutral displacmnt voltag across th systm s impdanc to arth. n th cas o a rsistanc arthd systm th impdanc to arth is th nutral point rsistanc in paralll to th phas to arth capacitancs. quation 6 givs th nutral displacmnt voltag which in cas o a solid arth ault quals th pr-ault phas to arth voltag o th aultd phas. 6

9 U n quation 6 + ( 3ωC ) Advantags o high rsistanc arthd systms: nabls high impdanc ault dtction in systms with wak capacitiv connction to arth Som phas-to-arth aults is sl-clard. Th nutral point rsistanc can b chosn to limit th possibl ovr voltag transints to.5 tims th undamntal rquncy maximum voltag []. Transints ar urthr discussd in sction blow. Disadvantags: Gnrats xtnsiv arth ault currnts whn combind with strong or modrat capacitiv connction to arth. Cost involvd sonant arthd systm To limit th ractiv part o th arth ault currnt in a powr systm a nutral point ractor can b connctd btwn th transormr nutral and th station arthing systm. A systm in which at last on o th nutrals is connctd to arth via an inductiv ractanc, a Ptrsn coil, and th currnt gnratd by th ractanc during an arth ault approximatly compnsats th capacitiv componnt o th singl phas arth ault currnt, is calld a rsonant arthd systm. Th systm is hardly vr xactly tund, i.. th ractiv currnt dos not xactly qual th capacitiv arth ault currnt o th systm. A systm in which th inductiv currnt is slightly largr than th capacitiv arth ault currnt is ovr compnsatd. A systm in which th inducd arth ault currnt is slightly smallr than th capacitiv arth ault currnt is undr compnsatd. Figur 9 shows th arth ault currnt phasors o a slightly ovr compnsatd systm. Figur 9, arth ault currnt phasors o a slightly ovr compnsatd powr systm Th nutral point ractor is otn combind with a nutral point rsistor. n a rsonant arthd systm th rsulting ractiv part o th arth ault currnt is too small or th rlay protction to masur. By using a nutral point rsistanc a masurabl rsistiv arth ault currnt is cratd as xplaind in th sction about rsistanc arthd systms. n addition to this, thr will always b activ losss in th nutral point gnrator, which contributs to th activ part o th arth ault currnt. Typical xampls o powr systms with strong capacitiv connction to arth, suitabl or rsonant arthing, ar systms consisting o an xtnsiv amount o cabls. th high capacitiv arth ault currnt o such systms is not compnsatd, th risk o dangrously high potntial ris o xposd parts o th powr systm is vidnt. 7

10 Figur shows an arth ault in a systm with a rsonanc arthd nutral. Figur shows th corrsponding Thévnin quivalnt. Figur, arth ault in a ntwork with a rsonant arthd nutral Figur, Thévnin quivalnt o a ntwork with a rsonant arthd nutral Th arth ault currnt is mad up o th capacitiv currnt du to th phas to arth capacitancs o th systm, th inductiv currnt gnratd in th nutral point ractor, th rsistiv currnt du to losss in th ractor paralll to th nutral point rsistor. quation 7 givs th singl-phas arth ault currnt in cas o a solid arth ault. ( + ) quation C j 3ωC ωc n cas o complt compnsation th solid arth ault currnt, givn by quation 8, is solly rsistiv. ( + ) quation 8 Th prsnc o ault rsistanc rducs th arth ault currnt as givn by quation 9. 8

11 X 3ωC ( + ) + X + ( + ) + ( X ) + quation 9 ω jx n cas o complt compnsation th arth ault currnt is solly rsistant as givn by quation. ( + ) + + quation sonanc arthing maks it possibl to mor or lss liminat th ractiv arth ault currnt. Th us o non-idal nutral point ractors and transportation o ractiv currnt dos howvr caus rsistiv losss. Th losss gnrat rsistiv arth ault currnts, which cannot b compnsatd. For powr systms with vry high capacitiv connction to arth and xtnsiv ractiv currnt transportation, ths rsistiv arth ault currnts may caus problm. On way to limit th rsistiv arth ault currnts is to us distributd compnsation. Chaptr x which is a rsum o an FOSK projct carrid out during th spring o 6 urthr looks into whn distributd compnsation might b ncssary and how it can b carrid out. Th rsistiv losss ar hard to prdict in such way analytical arth ault calculations is possibl to carry out. sults rom PSCAD simulations o simpl powr systms ar prsntd in th FOSK rsum chaptr. quation givs th nutral displacmnt voltag, th voltag across th systm s impdanc to arth. Th maximum, solid arth ault, nutral point displacmnt voltag o a rsonant arthd systm quals th pr-ault phas to arth voltag. n cas o high ault rsistanc th nutral point displacmnt voltag is highr than or corrsponding ault rsistanc in an isolatd systm [rror! rnc sourc not ound.]. U quation + 3ωC ω Advantags o rsonant arthd systms: Small ractiv arth ault currnt indpndnt o th phas to arth capacitanc o th systm. nabls high impdanc ault dtction. Disadvantags isk o xtnsiv activ arth ault losss Complicatd rlay protction 9

12 High costs associatd. ctivly arthd systms A systm in which th valu o th phas to arth voltag o th halthy phass during an arth ault, nvr xcd.39 tims th pr-ault phas to ground voltag is ctivly arthd, s quation. Th rlation btwn maximum voltag and prault voltag is calld th arth-ault actor. n practic most o th transormrs nutral points ar solidly arthd or arthd via vry small impdancs in an ctivly arthd systm []. n quation 3, th arth ault actor in cas o a solid arth ault is xprssd in trms o symmtrical componnts. Th colourd ara in Figur illustrats th rlation btwn /X and X /X in an ctivly arthd systm. U hp.39 quation X U hp, X.5 ± quation 3 ' X, X j.87 X ', ' ' ( ) + j ( X ) ' ' ( ) + j ( X ).39 Figur, Th colourd ara illustrats th rlation btwn /X (y-axis) and X /X (x-axis), in an ctivly arthd systm

13 Othr kinds o arthing Thr ar othr ways to labl arthing mthods. mpdanc arthd systms includs all systm arthd via an impdanc, intndd to limit th arth ault currnt []. Th systms can b arthd via a rsistanc, rsistanc arthd systm, a ractanc, ractanc arthd systm, or a combination o ths. n practic thr ar no such thing as an idal ractor and strictly ractanc arthd systms dos thror not xist in rality. Arc-supprssion-coil arthing as wll as Ptrsn arthd systms and systms with compnsatd nutrals ar othr nams or th rsonant arthd systms, th tund impdanc arthd systms xplaind abov. Comparison t is important to kp in mind that th arth ault currnt limitation countr acts th arth ault dtction, no mattr what arthing mthod is chosn. Th arthing dsign shall limit th arth ault currnt and nabl ault dtction whil kping th associatd costs as low as possibl. Th cost associatd to th arthing o isolatd systms is rlativly low. Whn possibl, powr systms ar thror otn isolatd. t is not suitabl isolat th nutral points o th systms i th systm s capacitiv connction to arth is ithr vry wak or quit strong. th capacitiv connction to arth is vry wak nutral point rsistors ar connctd to th systm. th capacitiv connction to arth on th othr hand is quit strong it is ncssary to connct nutral point ractors, Ptrsn coils, to compnsat or th capacitiv arth ault currnt. On o th aims o this thsis is to urthr look into whn it is suitabl to us a crtain arthing dsign. Sinc th Swdish distribution systms ar o spcial intrst in th PhD-projct, xtra attntion will b givn to th arthing mthods suitabl or Swdish conditions. Th arth ault currnt o an isolatd systm is small as long as th ntworks capacitiv connction to arth is limitd. n th past, a majority o th circuits in Swdish mdium voltag systms has bn ovrhad lins, which hav wk capacitiv connction to arth. Th Swdish MV systms hav thror to a larg xtnt bn isolatd. With th incrasd us o powr cabls, which has a strong capacitiv connction to arth, rsonant arthd systms ar bcoming incrasingly popular in th Nordic countris [] bcaus today rsonant arthing is th only arthing mthod that manags high capacitiv arth ault currnts.

14 rncs. htonn, M. & Hakola, T.: Nutral arthing and powr systm protction, SBN , ABB Transmit Oy, Vaasa 996. C multilingual dictionary, Sixth dition, 5 3. indahl, S., Mssing,., Östlund, S., Olsson, B. & Pttrson A.: Känsliga jordlsskydd. Bortkoppling av högrsistiva jordslutningar i ick dirkt jordad distributions- och transmissionssystm, Svnska lvrksörningn, Stockholm, Willhim,. & Watrs, M.: Nutral grounding in high voltag transmission, lsvir, Nw York, 956