DEPARTMENT OF ELECTRICAL ENGINEERING DIT UNIVERSITY HIGH VOLTAGE ENGINEERING

Transcription

1 DEPARTMENT F ELECTRICAL ENGINEERING HIGH VLTAGE ENGINEERING UNIT 1: BREAKDWN IN GASES 1.) Itrducti: I mder times, high vltages are used fr a wide variety f applicatis cverig the pwer systems, idustry ad research labratries. High vltages are applied i labratries i uclear research, i particle acceleratrs ad Va de Graff geeratrs. Fr trasmissi f large bulks f pwer ver lg distaces, high vltages are idispesable. Als vltages upt 100 kv are used i electrstatic precipitatrs, i autmbile igiti cils etc. X ray equipmet fr medical ad idustrial applicatis als uses high vltages. Mder high vltage test labratries emply vltages upt 6 MV r mre. The diverse cditis uder which a high vltage apparatus is used ecessitates careful desig f its isulati ad the electrstatic field prfiles. The priciple media f isulati used are gases, vacuum, slid ad liquid r a cmbiati f these. Fr achievig reliability ad ecmy, kwledge f the causes f deterirati is essetial ad the tedecy t icrease the vltage stress fr ptimum desig calls fr judicius selecti f isulati i relati t the dielectric stregth, cra discharges, ad ther relevat factrs. 2.) Cducti ad Breakdw i Gases: The simplest ad the mst cmmly fud dielectrics are gases. Mst f the electrical apparatus use air as the isulatig medium, ad i a few cases ther gases such as itrge, carb dixide, Fre ad sulphur hexafluride (SF6) are als used. Varius pheme ccur i gaseus dielectrics whe a vltage is applied. Whe the applied vltage is lw, small currets flw betwee the electrdes ad the isulati retais its electrical prperties. the ther had, if the applied vltages are large, the curret flwig thrugh the isulati icreases very rapidly ad a electrical breakdw ccurs. A strgly cductig spark frmed durig breakdw practically prduces a shrt circuit betwee the electrdes. The maximum vltage applied t the isulati at the mmet f breakdw is called the breakdw vltage. The electrical discharges i gases are f tw types i. N sustaiig discharges ii. Self sustaiig discharges The breakdw i a gas called spark breakdw is the trasiti f a sustaiig discharge it a self sustaiig discharge. The build up f high currets i a breakdw is due t the prcess kw as iizati i which electrs ad is are created frm eutral atms r mlecules, ad their migrati t the ade ad cathde respectively leads t high currets. Tw theries (i) Twsed thery ad (ii) Streamer thery are kw which explai the mechaism fr breakdw uder differet cditis. 3.) Cllisi Prcesses: A electrical discharge is rmally created frm uiized gas by cllisi prcesses. These prcesses are maily gas prcesses which ccur due t the cllisi betwee the charged particles ad gas atms r mlecules. These are f the fllwig tw types: Elastic cllisis: These are cllisis which whe ccur, chage takes place i the iteral eergy f the particles but ly their kietic eergy gets redistributed. These cllisis d t ccur i practice. A sigle electr mvig i a electric field gais very high eergy ad cllisi with a gas mlecule, it trasfers ly a part f its kietic eergy (beig light i weight) hece it mves at a much higher speed tha the is. Ielastic cllisis: These are the cllisis i which iteral chages i eergy takes place withi a atm r a mlecule resultig i decrease i ttal kietic eergy f the cllidig particle. The cllisi fte results i a chage i the structure f the atm. Thus all cllisis which ccur i practice are ielastic cllisis. 1

2 DEPARTMENT F ELECTRICAL ENGINEERING HIGH VLTAGE ENGINEERING 4.) Iizati Prcesses: A gas i its rmal state is almst a perfect isulatr. Hwever, whe a high vltage is applied betwee the tw electrdes immersed i a gaseus medium, the gas becmes a cductr ad a electrical breakdw ccurs. The prcesses which are respsible fr the breakdw f a gas are iizati by cllisi, pht-iizati ad secdary iizati prcesses. i. Iizati by Cllisi: The prcess f liberatig a electr frm a gas mlecule with the simultaeus prducti f a psitive i is called iizati. I this prcess f cllisi, a free electr cllides with a eutral gas mlecule givig rise t a ew electr ad psitive i. A few f the electrs are prduced at the cathde by ultra vilet light fallig it which the iize eutral gas particles prducig psitive is ad additial electrs. The additial electrs i tur themselves make iizig cllisis ad thus the prcess repeats itself. This represets a icrease i the electr curret sice the electrs reachig the ade per uit time are mre tha thse liberated at the cathde. I additi t this, the psitive is reachig the cathde give rise t secdary electrs by bmbardig the cathde. ii. iii. 2 Pht Iizati: Ather pheme which causes the breakdw f gases is phtiizati. I these pheme phts cllisi with the eutral gas atm r mlecule liberate free electrs ad frm psitive is whe the amut f radiati eergy absrbed by the atm r mlecule is mre tha its iizati ptetial. There are several prcesses by which radiati ca be absrbed by atms r mlecules. They are (a) Excitati f the atm t a higher eergy state. (b) Ctiuus absrpti by direct excitati f the atm r disassciati f diatmic mlecule r direct iizati. Just as a excited atm emits radiati whe the electrs retur t the lwer state r t the grud state, the reverse prcess takes place whe a atm absrbs radiati. This reversible prcess ca be expressed as * hν + A A h Iizati ccurs whe λ c where h is Plack s cstat, c is the velcity f light, V i λ is the wavelegth f the icidet radiati ad Vi is the iizati eergy f the atm. The higher the iizati eergy, the shrter will be the wavelegth f the radiati capable f causig iizati. It was bserved experimetally that a radiati havig a wavelegth f 1250 A0 is capable f causig pht iizati f almst all the gases. Secdary Iizati Prcesses: The iizati prcesses which prduce secdary electrs are the es which sustai a discharge after it is established due t iizati by cllisi ad pht iizati. They are as fllws: (a) Electr emissi due t psitive i impact: Iizati by cllisi r pht iizati creates psitive is which mve twards cathde. A psitive i apprachig a metallic cathde ca cause emissi f electrs frm the cathde by givig up its kietic eergy impact. If the ttal eergy f the psitive i is greater tha twice the wrk fucti f the metal the e electr will be ejected ad a secd electr will eutralize the i. The prbability f this prcess is measured by Twsed s secdary iizati cefficiet γ (b) Electr emissi due t phts: The phts f ultravilet light f suitable frequecy strikig the surface f cathde cause emissi f electrs whe the eergy f pht is mre tha the wrk fucti f the metallic cathde. If the icidet radiati has a frequecy greater tha the threshld frequecy, the the

3 DEPARTMENT F ELECTRICAL ENGINEERING HIGH VLTAGE ENGINEERING excess eergy is partly cverted t kietic eergy f emitted electr ad partly t heat the surface f the electrde. The threshld frequecy lies i the far ultravilet regi f the electrmagetic radiati spectrum. (c) Electr emissi due t metastable ad eutral atms: A metastable atm r mlecule is a excited particle whse lifetime is very large (10 3 s) cmpared t the lifetime f a rdiary particle (10 8 s). Electrs ca be ejected frm the metal surface by the impact f metastable atms prvided that their ttal eergy is sufficiet t vercme the wrk fucti. Neutral atms i the grud state als give rise t secdary electr emissi if their kietic eergy is high. (iv) Electr attachmet prcess: The types f cllisi i which electrs may becme attached t atms r mlecules t frm egative is are called attachmet cllisis. The prcess f electr attachmet depeds up the eergy f the electr ad the ature f the gas. All electrically isulatig gases such as C2, 2, Cl2, F2, C2F6, C3F8 ad SF6 exhibit this prperty. 5.) Twsed s Breakdw Mechaism: A iitial free electr at the surface f the cathde acquires sufficiet eergy i the presece f the field applied t release further electrs frm the gas atms r mlecules by cllisis as it mves twards the ade. These emitted electrs further acquire eergy frm the field ad release mre electrs frm eighburig gas atms by cllisi frm a srt f avalache as shw i fig. (a) A sigle avalache is attracted by the ade because f itese egative charge at its head with a velcity higher tha the drift velcity f psitive is as they are heavier. As cmpared t drift f electrs, the psitive is clud is almst statiary. The sigle avalache prcess becmes cmplete whe the iitial set f electrs reach the ade. Hwever sice the amplificati f electrs is ccurrig i the field, the prbability f additial ew electrs beig liberated i the gap by ther mechaisms icreases ad these ew electrs create further avalaches as shw i fig. (b). The ther mechaisms are (i) psitive is liberated cause liberati f electrs frm the cathde whe they impige it (ii) the excited atms r mlecules i avalaches may emit phts ad this will lead t the emissi f electrs due t phtemissi (iii) the metastable particles may diffuse back causig electr emissi. The electrs prduced by these prcesses are called secdary electrs. The secdary iizati cefficiet γ kw as Twsed s secdary iizati cefficiet is defied as the et umber f secdary electrs prduced per icidet psitive i, pht, excited particle r metastable particle. The ttal value f γ is the sum f the idividual cefficiets due t three differet prcesses i.e. γ γ1 + γ2 + γ3 ad it is a fucti f the gas pressure p ad E/p. Twsed s curret grwth equati: Let us assume that electrs are emitted frm the cathde. Whe e electr cllides with the eutral particle, a psitive i ad a electr are frmed. This is called iizig cllisi. Let α be the average umber f iizig cllisis 3

4 DEPARTMENT F ELECTRICAL ENGINEERING HIGH VLTAGE ENGINEERING made by a electr per cetimeter travel i the directi f the field. α is als kw as Twsed s first iizati cefficiet ad it depeds gas pressure p as well as E/p. Let the umber f electrs be x at ay distace x frm the cathde. Whe these x electrs travel a further distace f dx they rise t αxdx electrs. At x 0, x dx Als dx be d e. The average curret i the gap which is equal t the umber f electrs αx α xrx e the the umber f electrs reachig ade (x d) will travellig per secd will be I I e where I is the iitial curret at the cathde. Let umber f secdary electrs prduced due t secdary prcesses, electrs leavig the cathde the + The ttal umber f electrs reachig the ade becmes. " e ( + ) e ad γ [ ( + )] Elimiatig we get 0e 1 γ ( e α d Twsed s Criteri fr Breakdw: 1) I r I Ie α 1 γ ( e 1) d Ie α 1 γ ( e 1) d ttal umber f gives the ttal average curret i a gap befre the ccurrece f breakdw. As the distace betwee the electrdes d is icreased, the demiatr f the equati teds t zer, ad at sme critical distace d ds the term 1 γ ( e 1) 0. Fr values f d < ds, I is apprximately equal t I0 ad if the exteral surce fr the supply f I0 is remved, I becmes zer. If d ds the I appraches ifiity ad the curret will be limited ly by the resistace f the pwer supply ad the exteral circuit. This cditi is called Twsed s breakdw criteri ad ca be writte asγ ( e 1) 1. Nrmally e is very large ad hece we ca have γ ( e ) 1. Fr a give gap spacig ad at a give pressure the value f the vltage V which gives the values f α ad γ satisfyig the breakdw criteri is called the breakdw r sparkver vltage Vs ad the distace ds is called the sparkig distace. The Twsed s mechaism explais the pheme f breakdw ly at lw pressure crrespdig t pd (pressure p i trr X distace d i cm) values f 1000 trr-cm ad belw. Curret grwth i presece f secdary prcesses: Let us assume umber f secdary electrs prduced due t secdary prcesses " ttal umber f electrs leavig the cathde. " The + The ttal umber f electrs reachig the ade becmes " e ( + ) e Elimiatig " we get e Ie r I α 1 γ [ e 1] 1 γ [ e 1] d Breakdw i electregative gases: It has bee recgized that e prcess that gives high breakdw stregth t a gas is the electr attachmet i which free electrs get attached t eutral atms r mlecules t frm egative is. Sice egative is are t massive t prduce iizati due t cllisis, attachmet presets a effective way f remvig electrs which 4

5 DEPARTMENT F ELECTRICAL ENGINEERING HIGH VLTAGE ENGINEERING therwise wuld have led t curret grwth ad breakdw at lw vltage. The gases i which attachmet plays a active rle are called electregative gases. The mst cmm attachmet prcesses ecutered i gases are (a) the direct attachmet i which a electr directly attaches t frm a egative i ad (b) the dissciative attachmet i which the gas mlecule splits it its cstituet atms ad the electregative atm frms a egative i. These prcesses may be symblically represeted as: AB + e AB + hv direct attachmet prcess AB + e A + B + e dissciative attachmet prcess A simple gas f this type is xyge. ther gases are sulphur hexafluride, Fre, carb dixide ad flurcarbs. With such gases the Twsed s curret grwth equati is mdified t iclude iizati ad attachmet. A attachmet cefficiet η is defied as the umber f cllisis made by e electr driftig e cm i the directi f the field. 6.) Streamer s Thery f Breakdw i Gases: The Twsed s mechaism whe applied t breakdw at atmspheric pressure was fud t have certai drawbacks. Firstly, accrdig t the Twsed s thery, curret grwth ccurs as a result f iizati prcesses ly but i practice breakdw vltages were fud t deped the gas pressure ad the gemetry f the gap. The predicted time lags as per Twsed s thery were f the rder f 10 5 sec whereas i actual practice breakdw was bserved t ccur at very shrt times f 10 8 sec. The actual discharge was fud t be filametary ad irregular whereas as per Twsed s mechaism it was srt f diffused frm. The Twsed s mechaism failed t explai all these bserved phemea ad i 1940 Raether, Meek ad Leb idepedetly prpsed the streamer thery. It was bserved that whe charge ccetrati is betwee 10 6 ad 10 8, the grwth f the avalache becmes weak. the ther had, whe the charge ccetrati is higher tha 10 8, the avalache curret is fllwed by a steep rise i the curret betwee the electrdes leadig t the breakdw f the gap. The rigially applied uifrm field E is mdified by the space charge P as the avalache prgresses alg the gap as shw i fig. 1 + _ Er E(x) Er Fig.1 Field distrti i a gap due t space charge d 5

6 DEPARTMENT F ELECTRICAL ENGINEERING HIGH VLTAGE ENGINEERING It is assumed fr simplicity that the space charge at the head f the avalache is spherical vlume ctaiig egative charge at its tp because f higher electr mbility. Uder these cditis, the field gets ehaced at the tp f the avalache with field lies termiatig its head. Accrdig t the streamer thery, whe the avalache i the gap reaches a critical size, the cmbied applied field ad the space charge field causes itese iizati ad excitati f the gas particles i frt f the avalache. The istataeus recmbiati betwee psitive is ad electrs releases phts which i tur prduce secdary electrs by pht iizati. These secdary electrs uder the ifluece f the field i the gap develp it secdary avalaches as shw i fig.2. Cathde Ade Sice phts travel with the velcity f light, the pht-iizati prcess gives rise t rapid develpmet f cducti chaels acrss the gap. the basis f experimetal bservatis, Reather prpsed a empirical expressi fr the streamer spark criteri f the frm αx c l x c + l (E r /E) Where Er is the space charged field directed radially at the head f the avalache ad E is the applied field. The cditis fr the trasiti frm the avalache t streamer assumes that the space charged field appraches the exterally applied field ad hece the breakdw criteri becmes αx c l x c The miimum breakdw value fr a uifrm field gap by streamer mechaism is the btaied the assumpti that the trasiti frm a avalache t a streamer ccurs whe the avalache has just crssed a gap d. Thus, a miimum breakdw vltage by streamer mechaism ccurs ly whe a critical legth xc d. Accrdig t Meek, the electric field that trasfrms a avalache it a streamer is give by αx 7 αe Er V/cm ( x p) 1/ 2 where α is Twsed s first iizati cefficiet, p is the gas pressure i trr, ad x is the distace t which the streamer has exteded i the gap. Accrdig t Meek, the miimum breakdw vltage is btaied whe Er E ad x d i the abve equati. The pricipal pstulates f streamer breakdw theries are as fllws: a) There is a frmati f avalache by a iitiati electr by Twsed s α prcess (primary prcess). b) At the head f the avalache there is a large lcal ehacemet f the electric field due t i space charge. 6 Fig. 2

7 DEPARTMENT F ELECTRICAL ENGINEERING HIGH VLTAGE ENGINEERING c) The secdary prcesses result i large amut f pht iizati f gas mlecules i the space ahead f the avalache. 7.) Pasche s Law: Accrdig t Pasche s law the breakdw vltage is a fucti f prduct f pressure f the gas ad the distace betwee the electrdes. The breakdw criteri i gases is give as γ [ 1] 1 e α E E Where the cefficiets α ad γ are fuctis f E/p i.e. f1 adγ f 2. Substitutig p p p fr E i the expressis fr α ad γ we have V V f 2 [exp pdf1 1 1 pd pd The abve equati shws a relatiship betwee V ad pd, implyig that the breakdw vltage varies as the prduct pd varies. We ca write the equati as V f(pd). The Pashce s curve shws the relatiship betwee V ad pd beig liear i ature ad has a miimum value fr ay gas. V Breakdw Vltage (V) VS (VS)mi pd1 (pd) mi pd1 pd(trr-cm) 8.) Breakdw i N Uifrm Fields ad Cra Discharges: If the electric field is uifrm, a gradual icrease i vltage acrss a gap prduces a breakdw f the gap i the frm f a spark withut ay prelimiary discharges. If the field i uifrm, a icrease i vltage will first cause a discharge i the gas t appear at pits with highest electric field itesity, amely at sharp pits r where the electrdes are curved r trasmissi lies. This frm f discharge is called a cra discharge ad ca be bserved as bluish lumiescece. This pheme is always accmpaied by a hissig ise ad the air surrudig the cra regi becmes cverted it ze. Cra is als respsible fr csiderable lss f pwer frm high vltage trasmissi lies which leads t deterirati f isulati ad als causes radi iterferece. the high vltage cductrs at high pressures there is a distict differece i the visual appearace f the cra uder psitive ad egative plarities f the applied vltage. Whe the vltage is psitive, cra appears as a uifrm bluish white sheath ver the etire surface f the cductr. the ther had, whe the vltage is egative, the cra will appear like reddish glwig spts distributed alg the legth f the wire. It has bee bserved that whe a high vltage is applied acrss tw electrdes separated by air gap such that e electrde is plae 7

8 DEPARTMENT F ELECTRICAL ENGINEERING HIGH VLTAGE ENGINEERING shaped ad the ther electrde is pit shaped the the behaviur f cra is differet with applicati f differet plarity f applied vltage acrss the air gap. Whe the plarity f the pit electrde is egative the cra appears as curret pulses called Trichel pulses whse repetiti frequecy f these pulses icreases as the applied vltage is icreased ad decreases with decrease i pressure. the ther had, whe the plarity f the pit electrde is psitive the the cra curret icreases steadily with vltage. At sufficietly high vltage, curret amplificati icreases rapidly with vltage, upt a curret f abut 10-7 A, after which the curret becmes pulsed with repetiti frequecy f abut 1 khz cmpsed f small bursts. This frm f cra is called burst cra. The average curret the icreases steadily with applied vltage leadig t breakdw. 9.) Practical Csideratis i Usig Gases & Gas Mixtures fr Isulati Purpses: The majrity f the requiremets f a gd dielectric d t exist i a majrity f the gases. The fllwig are the geeral preferred prperties f a gaseus dielectric fr high vltage applicati: a) High dielectric stregth. b) Chemical stability, chemical iertess ad txicity. c) N flammability ad physilgical iertess. d) Evirmetally hazardus. e) Gd heat trasfer. f) Ready availability at mderate cst. g) Thermal stability ad gd thermal cductivity particularly whe the gas is als used as clig medium. h) Lw liquefacti temperature s that the gas ca be used at high pressure. 10.) Miimum Breakdw Vltage: The Twsed s first iizati cefficiet α is defied as the average umber f iizig cllisis made by e electr per cm drift i the directi f the electric field. Hwever t all the cllisis made by a electr while travellig twards ade will result i iizati. The mea free path f a electr i a gas is defied as the average distace it travels betwee successive cllisis (which eed t be ecessarily iizig). Frm the experimetal data Twsed gave the fllwig equati which related α, gas pressure p ad electric field E as fllws: α Bp / E Ae p Where A ad B are cstats depedig up the gas used. The abve equati was fud t be i gd agreemet with experimetal results ver a limited rage f E/p. The Twsed s secd iizati cefficiet γ is defied as the average umber f electrs emitted frm the cathde surface by the bmbardmet f e psitive i ad it depeds the ature f the gas, electrde material ad the average eergy gaied by the psitive i frm the electric field betwee cllisis. The Twsed s criteri fr breakdw is give by: 1 l 1 + (1) γ Bp E Bpd E Nw α Ape Ape. substitutig the value f α i equati (1) we get Bpd / 1 b Apde V l 1 + r γ Takig atural lg f bth the sides f equati (2) we get Bpd / Vb e Apd l ( 1+ 1γ) (2) 8

9 DEPARTMENT F ELECTRICAL ENGINEERING HIGH VLTAGE ENGINEERING Bpd Vb Apd l l ( 1+ 1γ) r Bpd V b (3) Apd l l ( 1+ 1γ) dv Fr miimum breakdw vltage Vbmi, b 0 d( pd) Differetiatig equati (3) with respect t pd ad equatig the derivative t 0 we get B B 0 r 2 Apd l Apd 1 l Apd l( 1+ 1γ) l( 1+ 1γ) l l( 1 1 ) + γ e r ( ) 1 pd + mi l 1 A γ Substitutig the value f pd frm abve equati i equati (3) we get B 1 B 1 V b mi e l l 1 + (4) A γ A γ The equatis (3) ad (4) maybe used fr the estimati f the breakdw vltage at a certai pd value ad the miimum breakdw vltage fr the gas with fair accuracy if mre exact data is t available. Example 1: The first iizati cefficiet fr air is apprximately represeted by α/p 14e 240p/E, where p pressure i mm f Hg ad E electric field i V/cm. fid the pressure at which the electr multiplicati is maximum. If this ccurs at 3 mm f Hg, fid the value f E ad the multiplicati btaied whe the electrdes are 5 mm apart. Sluti: The electr multiplicati is maximum whe α is maximum therefre dα d 240p E 0i. e. ( 14pe ) 0 dp dp 240p E E r 14e + p E 14p e 0r p E 240 Ad α 14pe 240p/E 14 x 3e ( 240x3 / 720) Therefre E p x x V/cm Therefre electr multiplicati e e 15.45x