Typical Limiting Values of Sub Station Electrical Equipments. 1. Transformer / Reactor: Sr. No. Equipment / test data Transformer oil a) !V "At "At t#e t#e tim timee of of fir first st c#ar c#argi ging ng "!uring 0, b) ,oisture content "At t#e time of first c#arging A) "!uring 0, c) Resisti5ity at 4% 6egree 7 6) Aci6ity e) -T at * 6egree 7 f) Tan 6elta at 4% 6egree 7 g) las# point Vibration le5el for reactors ) 7) !) E) ) () 9)
Permissible limits
Reference
$%% $%% &V &V '(ap '(ap *.+ *.+ mm) mm) ,i ,ini nim mum-S um-S 1$$ 1$$ +% &V '(ap *.+ mm) ,inimum -S 1$ -S 1$ 1+ 22, ',a3.) - S 1 $4 *+ 22, ',a3.) - S 1 % %.1"1%1* #m"7, ',in.) - S 1 1 %.* mg 89/gm ',a3.) - S 1 * %.%1 ;/, ',in.) - S 1 < %.*% ',a3.) - S 1 = 1*$ !eg. 7 ',in.) - S 1 + *%% ,icrons '2ea& to 2ea&) - S 1 $ $% ,icrons 'A5erage) - S 1 Tan 6elta for bus#ing at *% !eg. 7 %. % .% % > -E7 1< 7apacitance for bus#ing ? +@ 5ariation -E7 1< 1%%% ,"#m y +.%/1%.% &V -E7 1<4 -R 5alue for in6ing ,egger Tan 6elta for in6ings at *% !eg. %.%%> -EEE/7+.1*.4%.14% 7 7ontact resistance of bus#ing 1% ,. #m / 7onnector terminal connectors ;(7.B8 Recommen6atio Turret ;eutral 7T ratio errors <4 - S * %+
*. 7ircuit rea&ers Sr. No. Equipment / test data A)
!e point of S$ gas
) 7)
!e point of operating air 7 perating timings a) 7losing time ',a3.)
Permissible limits Reference !e point 5alues as per Anne3ure -"=+ !eg. 7 at AT,. 2ressure =%% &V **%8V 1+% ,S * % % ,S
b) Trip time ',a3.) c) 7lose/trip timeC 2ole 6iscrepancy 2#ase to 2#ase ',a3.) "rea& to brea& ',a3.) of same p#ase 2-R time 9EL ma&e
A ma&e !)
;(E ma&e ,0( ma&e TEL8 ma&e A ma&e '9V!7)
E) ) () 9) -) G)
8)
L)
*+ ,S
<+,S
<.<< ,S
<.<<,S
*.+ ,S
*.+,S
1*"1$ ,S
"1* ,S "1* ,S "1* ,S "1* ,S "1* ,S
,anufacturers Recommen6ations ,anufacturers Recommen6ations ,anufacturers Recommen6ations ,anufacturers Recommen6ations ,anufacturers Recommen6ations ,anufacturers Recommen6ations
2-R opening time prior to opening of main contacts 'AC 7(LC ,anufacturers ;(E ma&e 7s) + ,S ',in.) at rate6 pressure Recommen6ations 2ir an6 main contacts o5erlap time D9ELC ,0(C A 'importe6) ,anufacturers ma&e 7s $ ,S ',in.) at rate6 pressure Recommen6ations Tan 6elta of gra6ing capacitors %.%% at *% !eg. 7 Fit#in ? 1%@ / "+@ of t#e rate6 7apacitance of gra6ing capacitors 5alue -E7 <+4 7ontact resistance of 7 1+% ,. #m 7ontact resistance of 7 terminal connector 1% ,. #m per connector ;(7C B8 recommen6atio -R 5alue: 1%%% , #m ',in.) by +.% / 1%.% 1. 2#ase eart# &V ,egger 1%%% , #m ',in.) by +.%/1%.% &V *. Across open contacts ,egger +% , #m ',in.) by %.+ &V <. 7ontrol cables ,egger 2ressure sitc# settings "S$ gas pressure sitc#es Fit#in ? %.1 ar of set 5alue
,)
"perating air pr. Sitc#es "perating oil pr. Sitc#es !V of oil use6 for ,7 "At t#e time of filling "!uring 0,
Fit#in ? %.1 ar of set 5alue Fit#in ? %.1 ar of set 5alue =% &V at *.+ mm (ap ',in.) *% &V at *.+ mm (ap. ',in.)
,fgs. Recommen6ation ,fgs. Recommen6ation
Permissible limits
Reference
<. 7urrent Transformer Sr. No. Equipment / test data -R 5alue 1. 2rimary eart# A)
*. Secon6ary eart# <. 7ontrol cables
) 7) !) !)
Tan 6elta 5alue Terminal 7onnector 7T ratio errors 7T ratio errors
1%%% , #m ',in.) by +.%/1%.% &V ,egger +% , #m ',in.) by %.+ &V ,egger +% ,"#m ',in.) by %.+ &V ,egger %.%%> at *% !eg. 7 1% ,"#m per connector ;(7C B8 Recommen6atio ? <@ "2rotection cores - S * %+ ? 1@ ",etering cores - S * %$
=. 7apaciti5e Voltage Sr. No. Equipment / test data A) Tan !elta ) 7apacitance 7ontact resistance of terminal 7 connector -R Value 1. 2rimary eart# !)
*. Secon6ary eart# <. 7ontrol cables
E)
E,B tan& oil parameters a) !V ',in.) b) ,oisture content ',a3.) c) Resisti5ity at 4% !eg. 7
Permissible limits Reference %.%%> at *% !eg. 7 Fit#in ?1%@/"+@ of t#e rate6 5alue -E7 <+ 1% ,"#m per connector -R Value 1%%% , #m ',in.) by +.%/1%.% &V ,egger +% , #m ',in.) by %.+ &V ,egger +% ,"#m ',in.) by %.+ &V ,egger E,B tan& oil parameters <% &V '(ap. *.+ mm) <+ ppm %.1 1%1* #m. 7,
;(7C B8 Recommen6atio
- S 1 $$ "6o" "6o"
) )
6) Aci6ity e) -T at * !eg. 7 f) Tan 6elta at 4% !eg. 7 g) las# point 7VT 5oltage ratio errors 7VT 5oltage ratio errors
%.+ mg &9 /gm ',a3.) %.%1 ;/, ',in.) 1.% ,a3. 1*+ !eg. 7 ',in.) ? +@ protection cores ? %.+@ metering cores
"6o" "6o" "6o" "6o" -EEE/74<.1.144% -E7 1$
+. -solators Sr. No. A) )
Equipment / test data 7ontact resistance 7ontact resistance of terminal connector -R 5alue 1. 2#ase eart#
7)
*. Across open contacts <. 7ontrol cables
Permissible limits <%% ,"#m. ',a3.)
Reference
1% , #m per connector
;(7C B8 Recommen6ati
1%%% , #m ',in.) by +.%/1%.% &V ,egger ;(7C B8 Recommen6ati 1%%% , #m ',in.) by +.%/1%.% &V ,egger ;(7C B8 Recommen6ati +% ,"#m ',in.) by %.+ &V ,egger
$. Surge Arrester Sr. No. Equipment / test data A) Lea&age current ) -R 5alue
Permissible limits +%% ,"Amp. 'Resisti5e) 1%%% ,"#m. ',in.)
Reference 9itac#iC Gapan Recom. 9itac#iC Gapan Recom.
Permissible limits 1.% #m ',a3.)
Reference
. ,iscellaneous Sr. No. Equipment / test data A) Station eart# resistance T#ermo 5ision scanning Temp. up to 1+ !eg. 7 'abo5e ambient) ) Temp. abo5e 1+"+% !eg. 7 'abo5e ambient) Temp. abo5e +% !eg. 7 'abo5e ambient) 7) Terminal connectors 7ontact
;ormal Alert To be imme6iately atten6e6 1% ,"#m per connector
9(7C B8 Recommen6atio
resistance -R 5alues 1. All electrical motors !)
*. 7ontrol cables <. Lt. Transformers =. Lt. Sitc#gears
+% ,"#m ',in.) by %.+ &V ,egger +% ,"#m ',in) by %.+ &V ,egger 1%% ,."#m ',in.) by ,egger 1%% , #m ',in.) by %.+ &V ,egger
. atteries Sr. No. A) )
Equipment / test data Terminal connector resistance Specific gra5ity
Permissible limits 1% , #m ? *%@ 1*%% ? + (,/L at * !eg. 7
Reference A;S-/-EEE =+% 144
Temperature 7orrection actor for Tan !elta ,easurement Sr. No. 1 * < = + $ 4 1% 1% 11 11 1* 1* 1< 1<
Oil temperature Deg. C 1% 1+ *% *+ <% <+ =% =+ +% ++ $% $+ %
Correction factor(K %. %.4 1.% 1.1* 1.*+ 1.=% 1.++ 1.+ 1.4+ *. 1 *. = * *. % <. % %
-f Tan !elta of bus#ing/in6ing/7VT/7T is measure6 at oil temperature T !eg. 7. T#en Tan !elta at *% !eg. 7 s#all be as gi5en belo:
Tan !elta at *% !eg. 7 H Tan !elta at Temp T !eg. 7 / actor 8.
!e 2oint Limits for S$ (as in E9V 7ircuit rea&ers Sr. No Make Make of C.B C.B
1
B H EL
2
M&G
3
CGL
4
ABB
5
NGEF
De! point at rated Corresponding de! point Pr. Deg. C at "tmosp#eric .Pr. Remar$s "1+ "<$ At t#e time of commissioning " " "*4 !uring 0, "+ "+ "* 7ritical "<4 At t#e time of commissioning "<* !uring 0, "1 "1+ "<+ At t#e time of commissioning "1 "1% "<1 !uring 0, "1 "1+ "<+ At t#e time of commissioning "+ "+ " *$ !uring 0, "1 "1+ " <$ At t#e time of commissioning " " "*4 !uring 0,
Note: Dew point of SF6 ga !a"ie wit# p"e$"e at w#i%# ea$"eent i %a""ie' o$t( So it i to )e en$"e' t#at if ea$"eent i 'one at p"e$"e ot#e" t#an atop#e"i% p"e$"e* it nee' to )e %on!e"te' to t#e atop#e"i% p"e$"e(
Calculate Size of Capacitor Bank / Annual Saving & Payback Period April 1C *%1= $ 7omments •
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7alculate SiIe of 7apacitor an& Annual Sa5ing in ills an6 2aybac& 2erio6 for 7apacitor an&. Electrical Loa6 of '1) * ;oJs of 1.+8FC=1+V motor C4%@ efficiencyC%.* 2oer actor C'*) * ;oJs of .+8FC=1+V motor C4%@ efficiencyC%.* 2oer actorC'<) 1%8F C=1+V Lig#ting Loa6. T#e Targete6 2oer actor for System is %.4. Electrical Loa6 is connecte6 *= 9oursC Electricity 7#arge is 1%%Rs/8VA an6 1%Rs/8F. 7alculate siIe of !isc#arge Resistor for 6isc#arging of capacitor an&. !isc#arge rate of 7apacitor is +%5 in less t#an 1 minute. Also 7alculate re6uction in 8VAR rating of 7apacitor if 7apacitor an& is operate6 at frequency of =%9I instea6 of +%9I an6 -f perating Voltage =%%V instea6 of =1+V. 7apacitor is connecte6 in star 7onnectionC 7apacitor 5oltage =1+VC 7apacitor 7ost is $%Rs/85ar. Annual !eprecation 7ost of 7apacitor is 1*@.
7alculation: • • •
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or 7onnection '1): Total Loa6 8F for 7onnection'1) H8 / EfficiencyH'1.+K*) / 4%H=1.18F Total Loa6 8VA 'ol6) for 7onnection'1)H 8F /l6 2oer actorH =1.1 / %.*H+%.1 8VA Total Loa6 8VA 'ne) for 7onnection'1)H 8F /;e 2oer actorH =1.1 /%.4H =1.48VA Total Loa6 8VARH 8F'D'M1"'ol6 p.f)*) / ol6 p.f" D'M1"';e p.f)*) / ;e p.f) Total Loa6 8VAR1H=1.13'D'M1"'%.*)*) / %.*" D'M1"'%.4)*) / %.4) %otal &oad K'"R)*+.,- K'"R R tanN1HArcos'%.*)H%.$4 tanN*HArcos'%.4)H%.*% Total Loa6 8VAR1H 8F 'tanN1" tanN*) H=1.1'%.$4"%.*%)H*%.<+8VAR or 7onnection '*): Total Loa6 8F for 7onnection'*) H8 / EfficiencyH'.+K*) / 4%H1$.$$8F Total Loa6 8VA 'ol6) for 7onnection'1)H 8F /l6 2oer actorH 1$.$$ / %.
SiIe of 7apacitor an&: • • • •
Site of Capacitor 3an$),* K2ar. &eading K'"R supplied b4 eac# P#ase) K2ar/No of P#ase Lea6ing 8VAR supplie6 by eac# 2#ase H<*/
• • • • • • •
7apacitor 7#arging 7urrent '-c)H '1%.K1%%%)/'=1+/M<) 7apacitor 7#arging 7urrent '-c)H==.4Amp Capacitance of Capacitor ) Capacitor C#arging Current (5c/ 7c cH*K<.1=3f35H*K<.1=3+%3'=1+/M<)H+<$* 7apacitance of 7apacitorH==.4/+<$*H +.4$O Required , No8s of +. K2ar Capacitors and
Total SiIe of 7apacitor an& is <*85ar
2rotection of 7apacitor an& SiIe of 9R7 use for 7apacitor an& 2rotection: •
• •
Si9e of t#e fuse ):-; to *++; of Capacitor C#arging current. SiIe of t#e fuseH*K==.4Amp SiIe of t#e fuseH4%Amp
SiIe of 7ircuit rea&er for 7apacitor 2rotection: •
• • • • • • • •
Si9e of t#e Circuit 3rea$er ),-; to -+; of Capacitor C#arging current. SiIe of t#e 7ircuit rea&erH1.+K==.4Amp SiIe of t#e 7ircuit rea&erH$Amp T#ermal relay setting beteen 1.< an6 1.+of 7apacitor 7#arging current. T#ermal relay setting of 7.H1.+K==.4 Amp T#ermal relay setting of 7.H$ Amp ,agnetic relay setting beteen + an6 1% of 7apacitor 7#arging current. ,agnetic relay setting of 7.H1%K==.4Amp ,agnetic relay setting of 7.H==4Amp
SiIing of cables for capacitor 7onnection: •
• • • •
7apacitors can it#stan6 a permanent o5er current of <%@ ?tolerance of 1%@ on capacitor 7urrent. 7ables siIe for 7apacitor 7onnectionH 1.< 31.1 3 nominal capacitor 7urrent Cables si9e for Capacitor Connection ) .0, 6 nominal capacitor Current 7ables siIe for 7apacitor 7onnectionH1.=
,a3imum siIe of 6isc#arge Resistor for 7apacitor: •
7apacitors ill be 6isc#arge by 6isc#arging resistors.
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After t#e capacitor is 6isconnecte6 from t#e source of supplyC 6isc#arge resistors are require6 for 6isc#arging eac# unit it#in < min to + V or less from initial nominal pea& 5oltage 'accor6ing -E7"stan6ar6 $%<1). !isc#arge resistors #a5e to be connecte6 6irectly to t#e capacitors. T#ere s#all be no sitc#C fuse cut"out or any ot#er isolating 6e5ice beteen t#e capacitor unit an6 t#e 6isc#arge resistors.
*/ D2. */ D2 F#ere 7t H7apacitor !isc#arge Time 'sec) 7nH7apacitance ara6. Bn H Line Voltage !5H7apacitor !isc#arge 5oltage. ,a3imum !isc#arge resistance H$% / ''+.4$/1%%%%%%)3 log ' =1+3M* /+%)
Effect Effe ct of Decreasing 'oltage @ Arequenc4 on Rating of CapacitorB •
• • • • • • • • • •
T#e &5ar of capacitor ill not be same if 5oltage applie6 to t#e capacitor an6 frequency c#anges Re6uce6 in 85ar siIe of 7apacitor #en operating +% 9I unit at =% 9I Actual 8VAR H Rate6 8VAR 3'perating requency / Rate6 requency) Actual 8VAR H Rate6 8VAR 3'=%/+%) Actual 8VAR H %@ of Rate6 8VAR ence ,* K2ar Capacitor !or$s as +;6,*K2ar) *:.:K2ar Re6uce6 in 85ar siIe of 7apacitor #en operating =1+V unit at =%%V Actual 8VAR H Rate6 8VAR 3'perating 5oltage / Rate6 5oltage)P* Actual 8VAR H Rate6 8VAR 3'=%%/=1+)P* Actual 8VARH4<@ of Rate6 8VAR ence ,* K2ar Capacitor !or$s as ,;6,*K2ar) *,.+K2ar
Annual Sa5ing an6 2ay ac& 2erio6 efore 2oer actor 7orrection: • • • • • • • • •
%otal electrical load K'" (old) K'"1K'"*1K'", Total electrical loa6H +%.1?*%.%?11.$ Total electrical loa6H* 8VA Total electrical Loa6 8FH&F1?8F*?8F< Total electrical Loa6 8FH<?1+?1% Total electrical Loa6 8F H$*& Loa6 7urrentH8VA/VH%K1%%%/'=1+/1.<*) Loa6 7urrentH11=.1 Amp 8VA !eman6 7#argeH8VA 7#arge
• • • • • • •
8VA !eman6 7#argeH*3$%Rs 8VA !eman6 7#argeH14 Rs Annual Bnit 7onsumptionH8F3 !aily uses3<$+ Annual Bnit 7onsumptionH$*3*=3<$+ H+=<1*% 8# Annual c#arges H+=<1*%K1%H+=<1*%% Rs Total Annual 7ostH 14?+=<1*%%
Total Annual 7ost before 2oer actor 7orrectionH +=<4<4 Rs
After 2oer actor 7orrection:
•
%otal electrical load K'" (ne!) K'"1K'"*1K'", Total electrical loa6H =1.4+?1.%1?1%.*% Total electrical loa6H$4 8VA Total electrical Loa6 8FH&F1?8F*?8F< Total electrical Loa6 8FH<?1+?1% Total electrical Loa6 8F H$*& Loa6 7urrentH8VA/VH$4K1%%%/'=1+/1.<*) Loa6 7urrentH4$.* Amp 8VA !eman6 7#argeH8VA 7#arge 8VA !eman6 7#argeH$43$%Rs H$41$ RsQQQQ"'1) Annual Bnit 7onsumptionH8F3 !aily uses3<$+ Annual Bnit 7onsumptionH$*3*=3<$+ H+=<1*% 8# Annual c#arges H+=<1*%K1%H+=<1*%% RsQQQQQ'*) 7apital 7ost of capacitorH 85ar 3 7apacitor cost/85ar H * 3 $%H =414 RsQ'<) Annual -nterest an6 !eprecation 7ost H=414 3 1*@H+4% RsQ'=) Total Annual 7ostH $41$?+=<1*%%?=414?+4%
•
Total Annual 7ost After 2oer actor 7orrection H+=<%$ Rs
• • • • • • • • • • • • • • •
2ay ac& 2erio6: • • • •
Total Annual 7ost before 2oer actor 7orrectionH +=<4<4 Rs Total Annual 7ost After 2oer actor 7orrection H+=<%$ Rs Annual Sa5ingH +=<4<4"+=<%$ Rs
Annual Sa5ingH $4* Rs
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2aybac& 2erio6H 7apital 7ost of 7apacitor / Annual Sa5ing 2aybac& 2erio6H =41* / $4*
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2aybac& 2erio6 H .1 ears
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Calculate Technical Losses of Transission / !istribution Line" ,arc# 1C *%1= 4 7omments
5ntroductionB • • • •
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T#ere are to types of Losses in transmission an6 6istribution Line. '1) Tec#nical Losses an6 '*) 7ommercial Losses. -t is necessary to calculate tec#nical an6 commercial losses.;ormally Tec#nical Losses an6 7ommercial Losses are calculate6 separately .Transmission 'Tec#nical) Losses are 6irectly effecte6 on electrical tariff but 7ommercial losses are not implemente6 to all consumers. Tec#nical Losses of t#e !istribution line mostly 6epen6 upon Electrical Loa6C type an6 siIe of con6uctorC lengt# of line etc. LetJs try to calculate Tec#nical Losses of one of folloing 11 8V !istribution Line
E6ampleB • • •
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11 8V !istribution Line #a5e folloing parameter. ,ain lengt# of 11 8V Line is $.1 8ms. Total nos. of !istribution Transformer on ee6er *+ 8VAH < ;oC $< 8VA H< ;oC1%%8VAH1;o. *+8VA Transformer -ron Losses H 1%% FC 7opper LossesH *% FC A5erage LT Line LossH $
CalculationB %otal Connected &oad)No8s of Connected %ransformer. • •
Total 7onnecte6 Loa6H '*+K<) ? '$
Pea$ &oad ) .,* 6 &ine 'oltage 6
2ea& Loa6 H 1.<*31131* 2ea& Loa6 H** 8VA.
Di2ersit4 Aactor (DA ) Connected &oad (5n K'" / Pea$ &oad. • •
!i5ersity actor '!) H <$= /** !i5ersity actor '!) H1.1+
&oad Aactor (&A) =nit Sent Out (5n K!# / .,* 6 &ine 'oltage 6
Loa6 actor 'L)H=4%<<+ / 1.<*31131*3%.K$% Loa6 actor 'L)H%.<%
%$ &oss &oad Aactor (&&A) (+. 6 &A6 &A1 (+.* 6 &A • •
Loss Loa6 actor 'LL)H ' %. 3 %.<%$% 3 %.<%$% ) ? '%.* 3 %.<%$) Loss Loa6 actor 'LL)H %.1<$1
Calculation of 5ron lossesB •
• • • •
%otal "nnual 5ron loss in K!# )5ron &oss in atts 7 Nos of %C on t#e feeder 7:+ / +++ Total Annual -ron loss '*+8VA T7)H1%%3<3$% /1%%% H*$* 8# Total Annual -ron loss '$<8VA T7)H*%%3<3$% /1%%% H+*+$ 8# Total Annual -ron loss '1%%8VA T7)H*4%3<3$% /1%%% H*+=% 8# Total Annual -ron loss H*$*?+*+$?*+=% H1%=*=8#
Calculation of Copper lossesB •
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%otal "nnual Copper loss in K!# )Cu &oss in atts 7Nos of %C on t#e feeder &A7 &A 7:+ / +++ Total Annual 7opper loss '*+8VA T7)H*%3<3%.
% &ine &osses (K!#)+.+- 6 (Conn. &oad 6 * 6 &engt# 6 Resistance 6 &&A / ( &DA 6 DA 6 DA 6 * • •
9T Line LossesH 1.%+ 3'*$+K*) 3 $.1 3 %.+= 3 %.1<$1 /1.+ 3 1.1+ 31.1+ 3 * 9T Line Losses H <1 8#
Pea$ Po!er &osses) (, 6 %otal &% &ine &osses / (PP&6DA6DA6 +++ • •
2ea& 2oer LossesH < 3 '
&% &ine &osses (K!#) (PP&. 6 (&&A 6 :+ • •
LT Line Losses H < 3 %.1<$1 3 $% LT Line Losses H <<1+ 8#
%otal %ec#nical &osses) (% &ine &osses 1 &% &ine &osses 1 "nnual Cu &osses 1 "nnual 5ron &osses • •
Total Tec#nical Losses H ' <1? <<1+ ? 1%=*= ? $=4%) %otal %ec#nical &osses ) *+: K!#
; %ec#nical &oss) (%otal &osses / (=nit Sent Out "nnuall4 6 ++ •
@ Tec#nical LossH '*1%$1/=4%<<+) 31%%H 0.,+; 31%%H 0.,+;
; %ec#nical &oss)0.,+; Calculate #!$T over Current %elay Setting '(/')* ctober 11C *%1< 14 7omments •
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7alculate setting of -!,T o5er 7urrent Relay for folloing ee6er an6 7T !etail Aeeder Detail: Detail: ee6er Loa6 7urrent <= AmpC ee6er ault current ,in118A an6 ,a3 **8A. C% DetailB 7T installe6 on fee6er is $%%/1 Amp. Relay Error .+@C 7T Error 1%.%@C 7T o5er s#oot %.%+ SecC 7T interrupting Time is %.1 Sec an6 Safety is %.<< Sec. 5D<% Rela4 DetailB 5D<% Rela4 &o! Current settingB 5er settingB 5er Loa6 7urrent setting is 1*+@C 2lug setting of Relay is %. Amp an6 Time !elay 'T,S) is %.1*+ SecC Relay 7ur5e is selecte6 as ;ormal -n5erse Type. 5D<% Rela4 ig# Current setting B2lug B2lug setting of Relay is *.+ Amp an6 Time !elay 'T,S) is %.1%% SecC Relay 7ur5e is selecte6 as ;ormal -n5erse Type
Calculation of O2er Current Rela4 SettingB ( &o! o2er o2er Current SettingB (5F (5F •
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O2er &oad Current (5n ) Aeeder &oad Current 7 Rela4 setting ) ,0 7 *-; )0+ "mp Required O2er &oad Rela4 Plug Setting) O2er &oad Current (5n / C% Primar4 Current Require6 5er Loa6 Relay 2lug Setting H =% / $%% H %.
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Pic$ up Setting of O2er Current Rela4 (P
(* ig# o2er Current SettingB (5FF (5FF •
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Pic$ up Setting of O2er Current Rela4 (P
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perating Time of 2re5ious up Stream Relay H %.<= ? %.+ H %.+ Sec
Conclusion of CalculationB •
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2ic&up Setting of o5er current Relay '2,S) '-) s#oul6 be satisfie6 folloing To 7on6ition. ( 2ic&up Setting of o5er current Relay '2,S)'-) H 5er Loa6 7urrent '-n) / 7T 2rimary 7urrent (* T,S H ,inimum ault 7urrent / 7T 2rimary 7urrent or 7on6ition '1) %. H'=%/$%%) H %. H %.C F#ic# foun6 OK or 7on6ition '*) %.1*+ H 11%%%/$%% H %.1*+ H 1.<
Economical 'oltage for Po!er %ransmissionB •
Economic generation 5oltage is generally limite6 to folloing 5alues '7-2 ,anual).
Economic generation 2oltage (C35P
Economical 'oltage
Bp to +% 8VA
=1+ V
+% 8VA to *+%% 8VA
<.< 8V
*+%% 8VA to +%%% 8VA
$.$ 8V
•
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Abo5e +%%% 8VA 11 8V or 9ig#er (enerally terminal 5oltage of large generators is 11 &V in -n6ia. Step up 5oltage 6epen6s upon Lengt# of transmission line for interconnection it# t#e poer system an6 2oer to be transmitte6. 9ig# 5oltage increases cost of insulation an6 support structures for increase6 clearance for air insulation but 6ecreases siIe an6 #ence 7ost of con6uctors an6 line losses. ,any empirical relations #a5e been e5ol5e6 to appro3imately 6etermine economic 5oltages for poer e5acuation. An important component in transmission lines is labor costs #ic# are country specific.
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An empirical relation is gi5en belo.
'oltage in $' (line to line ) -.-6>+.:*& 1 $'"/-+ #ere &VA is total poer to be transmitte6U L is lengt# of transmission line in &m. American practice for economic line to line 5oltage &V 'base6 on empirical formulation) is
'oltage in $' line to line ) -.-6>+.:*& 1 ,P/++ or t#e purpose of stan6ar6iIation in -n6ia transmission lines may be classifie6 for operating at $$ &V an6 abo5e. << &V is sub transmissionC 11 &V an6 belo may be classifie6 as 6istribution. 9ig#er 5oltage system is use6 for transmitting #ig#er amounts of poer an6 longer lengt#s an6 its protection is important for poer system security an6 requires comple3 relay systems.
Requi Required red Po Po!er !er %ran %ransfe sferr (< (<
Dista Distanc ncee (K< (K< Econo Economi mica call 'olt 'oltag agee &e2 &e2el el (K< (K<
<+ %%
+% %
$+
+% %
=% %
=%%
1* %
1+ %
**%
%
+%
1<*
Aactor affected on 'oltage &e2el of s4stemB •
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2oer carrying capability of transmission lines increases roug#ly as t#e square of t#e 5oltage. Accor6ingly 6isconnection of #ig#er 5oltage class equipment from bus bars get increasingly less 6esirable it# increase in 5oltage le5els. 9ig# structures are not 6esirable in eart#qua&e prone areas. T#erefore in or6er to obtain loer structures an6 facilitate maintenance it is important to 6esign suc# sub"stations preferably it# not more t#an to le5els of bus bars.
Si9e of Cable according to S#ort circuit (for $',.,$' onl4 •
S#ort circuit 5erification is performe6 by using folloing formula:
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Cross Section area of Cable (mm*S ) 5 6>t / K
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F#ere:
• • • • • •
t H fault 6uration 'S) - H effecti5e s#ort circuit current '&A) 8 H %.%4= for aluminum con6uctor insulate6 it# L2E E6ampleB ault 6uration't)H %.*+secCault 7urrent '-) H *$.*= &A 7ross Section area of 7able H *$.*= 3 M '%.*+) / %.%4=H 1<4.$ sq. mm T#e selecte6 cross sectional area is - sq. mm.
Ground ClearanceB • •
Ground Clearance in
#ere K) ('oltJ,, / ,,
'oltage &e2el H<<8V $$8V 1<*8V **%8V =%%8V
Ground Clearance +.* ,eter +.=4 ,eter $.1% ,eter .% ,eter .= ,eter
'oltage Rise in %ransformers due to Capacitor 3an$B •
•
• • • • •
T#e 5oltage 6rop an6 rise on t#e poer line an6 6rop in t#e transformers. E5ery transformer ill also e3perience a 5oltage rise from generating source to t#e capacitors. T#is rise is in6epen6ent of loa6 or poer factor an6 may be 6etermine6 as follos:
; 'oltage Rise in %ransformer)(K2ar / K2a6 L
85ar HApplie6 85ar 85a H 85a of t#e transformer I H Transformer Reactance in @ E6ampleB <%% E6ampleB <%% 85ar ban& gi5en to 1*%% 8VA transformer it# +.+@ reactance. @ Voltage Rise in TransformerH'<%%/1*%%)3 +.+ H.0,; H.0,;
+lectrical Thub %ules,Part )*Guly *C *%1< <4 7omments • • •
7able 7apacity:
Aor Cu ire Current Capacit4 (=p to ,+ Sq.mm ) $ SiIe of Fire in Sq.mm E3. or *.+ Sq.mmH$K*.+H1+ AmpC or 1 Sq.mmH$K1H$ AmpC or 1.+ Sq.mmH$K1.+H4 Amp
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• • • •
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Aor Cable Current Capacit4 ) = SiIe of 7able in Sq.mm CE3. or *.+ Sq.mmH=K*.+H4 Amp. Nomenclature for cable Rating ) Bo/B ) Bo/B #ere BoH2#ase"(roun6 VoltageC BH2#ase"2#ase VoltageC BmH9ig#est 2ermissible Voltage
7urrent 7apacity of Equipments:
P#ase
Eart#ing Resistance: Eart#ing Resistance for Single Pit)+ Pit)+ CEart#ing (ri6H%.+ "s per NEC - Eart#ing Resistance s#oul6 Resistance s#oul6 be +. 'oltage bet!een Neutral and Eart# H* Eart# H* Volts Resistance bet!een Neutral and Eart# H1 Eart# H1 Creepage Distance)1 Distance)1 to **mm/8V ',o6erate 2ollute6 Air) or Creepage Distance)*+ Distance)*+ to <
5nsulation ResistanceB 5nsulation Resistance 'alue for Rotating
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• •
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Lig#ting Arrestor: "rrestor #a2e %!o Rating) '1) ,7VH,a3. 7ontinuous Line to (roun6 perating Voltage. '*) !uty 7ycle Voltage. '!uty 7ycle Voltage,7V).
Transformer: Current Rating of %ransformerH8VA31.= %ransformerH8VA31.= S#ort Circuit Current of %.C /Generator) 7urrent Rating / @ -mpe6ance No &oad Current of %ransformer)*@ %ransformer)*@ of Transformer Rate6 current Capacitor Current (5c)8VAR (5c)8VAR / 1.<*3Volt '2#ase"2#ase) Typically t#e local utility pro5i6es transformers rate6 up to -++$'" or ma3imum connecte6 loa6 of $ Typically t#e local utility pro5i6es transformers rate6 up to *-+$'" or ma3imum connecte6 loa6 of -+$. T#e 6i5ersity t#ey oul6 apply to apartments is aroun6 :+; ,a3imum 9T '11&V) connecte6 loa6 ill be aroun6 0.-<'" per circuit. circuit. =;o. eart# pits per transformer '*;o. for bo6y an6 *;o. for neutral eart#ing)C 7learancesC appro3.1%%%mm aroun6 T7 allo for transformer mo5ement for replacement.
Diesel GeneratorB
Diesel Generator Set Produces)<. Produces)<. Bnits '8F9) in 1 Litter of !iesel. Requirement "rea of Diesel Generator ) for ) for *+8F to =8FH+$ Sq.meterC 1%%8FH$+ Sq.meter. !( less t#an or equal to +++$'" must be in a canop4. !( greater +++$'" can +++$'" can eit#er be in a canopy or s&i6 mounte6 in an acoustically treate6 room !( noise le5els to be less t#an -d3" meter. !( fuel storage tan&s s#oul6 be a ma3imum of + &itter per unit Storage unit Storage tan&s abo5e t#is le5el ill trigger more stringent e3plosion protection pro5ision.
7urrent Transformer:
Nomenclature of C%B RatioB input RatioB input / output current ratio 3urden ('"B total ('"B total bur6en inclu6ing pilot ires. '*.+C +C 1%C 1+ an6 <%VA.) ClassB Accuracy ClassB Accuracy require6 for operation ',etering: %.*C %.+C 1 or
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"s per 5EEE Rela4ing (Protection C%B *.+71%% C%B *.+71%% Relaying 7T is accurate it#in *.+ percent if t#e secon6ary bur6en is less t#an 1.% o#m '1%% 5olts/1%%A). uic$ Electrical Calculation
192H%.=$8F 18FH1.<$92 1FattH%.=$ 8la/9r 1FattH<.=1 TB/9r TB/9r 18F9H<.$ ,G 17alH=.1$ G 1ToneH <+<% TB + Sq.ft loor AreaH1*%% TB 18calH=1$ Goule 18F9H$% 8cal 17alH=.1< Goule
Star Connection Line VoltageHM< 2#ase Voltage Line 7urrentH2#ase 7urrent Delta Connection Line VoltageH2#ase Voltage Line 7urrentHM< 2#ase 7urrent
.ver Current %elayType,Application,Connection*" %elayType,Application,Connection*" Ganuary 1C *%1< 1< 7omments
%4pes of protectionB •
2rotection sc#emes can be 6i5i6e6 into to maWor groupings:
1. Bnit sc# sc#emes *. ;on" ;on"un unit it sc#e sc#eme mess
=nit %4pe Protection •
•
Bnit type sc#emes protect a specific area of t#e systemC i.e.C a transformerC transmission lineC generator or bus bar. T#e unit protection sc#emes is base6 on 8erc#iefJs current la t#e sum of t#e currents entering an area of t#e system must be Iero. Any 6e5iation from t#is must in6icate an abnormal current pat#. -n t#ese sc#emesC t#e effects of any 6isturbance or operating con6ition outsi6e t#e area of interest are totally ignore6 an6 t#e protection must be 6esigne6 to be stable abo5e t#e ma3imum possible fault current t#at coul6 flo t#roug# t#e protecte6 area.
* Non unit t4pe protection •
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T#e non"unit sc#emesC #ile also inten6e6 to protect specific areasC #a5e no fi3e6 boun6aries. As ell as protecting t#eir on 6esignate6 areasC areas C t#e protecti5e Iones can o5erlap into ot#er areas. F#ile t#is can be 5ery beneficial for bac&up purposesC t#ere can be a ten6ency ten6enc y for too great an area to be isolate6 if a fault is 6etecte6 by 6ifferent non unit sc#emes. T#e most simple of t#ese sc#emes measures current an6 incorporates an in5erse time c#aracteristic into t#e protection operation to allo protection nearer to t#e fault to operate first. T#e non unit type protection system inclu6es folloing sc#emes: 'A) Time gra6e6 o5er current protection ') 7urrent gra6e6 o5er current protection '7) !istance or -mpe6ance 2rotection
(" O2er current protection • •
•
T#is is t#e simplest of t#e ays to protect a line an6 t#erefore i6ely use6. -t oes its application from t#e fact t#at in t#e e5ent of fault t#e current oul6 increase to a 5alue se5eral times greater t#an ma3imum loa6 current. -t #as a limitation t#at it can be applie6 only to simple an6 non costly equipments.
(3 Eart# fault protection •
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T#e general practice is to employ a set of to or t#ree o5er current relays an6 a separate o5er current relay for single line to groun6 fault. Separate eart# fault relay pro5i6e6 ma&es eart# fault protection faster an6 more sensiti5e. Eart# fault current is alays less t#an p#ase fault current in magnitu6e. T#ereforeC relay connecte6 for eart# fault protection is 6ifferent from t#ose for p#ase to p#ase fault protection.
'arious t4pes of &ine AaultsB No %4pe of Aault 1 2#ase to (roun6 fault 'Eart# ault) * 2#as 2#asee to to 2#a 2#ase se faul faultt ;ot ;ot it# it# (rou (roun6 n6 < !ouble p#ase to (roun6 fault
O2er current Rela4B
Operation of Rela4 Eart# ault Relay Rela Relate te66 2#a 2#ase se 5er 5er curr curren entt rela relayys Relate6 2#ase 5er current relays an6 Eart# ault relays
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A relay t#at operates or pic&s up #en itJs current e3cee6s a pre6etermine6 5alue 'setting 5alue) is calle6 5er 7urrent Relay. 5er current protection protects electrical poer systems against e3cessi5e currents #ic# are cause6 by s#ort circuitsC groun6 faultsC etc. 5er current relays can be use6 to protect practically any poer system elementsC i.e. transmission linesC transformersC generatorsC or motors. or fee6er protectionC t#ere oul6 be more t#an one o5er current relay to protect 6ifferent sections of t#e fee6er. T#ese o5er current relays nee6 to coor6inate it# eac# ot#er suc# t#at t#e relay nearest fault operates first. Bse timeC current an6 a combination of bot# time an6 current are t#ree ays to 6iscriminate a6Wacent o5er current relays.
O2er Current Rela4 gi2es Protection againstB aga instB 1. *. <. =. +. •
5er 5er current current inclu6es inclu6es s#ort" s#ort"cir circui cuitt protecti protection. on. S#or S#ortt cir circu cuit itss can can be 2#ase fau faults Eart# fau faults Fin6i n6ing fau faullts S#ort"circuit currents are generally se5eral times '+ to *%) full loa6 current. 9ence fast fault clearance is alays 6esirable on s#ort circuits.
Primar4 Requirement of O2er Current ProtectionB •
• •
T#e protection s#oul6 not operate for starting currentsC permissible o5er currentC current surges. To ac#ie5e t#isC t#e time 6elay is pro5i6e6 'in case of in5erse relays). T#e protection s#oul6 be co"or6inate it# neig#boring o5er current protection. 5er current relay is a basic element of o5er current protection.
Purpose of o2er current Protection • • • • • • •
!etect abnormal con6itions -solate faulty part of t#e system Spee6 ast operation to minimiIe 6amage an6 6anger !iscrimination -solate only t#e faulty section !epen6ability / reliability Security / stability 7ost of protection / against cost of potential #aIar6s
O2er Current Rela4 RatingsB
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-n or6er for an o5er current protecti5e 6e5ice to operate properlyC o5er current protecti5e 6e5ice ratings must be properly selecte6. T#ese ratings inclu6e 5oltageC ampere an6 interrupting rating. -f t#e interrupting rating is not properly. Selecte6C a serious #aIar6 for equipment an6 personnel ill e3ist. 7urrent limiting can be consi6ere6 as anot#er o5er current protecti5e 6e5ice ratingC alt#oug# not all o5er current protecti5e 6e5ices are require6 to #a5e t#is c#aracteristic 'oltage RatingB T#e RatingB T#e 5oltage rating of t#e o5er current protecti5e 6e5ice must be at least equal to or greater t#an t#e circuit 5oltage. T#e o5er current protecti5e 6e5ice rating can be #ig#er t#an t#e system 5oltage but ne5er loer. "mpere RatingB T#e RatingB T#e ampere rating of a o5er current protecting 6e5ice normally s#oul6 not e3cee6 t#e current carrying capacity of t#e con6uctors As a general ruleC t#e ampere rating of a o5er current protecting 6e5ice is selecte6 at 1*+@ of t#e continuous loa6 current
Difference 3et!een O2er current Protection @ O2er &oad ProtectionB •
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5er current protection protects against e3cessi5e currents or currents beyon6 t#e acceptable current ratingsC #ic# are resulting from s#ort circuitsC groun6 faults an6 o5erloa6 con6itions. F#ileC t#e o5erloa6 protection protects against t#e situation #ere o5erloa6 current causes o5er#eating of t#e protecte6 equipment. T#e o5er current protection is a bigger concept So t#at t#e o5erloa6 protection can be consi6ere6 as a subset of o5er o5 er current protection. T#e o5er current relay can be use6 as o5erloa6 't#ermal) protection #en protects t#e resisti5e loa6sC etc.C #oe5erC for motor loa6sC t#e o5er current relay cannot ser5e as o5erloa6 protection 5erloa6 relays usually #a5e a longer time setting t#an t#e o5er current relays.
%4pe of O2er Current Rela4B • • • • • • •
'A) -nstantaneous 5er 7urrent '!efine 7urrent) Relay ') !efine Time Time 5er 7urrent Relay '7) -n5erse Time 5er 7urrent Relay '-!,T Relay) ,o6erately -n5erse Very -n5erse Time E3tremely -n5erse '!) !irectional o5er 7urrent Relay.
(" 5nstantaneous O2er Current Rela4 (Define CurrentB
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!efinite current relay operate instantaneously #en t#e current reac#es a pre6etermine6 5alue. perates in a 6efinite time #en current e3cee6s its 2ic&"up 5alue. -ts operation criterion is only current magnitu6e 'it#out time 6elay). perating time is constant. T#ere is no intentional time 6elay.
7oor6ination of 6efinite"current relays is base6 on t#e fact t#at t#e fault current 5aries it# t#e position of t#e fault because of t#e 6ifference in t#e impe6ance beteen t#e fault an6 t#e source T#e relay locate6 furt#est from t#e source operate for a lo current 5alue T#e operating currents are progressi5ely increase6 for t#e ot#er relays #en mo5ing toar6s t#e source. -t operates in %.1s or less "pplicationB T#is "pplicationB T#is type is applie6 to t#e outgoing fee6ers
(3 Definite %ime O2er current Rela4sB •
-n t#is typeC to con6itions must be satisfie6 for operation 'tripping)C current must e3cee6 t#e setting 5alue an6 t#e fault must be continuous at least a time equal to time setting of t#e relay. ,o6ern relays may contain more t#an one stage of protection eac# stage inclu6es eac# on current curre nt an6 time setting.
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or peration of !efinite Time 5er 7urrent Relay operating time is constant -ts operation is in6epen6ent of t#e magnitu6e of current abo5e t#e pic&"up 5alue. -t #as pic&"up an6 time 6ial settingsC 6esire6 time 6elay can be set it# t#e #elp of an intentional time 6elay mec#anism. Easy to coor6inate. 7onstant tripping time in6epen6ent of in fee6 5ariation an6 fault location.
Dra!bac$ of Rela4B •
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T#e continuity in t#e supply cannot be maintaine6 at t#e loa6 en6 in t#e e5ent of fault. Time lag is pro5i6e6 #ic# is not 6esirable in on s#ort circuits. -t is 6ifficult to co"or6inate an6 requires c#anges it# t#e a66ition of loa6. -t is not suitable for long 6istance transmission lines #ere rapi6 fault clearance is necessary for stability. Relay #a5e 6ifficulties in 6istinguis#ing beteen ault currents at one point or anot#er #en fault impe6ances beteen t#ese points are smallC t#us poor 6iscrimination.
"pplicationB !efinite "pplicationB !efinite time o5er current relay is use6 as: • • •
ac& up protection of 6istance relay of transmission line it# time 6elay. ac& up protection to 6ifferential relay of poer transformer it# time 6elay. ,ain protection to outgoing fee6ers an6 bus couplers it# a6Wustable time 6elay setting.
(C 5n2erse %ime O2er current Rela4s (5D<% Rela4B •
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-n t#is type of relaysC operating time is in5ersely c#ange6 it# current. SoC #ig# current ill operate o5er current relay faster t#an loer ones. T#ere are stan6ar6 in5erseC 5ery in5erse an6 e3tremely in5erse types. !iscrimination by bot# XTimeJ an6 X7urrentJ. T#e relay operation time is in5ersely proportional to t#e fault current. -n5erse Time relays are also referre6 to as -n5erse !efinite ,inimum Time '-!,T) relay
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T#e operating time of an o5er current relay can be mo5e6 up 'ma6e sloer) by a6Wusting t#e Xtime 6ial settingJ. T#e loest time 6ial setting 'fastest operating time) is generally %.+ an6 t#e sloest is 1%. perates #en current e3cee6s its pic&"up 5alue. perating time 6epen6s on t#e magnitu6e of current. -t gi5es in5erse time current c#aracteristics at loer 5alues of fault current an6 6efinite time c#aracteristics at #ig#er 5alues An in5erse c#aracteristic is obtaine6 if t#e 5alue of plug setting multiplier is belo 1%C for 5alues beteen 1% an6 *% c#aracteristics c# aracteristics ten6 toar6s 6efinite time c#aracteristics. Fi6ely use6 for t#e protection of 6istribution lines. ase6 on t#e in5erseness it #as t#ree 6ifferent types.
( Normal 5n2erse 5n2erse %ime O2er O2er current Rela4B •
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T#e accuracy of t#e operating time may range from + to .+@ of t#e nominal operating time as specifie6 in t#e rele5ant norms. T#e uncertainty of t#e operating time an6 t#e necessary operating time may require a gra6ing margin of %.= to %.+ secon6s. use6 #en ault 7urrent is 6epen6ent on generation of ault not fault location Relati5ely small c#ange in time per unit of c#ange of current.
"pplicationB •
,ost frequently use6 in utility an6 in6ustrial circuits. especially applicable #ere t#e fault magnitu6e is mainly 6epen6ent on t#e system generating capacity at t#e time of fault
(* 'er4 5n2erse %ime O2er O2er current Rela4B • •
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(i5es more in5erse c#aracteristics t#an t#at of -!,T. Bse6 #ere t#ere is a re6uction in fault currentC as t#e 6istance from source increases. 2articularly effecti5e it# groun6 faults because of t#eir steep c#aracteristics. Suitable if t#ere is a substantial re6uction of fault current as t#e fault 6istance from t#e poer source increases. Very in5erse o5er current relays are particularly suitable if t#e s#ort"circuit current 6rops rapi6ly it# t#e 6istance from t#e substation. T#e gra6ing margin may be re6uce6 to a 5alue in t#e range from %.< to %.= secon6s #en o5er current relays it# 5ery in5erse c#aracteristics are use6. Bse6 #en ault 7urrent is 6epen6ent on fault location. Bse6 #en ault 7urrent in6epen6ent of normal c#anges in generating capacity.
(, E6tremel4 5n2erse 5n2erse %ime O2er O2er current Rela4B Rela4B •
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-t #as more in5erse c#aracteristics t#an t#at of -!,T an6 5ery in5erse o5er current relay. Suitable for t#e protection of mac#ines against o5er#eating. T#e operating time of a time o5er current relay it# an e3tremely in5erse time" current c#aracteristic is appro3imately in5ersely proportional to t#e square of t#e current T#e use of e3tremely in5erse o5er current relays ma&es it possible to use a s#ort time 6elay in spite of #ig# sitc#ing"in currents. Bse6 #en ault current is 6epen6ent on fault location Bse6 #en ault current in6epen6ent of normal c#anges in generating capacity.
"pplicationB
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Suitable for protection of 6istribution fee6ers it# pea& currents on sitc#ing in 'refrigeratorsC pumpsC ater #eaters an6 so on). 2articular suitable for gra6ing an6 coor6inates it# fuses an6 re closes or t#e protection of alternatorsC transformers. E3pensi5e cablesC etc.
(0 &ong %ime 5n2erse o2er current Rela4B •
T#e main application of long time o5er current relays is as bac&up eart# fault protection.
(D Directional O2er current Rela4s •
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F#en t#e poer system is not ra6ial 'source on one si6e of t#e line)C an o5er current relay may not be able to pro5i6e a6equate protection. T#is type of relay operates in on 6irection of current flo an6 bloc&s in t#e opposite 6irection. T#ree con6itions must be satisfie6 for its operation: current magnitu6eC time 6elay an6 6irectionality. T#e 6irectionality of current flo can be i6entifie6 using 5oltage as a reference of 6irection.
"pplication of O2er Current Rela4B • • • • • • •
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Connection of o2er current and Eart# Aault Rela4B ( , Nos O/C Rela4 for O2er Current and Eart# Aault ProtectionB ProtectionB • • •
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or <"p#ase faults t#e o5er current relays in all t#e <"p#ases act. or p#ase to p#ase faults t#e relays in only t#e affecte6 p#ases operate. or single line to groun6 faults only t#e relay in t#e faulty p#ase gets t#e fault current an6 operates. E5en t#en it# < 5er current RelayC t#e sensiti5ity 6esire6 an6 obtainable it# eart# lea&age o5er current relays cannot be obtaine6 in as muc# as t#e #ig# current setting ill #a5e to be necessarily a6opte6 for t#e 5er current Relay to a5oi6 operation un6er ma3imum loa6 con6ition.
5er current relays generally #a5e +%@ to *%%@ setting #ile eart# lea&ages o5er current relays #a5e eit#er 1%@ to =%@ or *%@ to %@ current settings. ne important t#ing to be note6 #ere is t#at t#e connection of t#e star points of bot# t#e 7.T. secon6aryJs an6 relay in6ings in6ings by a neutral con6uctor s#oul6 be ma6e. A sc#eme it#out t#e neutral con6uctor ill be unable to ensure reliable relay operation in t#e e5ent of single p#ase to eart# faults because t#e secon6ary current in t#is case 'it#out star"point interconnection) completes its circuit t#roug# relay an6 7.T. in6ings #ic# present large impe6ance. T#is may lea6 to failure of protection an6 s#arp 6ecrease in re6uction re 6uction of secon6ary currents by b y 7Ts. -t is not sufficient if t#e neutral of t#e 7Ts an6 neutral of t#e relays are separately eart#e6. A con6uctor s#oul6 be run as state6 earlier.
(* , No O/C Rela41 No E/A Rela4 for O2er Current Current and Eart# Aault ProtectionB •
T#e sc#eme of connection for < ;os 5er current Relay 1 ;o Eart# ault Relay is s#on in figure.
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Bn6er normal operating con6itions an6 t#ree p#ase fault con6itions t#e current in t#e <"p#ase are equal an6 symmetrically 6isplace6 by 1* !eg. 9ence t#e sum of t#ese t#ree currents is Iero. ;o current flo t#roug# t#e eart# fault relay. -n case of p#ase to p#ase faults 'say a s#ort beteen R an6 p#ases) t#e current flos from R"p#ase up to t#e point of fault an6 return bac& t#roug# XJ p#ase. T#us only /L relays in R an6 p#ases get t#e fault an6 operate. nly eart# faults cause currents to flo t#roug# E/L relay. A note of caution is necessary #ere. nly eit#er 7.T secon6ary star point of relay in6ing star point s#oul6 be eart#e6. Eart#ing of bot# ill s#ort circuit t#e E/L relay an6 ma&e it inoperati5e for faults.
(, * No O/C Rela4 1 No E/A Rela4 for O2er Current Current and Eart# Aault ProtectionB •
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T#e to o5er current relays in R0 p#ases ill respon6 to p#ase faults. At least one relay ill operate for fault in5ol5ing to p#ase.
or fault in5ol5ing groun6 reliance is place6 on eart# fault relay. T#is is an economical 5ersion of <"/L an6 1"E/L type of protection as one o5ercurrent relay is sa5e6. Fit# t#e protection sc#eme as s#on in igure complete protection against p#ase an6 groun6 fault is affor6e6
Current %ransformer Secondar4 ConnectionsB
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or protection of 5arious equipment of E3tra 9ig# Voltage classC t#e Star point on secon6aryJs of 7T s#oul6 be ma6e as follos for ensuring correct 6irectional sensiti5ity of t#e protection sc#eme %ransmission &ine 3us 3ar @ %ransformerB or Transmission Lines Line si6e or Transformers Transformer si6e
or us bar us si6e Generator ProtectionB (enerator 2rotection (enerator Si6e
T#e abo5e met#o6 #as to be folloe6 irrespecti5e of polarity of 7TJs on primary si6e. or e3ampleC in line protectionC if X21J is toar6s bus t#en XS*Js are to be s#orte6 an6 if X 2*J is toar6s bus t#en XS1Js are to be s#orte6.
Standard o2er Current @ Eart# Aault ProtectionB No
Name of t#e Equipment
Protection
1 11 8V ee6ers
'A) * ;o 5er 7urrent an6 one no Eart# ault -!,T relays ') * ;o -nstantaneous 5er current '#ig#est) an6 one no -nstantaneous Eart# fault relay
,VA 7apacity R To ' side B << 8V rea&er ' -n6i5i6ual or (roup 7ontrol it# < Transformer in a Sub Station 5er 7urrent an6 ne Eart# ault -!,T relays&' relays&' SideB ' -rrespecti5e of 7apacity) -n6i5i6ual 11 8V rea&ers it# < 5er 7urrent an6 ne Eart# * ault -!,T relays < ,VA ,VA 2oe 2oerr Tra Trans nsfo form rmer er nly one 2TR in a Sub = Station 'Less t#an ,VA)
!ifferential relays R RE relays on LV si6e ' Side B 9( B 9( fuse&' fuse&' Side B 11 B 11 8V rea&er it# < 5er 7urrent an6 one E/ -!,T relay
#pact of loating 0eutral in Po1er !istribution Guly *C *%1* + 7omments
-ntro6uction: •
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-f T#e ;eutral 7on6uctor opensC rea& or Loose at eit#er its source si6e '!istribution TransformerC (enerator or at Loa6 si6e '!istribution 2anel of 7onsumer)C t#e 6istribution systemJs neutral con6uctor ill YfloatZ or lose its reference groun6 2oint. T#e floating neutral con6ition can cause 5oltages to float to a ma3imum of its 2#ase 5olts R,S relati5e to groun6C subWecting to its unbalancing loa6 7on6ition. loating ;eutral con6itions in t#e poer netor& #a5e 6ifferent impact 6epen6ing on t#e type of SupplyC Type of installation an6 Loa6 balancing in t#e !istribution. ro&en ;eutral or Loose ;eutral oul6 6amage to t#e connecte6 Loa6 or 7reate #aIar6ous Touc# Voltage at equipment bo6y. ere e are tr4ing to understand t#e Aloating Neutral Condition in %J% distribution S4stem.
F#at is loating ;eutral[ •
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-f t#e Star 2oint of Bnbalance6 Loa6 is not Woine6 Woine6 to t#e Star 2oint of its 2oer Source '!istribution Transformer or (enerator) t#en 2#ase 5oltage 6o not remain same across eac# p#ase but its 5ary accor6ing to t#e Bnbalance6 of t#e loa6. As t#e 2otential of suc# an isolate6 Star 2oint or ;eutral 2oint is alays c#anging an6 not fi3e6 so itJs calle6 loating ;eutral.
;ormal 2oer 7on6ition 0 loating ;eutral 7on6ition
Normal Po!er ConditionB •
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n <"p#ase systems t#ere is a ten6ency for t#e star"point an6 2#ases to ant to Xbalance outJ base6 on t#e ratio of lea&age on eac# 2#ase to Eart#. T#e star"point ill remain close to %V 6epen6ing on t#e 6istribution of t#e loa6 an6 subsequent lea&age '#ig#er loa6 on a p#ase usually means #ig#er lea&age). T#ree p#ase systems may or may not #a5e a neutral ire. A neutral ire allos t#e t#ree p#ase system to use a #ig#er 5oltage #ile still supporting loer 5oltage single p#ase appliances. -n #ig# 5oltage 6istribution situations it is common not to #a5e a neutral ire as t#e loa6s can simply be connecte6 beteen p#ases 'p#ase" p#ase connection).
, P#ase , ire S4stemB T#ree p#ases #as properties t#at ma&e it 5ery 6esirable in electric poer systems. irstly t#e p#ase currents ten6 to cancel one anot#er 'summing to Iero in t#e case of a linear balance6 loa6). T#is ma&es it possible to eliminate t#e neutral con6uctor on some lines. Secon6ly poer transfer into a linear balance6 loa6 is constant. , P#ase 0 ire S4stem for
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T#is gets more complicate6 in t#ree p#ase poerC because no e #a5e to consi6er p#ase angleC but t#e concept is e3actly t#e same. -f e are connecte6 in Star connection it# a neutralC t#en t#e neutral con6uctor ill #a5e Iero current on it only if t#e t#ree p#ases #a5e t#e same current on eac#. -f e 6o 5ector analysis on t#isC a66ing up sin'3)C sin'3?1*%)C an6 sin'3?*=%)C e get Iero. T#e same t#ing #appens #en e are 6elta connecte6C it#out a neutralC but t#en t#e imbalance occurs out in t#e 6istribution systemC beyon6 t#e ser5ice transformersC because t#e 6istribution system is generally a Star 7onnecte6. T#e neutral s#oul6 ne5er be connecte6 to a groun6 e3cept at t#e point at t#e ser5ice #ere t#e neutral is initially groun6e6 'At !istribution Transformer). T#is can set up t#e groun6 as a pat# for current to tra5el bac& to t#e ser5ice. Any brea& in t#e groun6 pat# oul6 t#en e3pose a 5oltage potential. (roun6ing t#e neutral in a < p#ase system #elps stabiliIe p#ase 5oltages. A non"groun6e6 neutral is sometimes referre6 to as a Yfloating neutralZ an6 #as a fe limite6 applications.
loating ;eutral 7on6ition: •
2oer flos in an6 out of customersJ premises from t#e 6istribution netor&C entering 5ia t#e 2#ase an6 lea5ing 5ia t#e neutral. -f t#ere is a brea& in t#e neutral return pat# electricity may t#en tra5el by a 6ifferent pat#. 2oer flo entering in one 2#ase returns t#roug# remaining to p#ases. ;eutral 2oint is not at groun6 Le5el but it loat up to Line Voltage. T#is situation can be 5ery 6angerous an6 customers may suffer serious electric s#oc&s if t#ey touc# somet#ing #ere electricity is present.
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ro&en neutrals can be 6ifficult to 6etect an6 in some instances may not be easily i6entifie6. Sometimes bro&en neutrals can be in6icate6 by flic&ering lig#ts or tingling taps. -f you #a5e flic&ering lig#ts or tingly taps in your #omeC you may be at ris& of serious inWury or e5en 6eat#.
Voltage ,easurement beteen ;eutral to (roun6: •
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A rule"of"t#umb use6 by many in t#e in6ustry is t#at ;eutral to groun6 5oltage of *V or less at t#e receptacle is o&ayC #ile a fe 5olts or more in6icates o5erloa6ingU +V is seen as t#e upper limit. &o! Reading: Reading: -f ;eutral to groun6 5oltage is lo at t#e receptacle t#an system is #ealt#yC -f -t is #ig#C t#en you still #a5e to 6etermine if t#e problem is mainly at t#e branc# circuit le5elC or mainly at t#e panel le5el. ;eutral to groun6 5oltage e3ists because of t#e -R 6rop 6 rop of t#e current tra5eling t#roug# t#e neutral bac& to t#e ;eutral to groun6 bon6. -f t#e system is correctly ire6C t#ere s#oul6 be no ;eutral to (roun6 bon6 e3cept at t#e source transformer 'at #at t#e ;E7 calls t#e source of t#e Separately !eri5e6 SystemC or S!SC #ic# is usually a transformer). Bn6er t#is situationC t#e groun6 con6uctor s#oul6 #a5e 5irtually no current an6 t#erefore no -R 6rop on it. -n effectC t#e groun6 ire is a5ailable as a long test lea6 bac& to t#e ;eutral to groun6 bon6. ig# ReadingB A #ig# rea6ing coul6 in6icate a s#are6 branc# neutralC i.e.C a neutral s#are6 beteen more t#an one branc# circuits. T#is s#are6 neutral simply increases t#e opportunities for o5erloa6ing as ell as for one circuit to affect anot#er. Lero ReadingB A ReadingB A certain amount of ;eutral to groun6 5oltage is normal in a loa6e6 circuit. -f t#e rea6ing is stable at close to %V. T#ere is a suspect an illegal ;eutral to groun6 bon6 in t#e receptacle 'often 6ue to lose stran6s of t#e neutral touc#ing some groun6 point) or at t#e subpanel. Any ;eutral to groun6 bon6s ot#er t#an t#ose at t#e transformer source 'an6/or main panel) s#oul6 be remo5e6 to pre5ent return currents floing t#roug# t#e groun6 con6uctors.
Various actors #ic# cause ;eutral loating: •
T#ere are se5eral factors #ic# are i6entifying as t#e cause of neutral floating. T#e impact of loating ;eutral is 6epen6 on t#e position #ere ;eutral is bro&en
"t %#e %#ree P#ase Distribution %ransformerB • •
;eutral failure at transformer is mostly failure of ;eutral bus#ing. bus# ing. T#e use of Line Tap on transformer bus#ing is i6entifie6 as t#e main cause of ;eutral con6uctor failure at transformer bus#ing. bus #ing. T#e ;ut on Line Tap gets loose it# time 6ue to 5ibration an6 temperature 6ifference resulting in #ot connection. T#e con6uctor start melting an6 resulting bro&e off ;eutral.
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2oor or&mans#ip of -nstallation an6 tec#nical staff also one of t#e reasons of ;eutral ailure. A bro&en ;eutral on T#ree p#ases Transformer ill cause t#e 5oltage float up to line 5oltage 6epen6ing upon t#e loa6 balancing of t#e system. T#is type of ;eutral loating may 6amage t#e customer equipment connecte6 to t#e Supply. Bn6er normal con6ition current flo from 2#ase to Loa6 to Loa6 to bac& to t#e source '!istribution Transformer). F#en ;eutral is bro&en current from Re6 2#ase ill go bac& to lue or ello p#ase resulting Line to Line 5oltage beteen Loa6s. Some customer !ill e6perience o2er 2oltage !#ile some !ill e6perience &o! 2oltage.
* 3ro$en O2er#ead Neutral conductor in &' &ineB •
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T#e impact of bro&en o5er#ea6 ;eutral con6uctor at LV o5er#ea6 6istribution ill be similar to t#e bro&en at transformer. Supply 5oltage floating up to Line 5oltage instea6 of p#ase Voltage. T#is type of fault con6ition may 6amage customer equipment connecte6 to t#e supply.
, 3ro$en of Ser2ice Neutral ConductorB •
A bro&en ;eutral of ser5ice con6uctor ill only result of loss of supply at t#e customer point. ;o any 6amages to customer equipments.
0 ig# Eart#ing Resistance of Neutral at Distribution %ransformerB •
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(oo6 Eart#ing Resistance of Eart# 2it of ;eutral pro5i6e lo resistance pat# for neutral current to 6rain in eart#. 9ig# Eart#ing Resistance may pro5i6e #ig# resistance 2at# for groun6ing of ;eutral at !istribution Transformer. Limit eart# resistance sufficiently lo to permit a6equate fault current for t#e operation of protecti5e 6e5ices in time an6 to re6uce neutral s#ifting.
- O2er &oading @ &oad =nbalancingB •
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!istribution ;etor& 5erloa6ing combine6 it# poor loa6 6istribution is one of t#e most reason of ;eutral failure. ;eutral s#oul6 be properly 6esigne6 so t#at minimum current ill be flo in to neutral con6uctor. T#eoretically t#e current flo in t#e ;eutral is suppose6 to be Iero because of cancellation 6ue to 1*% 6egree p#ase 6isplacement of p#ase current. 5N) 5RM+ 1 5M*+ 1 53MJ*+. -n 5erloa6e6 Bnbalancing ;etor& lot of current ill flo in ;eutral #ic# brea& ;eutral at its ea&est 2oint.
: S#ared neutrals
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Some buil6ings are ire6 so t#at to or t#ree p#ases s#are a single neutral. T#e original i6ea as to 6uplicate on t#e branc# circuit le5el t#e four ire 't#ree p#ases an6 a neutral) iring of panel boar6s. bo ar6s. T#eoreticallyC only t#e unbalance6 current ill return on t#e neutral. T#is allos one neutral to 6o t#e or& for t#ree p#ases. T#is iring s#ortcut quic&ly became a 6ea6"en6 6ea6"en 6 it# t#e grot# of single"p#ase non"linear loa6s. T#e problem is t#at Iero sequence current rom nonlinear loa6sC primarily t#ir6 #armonicC ill a66 up arit#metically an6 return on t#e neutral. -n a66ition to being a potential safety problem because of o5er#eating of an un6ersiIe6 neutralC t#e e3tra neutral current creates a #ig#er ;eutral to groun6 5oltage. T#is ;eutral to groun6 5oltage subtracts from t#e Line to ;eutral 5oltage a5ailable to t#e loa6. -f youJre starting to feel t#at s#are6 neutrals are one of t#e orst i6eas t#at e5er got translate6 to copper.
Poor !or$mans#ip @
;ormally LV netor& are mostly not gi5en attention by t#e ,aintenance Staff. Loose or ina6equate tig#tening of ;eutral con6uctor ill effect on continuity of ;eutral #ic# may cause floating of ;eutral.
9o to 6etect loating ;eutral 7on6ition 7on6 ition in 2anel: •
Let us Ta&e one E3ample to un6erstan6 ;eutral loating 7on6ition.Fe #a5e a Transformer #ic# Secon6ary is star connecte6C 2#ase to neutral H *=%V an6 2#ase to p#ase H ==%V.
Condition (B Neutral is not Aloating •
F#et#er t#e ;eutral is groun6e6 t#e 5oltages remain t#e same *=%V beteen p#ase 0 ;eutral an6 ==%V beteen p#ases. T#e ;eutral is not loating.
Condition (*B Neutral is Aloating •
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"ll "ppliances are connectedB -f connectedB -f t#e ;eutral ire for a circuit becomes 6isconnecte6 from t#e #ouse#ol6Js main poer supply panel #ile t#e 2#ase ire for t#e circuit still remains connecte6 to t#e panel an6 t#e circuit #as appliances plugge6 into t#e soc&et outlets. -n t#at situationC if you put a 5oltage Tester it# a neon lamp onto t#e ;eutral ire it ill glo Wust as if it as Li5eC because it is being fe6 it# a 5ery small current coming from t#e 2#ase supply 5ia t#e plugge6"in appliance's) to t#e ;eutral ire. "ll "ppliances are DisconnectedB -f DisconnectedB -f you unplug all appliancesC lig#ts an6 #ate5er else may be connecte6 to t#e circuitC t#e ;eutral ill no longer seem to be Li5e because t#ere is no longer any pat# from it to t#e 2#ase supply. P#ase to P#ase 'oltageB T#e 'oltageB T#e meter in6icates ==%V A7. ';o any Effect on < 2#ase Loa6) P#ase to Neutral 'oltageB T#e meter in6icates 11%V A7 to <<%V A7. Neutral to Ground 'oltageB T#e meter in6icates 11%V.
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P#ase to Ground 'oltageB T#e meter in6icates 1*%V. T#is is because t#e neutral is YfloatsZ abo5e groun6 potential '11%V ? 1*%V H *<%VA7). As a result t#e output is isolate6 from system groun6 an6 t#e full output of *<%V is reference6 beteen line an6 neutral it# no groun6 connection. -f su66enly 6isconnect t#e ;eutral from t#e transformer ;eutral but &ept t#e loa6ing circuits as t#ey areC T#en Loa6 si6e ;eutral becomes loating since t#e equipment t#at are connecte6 beteen 2#ase to ;eutral ill become beteen 2#ase to 2#ase ' R to C to )C an6 since t#ey are not of t#e same ratingsC t#e artificial resulting neutral ill be floatingC suc# t#at t#e 5oltages present at t#e 6ifferent equipments ill no longer be *=%V but some#ere beteen % 'not e3actly) an6 t#e ==% V 'also not e3actly). ,eaning t#at on one line 2#ase to 2#aseC some ill #a5e less t#an *=%V an6 some ill #a5e #ig#er up to near =1+. All 6epen6s on t#e impe6ance of eac# connecte6 item. -n an unbalance systemC if t#e neutral is 6isconnecte6 from t#e sourceC t#e neutral becomes floating neutral an6 it is s#ifte6 to a position so t#at it is closer to t#e p#ase it# #ig#er loa6s an6 aay from t#e p#ase p#as e it# smaller loa6. Let us assume an unbalance < p#ase system #as < 8F loa6 in R"p#aseC * 8F loa6 in " p#ase an6 1 8F loa6 in "p#ase. -f t#e neutral of t#is system is 6isconnecte6 from t#e mainC t#e floating neutral ill be closer to R"p#ase an6 aay from " p#ase. SoC t#e loa6s it# "p#ase ill e3perience more 5oltage t#an usualC #ile t#e loa6s in R"p#ase ill e3perience less 5oltage. Loa6s in "p#ase ill e3perience almost same 5oltage. T#e neutral 6isconnect for an unbalance6 system is 6angerous to t#e loa6s. ecause of t#e #ig#er or loer 5oltagesC t#e equipment is most li&ely to be 6amage6. ere !e obser2e t#at Neutral Aloating condition does not impact on , P#ase &oad but 5t impacts onl4 P#ase &oad onl4
9o to Eliminate ;eutral loating: •
T#ere are Some 2oint nee6s to be consi6er to pre5ent of ;eutral loating.
a =se 0 Pole 3rea$er/E&C3/RC3O in Distribution PanelB •
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A floating neutral can be a serious problem. Suppose e #a5e a brea&er panel it# < 2ole rea&er for T#ree 2#ase an6 us bar for ;eutral for < 2#ase inputs an6 a neutral '9ere e #a5e not use6 = 2ole rea&er). T#e 5oltage beteen eac# 2#ase is ==% an6 t#e 5oltage beteen eac# 2#ase an6 t#e neutral is *<%. Fe #a5e single brea&ers fee6ing loa6s t#at require *<%Volts. T#ese *<%Volt loa6s #a5e one line fe6 by t#e brea&er an6 a neutral. ;o suppose t#e ;eutral gets loose or o3i6iIe6 or some#o 6isconnecte6 in t#e panel or maybe e5en out #ere t#e poer comes co mes from. T#e ==%Volt loa6s ill be unaffecte6 #oe5er t#e *<%V loa6s can be in serious trouble. Fit# t#is loating neutral con6ition you ill 6isco5er t#at one of t#e to lines ill go from *<%Volts up to <=% or <+% an6 t#e ot#er line ill go 6on to 11% or 1*% 5olts. 9alf of your *<%Volt equipment ill go up in #ig# 6ue to o5er5oltage an6 t#e
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ot#er #alf ill not function 6ue to a lo 5oltage con6ition. SoC be careful it# floating neutrals. Simply use EL7C R7 or = 2ole 7ircuit rea&er as income in t#e
b =sing 'oltage Stabili9erB •
F#ene5er neutral fails in t#ree p#ase systemC t#e connecte6 loa6s ill get connecte6 beteen p#ases oing to floating neutral. 9ence 6epen6ing on loa6 resistance across t#ese p#asesC t#e 5oltage &eeps 5arying beteen *<%V to =%%V.A suitable ser5o stabiliIer it# i6e input 5oltage range it# #ig# 0 lo cutoff may #elp in protecting t#e equipments.
c Good !or$mans#ip @
(i5e #ig#er 2riority on ,aintenance of LV netor& . Tig#t or apply a6equate Torque for tig#tening of ;eutral con6uctor in LV system
7onclusion: •
A loating ;eutral '!isconnecte6 ;eutral) fault con6ition is 'ER =NS"AE because -f Appliance is not or&ing an6 someone #o 6oes not &no about abou t t#e ;eutral loating coul6 easily touc# t#e ;eutral ire to fin6 out #y appliances 6oes not or& #en t#ey are plugge6 into a circuit an6 get a ba6 s#oc&. Single p#ase Appliances are 6esign to or& its normal 2#ase 2#as e Voltage #en t#ey get Line Voltage Appliances may !amage .!isconnecte6 ;eutral fault is a 5ery unsafe con6ition an6 s#oul6 be correcte6 at t#e earliest possible by troubles#ooting of t#e e3act ires to c#ec& an6 t#en connect properly.
Types of 0eutral +arthing in Po1er !istribution Ganuary *1C *%1* 1= 7omments
Types of ;eutral Eart#ing in 2oer !istribution: -ntro6uction: •
-n t#e early poer systems ere mainly ;eutral ungroun6e6 6ue to t#e fact t#at t#e first groun6 fault 6i6 not require t#e tripping of t#e system. An unsc#e6ule6 s#ut6on on t#e first groun6 fault as particularly un6esirable for continuous process in6ustries. T#ese poer systems s ystems require6 groun6 6etection systemsC but locating t#e fault often pro5e6 6ifficult. Alt#oug# ac#ie5ing t#e initial goalC t#e ungroun6e6 system pro5i6e6 no control of transient o5er"5oltages.
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A capaciti5e coupling e3ists beteen t#e system con6uctors an6 groun6 in a typical 6istribution system. As a resultC t#is series resonant L"7 circuit can create o5er"5oltages ell in e3cess of line"to"line 5oltage #en subWecte6 to repetiti5e re"stri&es of one p#ase to groun6. T#is in turnC re6uces insulation life resulting in possible equipment failure. ;eutral groun6ing systems are similar to fuses in t#at t#ey 6o not#ing until u ntil somet#ing in t#e system goes rong. T#enC li&e fusesC t#ey protect personnel an6 equipment from 6amage. !amage comes from to factorsC #o long t#e fault lasts an6 #o large t#e fault current is. (roun6 relays trip brea&ers an6 limit #o long a fault lasts an6 ;eutral groun6ing resistors limit #o large t#e fault current is.
-mportance of ;eutral (roun6ing: •
1. *. <. =. +. $. .
T#ere are many neutral groun6ing options a5ailable for bot# Lo an6 ,e6ium 5oltage poer systems. T#e neutral points of transformersC generators an6 rotating mac#inery to t#e eart# groun6 netor& pro5i6es a reference point of Iero 5olts. T#is protecti5e measure offers many a65antages o5er an ungroun6e6 systemC li&eC Re6uce Re6uce66 magnit magnitu6e u6e of trans transien ientt o5er o5er 5oltage 5oltagess Simpli Simplifie fie66 groun6 groun6 fault fault locat location ion -mpro5e -mpro5e66 system system an6 equip equipmen mentt fault fault protect protection ion Re6uce Re6uce66 maint maintena enance nce time time an6 e3pense e3pense (rea (reate terr safet safetyy for for pers personn onnel el -mpro5e -mpro5e66 lig# lig#tni tning ng protect protection ion Re6uct Re6uction ion in frequen frequency cy of of fault faults. s.
,et#o6 of ;eutral Eart#ing: •
T#ere are fi5e met#o6s for ;eutral eart#ing.
1. Bnea Bneart rt#e #e66 ;eut ;eutra rall Syst System em *. Soli6 Soli6 ;eutra ;eutrall Eart#e Eart#e66 Syst System. em. <. Resistance Resistance ;eutral ;eutral Eart#ing Eart#ing System.Res System.Resonant onant ;eutral ;eutral Eart#ing Eart#ing System. System. 1. Lo Lo Resi Resist stan ance ce Ear Eart# t#in ing. g. *. 9ig# 9ig# Resi Resist stan ance ce Ear Eart# t#in ing. g. =. Reson Resonan antt Eart Eart#i #ing ng Syste System. m. +. Eart#i Eart#ing ng Transfo Transforme rmerr Eart#in Eart#ing. g.
'1) Bngroun6e6 ;eutral Systems: •
-n ungroun6e6 system t#ere is no internal connection beteen t#e con6uctors an6 eart#. 9oe5erC as systemC a capaciti5e coupling e3ists beteen t#e system con6uctors an6 t#e a6Wacent groun6e6 surfaces. 7onsequentlyC t#e Yungroun6e6
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systemZ isC in realityC a Ycapaciti5e groun6e6 systemZ by 5irtue of t#e 6istribute6 capacitance. Bn6er normal operating con6itionsC t#is 6istribute6 capacitance causes no problems. -n factC it is beneficial because it establis#esC in effectC a neutral point for t#e systemU As a resultC t#e p#ase con6uctors are stresse6 at only line"to" neutral 5oltage abo5e groun6. ut problems can rise in groun6 fault con6itions. A groun6 fault on one line results in full line"to"line 5oltage appearing t#roug#out t#e system. T#usC a 5oltage 1.< times t#e normal 5oltage is present on all insulation in t#e system. T#is situation can often cause failures in ol6er motors an6 transformersC 6ue to insulation brea&6on.
"d2antageB
1. After t#e t#e first groun6 faultC faultC assuming assuming itit remains remains as a single single faultC faultC t#e t#e circuit circuit may continue in operationC permitting continue6 pro6uction until a con5enient s#ut 6on for maintenance can be sc#e6ule6. •
Disad2antagesB
1. T#e interacti interaction on beteen beteen t#e faulte6 faulte6 system system an6 its its 6istribute 6istribute66 capacitance capacitance may may cause transient o5er"5oltages 'se5eral times normal) to appear from line to groun6 6uring normal sitc#ing of a circuit #a5ing a line"to groun6 fault 's#ort). T#ese o5er 5oltages may cause insulation failures at points ot#er t#an t#e original fault. *. A secon6 fault on anot#er anot#er p#ase may may occur before before t#e first first fault fault can be cleare6. cleare6. T#is can result in 5ery #ig# line"to"line fault currentsC equipment 6amage an6 6isruption of bot# circuits. <. T# T#ee cost cost of equi equipm pment ent 6am 6amage age.. =. 7omplicate 7omplicate for for locating locating fault's)C fault's)C in5ol5ing in5ol5ing a te6ious te6ious process of of trial an6 error: first isolating t#e correct fee6erC t#en t#e branc#C an6 finallyC t#e equipment at fault. T#e result is unnecessarily lengt#y an6 e3pensi5e 6on 6ontime.
'*) Soli6ly ;eutral (roun6e6 Systems:
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Soli6ly groun6e6 systems are usually use6 in lo 5oltage applications at $%% 5olts or less. -n soli6ly groun6e6 systemC t#e neutral point is connecte6 to eart#. Soli6ly ;eutral (roun6ing slig#tly re6uces t#e problem of transient o5er 5oltages foun6 on t#e ungroun6e6 system an6 pro5i6e6 pat# for t#e groun6 fault current is in t#e range of 25 to 100% of the system three phase fault current. 9oe5erC if t#e reactance of t#e generator or transformer is too greatC t#e problem of transient o5er 5oltages ill not be sol5e6. F#ile soli6ly groun6e6 systems are an impro5ement o5er ungroun6e6 systemsC an6 spee6 up t#e location of faultsC t#ey lac& t#e current limiting ability of resistance groun6ing an6 t#e e3tra protection t#is pro5i6es. To maintain systems #ealt# an6 safeC Transformer neutral is groun6e6 an6 groun6ing con6uctor must be e3ten6 from t#e source to t#e furt#est point of t#e system it#in t#e same raceay or con6uit. -ts purpose is to maintain 5ery lo impe6ance to groun6 faults so t#at a relati5ely #ig# fault current ill flo t#us insuring t#at circuit brea&ers or fuses ill clear t#e fault quic&ly an6 t#erefore minimiIe 6amage. -t also greatly re6uces t#e s#oc& #aIar6 to personnel
-f t#e system is not soli6ly groun6e6C t#e neutral point of t#e system oul6 YfloatZ it# respect to groun6 as a function of loa6 subWecting t#e line"to"neutral loa6s to 5oltage unbalances an6 instability. T#e single"p#ase eart# fault current in a soli6ly eart#e6 system may e3cee6 t#e t#ree p#ase fault current. T#e magnitu6e of t#e current 6epen6s on t#e fault location an6 t#e fault resistance. ne ay to re6uce t#e eart# fault current is to lea5e some of t#e transformer neutrals uneart#e6. "d2antageB
1. T#e main main a65antage a65antage of soli6ly soli6ly eart#e6 eart#e6 systems systems is lo o5er 5oltagesC 5oltagesC #ic# #ic# ma&es ma&es t#e eart#ing 6esign common at #ig# 5oltage le5els '9V). •
Disad2antageB
1. T#is system system in5ol5es in5ol5es all all t#e 6rabac&s 6rabac&s an6 an6 #aIar6s of of #ig# eart# eart# fault fault current: current: ma3imum 6amage an6 6isturbances.
*. T#ere T#ere is no ser5ic ser5icee continu continuity ity on t#e t#e faulty faulty fee6er. fee6er. <. T#e 6anger 6anger for personnel personnel is #ig# 6uring 6uring t#e fault since since t#e touc# touc# 5oltages 5oltages create6 create6 are #ig#. •
"pplicationsB
1. !istri !istribut bute6 e6 neutra neutrall con6uct con6uctor. or. *. <"p#ase <"p#ase ? neut neutral ral 6istri 6istribut bution ion.. <. Bse of t#e t#e neutral neutral con6uctor con6uctor as a protecti protecti5e 5e con6uctor con6uctor it# it# systematic systematic eart#in eart#ingg at eac# transmission pole. =. Bse6 Bse6 #en t#e t#e s#ort"ci s#ort"circui rcuitt poer poer of t#e source source is lo. lo.
'<) Resistance eart#e6 systems: •
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Resistance groun6ing #as been use6 in t#ree"p#ase in6ustrial applications for many years an6 it resol5es many of t#e problems associate6 it# soli6ly groun6e6 an6 ungroun6e6 systems. Resistance (roun6ing Systems limits t#e p#ase"to"groun6 fault currents. T#e reasons for limiting t#e 2#ase to groun6 ault current by resistance groun6ing are:
1. To re6uce burning burning an6 melting melting effects effects in faulte faulte66 electrical electrical equipme equipment nt li&e sitc#gearC transformersC cablesC an6 rotating mac#ines. *. To re6uce mec#anical mec#anical stresses stresses in circuits/Equi circuits/Equipments pments carryin carryingg fault currents. currents. <. To re6uce re6uce electrica electrical"s#oc& l"s#oc& #aIar6s #aIar6s to to personnel personnel cause6 cause6 by stray groun6 groun6 fault. fault. =. To re6uce re6uce t#e arc blas blastt or flas flas## #aIar6. #aIar6. +. To re6uce re6uce t#e mome momenta ntary ry line"5 line"5olt oltage age 6ip. 6ip. $. To secure secure control control of t#e transien transientt o5er"5oltages o5er"5oltages #ile #ile at t#e t#e same time. time. . To impro5e impro5e t#e t#e 6etection 6etection of t#e eart# fault fault in in a poer poer system. system. •
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(roun6ing Resistors are generally connecte6 beteen groun6 an6 neutral of transformersC generators an6 groun6ing transformers to limit maximum fault current as per Ohms Law to a value which will not damage the equipment in t#e poer system an6 allo sufficient flo of fault current to 6etect an6 operate o perate Eart# protecti5e relays to clear t#e fault. Alt#oug# it is possible to limit fault currents it# #ig# resistance ;eutral groun6ing ResistorsC eart# s#ort circuit currents can be e3tremely re6uce6. As a result of t#is factC protection 6e5ices may not sense t#e fault. T#ereforeC it is t#e most common application to limit single p#ase fault currents it# lo resistance ;eutral (roun6ing Resistors to appro3imately rate6 current of transformer an6 / or generator. -n a66itionC limiting fault currents to pre6etermine6 ma3imum 5alues permits t#e 6esigner to selecti5ely coor6inate t#e operation of protecti5e 6e5icesC #ic# minimiIes system 6isruption an6 allos for quic& location of t#e fault. T#ere are to categories of resistance groun6ing:
'1) Lo resistance (roun6ing.
'*) 9ig# resistance (roun6ing. •
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(roun6 fault current floing t#roug# eit#er type of resistor #en a single p#ase faults to groun6 ill increase t#e p#ase"to"groun6 5oltage of t#e remaining to p#ases. As a resultC conductor insulation and surge arrestor ratings must e ased on line!to!line voltage . T#is temporary increase in p#ase"to"groun6 5oltage s#oul6 also be consi6ere6 #en selecting to an6 t#ree pole brea&ers installe6 on resistance groun6e6 lo 5oltage systems. T#e increase in p#ase"to"groun6 5oltage associate6 it# groun6 fault currents also preclu6es t#e connection of line"to"neutral loa6s 6irectly 6 irectly to t#e system. -f line"to neutral loa6s 'suc# as *V lig#ting) are presentC t#ey must be ser5e6 by a soli6ly groun6e6 system. T#is can be ac#ie5e6 it# an isolation transformer t#at #as a t#ree"p#ase 6elta primary an6 a t#ree"p#aseC four"ireC ye secon6ary
;eit#er of t#ese groun6ing systems 'lo or #ig# resistance) re sistance) re6uces arc"flas# #aIar6s associate6 it# p#ase"to"p#ase faultsC but bot# systems significantly re6uce or essentially eliminate t#e arc"flas# #aIar6s associate6 it# p#ase"to" groun6 faults. ot# types of groun6ing systems limit mec#anical stresses an6 re6uce t#ermal 6amage to electrical equipmentC circuitsC an6 apparatus carrying faulte6 current. T#e 6ifference beteen Lo Resistance (roun6ing an6 9ig# Resistance (roun6ing is a matter of perception an6C t#ereforeC is not ell 6efine6. "enerally spea#ing high!resistance grounding refers to a system in which the $" let! through current is less than 50 to 100 &. Low resistance grounding indicates that $" current would e aove 100 & . & .
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A better 6istinction beteen t#e to le5els mig#t be alarm only an6 tripping. An alarm"only system continues to operate it# a single groun6 fault on t#e system for an unspecifie6 amount of time. -n a tripping system a groun6 fault is automatically remo5e6 by protecti5e relaying an6 circuit interrupting 6e5ices. Alarm"only systems usually limit ;(R current to 1% A or less. Rating of %#e Neutral grounding resistorB
1. . Voltage: Line"to"neutral 5oltage of t#e system to #ic# it is connecte6. *. *. -nitial 7urrent: T#e initial current #ic# ill flo t#roug# t#e resistor it# rate6 5oltage applie6. <. ,. Time: T#e Yon timeZ for #ic# t#e resistor can operate it#out e3cee6ing t#e alloable temperature rise.
'A).Lo Resistance (roun6e6: •
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Lo Resistance (roun6ing is use6 for large electrical systems #ere t#ere is a #ig# in5estment in capital equipment or prolonge6 loss of ser5ice of equipment #as a significant economic impact an6 it is not commonly use6 in lo 5oltage systems because t#e limite6 groun6 fault current is too lo to reliably operate brea&er trip units or fuses. T#is ma&es system s ystem selecti5ity #ar6 to ac#ie5e. ,oreo5erC lo resistance groun6e6 systems are not suitable for ="ire loa6s an6 #ence #a5e not been use6 in commercial mar&et applications A resistor is connecte6 from t#e system neutral point to groun6 an6 generally siIe6 to permit only 200& to 1200 amps of groun6 fault current to flo. Enoug# current must flo suc# t#at protecti5e 6e5ices can 6etect t#e faulte6 circuit an6 trip it off"line but not so muc# current as to create maWor 6amage at t#e fault point.
Since t#e groun6ing impe6ance is in t#e form of resistanceC any transient o5er 5oltages are quic&ly 6ampe6 out an6 t#e #ole transient o5er5oltage p#enomena is no longer applicable. Alt#oug# t#eoretically possible to be applie6 in lo 5oltage systems 'e.g. =%V)Csignificant amount of t#e system 5oltage 6roppe6 across t#e groun6ing resistorC t#ere is not enoug# 5oltage across t#e arc forcing current to floC for t#e fault to be reliably 6etecte6. or t#is reason reason lo! resistance grounding is not used for lo! 2oltage s4stems 'un6er s4stems 'un6er 1%%% 5olts line to"line). "d2antagesB
1. Limits Limits p#ase" p#ase"to" to"grou groun6 n6 current currentss to *%%"=%%A *%%"=%%A.. *. Re6uces Re6uces arcing current current an6C an6C to some some e3tentC e3tentC limits limits arc"flas# arc"flas# #aIar6s #aIar6s associate6 associate6 it# p#ase"to"groun6 arcing current con6itions only. <. ,ay limit limit t#e t#e mec#anical mec#anical 6amage 6amage an6 t#ermal t#ermal 6amage 6amage to to s#orte6 transformer transformer an6 rotating mac#inery in6ings. •
Disad2antagesB
1. *. <. =. •
!oes !oes not pre5ent pre5ent opera operatio tionn of o5er curren currentt 6e5ices. 6e5ices. !oes !oes not requir requiree a groun6 faul faultt 6etecti 6etection on system system.. ,ay be util utiliIe6 iIe6 on me6i me6ium um or #ig# #ig# 5oltage 5oltage syste systems. ms. 7on6uctor 7on6uctor insulation insulation an6 an6 surge arrestors arrestors must must be rate6 rate6 base6 on t#e line line to"line to"line 5oltage. 2#ase"to"neutral loa6s must be ser5e6 t#roug# an isolation transformer. =sedB Bp to =%% amps for 1% sec are commonly foun6 on me6ium 5oltage systems.
').9ig# Resistance (roun6e6: •
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9ig# resistance groun6ing is almost i6entical to lo resistance groun6inge3cept t#at t#e groun6 fault current magnitu6e is typically limite6 to 10 amperes or less . 9ig# resistance groun6ing accomplis#es to t#ings. T#e first is t#at t#e ground fault fault current current magnitude magnitude is sufficiently sufficiently low low enough such t#at no appreciable 6amage is 6one at t#e fault point. T#is means t#at t#e faulte6 circuit nee6 not be trippe6 off"line #en t#e fault first occurs. ,eans t#at once a fault 6oes occurC e 6o not &no #ere t#e fault is locate6. -n t#is respectC it performs Wust li&e an ungroun6e6 system. T#e secon6 point is it can control the control the transient overvoltage phenomenon present on ungroun6e6 systems if engineere6 properly. Bn6er eart# fault con6itionsC t#e resistance must 6ominate o5er t#e system c#arging capacitance but not to t#e point of permitting e3cessi5e current to flo
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9ig# Resistance (roun6ing '9R() systems limit t#e fault current #en one p#ase of t#e system s#orts or arcs to groun6C but at loer le5els t#an lo resistance systems. -n t#e e5ent t#at a groun6 fault con6ition e3istsC t#e 9R( typically limits t#e current to +"1%A. 9R(Js are continuous current rate6C so t#e 6escription of a particular unit 6oes not inclu6e a time rating. Bnli&e ;(RJsC groun6 fault current floing t#roug# a 9R( is usually not of significant magnitu6e to result in t#e operation of an o5er current 6e5ice. Since t#e groun6 fault current is not interrupte6C a groun6 fault 6etection system must be installe6. T#ese systems inclu6e a bypass contactor tappe6 across a portion of t#e resistor t#at pulses 'perio6ically opens an6 closes). F#en t#e contactor is openC groun6 fault current flos t#roug# t#e entire resistor. F#en t#e contactor is close6 a
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portion of t#e resistor is bypasse6 resulting res ulting in slig#tly loer resistance an6 slig#tly #ig#er groun6 fault current. %o a2oid transient o2erJ2oltages an RG resistor must be si9ed so t#at t#e amount of ground fault current t#e current t#e unit ill allo to flo e3cee6s t#e electrical systemJs c#arging current. As a rule of t#umbC c#arging current is estimate6 at 1A per *%%%8VA of system capacity for lo 5oltage systems an6 *A per *%%%8VA of system capacity at =.1$&V. T#ese estimate6 c#arging currents increase if surge suppressors are present. Eac# set of suppressors installe6 on a lo 5oltage system results in appro3imately %.+A of a66itional c#arging current an6 eac# set of suppressors installe6 on a =.1$&V system a66s 1.+A of a66itional c#arging current. A system it# <%%%8VA of capacity at =% 5olts oul6 #a5e an estimate6 c#arging current of 1.+A.A66 one set of surge suppressors an6 t#e total c#arging current increases by %.+A to *.%A. A stan6ar6 +A resistor coul6 be use6 on t#is system. ,ost resistor manufacturers publis# 6etaile6 estimation tables t#at can be use6 to more closely estimate an electrical systemJs c#arging current. "d2antagesB
1. Enables Enables #ig# impe6ance impe6ance fault fault 6etecti 6etection on in systems systems it# it# ea& capacit capaciti5e i5e connection to eart# *. Some Some p#ase"to p#ase"to"ea "eart# rt# fault faultss are self"cl self"clear eare6. e6. <. T#e neutral neutral point point resistance resistance can be be c#osen to to limit limit t#e possibl possiblee o5er 5oltage 5oltage transients to *.+ times t#e fun6amental frequency ma3imum 5oltage. =. Limits Limits p#ase p#ase"to "to"gro "groun6 un6 curre currents nts to to +"1%A. +"1%A. +. Re6uces Re6uces arcing current current an6 an6 essentially essentially elimi eliminates nates arc"flas# arc"flas# #aIar6s #aIar6s associate6 associate6 it# p#ase"to"groun6 arcing current con6itions only. $. Fill eliminate eliminate t#e t#e mec#anical mec#anical 6amage 6amage an6 may limit limit t#ermal t#ermal 6amage 6amage to s#orte6 s#orte6 transformer an6 rotating mac#inery in6ings. . 2re5ents 2re5ents operation operation of o5er current current 6e5ices 6e5ices until until t#e fault can can be locate6 locate6 '#en '#en only one p#ase faults to groun6). . ,ay be utiliI utiliIe6 e6 on lo 5oltag 5oltagee systems systems or me6ium me6ium 5oltage 5oltage systems systems up to +&V. +&V. -EEE Stan6ar6 1=1"144< states t#at Y#ig# resistance groun6ing s#oul6 be restricte6 to +&V class or loer systems it# c#arging currents of about +.+A or less an6 s#oul6 not be attempte6 on 1+&V systemsC unless proper groun6ing relaying is employe6Z. 4. 7on6uctor 7on6uctor insulation insulation an6 an6 surge arrestors arrestors must must be rate6 rate6 base6 on t#e line line to"line to"line 5oltage. 2#ase"to"neutral loa6s must be ser5e6 t#roug# an isolation transformer. •
Disad2antagesB
1. (enerates (enerates e3tensi5e e3tensi5e eart# eart# fault currents currents #en #en combine6 combine6 it# it# strong strong or mo6erate mo6erate capaciti5e connection to eart# 7ost in5ol5e6. *. Requires Requires a groun6 fault 6etect 6etection ion system system to notify notify t#e facili facility ty engineer engineer t#at t#at a groun6 fault con6ition #as occurre6.
'=) Resonant eart#e6 system:
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A66ing in6ucti5e reactance from t#e system neutral point to groun6 is an easy met#o6 of limiting t#e a5ailable groun6 fault from somet#ing near t#e ma3imum < p#ase s#ort circuit capacity 't#ousan6s of amperes) to a relati5ely lo 5alue '*%% to %% amperes). To limit t#e reacti5e part of t#e eart# fault current in a poer system a neutral point reactor can be connecte6 beteen t#e transformer transf ormer neutral an6 t#e station eart#ing system. A system in #ic# at least one of t#e neutrals is connecte6 to eart# t#roug# an
1. -n6uc -n6ucti ti5e 5e reac reacta tance nce.. *. 2etersen 2etersen coil / Arc Suppression Suppression 7oil 7oil / Eart# Eart# ault ault ;eutral ;eutraliIer. iIer. •
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T#e current generate6 by t#e reactance 6uring an eart# fault appro3imately compensates t#e capaciti5e component of t#e single p#ase eart# fault currentC is calle6 a resonant eart#e6 system. T#e system is #ar6ly e5er e3actly tune6C i.e. t#e reacti5e current 6oes not e3actly equal t#e capaciti5e eart# fault current of t#e system. A system in #ic# t#e in6ucti5e current is slig#tly larger t#an t#e capaciti5e eart# fault current is o5er compensate6. A system in #ic# t#e in6uce6 eart# fault current is slig#tly smaller t#an t#e capaciti5e eart# fault current is un6er
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9oe5erC e3perience in6icate6 t#at t#is in6ucti5e reactance to groun6 resonates it# t#e system s#unt capacitance to groun6 un6er arcing groun6 fault con6itions an6 creates 5ery #ig# transient o5er 5oltages on t#e system. To control t#e transient o5er 5oltagesC t#e 6esign must permit at least $%@ of t#e < p#ase s#ort circuit current to flo un6ergroun6 fault con6itions. E3ample. A $%%% amp groun6ing reactor for a system #a5ing 1%C%%% amps < p#ase s#ort circuit capacity a5ailable. !ue to t#e #ig# magnitu6e of groun6 grou n6 fault current require6 to control transient o5er 5oltagesC in6uctance groun6ing is rarely used within industry.
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2etersen 7oils: A 2etersen 7oil is connecte6 beteen t#e neutral point of t#e system an6 eart#C an6 is rate6 so t#at t#e capaciti5e current in t#e earth fault is compensated y an inductive current passed y the 'etersen (oil . A small resi6ual current ill remainC but t#is is so small t#at any arc beteen t#e faulte6 p#ase an6 eart# ill not be maintaine6 an6 t#e fault ill e3tinguis#. ,inor eart# faults suc# as a
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bro&en pin insulatorC coul6 be #el6 on t#e system it#out t#e supply being interrupte6. Transient faults oul6 not result in supply interruptions. Alt#oug# t#e stan6ar6 X2eterson coilJ 6oes not compensate t#e entire eart# fault current in a netor& 6ue to t#e presence of resisti5e losses in t#e lines an6 coilC it is no possible to apply Xresi6ual current compensationJ by inWecting an a66itional 1%\ out of p#ase current into t#e neutral 5ia t#e 2eterson coil. T#e fault current is t#ereby re6uce6 to practically Iero. Suc# systems are &non as XResonant eart#ing it# resi6ual compensationJC an6 can be consi6ere6 as a special case of reacti5e eart#ing. Resonant eart#ing can re6uce E2R to a safe le5el. T#is is because t#e 2etersen coil can often effecti5ely act as a #ig# impe6ance ;ERC #ic# ill substantially re6uce any eart# fault currentsC an6 #ence also any correspon6ing E2R #aIar6s 'e.g. touc# 5oltagesC step 5oltages an6 transferre6 5oltagesC inclu6ing any E2R #aIar6s impresse6 onto nearby telecommunication netor&s). "d2antagesB
1. Small reacti5e reacti5e eart# fault fault current current in6epen6en in6epen6entt of t#e p#ase p#ase to eart# eart# capacitan capacitance ce of t#e system. *. Enable Enabless #ig# impe6a impe6ance nce fault fault 6etect 6etection ion.. •
Disad2antagesB
1. Ris& Ris& of e3tensi e3tensi5e 5e acti5e acti5e eart# eart# fault fault losse losses. s. *. 9ig# 9ig# cost costss asso associ ciat ate6 e6..
'+) Eart#ing Transformers: •
or cases #ere t#ere is no neutral point a5ailable for ;eutral Eart#ing 'e.g. for a 6elta in6ing)C an eart#ing transformer may be use6 to pro5i6e a return pat# for
single p#ase fault currents •
-n suc# cases t#e impe6ance of t#e eart#ing transformer may be sufficient to act as effecti5e eart#ing impe6ance. A66itional impe6ance can be a66e6 in series if require6. A special XIig"IagJ transformer is sometimes use6 for eart#ing 6elta in6ings to pro5i6e a lo Iero"sequence impe6ance an6 #ig# positi5e an6 negati5e sequence impe6ance to fault currents.
7onclusion:
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Resistance (roun6ing Systems #a5e many a65antages o5er soli6ly groun6e6 systems inclu6ing arc"flas# #aIar6 re6uctionC limiting mec#anical an6 t#ermal 6amage associate6 it# faultsC an6 controlling transient o5er 5oltages. 9ig# resistance groun6ing systems may also be employe6 to maintain ser5ice continuity an6 assist it# locating t#e source of a fault. F#en 6esigning a system it# resistorsC t#e 6esign/consulting engineer must consi6er t#e specific requirements for con6uctor insulation ratingsC surge arrestor ratingsC brea&er single"pole 6uty ratingsC an6 met#o6 of ser5ing p#ase"to"neutral loa6s.
7omparison of ;eutral Eart#ing System: Sy stem: Condition
=n grounded
Solid &o! Resistance Grounded Grounded
ig# Resistance Reactance Grounded Grounding
-mmunity to Transient 5er Forse (oo6 (oo6 est est 5oltages <@ -ncrease in Voltage Stress Bn6er 2oor est (oo6 2oor Line"to"(roun6 ault 7on6ition Equipment 2rotecte6 Forse 2oor etter est est Safety to 2ersonnel Forse etter (oo6 est est Ser5ice Reliability Forse (oo6 etter est est ,aintenance 7ost Forse (oo6 etter est est Ease of Locating Forse (oo6 etter est est irst (roun6 ault 2ermits !esigner to 7oor6inate2rotecti5e ;ot 2ossible (oo6 etter est est !e5ices Re6uction in Forse etter (oo6 est est requency of aults Bngroun6e6neutral (roun6e6" Bngroun6e6neutral Bngroun6e6neutral Bngroun6e6neu Lig#ting Arrestor type neutraltype type type type 7urrent for p#ase"to VariesC groun6 fault in may be Less t#an 1@ + to *%@ Less t#an 1@ + to *+@ percent oft#ree" 1%%@ or p#ase fault current greater ReferenceB •
y ,ic#ael !. SealC 2.E.C (E Senior Specification Engineer.
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-EEE Stan6ar6 1=1"144
Abstract of 0ational +lectrical Code for Transforer2s Protection ;o5ember
Abstract of ;ational Electrical 7o6e for TransformerJs 2rotection: ;E7C 7o6e =+%.=: '7alculate '7alculate o5er current 2rotection on t#e 2rimary) •
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Accor6ing to ;E7 =+%.=C Yeac# transformer $%% 5oltsC nominalC or less s#all be protecte6 by an in6i5i6ual o5er current 6e5ice installe6 in series it# eac# ungroun6e6 input con6uctor. Suc# o5er current 6e5ice s#all be rate6 or set at not more t#an *-; of *-; of t#e rate6 full"loa6 input current of t#e auto transformer. urt#erC accor6ing to ;E7 Table =+%.<')C if t#e primary current of t#e transformer is less t#an 4 ampsC an o5er current 6e5ice rate6 or set at not more t#an :; of :; of t#e primary current s#all be permitte6. F#ere t#e primary current is less t#an * ampsC an o5er current 6e5ice rate6 or set at not more t#an ,++; s#all ,++; s#all be permitte6. E3ample: !eci6e SiIe of circuit brea&er 'o5er current protection 6e5ice) is require6 on t#e primary si6e to protect a -$2a ==%5"*<%5 -$2a ==%5"*<%5 <] transformer. +&5a 3 1C%%% H +C%%%5a +C%%%5a / '==%V 3 M<) H 4.=1 amps. T#e current 'amps) is more t#an 4 amps so use 1*+@ rating. 4.=1 amps 3 1.*+ H 1*
;E7C 7o6e =+%.<:'7alculate =+%.<:'7alculate o5er current 2rotection 2rotection on t#e Secon6ary) •
Accor6ing to ;E7 Table =+%.<')C #ere t#e secon6ary current of a transformer is 4 amps or more an6 *-; of *-; of t#is current 6oes not correspon6 to a stan6ar6 rating of a fuse or circuit brea&erC t#e ne3t #ig#er stan6ar6 rating s#all be require6. F#ere t#e secon6ary current is less t#an 4 ampsC an o5er current 6e5ice rate6 or set at not more t#an :; of :; of t#e secon6ary current s#all be permitte6.
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E3ample: !eci6e SiIe of circuit brea&er 'o5er current protection 6e5ice) is require6 on t#e secondary si6e to protect a -$2a ==%5"*<%5 -$2a ==%5"*<%5 <] transformer. Fe #a5e 7alculate t#e secon6ary o5er current protection base6 on t#e siIe of t#e transformerC not t#e total connecte6 loa6. +&5a 3 1C%%% H +C%%%5a +C%%%5a / '*<%V 3 M<) H 1.* amps. ';ote: *<%V <] is calculate6) T#e current 'amps) is more t#an 4 amps so use 1*+@ rating. 1.* amps 3 1.*+ H *<+.<= amps. T#erefore: Bse ,++amp ,Jpole circuit brea$er 'per brea$er 'per ;E7 *=%.$).
;E7C Section =+%"<'a):'Transforme =+%"<'a):'Transformers rs o2er :++ 2olts Nominal •
or primary an6 secon6ary protection it# a transformer impe6ance of $@ or lessC t#e primary fuse must not be larger t#an ,++; of ,++; of primary ull Loa6 Amps '.L.A.) an6 t#e secon6ary fuse must not be larger t#an *-+; of *-+; of secon6ary .L.A.
;E7C Section =+%"<'b):'Transformers =+%"<'b):'Transformers o5er $%% 5oltsC ;ominal ;ominal •
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or primary protection onlyC t#e primary fuse must not be larger t#an *-; of *-; of primary .L.A. or primary an6 secon6ary protection t#e primary fee6er fuse must not be larger t#an *+%@ of primary .L.A. if t#e secon6ary fuse is siIe6 at *-; of *-; of secon6ary .L.A.
;E7C Section =+%"<'b):'2otential =+%"<'b):'2otential 'Voltage) Transformer Transformer •
T#ese s#all be protecte6 it# primary fuses #en installe6 in6oors or enclose6
;E7C Section *<%"4+'(roun6"ault *<%"4+'(roun6"ault 2rotection of Equipment). •
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T#is section s#o t#at */=% 5olt YyeZ only connecte6 ser5icesC +++ amperes an6 amperes an6 largerC must #a5e groun6 fault protection in a66ition to con5entional o5er current protection. T#e groun6 fault relay 'or sensor) must be set to pic& up groun6 faults #ic# are 1*%% amperes or more an6 actuate t#e main sitc# or circuit brea&er to 6isconnect all ungroun6e6 con6uctors of t#e faulte6 circuit.
;E7C Section 11%"4 -nterrupting -nterrupting 7apacity. •
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Any 6e5ice use6 to protect a lo 5oltage system s#oul6 be capable of opening all fault currents up to t#e ma3imum current a5ailable at t#e terminal of t#e 6e5ice. ,any o5er current 6e5icesC to6ayC are use6 in circuits t#at are abo5e t#eir interrupting rating.
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y using properly siIe6 7urrent Limiting uses a#ea6 of t#ese 6e5icesC t#e current can usually be limite6 to a 5alue loer t#an t#e interrupting capacity of t#e o5er current 6e5ices.
;E7C Section 11%"1% 7ircuit 7ircuit -mpe6ance an6 t#er 7#aracteristics. 7#aracteristics. •
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T#e o5er current protecti5e 6e5icesC along it# t#e total impe6anceC t#e component s#ort"circuit it#stan6 ratingsC an6 ot#er c#aracteristics of t#e circuit to be protecte6 s#all be so selecte6 an6 coor6inate6 so t#at t#e circuit protecti5e 6e5ices use6 to clear a fault ill 6o so it#out t#e occurrence of e3tensi5e 6amage to t#e electrical components of t#e circuit. -n or6er to 6o t#is e must select t#e o5er current protecti5e 6e5ices so t#at t#ey ill open fast enoug# to pre5ent 6amage to t#e electrical components on t#eir loa6 si6e.
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