Experience in on-line relaxation of XLPE insulated high
Transcription
Experience in on-line relaxation of XLPE insulated high
Close and Return 45 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.