Selecting Arrester MCOV and Uc Part 1 of Arrester Selection Guide
Prepared by Jonathan Woodworth Consulting Engineer ArresterWorks Feb 12 200!
Selecting Arrester MCOV and Uc
Selecting Arrester MCOV and U c Contents •
Introducti on Relevant Defnition s Syste Confgura tions •
Deterining !ine" #nd Voltage Deterining Voltage Rise due to Fault Using t$e %OV Curve •
%ransission !ine Arresters Suary Recoended Ratings %a&les •
applied continosly between the terminals of an arrester. #uty*cycle +oltage rating ("EEE) # The designated maximm permissible voltage between its terminals at which an arrester is designed to perform its dty cycle test. ,&' Cur+e 'A graph that shows the power fre&ency withstand voltage vs. time for arrester from .) sec to )* sec ("EEE* "EC!
-round Fault 'An event There are several necessary where crrent steps that need to be taken when flows from the selecting an arrester for an power system application. An early step in this selection is the determination of to grond the voltage rating of the arrester. when a system phase The only voltage rating of an arrester that is important is the condctor is MCOV (Maximm Continos connected to Operating Voltage "EEE ! and "c earth either (Continos Operating Voltage throgh a "EC!. This MCOV # "c rating direct contact however is not always obvios or throgh an withot a fairly good arc. ("EEE* "EC! nderstanding of the system to which it is to be applied. The ob$ective of this Arrester%acts is to make this decision clearer and nderstandable.
#e$initions %C&' rating ("EEE) # The maximm designated root#mean s&are (rms! vale of power fre&ency voltage that may be
.c * Continuous operating +oltage ("EC) The designated permissible r.m.s. vale of power# fre&ency voltage that may be applied continosly between the arrester terminals indefinitely. .r * /ated +oltage o$ an arrester ("EC) Maximm permissible r.m.s. vale 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 dty 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 dty test after high#crrent or long#dration implses. Tests sed to establish the voltage rating in ,C /00#)* as well as some national standards* involve the application of repetitive implses at nominal crrent with power fre&ency voltage applied. Attention is drawn to the fact
that these two methods sed to established rating do not necessarily prodce e&ivalent vales.
phase to phase mst be taken voltage divided into by ).23. %or consideration. f example* on a the reglation is #eterining ine*-nd 2/kV )6* then for 'oltage and %iniu 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 grond voltage switching srges* they are voltage is 2/kV cold be 44x 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 wold be 44kV. "c or an arrester mst be e&al or higher to the 5ince all for this system at continos voltage between the systems have a minimm phase and earth. On three some reglation shold be phase systems* the line to error* this too 489kV. grond voltage is e&al to the Arrester *00+" -onat$an -. (ood/ort$ Co'yrig$t (or)s *00, age*
Selecting an Arresters MCOV or Uc needed for selection is to Typic know how the al IEC Typical IEEE system netral Syste System m Voltages condctor is Volta om Ma! Ma! sed in the ges "ine "ine circit if there "ine to Ma! to "ine to Grnd ominal Typical is one. The "ine "ine Voltage Voltage Ma! Voltage power sorce "ine to "ine Minimum to Grnd #V rms #V rms #V transformer rms "ine to "ine Uc Voltage *.0 *.2* 1.6 and the Voltage Voltage .16 .34 *.2* netral #V rms #V rms #V rms #V rms .+0 2.0 *.,1 bonding 3.4 *.1 3.3 *.1 6.,0 4.*2 .1, scheme 6.6 .* 4.3 .* determine how +.3* +.4 2.02 6.6 10.0 11.2 6.6 high the line to 1*.0 1*.6 4.*+ 6., 11.0 1*.0 6., grond 1*.2 13.1 4.24 16. 1+.0 10. 10. voltage of the 13.* 13., +.01 **.0 *.0 13., 13., nfalted 13.+ 1.2 +.3+ 33.0 36.3 *1.0 *1.0 phases *0.+ *1.+ 1*.6 30.1 4.0 2* 30.1 **., *.0 13., 1.6 66.0 4* 1.6 *3.0 *.* 1.0 ,1.0 100 24.+ 24.+ *., *6.* 12.1 110.0 1*3 41.1 41.1 *4.6 *,.0 16.+ 13*.0 12 +3.+ +3.+ 3.2 36.* *0., 122.0 140 ,+.3 ,+.3 6.0 +.3 *4., **0.0 *2 1* 1* 6,.0 4*.2 1., *42.0 300 143 143 112.0 1*1 6,.+ 330.0 36* *0, *0, 13+.0 12 +3.+ 00.0 *0 *3 *3 161.0 16, ,+ *30.0 32.0 200.0 462.0
** 36* 2*2 +00
10 *0, 303 6*
yste Con$igurations Once the system voltages are nderstood* the next step in the selection process is to determine the system configration to which the arrester will be applied. n other words* one mst determine if it is a wye or delta system (star or delta in the ,C world!. Also
will rise dring a grond falt. %ortnately the nmber of system configration s are limited. The most common ,,, configration is the 4 wire solid mlti# gronded netral as shown in figre -a. This is also known as an
effectively gronded system.
$igure %a Solidly Multi& grounded ' (ire system
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Selecting an Arresters MCOV or Uc and -d show these two A common systems. transmission line configration is the single gronded 1ye as seen in %igre -d .
$igure %) Impedance or *esonant Grounded System
A common indstrial and very common ,C configration is the 3 wire impedance gronded wye (or star!. The reason for poplarity of this system is that the falt crrent to earth is limited by the impedance. 1hen low impedance is sed* it can limit the falt crrent to levels that allow for lower falt crrent rated e&ipment to be sed on the system. This is often a cost savings configration. 1hen the impedance is high* a :etersen coil is sed which can offer falt extingishing capabilities withot sing breakers to break the falt. This is sometimes referred to as a resonant gronded system. A third common system configration is an isolated or ngronded system. This can be either delta or wye configred. %igre -c
$igure %c Ungrounded systems +isolated neutral,
$igure %d Single grounded neutral system +Uni& grounded system,
#eterini ng Phase 'oltage /ise due to Earth or -round Faults 1hen a three phase power system experiences a falt to earth on any one of its phases* the two nfalted phases experience an increase in the voltage between the phase and grond. 5ince arresters are most often applied between the phase
condctor and earth* vale is then they also see this desired. There are some rles increase in voltage across their terminals. of thmb and This increase in voltage graphs that will remain across the can be sed* arrester ntil a system bt these are breaker operates and &it crde and breaks or interrpts the difficlt at best to se. Annex falt. This is a very significant event in the C of ,,, life of an arrester and standard mst be acconted for C/-.-- and Annex A of dring the voltage rating selection of an ,C /00#9 cover this arrester. sb$ect.
where the system and transformer impedances are relatively nknown* a worst case scenario is sed for each type of system. The voltage rise dring a falt in these cases is determined by mltiplying the line to grond voltage by
The determination of a %or voltage rise dring a grond falt is not an distribtion easy task if a precise systems Arrest )s -onat$an -. Co'yrig$t er(or *00, (ood/ort$ age
a grond falt factor or earth falt factor. %igre 3 lists the grond falt factors sed to determine the nfalted phase voltage rise dring a grond falt.
Selecting an Arresters MCOV or Uc
%i3ed Con$iguration s
t is also important to note that the gronding of the %y'e o5 Syste netral at the sorce Solidly #rounded transformer is the /ire configratio systes Uni"grounded 3 /iren referred to in systes determining I'edance grounded the voltage systes rise of the Isolated #round system. Systes and Delta Systes %or example $igure - Ground $ault as seen in %igre 9* a $actors delta;delta transformer %or example in a is tied to a )3.8kV mlti#gronded solidly system* the maximm gronded continos line to wye system. grond voltage is n this case 8.38kV. The voltage MOV) dring a grond falt shold be on the nfalted sieca se of this isse* for all syste ms other than the mlti gron ded syste m* the MCO V or "c of the arrest er is select ed to e&al or excee d the line to line voltag e. Most man factr ers also
o f f e r a & i c k l o o k p t a b l e t o s e l e c t t h e a r r e s t e r r a
tin g ba se d on th e sy st e m to wh ich it is att ac he d. 5e e %i g re 8 for thi s re co m m en da tio n. F or su bs ta ti o n ap pli ca ti o
ns* the comp ariso n of the poten tial syste m overv oltag e and the arrest er overv oltag e withst and capa bility is essen tial in select ing the arrest er MCO V or "c. n the case of trans missi on syste ms and sbst ations * the expec ted syste m overv oltag e
m a g n i t d e a n d d r a t i o n a r e k n o w n & a n t i t i e s s o t h i s c
o m pa ris on is & ite ac c rat e. Th e be st me an s of obt ain ing the ex pe cte d ov erv olt ag e dr ing a fa lt on a tra ns mi ssi on sys te m is to as
k the person s respon sible for relay setting s. They have sall y model ed the syste m extens ively with proven softwa re* they can spply both magnit de and dratio ns of falts at most locatio n on the syste m. "se this inform ation to
s e l e c t i o n o f t r a n s m i s s i o n l i n e a r r e s t e r s ( T ? A !
M C compare against the O target arr crve. V
,ransission ine Arresters The
r a t i
ng or "c rat in g is dif fer en t th an a dis tri b tio n or s bs tat io n arr es ter . n the case of T?A@sonly the ob$e protect inslators from the ndesirable bac kfla sh dr ing a swi tchi ng or ligh tnin g sr ge. 5in ce ove rhe ad
inslat ors are genera lly a self# restori ng type of inslati on it is not impera tive to have the lowest
p o s s i b l e c l a m p i n
g volt ag e for the arr est er to miti gat e
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Selecting an Arresters MCOV or Uc
flashover. 5ometimes it 5election of an is also desirable to si