Experience in on-line relaxation of XLPE insulated high

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A.2.3. Experience in on-Iine relaxation of XLPE insulated high voltage
cables
KOUTI T., Nokia Cables, Pikkala , Finland
KARPPO J., HUOTARI P., HONGISTO A..
Nokia-Maillel er, Vantaa . Finland
A.2.3. Expérience de relaxation en
ligne de câbles haute tension à
isolation polyéthylène réticulé
KOUTI T., Nokia Cables, Pikkala, Finland e
KARPPO J., HUOTARI p.. HONGISTO A.,
Nokia-Maillefe r, vantaa , Finlande
Ab st ract
Une méthode de relaxation directe a été d éveloppé pou r
redu ire d es c harges mécani qu es dans l'is olan t PR et po ur
diminue r le retrait axial. La m ét hod e est ba s ée su r u ne zo ne d e
cha uffage s upp léme nt aire a u milie u du sect ion d e
refroidis s em en t dan s la chaîne d e vulca nisatio n en co nt inu . La
surface d 'is olati on est cha uffée et refr oidie enco re un e fois . Ce
trait ement diminue d'une maniè re sig nificative les charges
méca niques intérieu rs . La frict ion e nt re la co uche serni-co nductive et le co nduc teu r a ug me nte aussi.
La méthode de relaxat ion direct e a été essaiée da ns la nouvelle
ligne vertic ale COCC haute-t en s ion . Plusie urs câbles ont été
analysés en es sai s ta nda rd et qu elq ues essais ad hoc o nt été
développé s pecialeme nt pour déte rminer les charges intérieu rs
de l'is olati on a u polyéthylèn e rét iculé . Ces ess ais mo nt re nt
cla ireme nt qu e le retr ait axial et les charges m écaniqu es int érie urs pe uve nt être réduit s d'une m an ièr e s ignifica tive .
Introdu cti on
The s pe cifie vo lum e of XLPE insul ati on increases with
incr easing t em perature (11. The sp ec ifie vo lume of XLPE as a
func tion of tempe rature is s how n in Figure 1. Ouring insu latin g
of high vo ltage cables th e temper ature d ep end en ce res ulta in
un even m echa nical s t resses in t he XLPE insu lat ion. Sh rinkback
ca us ed by m echanical s tress es ma y cau s e problem s in ca ble
te rmin ations and joints .
An on-fine relaxati on metho d has bee n d evelop ed to rmmrmze
me c ha nica l st resses in th e XLPE ins ulation and ta d ecr eas e
axia l s hrinkbac k. The met hod ls based on a n additional heatin g
zone in th e middle of t he coo ling section of the Continuou s
Vulca nizing (CV) line . lnsu lat ion su rface is heated up and
cooled down again . This treatm en t re duces internai m ech anical
s t res s es sig nifica ntly . Frict io n bet w een th e co nd ucto r a nd t he
co nductor sc ree n le also incr ea s ed .
The on-tine relaxati on metho d has bee n exami ne d in a new
vertical hiqh-voltaqe Comp letely Dry Curing and Coo ling
(CDCCI line . Seve ral cables have been te st ed using s ta ndard
tests as weil as so rne ad hoc te st s deve lope d speci ally for
determining inte rnaI st resses in XLPE ins ulation . Th es e tests
s ho w clea rly t hat bo th s hrinkba ck and inte rnai mechanical
stresses ca n be s ign ifica nt ly red uced .
This pape r dis cu s s es :
•
Mech an ical stresses of XLPE ins ulatio n
•
The o n-fi ne relaxati on me t hod invent ed to reduce
m ech an ical st resse s and s hrinkbac k of the XLPE
ins ulat ion in high vo ltage ca bles
•
Compa riso n bet w een non -relaxed and relaxed high
vo ltage ca bles. Visual ins pecti on , insp ect ion of
mechani cal forc es , s hrinkbac k a nd ele ct rice! tests
have been ca rried out .
20
Mech ani ca l S tr es s es of XLPE
%
15
V~
XLPE -
/
Tem peratu re de pe nda nce of XLPE resu lts in uneven
mechan ica l s tresses ins ide t he ca ble insul ati o n. T his
phen om enon can b e exp lained as fcüows .
ln t he Co ntinuous Vulca nizing (CV) pr oc es s , t he c ond uc to r is
first cove red with ex t rude d XLPE, Then it is heated up in a
pressu rise d atmosphe re to activate c ross-li nking of XLPE
ins ulation materiel . The s pecifie vol ume of t he XLPE ins uletion
is now mu ch higher t ha n at a room te m pe rature .
..... <, PE
)/
10
5
o
o
/
/
20
/
50
100
oc
150
Figur e 1 Cha nge in the s pe cifie vo lume of XLPE and LOPE as
a fun ction of tempe ratu re
Finally, XLPE ins ulatio n is cooled dow n from th e outsi de be fore
leaving t he CV tu be . In t he begin ning of cooling th e ins ulatio n
s urfa ce cools, crys t allizes and becomes ha rd firs t. w hile t he
out er d iam et er of t he ins ulatio n ls s till mu ch lar ger t ha n at a
room temper atu re . The inter ior is s t ill hot , so ft a nd ex panded .
Then the inte rior cools dow n, crys t allizes and tr ies to shrink.
How ever, lar ge diameter of th e har d ins utatio n surface t ries ta
preven t t his s hrinkage (Figu re 2). As a resutt , radial strain
a ppea rs in th e cable insulatio n.