OH Transmisssion Lines
Transcript of OH Transmisssion Lines
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Univ
ersityo
fSydne
y
SchoolofE
lectrical&Inform
ationEngineering
DrKeithMit
chell
ELEC5
205
-Hig
hVoltage
Engineer
ing
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UnitOverview
An
introductiontothe
practica
l
applicationofthesupplyofelectric
energyoverhighvoltagetra
nsmission
systemstoelectricityusers
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Topics
Covered
inThis
Unit
M
McAlpine:
-
HighVoltage
Engineering
K
Mitchell:
-
OverheadTransmissionLines.
-
Underground
Cables.
-
Switchgear.
-
SwitchyardE
arthing
-
TestingTechniques
-
EnvironmentalImpacts
-
UseofStandards
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TeachingFormat
&Assessment
Format:On
e2-hour
lectureplus3-hour
tutorial/labsessionperweek
Assessment:
Mid-semesterassignment
plusFina
lexam
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ReferenceTextBook
s
PowerSystemAnalysis&
Design-JGlov
er,MSarma,
2nded.
ElectricPo
werEngineeringHandbookE
lectricPower
Generation
,Transmission&Distribution.
LLGrigsbey
(Ed).
CableSystemsforHighan
dEHVoltage
EPeschke&R
vonOlshau
sen,PirelliCab
les
Transmissi
on&DistributionElectricalEn
gineeringC
Bayliss&BHardy
HighVolta
geTestTechniquesDKumud&KFeser
Purcha
seofthesebooksisNOTbeing
recommended,buttheydo
provideusefulad
ditionalbackgroundinformatio
n.
Other
referencesand
materialappea
randareackn
owledgedinth
e
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T
he
Lecturer
D
rKeithM
itchell
WhoamI?
Whatismyb
ackground
?
Contactdeta
ils?
Room
40611-12A
MTues
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E
LEC52
05
-H
igh
Voltage
E
nginee
ring
1.OverheadTrans
missionLines
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Pow
erSyste
mOutlin
e
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Power
SystemC
omponen
ts
Anelectricpowersystemconsis
tsofthreebasic
ele
ments:
On
eormoregen
erators,almo
stuniversally
ofthe
synchronousma
chinetypein
powerstation
scloseto
sourcesofprima
ryenergy
Atransmissionanddistributio
nnetwork,w
hich
transmitspower
inbulkfrom
generationpo
intstoload
centresandthen
stepsthisdownanddistrib
utesitto
ultimatecustomers
Thecustomersw
houseelectr
icenergybywayof
connectingloadstothenetwo
rk.
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TypicalPowerRequirem
ents
Lightbulbs-40to100
Watts;energyefficientbulb15Watts
Vacuumcleaner-600t
o1,000Watts
Electricoven-4.8kW(4,800Watts)
WaterHeater-4.8kW
Avera
gedemand(divers
ified)ofdomesticresidence-4kW
Equiv
alentpeakpowero
fmediummotorcar-100to150kW
WestfieldShoppingtown,Parramatta-30
MW
Clyde
OilRefinery-30
MW
ParramattaCBDload-
150MW
SydneycityCBDload
600MW
Startingpowerof8-car
Tangaracommutertrain-4.5MW;runningpower
about
1.5MW.
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RoleofTransmission
Bulktransmis
sionofmajoramountsofpower
frombase-loadpowerstationstotransmission
su
bstations(andultimatel
ydowntothe
cu
stomer)
Highcapacity
(upto~120
0MW),hencehigh
vo
ltage
M
ustbehighlyreliable,h
encemeshedstructure
withbuilt-inredundancy
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T
ransGrid500kV
and330kVOH
Double-CircuitLines
NotetheuseofV-insulators
onthe
500kVline,toreduceeasem
entsize.
The
wood-polelineinth
ebackground
isa
single-cct132kVline.
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1
32kVTr
ansmissio
nLine(D
ouble-
Circu
it)
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33
kV
Subtran
smission
Linewith11kV
and415V
distribution
underneath
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HVW
ood-
poleSingle
Circuit
Line
Source:Re
f(1)
About220kV.Note
thetransitio
nfrom
bundledtosingle
conductor.
Thereare2
earthwires
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Revision:Representatio
nof
TransmissionLines
Bo
thoverheada
ndundergrou
ndcablesare
transmissionlinesintheele
ctricalsense
Distributed(true
)modelalwaysusedinhig
hfrequency
(co
mmunication
s)analysis;onlyinpowersystem
analysisforhigh
voltagesand
verylonglen
gths(>250
km
)
M
ediumlengt
horpimod
elforsub-
transmission/transmissionlinesoverabout
80km
Shortlineswithasimplese
riesRandjX
for
dis
tributionlinesandsub-tran
smissionlines
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True
Representationo
fa
TransmissionLine
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Medium
LengthTransm
ission
Lin
e
R
X
Y
Y
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ShortTransm
issionL
ine
R
X
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TransmissionL
ineModels.
Z=R+jL
/mseriesimpeda
nceperunitlength
y
=G+jc
V/mshuntadmittanceperunitlengt
h
Z=Zl
l=Linelengthinm
eters
Y=yl
V
Z=sinhl
Zc=
z/y
Characteristic
Impedance
Y/2=tanh(l/
2)
Zc
=zym
-1
Propagation
Constant
y
y
y
y
y
z
z
z
z
Shortline
80kmorless
Mediumlengt
hline
80kmto250km
Longline250kmor
more
ZZ
Z
VsVs
Vs
VR
VR
VRI
R
IR I
R
IsIsIs
Y/2
Y/2
Y
/2
Y
/2
TransmissionLines
ComparisonofShort,
Mediu
mandLong
models
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MediumandS
hortLineAp
proximations
Using
ABCDparam
eters
Stan
dardShortLine
piEquivalent
Modelforatransmissionline
VS=A
VR+BIR
IS=CVR+DIR
A,B,C,D
areparametersde
pend
onthetransmissionline
parameter
s:R,L,C,G
TheABCDparametersare
complexn
umbers.
Solvingth
esystemof2
equations:
AD-BC=1
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Med
iumLength
Transmissi
onLineD
etermining
theABCD
Medium-Lengthtransmissionline
nominalpicircuit
Mediumlinelengthfrom80to250km
lcjG
yxl
Y
ZY
Y
C
Z
B
ZY
D
A
ZI
ZY
V
YV
IZ
V
V
YV
I
I
R
R
R
R
R
S
R
R
S
)
( )2
1(
2
1
)21(
)2
( 2
+
=
=
+
= =
+
=
=
+
+
=
+
+
=
+
=
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M
eshedV
sRadial
PowerN
etwork
Arrangements
Majortransmissionlinesservemanycustomersandare
me
shedsub-sy
stemshaveredundantcapa
city
automaticswitching;nolossofsupply
Majortransmissionsubstation
shaveredundant
transformercapacity
Localdistributio
nlinesservefewercustom
erseach
andareradialLocaldistributionsubstationshave
sim
pleswitchgearandnored
undancyma
nualre-
switching;some
lossofsupply
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Reliabilitybase
dDesignand
DeliberateRedundancy
M
odernsystemsgiveoutageratesof~1hour
pe
rcustomerperyear(99
+%availab
ility)
Keytodesign
philosophy
is(Probabilityof
ou
tage)X(numberofcus
tomersaffected)
M
ajortransmissionassets
->morecu
stomers
su
pplied->higherreliabilityrequired
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Mes
hedPow
erSystem
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Ex
ampleof
MeshedNetwork
Integral
Ener
gySub-transmissio
n
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Ring-
MainPo
werSystem
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W
hyOverheadHV
Transm
ission?
Greaterpowercapacitypereasement
(dueto
bu
ndledconductorsandair-cooling)
UG/OHcostr
atiosupto15:1forvoltagesof
30
0kVandabove
EH
Vcableno
tavailableuntilfairlyrecently
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WhyDouble-CircuitO
H?
M
oreefficientuseofspac
e(easementspacefor
EHVOHlinesisverydifficulttoget).
ReliabilityofOHHVline
sishighco
mpared
po
tsamelengthoflowervoltageline.A
do
uble-circuitoutageisveryrare.
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Conductors
Used
inHV
Transm
ission
Source:R
ef(1)
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Typic
alASCR
Conductor
Source:ref(1).Theh
igher-resistance
steelinnercoreandskineffect
forcecurrentmainlyin
totheAlouterlayers.
Straigh
tAlisusedins
ub-transmission
anddistributionlines
Strandingaddsabout1
5%toresistance
Skineffectcanaddup
to30%forlargeconductors.
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Resistan
ceofOH
Transm
ission
Lines
Fo
rmulas:
Rdc=l/A
hot=cold*(Thot+Tref)/(Tcold+Tref)
Rac=Rdc*Fs
kin*Fstrand
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ResistanceStra
ndingFactor
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LandC
ofTransmission
Lines
r
D
=
GMR
GMD
x
L
ln.10
2
7
.
ln
2
=
GMR
GMD
C
Equalspacing,GMD=D
Singlewire,GMR=rforL
GMR=rfor
C
n
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InternalFlux
Linkage
Consideruniformcurrentdensity.Atinternaldistancex,thelinkedcurrentI(x)=
I(x/r)2,withthefieldintensity
H(x).Theinductanc
eisconstantregardle
ssofradiusr.
Atinternaldistancex,
H(x)=I(x)/2x
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Flu
xLinkageBetwe
en2Conductors
12=2x10-7.I.ln(D
2/D1)
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SelfandMutualL
for2W
ires
Consid
erfluxlinkagesbetw
eenconductorxan
dconductory,(bothinternaland
externallinkages)andthenrepeatprocessfrolinkagesbetweencondu
ctory
andconductorx,andsum.
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SelfGM
R(r)
ofConductors
Simplifiesind
uctancecalcsbyincorp
orating
effectofinternalflux
Is
radiusofeq
uivalenthollowconduc
tor
r
=r.exp(1/4)whererisradiusofsm
ooth
co
nductor
Ty
picalvalues:Al/Cu0.7
26(7-strand
)to0.779
(solidconductor);ACSR0.68(6-strand)to
0.81(54strand
)
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ExtendedforBalanced3-Phase
Considerthefluxlink
ageswithaphase,duetoi)itself,
ii)bphaseandiii)cphase,thens
umup.Note:c
urrents
balancedinallphasesandsumtozer
o.
Duetosymmetry,the
sameresultswillapplytothe
other
twophases.
a=
2x10-7.I.ln(D/r)
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FluxLinkageM
Conductors
Consider
fluxlinkagesfromallMconductorsandpointP,thenmovePtoinfinity,
atthesam
etimesettingsumofcurrentstozero(balancedsystem).
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ExtendedforN
Conduc
tors
This
systemhasNforw
ardandMreturnc
onductors,sharing
atot
alcurrentI.Dkmrepresentsthedistancefromconductorkand
cond
uctorm;Dkkrepresentstheselfinductanceofconductork.
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E
xtended
forBund
ledConductors
Ex
tendinggen
eralformul
afrompreviousslide,
thefluxlinkagewithcom
positeconductorxis
an
dhencethe
inductance
is
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C
oncepto
fGMR
Usinggeneralform
ulaforNconductorsdeve
loped
previously,thereisatermforth
eselfGMDamong
allconductorsand
themselves.Thisisdefinesasthe
GMR
(geometricmeanradius).
Form
ula:
(prov
eastutorialexercise)
DetermineGMRs
for2-,3-and
4-wirebundles
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Advantage
sofBun
dledConductors
Increasedrating-lessweightthanan
equivalent
singleconductor
Le
ssskineffectlosses
Reducedserie
sL-improvespower
transmissionc
apacity
Reduceselectricfieldstre
ngtharound
co
nductors-reducescoro
na
Se
etutorialexercise
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Transpo
sitionof
Conductors
(un
equalSp
acing)
Reallinesarerarelygeometricallyarranged.Theune
qualspacingcausesunbalanced
inductances.Solvedbyrotatingconductorsabouteve
rykmorsotoaverageoutspacings.
Averagefluxlinkagesarethenth
eaverageofthelogs
(seeformulaabove)
-thiscreatesa
geometricm
eandistanceorGM
D=3D1.D2.D3
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Effe
ctof
Earthin
LineSeries
Impe
dance
Earthreturncurrentscanbereplaced
withequivalentconductorsunderthe
surfacecarryingthenegativecurrent
oftheoverhead
conductor(Carsons
equations).
Dkk=Dkkm(GMRsthesame)
Dkk=658.5(
/f)m
Rk=9.869x10-7f/m
InAustralia,f=
50hz
Imagesofthe3
phaseandN
O/Hneutrals
3phases(a,b,c
)
andNO/H
neutrals(n1-nN)
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Complete
Equ
ivCct
fo
rall
Conductors
andImages
Note:Onlyphaseconductors
havedrivingvoltages(phase
volts);theneutralshaveno
drivingvoltag
es
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Deter
mination
ofPhaseand
Sequence
Imped
ances(1)
Reference:Glo
ver&Sarma
(Ref1),pp176
-182.
Equationsfora
systemof3
phasesandNO
/Hneutrals.
Theimpedance
matrixisoften
referredtoasth
eprimitive
matrix.
TheRk,resistanceofthe
earthimageofconductork,is
givenbythepreviousslide.
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Deter
mination
ofPhaseand
Sequence
Imped
ances(2)
Reference:Glo
ver&Sarma
(Ref1),pp176
-182.
Theprimitiveimpedance
matrixissimplifiedby
partitioningand
transformation
intoaphase
matrix,ZP(Kronreduction)
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Deter
mination
ofPhaseand
Sequence
Imped
ances(3)
Reference:Glo
ver&Sarma
(Ref1),pp176
-182.
ThephasematrixZPcanbe
convertedintoasequence
matrixZSforanalysisby
symmetricalco
mponents,
whichisnorma
llyeasier.
OnlyworksifZ
Pis
symmetrical.T
hisis
approximatelytrueforaline
withtransposed
conductors.
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Deter
mination
ofPhaseand
Sequence
Imped
ances(4)
Reference:Glo
ver&Sarma
(Ref1),pp176
-182.
IfZPisassumedsymmetrical
(by,egtakinga
veraged
values),thenth
e
transformation
intothe
sequencematrixZSisalot
easier.
Refertotutoria
lexample.
Therearealsocomputer
programsthatd
othese
calculationsforyou.
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ElecField
andPotentialDiff-
S
ingleWire
Consideracylinderwitharadial
electricfield.
GuasssLawis
thenapplied.
Foralengthlandacircularpath
ofradiusx,
.E
(x).2x.l=q.l,for
chargeofqperunitlength.
ThusE(x)=q/2xV/m
Note:thefield
is>0onlyoutside
thecharge.
Refertutorialexercise,andshow
thatV12=(q/2
).ln(D2/D1)
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ExtendedforM
Conductors
Extendthepreviousslideto
determinethepotentialdifference
betweenconductorskandi,due
toa
chargeqmonconductorm.Thus
,
Vki,m=(qm
/2).ln(Dim/Dkm)
Now,sum
theeffectofallm
conductors.
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Capacitanceof2
-WireSystem
D
r
Apply
thepreviousslideto
caseofM=2,radiu
srandspacingD.
Considerpotentialdifferen
cebetweenconducto
rs1and2duetocha
rge+qon
condu
ctor1,andthenaddthepotentialdifferencebetweenconductors2and1,
dueto
charge-qonconduc
tor2.Thus,
V12=
(1/2)[q.ln(D12/D11-q.ln(D22/D21)]
V12=
(1/).q.ln(D/r),as
D12/D21=D;D11/D22=r.(wheredidthe2goto?)
ThusC=q/V=
/ln(D/r)Note:forcapacitance,user,notr.W
hy?
Capac
itanceperphaseton
eutralC=2
/ln(D/r)F/m.Whythe2?
Refer
tutorialexercise.
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Extended
forBalanced3-Phase
Balanced3-wiresystem,ra
diusr,spacingD
Extendthe
2-wiresystemtoa
balanced3-wiresystem(orusethe
Mconductorscase).Consider
potentialdifferenceonconductor1
duetochargesonconductors2and
3.Notethatthechargesandelectric
fieldcontributionshavetobe
considered
asphasors.
Resultis:C
=2
/ln(D/r)F/m
(phaseton
eutral).
Refertotutorialexercise.
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Capacitanc
eofBun
dledConductors
Considera3-phsystemwithatwin-conductorbundle.
Thereisachargeof
q/2oneach
conductor.
Thepotentialdifferencebetweensayconductorsaand
bcanbedetermined
asbeforeby
consideringallchargesonallconductors.
ThecapacitancecanbeshowntobeC=2
/ln(GMD
/GMRc)
F/m.
TheconductorGMDisthesameaswithinductanceca
lcs.However,GMR
cusesthe
conductorradiusr,notr.Why?
Refertutorialexercise.
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EffectofEarthonElecField
Theearthisaconductingplaneandeffectstheelectricfield.Theeffe
ctisthesameifall
conduc
torswerereplacedb
yimageconductorsundertheearthssurface.Eachimage
conduc
torhasthesameradiusandisthesamedistancebelowground
asitscorresponding
realconductorisaboveearth.Notethattheseim
ageconductorsared
ifferentfromthe
imageconductorsusedwith
inductancecalculations..
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E
ffectofEarthon
Capacitance-2
WireSystem
Considerthe2-wiresystemshow
n,ofconductorsxandyandtheirrespectiveimage
conductorsxandy.Ifconductorxhasachargeof+q,thenywillhavea
chargeof-q.
Imagecond
uctorshavereversec
hargesieimagexhascharge-qandimag
eyachargeof+q.
Thevoltage
differencebetweenconductorsxandy,Vxy,canbedetermine
dbyconsidering
theelectricfieldcontributionsfromallconductorsinc
ludingimages.
TheresultisVxy=(q/)[ln(D/r)-ln(Hxy/Hxx)]andthuscapacitance
Cxy=/[ln
(D/r)-ln(Hxy/Hxx)]
F/m.Theeffectis
minimalexceptforh
orizontal
arrangemen
tsclosetoearth.Re
fertutorialexercise.
x
y
x
y
Earthplan
e
D
Hxx
Hxy
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Determ
inationofPhase
and
SequenceCapacitances
(1)
Considerno
wageneralsystem,of3phase
conductorsa,b,c;Noverheadneutralsn1..
nN;andtheirrespectiveearthim
ages.
Forconductork,thethepotentia
l
differenceV
kkbetweenitanditsimage
conductork
,duetoallmconductors,isthe
sumofeffectsofallconductorsandtheir
images.
Thus,Vkkisgivenby-
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Determination
ofPh
aseand
Seq
uence
Capacitances
(2)
ReferGlover&
Sarma
(ref1),pp194-197.
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Determination
ofPh
aseand
Seq
uence
Capacitances
(3)
ReferGlover&
Sarma
(ref1),pp194-197.
Theprocedure
is
similartotheline
inductances,creatinga
potentialmatrixP
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Determination
ofPh
aseand
Seq
uence
Capacitances
(4)
ReferGlover&Sarma
(ref1),pp194
-197.
Asbefore,thismatrix
istransformed
intoa
phasecapacita
nce
matrixCPand
thence
intoasequenc
ematrix
CS.
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Determination
ofPh
aseand
Seq
uence
Capacitances
(5)
ReferGlover&Sarma
(ref1),pp194
-197.
Again,thepha
se
capacitancem
atrix
mustbesymm
etrical
toenableitseasy
conversionint
othe
sequencematr
ixand
useinsymmetrical
componentsanalysis.
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Determin
ingSequ
enceImpedances
Asa
llHVtransmissionlinesaretra
nsposed,thenw
ecan
aver
agephaseimpedancematricesandconvertinto
sequence
impedances.
Fortransmissionlin
esandcables,n
egativesequenc
eimpedance
Z2=
positivesequenceimpedanceZ
1.
Positivesequenceim
pedances(LandC)canberea
dily
calculatedbytheno
rmalmethod,ig
noringearthforLandonly
consideringearthfo
rCwithhorizontalconductora
rrangements
reasonablyclosetotheground.
Zero
sequenceimpe
dancesZ0howe
verrequiretheconsideration
ofallconductorsandtheirearthimages,forbothR,LandCand
matrixtransformation.
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ThermalRatingsofO
H
TransmissionLines
Thermalratingo
foverheadlinesdependso
n(i)the
ma
ximumallow
ableconducto
rtemperature,beforeit
anneals(softens)andfallsdown,and(ii)th
emaximum
sag
permissible,
beforethelin
esgettooclo
setolines
underneath,structure,humans,etc
Thefirstfactori
softenreferredtoasampacity
Theampacityof
alinedependsonambient
tem
perature,wind,solarradia
tionandcond
uctor
surfacecondition.
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MainFactorsAffectingR
atings
Eq
uations:
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R
atingEquation
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TypicalRatin
gs
ofOHLines
Availablefromconductormanufactu
rers.
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ShortT
ermand
RealTime
Ratin
gs
Itw
illtakealittlewhilefora
coldconductortoreach
wo
rkingtempfo
llowingastepincreaseincurrent.
Thisallowstheapplicationof
short-time,emergency
ratingstooverheadlines.
Usualemergencyratingisabout10minutes.
Iflineratingsareconstrained
bysag,often
are-
ten
sioningand/o
rliftingconductorshigher
offthe
gro
undwillallowanincreasedrating.
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InsulatorsU
sedonO
HTrans
mission
Lines
StandardC
levis
Disk
LongRod
Insulators
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In
sulatorM
aterials
To
ughenedGlass(Clevis
disks)
Po
rcelain(Cle
visdisks,cabletermina
ting
structures,transformerbu
shings,CTs
Ep
oxy(posto
rstand-offinsulators)
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Advantages
of
Stand-
off
Insulators
Source:Ref(1).Theuseofstand-off
insulatorsreduc
esconductorswing,
enablingtheuse
oflargerconductors
andincreasedvo
ltages,aswithlessswing,
conductorcleara
ncescanbemaintained.
Inthisexample,
theMVAratinghasbeen
increasedbyabo
utafactorof3times.
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Typic
alHV
Line
Design
Param
eters
Source:Ref(3)
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Lightnin
g&SwitchingSurges
Lightningisafastwavefront(fewmicrosec)
Sw
itchingcause
sslowwav
efronts(few
milliseconds)
Insulatorscanw
ithstandhigh
erlevelsoffastsurges
Lightningsurgestendtobefixed,duetoth
elightning
processitself,in
dependentof
linevoltage
Sw
itchingsurge
scanbeashighas2xline
volts
Lightningmore
ofaproblem
3
00kV
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Lig
htningS
urges
Typical1/50Lightnin
gsurge(travellingwave)
Lightnin
g(directstroke,induce
d,earthpotentialrise)
Switching(suddenenergisation
)
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Lightnin
gMitiga
tionMethods
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L
ightning
Ground
FlashD
ensity
Where-
GFD=groundflashdensity,
strokes/km2/yr
TD=thunder-daylevel=35/yearinSydneyregion
TH=thunderhours/year
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LightningStrok
eIncidence
Where-
Ns=no
ofstrokesper100kmofline/year
h=heig
htofconductorabov
eearth
GFD=groundflashdensity
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LightningI
nductive
Overvoltageon
Tower
Where-
l=lengthofline
h=heightofconductoraboveearth
r=conductorradius
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LightningOutage
sonShielded
Line
s
Source:Ref2
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Lightning
Outages
onUnsh
ielded
Line
s
Source:Ref2
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Earth
WireShieldAngle
The30d
egangle
hasbeen
determin
edfrom
experien
ce
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TransmissionLin
eSIL(orBIL)
Th
eSIL(standardinsulatio
nlevel)ofan
OH
transmissionlinevarieswithlinenominalvoltage
De
terminedbypeakvalueof
standard1.2x50surge
SetbyASandIEC(alsoIEEEetc)standards.For
example(IEC)-
36
kVline:
70
kV170kV
245kVline:46
0kV1050kV
525kVline:1175kV1550k
V
(sh
ort-timepowerfreq/lightningimpulsew
ithstands)
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Ty
picalClearances
Th
eSILdeterminesclearance
sinair
De
terminedbysafety,maxelectricfieldlevels
SetbyASandIECstandards.
52
kVline:
0.17m(powerfreq)/0.7m(surges)
245kVline:0.69m/2.0m
765kVline:2.3
m/5.6m
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Atmospheric
Pollution
Dust,plantpollens,industrialparticles,salt(nearco
ast)get
coatedonoutsideof
insulators
Whendamp(eginlightrain),thesearesemi-conductors
Caus
epartialdischargesandeventuallyflash-over
PDs
causeaudibleandelectricalnoi
se
Skirtdesignoninsulatorsincrease
strackingpath
length
Artif
icialrainteston
insulatorsets
Creepagedistances16-31mm/peakkV(basedonpo
llution
class
ification)
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IECHVEquipmentC
reepageDistances
Ref:AS6
0044.1
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OtherO
HLineDesignIssues
Fla
sh-overpathinairmustbe
longenough
to
extinguishpowe
rfrequencyarc
Sag-tensionofconductors-notexceedtensilestrength
(whencold)ors
agexcessively(groundclearance)
wh
enhot.
Mid-spancondu
ctorclearances(allowfors
wing)
Clearancestostructures,grou
ndhighenoughfor
surgesandsafeworking.
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Ageing&FailureMechanisms
M
echanicalFailure
(P
orcelain)Insulator(glazeandporce
lain)
pu
nch-through
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ElectricalCorona
OnsetFormula
Whe
re-
Eciscriticalelectricalfie
ld
aisconductorradius
mis
is
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Au
dible
Noise(AN)
Coronas
Source:Ref2
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Rad
io
Interference
(RI)
Coro
nas
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ElectricFieldStrength
on
Conduc
tors
Refertutorialexercise
Singleconductor
Averageofn
conductors
Q=C
V
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ElectricFieldStrengthatEarth
Refertutorialexercise
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EnvironmentalImpactsofHVOH
Line
s
Aesthetics-th
eyrebig!
El
ectromagneticinterferencewithele
ctronic
eq
uipment(50
hzinduction,coronan
oise)
Lo
wfrequenc
yelectroma
gneticradia
tion
(healthconcerns)
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MagneticFieldUnderOH
TransmissionLine
Calculatehorizo
ntalandverticalcomponen
tsof
ma
gneticfluxdensityatacho
senpoint,basedon
dis
tancetoeach
phaseconduc
tor
Ad
deffectsofall3phases(th
esearephaso
rs)
ResultantHand
Vcomponentsarealsophasors
Squareandadd,
converttoanewphasoran
dtakesqrt
Refertutorialex
ample.Field
ishighestdirectlyunder
the
lines(ordire
ctlyabove,in
thecaseofU
Gcables).
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PowerFrequencyEM
R
ReportbyKarolin
skaInstitute(Sweden)inearly
1970ssuggested
apossiblelow
-levellinkbetween
pow
erfrequencymagneticfieldsandchildho
od
leuk
aemia
Noeffectnotedbyelectricfields
Late
rreportsinU
KandUSsee
medtosuppo
rtthe
earlierreport
Late
ststudieshav
enotprovenanydirectlink.If
present,itseemsveryweak.
Thereishowever,moreofaconcernwithRFfields
near
thehead-he
nceworriesw
ithmobileph
oneuse.
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OHLineT
esting&
Comm
Des
ignchecks,etcforstructures
Cur
rentinjectionte
st(forimpedances,earthres
Insulatortests(doneoninsulatorse
ts)-
-
Powervoltage
withstand
)
-
Fastandslo
wsurgetest)valuessetb
-
Artificialrain(pollutiontest)
-
Powerarctest
)
-
RIstrengthtest
)
-
Coronaonsettest
)asagr
eedwithsup