Transient Over-Voltages in Crossbonded Cable Systems

TR~NSIENT

OVER-VOLT ACES IN CROSSBONDED CABlf SYSTEMS

c.S. Indulkar Dep.rt • .nt of ' Electrical Engineering, Indi.n Inatitute of Technology, N.. Delhi, India S. Thiruv.ngedea Dep.rt.eAt of Electric.l Engineering, Ragionel Engineering Collage CaUout, India Abstr.ct. In this paper, tran.il!lF'lt overvoU.g .. in cra. .bonded buried csble syst~s .re comput~ uSing the modified fourier transfor_ .ethod. The .ffect of v.rying th~ nUllbe~ of •• jor aections and earth r .. i.tivity on the tranSient ovarvoltage i. i:leterlllined. The study i. e)(tended to • long cable .ystem coapri.ing twenty lIIajor .ections. The effect of includon of capacitora et the croe.bonded joint. is alao considered. Keyword •• Swit'ching overvoltag .. , crossbonded cables, fourier traneform .athod, .ajor .action•• INTRODU er ION While cro.sbonding in EHV cables is very de.irabla fra • • staady-atata point of lIi .. , it giv .. ri.e to very und ..1r.ble tran.ient overvo!tag ..
DESCRIPTION

The .y.t .... is coaposed of three .ingle-core c.bl .. buried fl8t below a depth of 30.48 c., the centre-to-centre spacing bet.,een adjacent cabl .. baing 30.48 c.. The sy.tall i •••• Ulled to coaprise of 20 ~.jor .ection•• The length of a .ajor .action i. 1371.6 011. Single-core • l~iniue-.h ..thed cabl .. with the following dimen.ion. ara used I

Core diameter ... 3.985 cm Inner diameter of .heath = 7.115 c. outer diameter of eheath _ 7.412 CIII Diameter over sheath insulation ~ 8.356

CIII

NUMERIC~l·RtsUlTS

The trsnsient voltsge waveforms are obtained by applying s unit .tep 1I0ltage between the core of en outer cabl. and earth. fig.' .hows the tr.n.ient voltage w.vefor. on the energised conductor at the beginning of the last .ajor section. The nuMber of lIIajor .ection i. varied frolll 10 to 60 in steps of tAn.llith an increa.e in the nuaber of major .actions, the rete at' riae of voltage is reduced, and the occurrence of the pe.k overvoltage i. delayed. . Three values of earth reai.tivity. nSlllely, 20 0,",--, 2,000 Otw-III and 20,000 ohe-ll, covering the three CaS .. of v~y low, .ediUllt and very hiqh resistivity were used in the coaputations. It cen be seen from fig.2 that a higher esrth resietivity yields a lower value of pe.k voltage. However, the reduction i" the peelc.OVCVOltage and the chenge in the wl!IVeshepe with higher value of earth reaistivity are only marginal. CApaCitors of velues 0.5, 6 and 8 pF were used in turn to bridge each cras.bonding point ot' the systBIII. The volteqe waveforme at the b89inning of the lest ... jor .action are plotted in fig.3. It is notleed thst the voltage waveforlll, the peek overvoltage and the rate or rise of voltage e.e not appreciably altered with the insertion of low value capacitors. HowlIVer, with c.p.citor • of value 6 ~f and above. the rate of rise of voltage drops considerably, and the occurrence of the peak overvoltage is delayed.. This

266

i. b.,ef1ciel rrom the point or view or reduction the voltage atresaes that develop in the insulation. It ia worth noting that the bridging capacitors muat have. surge rating equal to the ~sxiMUm permiasible tranaient voltage acrosa the aheath insulator and have. ahort-time po..,ar-rrequ.,cy r.ting surricient to ..,itnstand rault voltagee.

0'

CONCWSIONS It is .allO that tha peak ovarvolt8ge at the remote end or a croasbondad cable syst em vari .. with the number or major sections and shows s .. inima ror 8 particular vslue or the number ot major aectiona. For the aystem studied this number ia 50. The errect or higher .arth resistivity is to lower tha values or peak overvoltase. although the reduction in the peak value is only marginal. finally it haa been, ahown tllet the insertion of' capacitora across tile crDssbonding joints reduces tha rat e of' rise or voltage and delaya the occurrancO or the peak overvoltaga.

RErERENcrs Watson. W•• and (rvan, C.(196J).'Surge potent1all on underground cable sh.the .nd joint insulation'. l£!n!.. A.I.E.E.,!!. 239-243.

l.".,

Wadepohl, and Wllcox, D.J.(1973)'Trenaiant analysis or underground power transmission syeta-sl syatem ~d.l and "'alle-propagation cl'laracteristics'. ~. ~. 120. 253-260.

t

100 t2CO t600 ,tM£cA)N:"UIII~tr .f ... jet"

2CD)

.. eti.1S

Fi, ' t . '''''NSIlNT YOLTAG£ *Y£fOIMS

2·'

.6: 20000.fl.-m

t

_1 ·6 ~

a.

~:

Earth

Rests fIlii ty

oO~------~~---------7.'ObO~-----~OO~O~-----~~----~' TIME ()I S ) Fig 2 , TRANSIENT VOLTAGE WAVEFORMS

2·'

t

c

: :

~I · 6 a.

:

W

Cl

:! O. ...J

0

> O~----~-L

o

________

Fig .3 . Transient

~~

____

~~

roo

______

TIME ( ) l s ) YOtloge wove form

~~

800

______

~.

~oo