POWER SYSTEMS ON SAG AND CORONA PPT UNIT 4.pdf

SAG,CORONA

1. Sag and tension calculations
Sag and tension calculations with equal
heights of towers
Sag and tension calculations with unequal
heights of towers
Effect of wind and ice on weight of
conductor.

2. Whileerectinganoverheadline,itisveryimportant
thatconductorsareundersafetension.
Iftheconductorsaretoomuchstretchedbetween
supportsinabidtosaveconductormaterial,the
stressintheconductormayreachunsafevalueand
incertaincasestheconductormaybreakdueto
excessivetension.
Inordertopermitsafetensionintheconductors,
theyarenotfullystretchedbutareallowedtohave
adiporsag.
“Thedifferenceinlevelbetweenpointsof
supportsandthelowestpointontheconductor
iscalledsag”

3. When the conductor is suspended between two supports at
the same level, it takes the shape of catenary.However, if the
sag is very small compared with the span, then sag-span
curve is like a parabola.
The tension at any point on the conductor acts tangentially.
Thus tension To at the lowest point O acts horizontally as
shown in Fig.
Thehorizontalcomponentoftensionisconstantthroughout
thelengthofthewire.
Thetensionatsupportsisapproximatelyequaltothe
horizontaltensionactingatanypointonthewire.ThusifTis
thetensionatthesupportB,thenT=TO.

4. Conductor sag and tension
•Thisisanimportantconsiderationinthemechanical
designofoverheadlines.Theconductorsagshouldbe
kepttoaminimuminordertoreducetheconductor
materialrequiredandtoavoidextrapoleheightfor
sufficientclearanceabovegroundlevel.
•Itisalsodesirablethattensionintheconductorshould
belowtoavoidthemechanicalfailureofconductorand
topermittheuseoflessstrongsupports.However,low
conductortensionandminimumsagarenotpossible.
•Itisbecauselowsagmeansatightwireandhigh
tension,whereasalowtensionmeansaloosewireand
increasedsag.Therefore,inactualpractice,a
compromiseinmadebetweenthetwo.

5. Sag-supports are at equal levels

7. Sag-When supports are at unequal
levels

9. Effect of wind and ice loading
Theformulaeforsagaretrueonly
instillairandatnormal
temperaturewhentheconductor
isactedbyitsweightonly.
However,inactualpractice,a
conductormayhaveicecoating
andsimultaneouslysubjectedto
windpressure.Theweightofice
actsverticallydownwardsi.e.,in
thesamedirectionastheweight
ofconductor.
Theforceduetothewindis
assumedtoacthorizontallyi.e.,
atrightangletotheprojected
surfaceoftheconductor.
•Hence,thetotalforceonthe
conductoristhevectorsumof
horizontalandverticalforces