,[}

'(;.\

~~J

o

~

JI .

,\.I ",

\ f',

'

,

'"

,

. . .

,

"t,

'

J ".J'11~

li,!

~

~

8

.§

~

<li! o:~11

~-" ~111r'

~ !li.IIÀ

i1I I~

Ifii

li!I

m~

..

.,ti

~II

iTJ

;11

,~ ..l1li ,IiI/IíI ,!!lI~

li\'!!!:!"" ~~~... ....

= g!!i

m

!fj 111fi

1111:li

l1li

8

..

ti..I

21, rue d'Artois, F-75008 PARIShttp://www.cigre.org

81 201 CIGRE 2006

HIGH VOLT AGE OF (OIL FILLED) CABLES- OPTIMIZA TION OF THE TIME FORGROUNDING CONSIDERING SAFETY ASPECTS

H. TATIZA WA1(*),E.W. CITRON2,P. F. OBASE\ N.M.MATSU011Instituto de Eletrotécnica e Energia da USP -IEEUSP

2Companhia de Transmissão de Energia Elétrica Paulista - CTEEPBrazil

SUMMARYWhen a maintenance procedure in a transmission class Oil Filled Cable or in its associated equipmentis performed, a common situation found in practice is the necessity of to ground the electricalequipment, considering safety aspects of the maintenance crew and equipment. In those situations,afier the disconnection ofthe OF Cable from power source, a minimum amount oftime is required forsafely ground transmission class (88/138kV, 230kV and 345kV) Oil Filled AC Cables (OF Cables), byconnecting the live parts of the circuit to the substation grounding grid. This time for grounding arepracticed by many utilities around the world and can be a cost1y procedure, reaching around a fewhours in some cases. The aim of this work is to optimize this time for grounding, by means of studyingthe discharging behavior oftransmission class Oil Filled AC Cables (OF cables), afier disconnectionfrom power source, considering the electrical modelling of cable installations,1Uld measurements atthe field. Considering the electrical modelling of the cable and associated network elements, computersimulation using the Alternative Transients Programs - ATP [1] were performed, in order totheoretically estimate the discharging time of the cable.For the measurements, a special arrangement was conceived, for not to change the circuits' timeconstant. For this purpose, a conventional capacitive voltage divider was employed. Considering thenormal procedures of cable operation, situations are present in which on1y load of capacitive nature, orno other equipment, are direct1y connected to the cable during disconnection. Realizing that thesituations present in this research consider measurements of DC voltages (and very low ftequencies), aconventional arrangement of capacitive voltage divider wouldn't measure properly, an adaptation hadto be made. In this way, in the secondary arm of the capacitive voltage divider an electrostaticvoltmeter where employed, considering its negligible leakage current. In some circuit types, where itwas expected higher frequency voltage signals (kilohertz range), a conventional transient recorder wasemployed.The results obtained in the computer modelling and simulation using the program ATP - AltemativeTransients Program, showed good agreement with field measurements, as expected.Considering the obtained results, the period of time for grounding the equipment, put into practice atpresent, would possibly be safely reduced. The worst situation would be the case of a long (Le., a fewkilometers long) underground transmission line, 345 kV class. It was demonstrated, that this safe timefor grounding, could be reduced ftom 4 hours, in the worst case as practiced now, to a few minutes.KEYWORDSOil Filled cable; underground cable, underground transmission line.

.(*)Hédio TatizawaIEEUSPhedio@iee.usp.br

.

.