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Arresterfacts 016 Selecting Arrester Mcov-uc

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ArresterFacts 016 Selecting Arrester MCOV and Uc Part 1 of Arrester Selection Guide Prepared by Jonathan Woodworth Consulting Engineer  ArresterWorks Feb 12 200! ArresterFacts 016 Selecting Arrester MCOV and Uc Selecting Arrester MCOV and U c Contents • Introducti on Relevant Defnition s Syste Confgura tions • • Deterining !ine" #nd Voltage Deterining Voltage Rise due to Fault Using t$e %OV Curve • • • •  %ransission !ine Arresters Suary Recoended Ratings  %a&les • • applied continosly between the terminals of an arrester. #uty*cycle +oltage rating ("EEE) # The designated maximm permissible voltage between its terminals at which an arrester is designed to perform its dty cycle test. ,&' Cur+e 'A graph that shows the power fre&ency withstand voltage vs. time for arrester from .) sec to )* sec ("EEE* "EC! -round Fault  'An event There are several necessary where crrent steps that need to be taken when flows from the selecting an arrester for an power system application. An early step in this selection is the determination of to grond the voltage rating of the arrester. when a system phase The only voltage rating of an arrester that is important is the condctor is MCOV (Maximm Continos connected to Operating Voltage "EEE ! and "c earth either (Continos Operating Voltage throgh a "EC!. This MCOV # "c rating direct contact however is not always obvios or throgh an withot a fairly good arc. ("EEE* "EC! nderstanding of the system to which it is to be applied. The ob$ective of this Arrester%acts is to make this decision clearer and nderstandable. "ntroduction #e$initions %C&' rating ("EEE) # The maximm designated root#mean s&are (rms! vale of power fre&ency voltage that may be .c * Continuous operating +oltage ("EC) The designated permissible r.m.s. vale of power# fre&ency voltage that may be applied continosly between the arrester terminals indefinitely. .r * /ated +oltage o$ an arrester ("EC) Maximm permissible r.m.s. vale of power# fre&ency voltage between its terminals at which it is designed to operate correctly nder temporary overvoltage conditions as established in the operating dty tests. +OT, ) The rated voltage is sed as a reference parameter for the specification of operating characteristics. +OT, The rated voltage as defined in this standard is the ) s power#fre&ency voltage sed in the operating dty test after high#crrent or long#dration implses. Tests sed to establish the voltage rating in ,C /00#)* as well as some national standards* involve the application of repetitive implses at nominal crrent with power fre&ency voltage applied. Attention is drawn to the fact that these two methods sed to established rating do not necessarily prodce e&ivalent vales. phase to phase mst be taken voltage divided into by ).23. %or consideration. f example* on a the reglation is #eterining ine*-nd 2/kV )6* then for 'oltage and %iniu transmission example* on the %C&' or .c system* the above system* 1hen arresters are applied to nominal system the line to protect systems from lightning or  phase to phase grond voltage switching srges* they are voltage is 2/kV cold be 44x installed between the phase and therefore the line ).) 7 489kV. earth. %or this application* the to earth voltage The MCOV or MCOV of the installed arrester wold be 44kV. "c or an arrester  mst be e&al or higher to the 5ince all for this system at continos voltage between the systems have a minimm phase and earth. On three some reglation shold be phase systems* the line to error* this too 489kV. grond voltage is e&al to the Arrester *00+"  -onat$an -. (ood/ort$ Co'yrig$t (or)s *00, age* ArresterFacts 016 Selecting an Arresters MCOV or Uc needed for selection is to Typic know how the al IEC Typical IEEE system netral Syste System m Voltages condctor is Volta om Ma! Ma! sed in the ges "ine "ine circit if there "ine to Ma! to "ine to Grnd ominal Typical is one. The "ine "ine Voltage Voltage Ma! Voltage power sorce "ine to "ine Minimum to Grnd #V rms #V rms #V transformer rms "ine to "ine Uc Voltage *.0 *.2* 1.6 and the Voltage Voltage .16 .34 *.2* netral #V rms #V rms #V rms #V rms .+0 2.0 *.,1 bonding 3.4 *.1 3.3 *.1 6.,0 4.*2 .1, scheme 6.6 .* 4.3 .* determine how +.3* +.4 2.02 6.6 10.0 11.2 6.6 high the line to 1*.0 1*.6 4.*+ 6., 11.0 1*.0 6., grond 1*.2 13.1 4.24 16. 1+.0   10. 10. voltage of the 13.* 13., +.01 **.0 *.0   13., 13., nfalted 13.+ 1.2 +.3+ 33.0 36.3   *1.0 *1.0 phases *0.+ *1.+ 1*.6   30.1 4.0 2* 30.1 **., *.0 13.,   1.6 66.0 4* 1.6 *3.0 *.* 1.0 ,1.0 100   24.+ 24.+ *., *6.* 12.1 110.0 1*3   41.1 41.1 *4.6 *,.0 16.+ 13*.0 12   +3.+ +3.+ 3.2 36.* *0., 122.0 140   ,+.3 ,+.3 6.0 +.3 *4., **0.0 *2   1* 1* 6,.0 4*.2 1., *42.0 300   143 143 112.0 1*1 6,.+ 330.0 36*   *0, *0, 13+.0 12 +3.+ 00.0 *0   *3 *3 161.0 16, ,+ *30.0 32.0 200.0 462.0 ** 36* 2*2 +00 10 *0, 303 6* yste Con$igurations Once the system voltages are nderstood* the next step in the selection process is to determine the system configration to which the arrester will be applied. n other words* one mst determine if it is a wye or delta system (star or delta in the ,C world!. Also will rise dring a grond falt. %ortnately the nmber of system configration s are limited. The most common ,,, configration is the 4 wire solid mlti# gronded netral as shown in figre -a. This is also known as an effectively gronded system. Co'yrig$t Arrest er(or $igure %a Solidly Multi& grounded ' (ire system )s  -onat$an -. *00, (ood/ort$ age3 ArresterFacts 016 Selecting an Arresters MCOV or Uc and -d show these two  A common systems. transmission line configration is the single gronded 1ye as seen in %igre -d . $igure %) Impedance or *esonant Grounded System  A common indstrial and very common ,C configration is the 3 wire impedance gronded wye (or star!. The reason for poplarity of this system is that the falt crrent to earth is limited by the impedance. 1hen low impedance is sed* it can limit the falt crrent to levels that allow for lower falt crrent rated e&ipment to be sed on the system. This is often a cost savings configration. 1hen the impedance is high* a :etersen coil is sed which can offer falt extingishing capabilities withot sing breakers to break the falt. This is sometimes referred to as a resonant gronded system.  A third common system configration is an isolated or ngronded system. This can be either delta or wye configred. %igre -c $igure %c Ungrounded systems +isolated neutral, $igure %d Single grounded neutral system +Uni& grounded system, #eterini ng Phase 'oltage /ise due to Earth or  -round Faults 1hen a three phase power system experiences a falt to earth on any one of its phases* the two nfalted phases experience an increase in the voltage between the phase and grond. 5ince arresters are most often applied between the phase condctor and earth* vale is then they also see this desired. There are some rles increase in voltage across their terminals. of thmb and This increase in voltage graphs that will remain across the can be sed* arrester ntil a system bt these are breaker operates and &it crde and breaks or interrpts the difficlt at best to se. Annex falt. This is a very significant event in the C of ,,, life of an arrester and standard mst be acconted for C/-.-- and  Annex A of dring the voltage rating selection of an ,C /00#9 cover this arrester. sb$ect. where the system and transformer impedances are relatively nknown* a worst case scenario is sed for each type of system. The voltage rise dring a falt in these cases is determined by mltiplying the line to grond voltage by The determination of a %or voltage rise dring a grond falt is not an distribtion easy task if a precise systems Arrest )s  -onat$an -. Co'yrig$t er(or *00, (ood/ort$ age ArresterFacts 016 a grond falt factor or earth falt factor. %igre 3 lists the grond falt factors sed to determine the nfalted phase voltage rise dring a grond falt. Selecting an Arresters MCOV or Uc %i3ed Con$iguration s t is also important to note that the gronding of the  %y'e o5 Syste netral at the sorce Solidly #rounded  transformer is the /ire configratio systes Uni"grounded 3 /iren referred to in systes determining I'edance grounded the voltage systes rise of the Isolated #round system. Systes and Delta Systes %or example $igure - Ground $ault as seen in %igre 9* a $actors delta;delta transformer %or example in a is tied to a )3.8kV mlti#gronded solidly system* the maximm gronded continos line to wye system. grond voltage is n this case 8.38kV. The voltage MOV) dring a grond falt shold be on the nfalted sieca se of  this isse* for all syste ms other than the mlti gron ded syste m* the MCO V or "c of the arrest er is select ed to e&al or excee d the line to line voltag e. Most man factr  ers also o f  f  e r  a &  i c k l o o k  p t a b l e t o s e l e c t t h e a r  r  e s t e r  r  a tin g ba se d on th e sy st e m to wh ich it is att ac he d. 5e e %i g re 8 for  thi s re co m m en da tio n. F or  su bs ta ti o n ap pli ca ti o ns* the comp ariso n of the poten tial syste m overv oltag e and the arrest er overv oltag e withst and capa bility is essen tial in select ing the arrest er MCO V or "c. n the case of trans missi on syste ms and sbst ations * the expec ted syste m overv oltag e m a g n i t  d e a n d d  r  a t i o n a r  e k n o w n &  a n t i t i e s s o t h i s c o m pa ris on is & ite ac c rat e. Th e be st me an s of obt ain ing the ex pe cte d ov erv olt ag e dr  ing a fa lt on a tra ns mi ssi on sys te m is to as k the person s respon sible for relay setting s. They have sall y model ed the syste m extens ively with proven softwa re* they can spply both magnit de and dratio ns of falts at most locatio n on the syste m. "se this inform ation to s e l e c t i o n o f  t r  a n s m i s s i o n l i n e a r  r  e s t e r  s ( T ?  A ! M C compare against the O target arr crve. V ,ransission ine Arresters The r  a t i ng or  "c rat in g is dif  fer  en t th an a dis tri b tio n or  s bs tat io n arr  es ter  . n the case of T?A@sonly the ob$e protect inslators from the ndesirable bac kfla sh dr  ing a swi tchi ng or ligh tnin g sr  ge. 5in ce ove rhe ad inslat ors are genera lly a self# restori ng type of  inslati on it is not impera tive to have the lowest Co'yrig$t p o s s i b l e c l a m p i n Arrest er(or g volt ag e for the arr  est er to miti gat e )s *00, -onat$an -. (ood/ort$ age6 ArresterFacts 016 Selecting an Arresters MCOV or Uc uary flashover. 5ometimes it 5election of an is also desirable to si