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C H A P T E R 19Paper Insu la ted D i s tr ibut ion C ab les

F o r d i s t r i b u t i o n a n d t r a n s m i s s i o n p u r p o s e s i m p r e g n a t e d p a p e r i n s u l a t e d c a b l e s h a v eh a d a n i m p r e ss i ve r e c o r d o f r e li a bi li ty t h r o u g h o u t t h e 2 0 t h c e n t u r y , e x c e p t f o r t h ed e v e l o p m e n t p h a s e s in t h e 192 0s a n d 1 93 0s w h e n t h e u s e o f b e l t e d c a b le w a s a t t e m p t e da t 3 3 - 6 6 k V ( c h a p t e r 2 ) . T h e r e l i a b il i ty o f p a p e r c a b l e s is i n d i c a t e d b y t h e f a c t t h a t t h eU K s u p p l y i n d u s t r y d e p r e c ia t e s p a p e r d i s t r ib u t i o n c a b le s o v e r a 4 0 y e a r life .

A l t h o u g h t h e ba s ic c o n s t r u c t i o n h a s c h a n g e d l it tl e s in c e i m p r e g n a t e d p a p e r w a s f ir sti n t ro d u c e d , t h er e h a v e b e e n c o n t i n u o u s i m p r o v e m e n t s in m a t e r i a ls a n d m a n u f a c t u r i n gt e c h n i q u e s w h i c h h a v e l e d o v e r t h e y e a r s t o h i g h q u a l i t y a n d r e d u c t i o n s i n d i m e n s i o n s .P r o b a b l y t h e m o s t s ig n i fi c an t c h a n g e w a s t h e i n t r o d u c t i o n o f n o n - d r a i n i n g i m p r e g n a n t si n t h e 1950s. T h i s o v e r c a m e t h e p r o b l e m o f m i g r a t i o n o f i m p r e g n a n t w h i c h l e f t ar e l a t i v e l y ' d r y ' d i e l e c t r i c .

M u c h i n f o r m a t i o n o n t h e p r o p e r t i e s o f a ll t h e m a t e r ia l s u s e d a n d o f th e i m p r e g n a t e dp a p e r d i e l e c tr i c h a s b e e n i n c l u d e d i n c h a p t e r 3 , w h i c h s h o u l d b e r e a d i n c o n j u n c t i o nw i t h t h i s c h a p t e r .

C O N S T R U C T I O NP a p e r c a b l e s i n t h e 1 - 3 3 k V r a n g e a r e o f t e n r e f e r r e d t o a s ' s o l i d t y p e ' a s t h e y a r ed e s i g n e d t o o p e r a t e w i t h o u t i n t e r n a l o r e x t e r n a l p r e s s u r e .

T h e i n s u l a t i o n c o n s i st s o f h e l ic a l ly a p p l i e d p a p e r t a p e s w i t h a s m a l l g a p b e t w e e nt u r n s ( fig . 1 9.1 ). T h e r e g i s t r a t i o n o f t a p e s in r e l a t i o n t o e a c h o t h e r is i m p o r t a n t t o a v o i ds u c c e s s i v e b u t t g a p s i n a r a d i a l d i r e c t i o n ( c h a p t e r 3 ) . W h e n c a b l e s a r e b e n t f o rd r u m m i n g a n d l ay i ng , t h e p a p e r t a p e s h a v e t o s li de o v e r e a c h o t h e r w i t h o u t u n d u ec r e a s i n g , w r i n k l i n g o r t e a r i n g a n d a r e th e r e f o r e a p p l i e d w i t h a g a p b e t w e e n t u r n s . T h eg a p w i d t h m u s t b e s u c h t h a t , w h e n t h e c a b l e is b e n t t o t h e s m a l l e s t p e r m i s s i b l e r a d i u s , i tw ill n o t c lo s e c o m p l e t e l y a n d c a u s e w r in k l in g o f th e p a p e r . F o r b e n d i n g r e a s o n s t h em e c h a n i c a l d e s i g n r e q u i r e m e n t s a r e a s i m p o r t a n t a s e l e c t r i c a l a s p e c t s i n r e l a t i o n t oi n s u l a t io n t h ic k n e ss , c e r t a in l y f o r lo w v o l t a g e c ab le s . T h e s e m e c h a n i c a l r e q u i r e m e n t sc o v e r s u c h f e a tu r e s as t h e a n g le a n d l a p p i n g t e n s i o n d u r i n g p a p e r a p p l i c a t io n , t h e w i d t ho f t h e t a p e ( g e n e r al ly 1 2 -2 8 m m ) , t h e t h ic k n e s s o f t h e p a p e r ( 0 . 0 7 - 0 . 1 9 m m ) , t h e p a p e rd e n s i t y a n d t e n s i l e s t r e n g t h .

3 1 6

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Fig . 19 ,1 S ingle-core , 30 0ra m 2, 600/1000V, pa pe r insula ted lead sh eathed cable w i th P VCovershea th

T h e c o n d u c t o r s i n m u l t ic o r e c a b l es a r e u s u a l l y s e c t o r s h a p e d u p t o 11 k V a n d o v a l f o r3 3 k V . S o li d a l u m i n i u m is u s e d e x t e n si v e ly a t 1 k V . S t r a n d e d c o n d u c t o r s a r e n o r m a l l yp r e - s p ir a l le d ( c h a p t e r 4 ) to r e d u c e t h e p o s s i b i l i t y o f d a m a g e t o th e i n s u l a t i o n b yt w i s t i n g d u r i n g t h e l a y i n g - u p o p e r a t i o n b u t t h i s i s n o t u n i v e r s a l f o r t h e s m a l l e r s i z e s o f1 k V c a b l e .

B e l t e d c o n s t r u c t i o n

T h e c a b l e d e s i g n w i t h a ' b e l t ' o f i n s u l a t i o n o v e r t h e l a i d - u p c o r e s ( fig . 1 9 .2 ) i s t h e m o s te c o n o m i c a l in t e r m s o f t o t a l m a t e r i a l co s t . S u c h c a b l e s a r e n e a r l y a l w a y s u s e d u p t o6 .6 k V a n d a r e t h e m o s t c o m m o n t y p e a t 11 k V .

T h e s p a c e s b e t w e e n t h e c a b l e c o r e s u n d e r t h e b e l t a r e f il le d w i t h j u t e o r p a p e r .W h e r e a s t h e m a i n i n s u l a t i o n c o n s i s t s o f p a p e r t a p e s p r e c i s e l y a p p l i e d , t h e fi ll eri n s u l a t io n h a s t o b e s o f t e r a n d le ss d e n s e s o a s to b e c o m p r e s s e d i n t o t h e s p a c e a v a i l a b l ea n d is w e a k e r e le c t r i c a ll y . S t r e s s e s i n t h e f il le r s h a v e t o b e l i m i t e d t o a n a c c e p t a b l e l e v e la n d t h e r e f o r e b e l t e d c a b le s a r e n o t g e n e r a l l y u s e d a t v o l t a g e s g r e a t e r t h a n 11 k V .

A l t h o u g h a n y d i s c h a r g e b e t w e e n t h e o u t si d e o f t h e i n s u l a t i o n a n d t h e m e t a l l i c s h e a t hm a y p r o d u c e d i s c o l o r a t i o n o f t h e i m p r e g n a t i n g c o m p o u n d , i t h a s l it tle e ff e ct o n t h e l if eo f t h e c ab l e . H o w e v e r , w h e n B S 6 4 80 w a s r e v i s e d in 1 96 9 it w a s d e c i d e d t o s p e c i f y th ei n cl u si o n o f a s e m i c o n d u c t i n g c a r b o n p a p e r t a p e o v e r t h e i n s u l a t io n f o r 11 k V b e l te dc a b l e s, o p t i o n a l i n I E C 5 5.

Fig . 19 .2 4-core , 70 m m 2, 600/1000V, pa pe r insula ted lead shea thed cable w i th ST A andbituminous finish

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Fig. 19.3 3-core, 150 m m 2, 6.35/11 kV, screen ed PIL S cab le with P V C ov ers he ath

S c r e e n e d c o n s t r u c t io nT h e e l ec tr ic s t r e n g th o f i m p r e g n a t e d p a p e r is w e a k e r i n th e t a n g e n t i a l d i r e c t i o n t h a n int h e r a d i a l d i r e c t i o n a n d f o r c a b le s a t v o l t a g e s a b o v e 11 k V i t is n e c e s s a r y t o e n s u r e t h a tt h e e l e c tr i c a l f ie l d i s r a d i a l ( c h a p t e r 2 ). A s o p e r a t i n g v o l t a g e s w e r e r a i s e d w i t h 3 - c o r ec a b l e s in t h e 1 9 20 s a n d e a r l y 1 9 30 s, n o n - r a d i a l f ie l ds w e r e t h e c a u s e o f e x t e n s i v e c a b l ef a il u r e s o f b e l t ed c a b le s . H o c h s t a d t e r fi rs t p o i n t e d t o t h e n e e d f o r s c r e e n i n g a n d t h ed e s i g n is s ti ll s o m e t i m e s r e f e r r e d t o a s H t y p e . S c r e e n i n g c o n s i s t s o f a t h i n m e t a l l i c l a y e ri n c o n t a c t w i t h t h e m e t a l l i c s h e a t h ( fi g. 1 9 .3 ). A s i t c a r r i e s o n l y a s m a l l c h a r g i n g c u r r e n t ,t h e th i ck n e s s is u n i m p o r t a n t b u t i t is n e c e s s a ry to h a v e s m o o t h c o n t a c t w i th t h ei n s u la t io n t o g e t h e r w i th a n a b i li ty t o w i t h s t a n d c a b l e b e n d i n g w i t h o u t d a m a g e . Ap a p e r / a l u m i n i u m f oi l l a m in a te , p i n h o l e d t o a ll ow p e n e t r a t i o n o f c o m p o u n d d u r i n gi m p r e g n a t i o n ( c h a p t e r 2 5 ) a n d a p p l i e d w i t h a n o v e r l a p , i s n o r m a l l y u s e d b u t a t h i nc o p p e r o r a l u m i n i u m t a p e i s o f t e n p r e f e r r e d b e c a u s e i t i s l e s s s u s c e p t i b l e t o d a m a g e .M u l t i c o r e c a b l e s u s u a l l y r e q u i r e a b i n d e r o v e r t h e l a i d - u p c o r e s a n d t h i s i s n o r m a l l y ac o p p e r w o v e n f a b r i c ( C W F ) t a p e . F i n e c o p p e r w i r e s a r e i n c l u d e d i n t h e f a b r i c t op r o v i d e e le c t ri c a l c o n t a c t b e t w e e n t h e s c r ee n a n d t h e m e t a l l i c s h e a t h .

W h e r e s c r e e n i n g i s o p t i o n a l , i .e . a t 11 k V , t h e d i s a d v a n t a g e s c o n c e r n t h e e x t r ac o m p l e x i t y a n d c o s t o f p r o v i d i n g s c r e e n e d j o i n ts . T h e a d v a n t a g e o f s c r e e n in g is i m p r o v e de l e c t r i c a l q u a l i t y , p e r m i t t i n g o p e r a t i o n t o a h i g h e r t e m p e r a t u r e a n d h e n c e i n c r e a s e dr a t in g . M o s t u s e rs , h o w e v e r , p r e f e r t h e b e l t e d d e s i g n b e c a u s e o f s im p l e r j o i n t i n g .

A t v o l t a g e l e ve ls w h e r e i t is n e c e s s a r y t o a d o p t i n s u l a t i o n s c r e e n i n g a s c r e e n i n g l a y e ro v e r th e c o n d u c t o r i s a l so r e q u ir e d . T h i s p r o v i d e s a s m o o t h i n t e r f a c e b e tw e e n t h e w i r eso f a s t r a n d e d c o n d u c t o r a n d t h e in s u l a ti o n , t h u s l i m i ti n g d i s c h a r g e w h i c h m a y a r i se d u et o e l e c t r i c a l s t r e s s e n h a n c e m e n t o n t h e s t r a n d s o r v o i d s a t t h e i n t e r f a c e . C o n v e n t i o n a lp r a c t i c e i s t o a p p l y t w o s e m i c o n d u c t i n g c a r b o n p a p e r t a p e s o v e r t h e c o n d u c t o r s .

S L a n d S A s c r ee n e d c a b l e sT h e s e a r e r a d i a l f i el d s i n g le - c o r e m e t a l l i c s h e a t h c a b l es w i t h e l e c t r o s t a t i c t a p e a c t i n g a st h e i n s u l a t i o n s c re e n . S L a n d S A r e f e r t o s h e a t h i n g w i t h l e a d a n d a l u m i n i u mr e s p ec t iv e l y . T h e t h r e e c o r r o s i o n p r o t e c t e d c o r e s o f S L c a b l e s a r e l a i d u p t o g e t h e r ,a r m o u r e d a n d f i n is h e d w i t h f u r t h e r c o r r o s i o n p r o t e c t i o n (fig. 1 9.4 ). S A c a b l e s a r e l a idu p s i m i la rl y w i t h a P V C o v e r s h e a t h o n e a c h c o r e b u t a r e n o t n o r m a l l y a r m o u r e d .

A l t h o u g h t h e a m o u n t o f m e t a l in t h e t h r e e i n d i v i d u a l s h e a t h s is l it tl e d i f f e r e n t f r o mt h a t i n a c a b l e h a v i n g t h r e e c o r e s w i t h i n a s in g le s h e a t h , t h e g r e a t e r d i a m e t e r r e s u lt s in

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Fig. 19.4 3-core, 150mm2, 19/33kV, SL cable

extra bedding and armouring material, thereby increasing the total cable cost.However, jointing and terminating is more convenient. Some users prefer to installthree separate single-core cables rather than a multicore cable, but the multicore SL andSA designs provide some of the advantages of single-core cable and avoid the highmagnetic losses which would occur with steel armour.SL and SA cables are not commonly used in the UK but are extensively employed incontinental Europe and in other countries following European practice. A factor whichhas been associated with individual preference relates to the draining of oil-rosinimpregnant on hilly routes, as there is less compound under the sheath with the single-core construction. In the UK the problem was overcome by the use of non-draining(MIND) insulation.

INSULATION CHARACTERISTICSThe characteristics of impregnated paper insulation, including moisture sensitivity andneed for metallic sheathing, electric strength and resistance to discharge in butt-gapspaces, are reviewed in chapter 3.It is not only the electric strength which defines insulation thickness. Particularly atthe lower end of the voltage range mechanical requirements predominate and at theupper end allowance has to be made for the condition of the dielectric resulting fromimpregnant movement during load cycles to defined temperature. An indication ofinsulation thickness and operating stresses is shown in table 19.1 (see also chapter 18).

Even in the 6-15kV range the electrical stress is not a primary factor and theoperating stress of 2-4MV/m is well below the capability of impregnated paper.However, because of the possibility of ionisation in voids it is necessary to ensure a highlevel of filling with impregnant.The electrical stress becomes important at 33 kV and, partly to reduce stress at theconductor surface, oval conductors are usually used in preference to sector-shapedconductors. The use of oval conductors for multicore cables also improves the bendingperformance of cable with the thicker insulation. For the same reason, more insulationis required on very small conductor sizes, in contrast with the increase in insulationthickness with conductor size on lower voltage cables.

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3 2 0 Electric Cables HandbookT a b l e 19 .1 Insu la t ion th ickness (min imum ) and e lect rica l s tress (max im um ) fo r p ape r insu la tedcablesVo l t a g e Co n d u c to r(kV) size ( I n l n 2 ) Belted design insula t ion th ickness Single-core and screened designBe twe e n Co n d u c to r I n s u l a ti o n S t re s sc o n d u c to r s s h e a th (mm) (M V /m)( m m ) ( m m )0.6/1 50

1 0 0 01.9/3.3 5010003.8/6.6 501 0 0 06.35/11 5010008.7/15 50100012.7/22 50100019/33 501 0 0 0

1 .4 1 .2

2.4 1.84.2 2.75.6 3.4

1.2 0.242.0 0.131.8 1.32.0 1.02.4 2.02.4 1.72.8 2.92.8 2.43.6 3.33.6 2.64.9 3.84.9 2.87.3 4.46.8 3.2

M A S S - I M P R E G N A T E D N O N - D R A I N I N G ( M I N D ) C A B L E SS p ec ia l i m p r e g n a n t s a r e u s ed t o p r e v e n t d r a in a g e o f th e c o m p o u n d w h e n c a bl es a r ei n s t a ll e d v e r ti c a l ly o r o n s lo p e s . S u c h c a b l e s w e r e p i o n e e r e d b y B I C C i n 1 9 49 , i n i ti a l lya t 1 k V . F o l l o w i n g e x te n s iv e d e v e l o p m e n t w o r k in c o l l a b o r a t i o n w i t h D u s s e k B r o s L t d ,d u r i n g t h e n e x t t w o d e c a d e s t h e y s u b s e q u e n t l y b e c a m e a l m o s t u n i v e r s a ll y s t a n d a r d i s e di n t h e U K o v e r t h e w h o l e v o l t a g e r an g e .W h e n c a bl es w i t h o i l - r o s i n i m p r e g n a n t a r e i n s ta l le d o n r o u t e s h a v i n g d i f fe r e n c es i nl ev el, t h e i m p r e g n a n t m i g r a t e s t o w a r d s t h e l o w e r e n d a n d , u n le s s g r e a t c a r e is t a k e n i nm a k i n g j o in t s a n d t e r m i n a t i o n s , t h e r e m a y b e l e ak a g e o f t he c o m p o u n d . O n s te e pg r a d i e n ts b a r r i e r j o i n t s m a y b e n e c e s s a r y a n d t h e r e is a ls o a d a n g e r o f l ea d s h e a t he x p a n s i o n r e s u lt in g in f ra c t u r e . M o r e i m p o r t a n t l y , t h e l os s o f c o m p o u n d r e d u c e s t h ee l ec tr ic a l s t r e n g th o f t h e i n s u l a ti o n a t t h e h i g h e r e n d , a n d t h e c r e a t i o n o f a v a c u o u sc o n d i t i o n m a y l e a d t o i n gr e ss o f m o i s t u r e i f t e r m i n a t i o n s a r e n o t c o m p l e t e l y s e al ed . A tv o l ta g e s o f 11 k a n d a b o v e , t h e lo s s o f c o m p o u n d c a n c a u s e a h i g h le v el o f d i s c h a rg ew i t h i n t h e c a b le a n d e x p e r i e n c e h a s s h o w n t h a t t hi s m a y b e i m p o r t a n t e v e n f o r q u i tem o d e s t v e r t ic a l s e c ti o n s s u c h a s a t p o l e te r m i n a t i o n s . B e f o r e th e a d v e n t o f M I N Di n s u l a t i o n i t w a s n e c e s s a r y f o r c a b l e s o n s l o p e s e i t h e r t o b e p a r t ia l l y d r a i n e d a n d t o h a v ec o n s i d e r a b l y i n c r e a s e d i n s u l a t i o n t h i c k n e s s t o r e d u c e t h e s t r e s s o r , a l t e r n a t i v e l y , t o b eo f th e p r e - im p r e g n a t e d p a p e r t y p e o f n o n - d r a i n i n g c o n s t r u c ti o n .

D u r i n g i m p r e g n a t io n , t h e M I N D c o m p o u n d i s v e r y f lu id a n d r e a d il y s a t u ra t e s t h ei n s u la t io n . O n c o o l i n g t o a m b i e n t t e m p e r a t u r e , t h e c h a n g e o f s t a te t o a s o f t s o li d f o r mis a c c o m p a n i e d b y a s li gh t c o n t r a c t i o n . W i t h M I N D i n s u l a t io n i t is e v e n m o r ei m p o r t a n t t h a n w i t h o i l - ro s i n t o e n s u r e t h a t t h e c o o l i n g p r o c e s s is s lo w , th e c o m p o u n d

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Paper Insulated Distribution Cables 321being circulated through a cooling system to obtain temperature uniformity. This alsoprevents the formation of a skin on the compound surface which could act as a barrierto further compound entry. Even so, the volume impregnation factor is of the order of93% in comparison with 96% for oil-rosin. Later MIND compounds have been betterthan the original ones with respect to coefficient of contraction.

In service the MIND compound will remain in position whereas oil-rosin compoundmay readily drain away and leave more voids in the insulation. However, when cablesare tested immediately following manufacture, MIND insulation shows moreionisation, i.e. greater difference in power factor between one-half working voltageand twice working voltage. When BS 480 was amended in 1954, the limit for multicore33 kV cables was raised from 0.0006 for oil-rosin to 0.006 for MIN D to allow for thisfeature and further revision is detailed in BS6480: 1988. In addition, the impulsestrength of MIND cable is slightly below that of virgin oil-rosin cable. Nevertheless,the BS6480:1988 requirements of 194kV at 33kV and 95kV at llkV are readilymet, the latter being above the IEC 55 requirement of 75 kV.

Initial ionisation is not of importance and MI ND cable provides excellent long-termperformance, as shown by a stability test with daily load cycles to a temperature5C above the maximum permitted continuous limit and a voltage of 1.33 times theworking level.

Periodic measurements of the power factor at 1.5 and 2.0 times working voltage showthat both oil-rosin and MIND cables reach a peak and then stabilise. MIND cablesreach this peak more quickly, generally after a few cycles, and the level may subsequentlydecrease. At working voltage, however, in contrast with oil-rosin insulation, MINDinsulation shows little or no ionisation after a prolonged period of testing.

It has been argued in some countries that, under short-circuit conditions, MINDinsulation could be inferior because the conductor temperature permitted is such thatthe compound may change to the liquid state. This has been difficult to establishexperimentally and it is apparent that the temperature of the mass of cable is well belowthat of the conductor. MIND cables have shown a very satisfactory service performancefor over 30 years. Any shortcomings in this and any o the r respects arising in non-drainingcables made elsewhere have no doubt been due to lack of recognition of the expertisenecessary in formulating satisfactory compounds and correct processing techniques.

METALLIC SHEATHSPaper insulated cables are sheathed with either lead or aluminium andcharacteristics of these two metallic sheaths have been discussed in chapter 3.

the

ALUMINIUM SHEATHED PAPER INSULATED CABLESIn the early 1970s, many users th roughout the world were considering changing to MVpolymeric insulated cables. British manufacturers however, conscious that the servicereliability of such cables appeared to be much lower than that of paper cables, drewattention to a further economy which could be obtained with paper cables whilstfurther development of polymeric insulation was proceeding. Substitution of an

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3 2 2 Electric Cables Handbooka l u m i n i u m s h e a t h f o r a le a d s h e a t h a n d a r m o u r p r o v i d e d a p r i ce s a v i n g o f a r o u n d 2 5 % .S e r v ic e t ri a ls w e r e q u i c k l y a r r a n g e d a n d b y 1 97 5 n e a r l y a l l t h e U K e l e c t r ic i t y u t il it ie sh a d c h a n g e d t o t h is c o n s t r u c t i o n f o r t h ei r 11 k V n e t w o r k s .

O n e p r o b l e m i n th e e a r l y y e a r s r e l a t e d t o t h e m u l t i p l i c it y o f d e s ig n s a v a i l a b l e , a lla d d i n g t o t h e c a b l e m a k e r s ' s t o ck s . In a d d i t i o n t o l e a d s h e a t h e d c a b le s n o t b e i n gs u p e r s e d e d f o r m a n y y e a r s, t h e a l u m i n i u m s h e a t h c o u l d b e s m o o t h , w h i c h w a s s li g h tl yc h e a p e r , o r c o r r u g a t e d ( f i g . 1 9 . 5 ) a n d , a s w i t h l e a d s h e a t h e d c a b l e s , s o m e a u t h o r i t i e sf a v o u r e d a b e l te d a n d o t h e r s a s c r e e n e d d e s i g n . A r e v i e w w a s c a r r i e d o u t b y t h e u t i l it ie sa n d i n 1 978 it w a s c o n s i d e r e d t h a t t h e m u c h b e t t e r h a n d l i n g c h a r a c t e r i s t i c s o f t h e c a b l ew i t h c o r r u g a t e d s h e a t h h a d a d i s ti n c t a d v a n t a g e . I t w a s d e c i d e d t o s t a n d a r d i s e t h eb e l t e d c a b le w i t h c o r r u g a t e d s h e a t h i n c o n d u c t o r s iz es o f 9 5 , 1 85 a n d 3 0 0 m m 2, t h o u g ht h e u t i l i t i e s w h i c h h a d t r a d i t i o n a l l y u s e d s c r e e n e d c a b l e r e m a i n e d w i t h t h i s t y p e , a sd e t a i l e d i n E A 0 9 - 1 2 .

A s c a b l e w e i g h t w a s r e d u c e d b y 5 0 % , s m o o t h s h e a t h d e s ig n s u p t o 185 m m 2 w e r e n om o r e d i f fi cu l t t o in s t a ll t h a n l e a d s h e a t h e d c a b l e b u t p r o b l e m s c o u l d a r i s e w i t h l a r g e rs iz e s i n d u c t i n s t a l l a t i o n s b e c a u s e o f t h e s ti f fn e s s o f t h e c a b l e . T h i s e x t r a r i g i d i t y w a s a l s od e t r i m e n t a l i n s e t t i n g c a b l e e n d s i n t o e q u i p m e n t w h e n s p a c e w a s l i m i t e d i n s u b s t a t i o n s .T h e c o r r u g a t e d s h e a t h e d c a b le w a s h o w e v e r e a s y t o i ns ta ll a n d h a d a d i st i n c t a d v a n t a g eo v e r l e a d s h e a t h e d c a b l e .

T h e i n c r e a s e d r ig i d i t y o f s m o o t h s h e a t h d e s i g n s , t o g e t h e r w i t h t h e h i g h c o e f f i c ie n t o fe x p a n s i o n o f a l u m i n i u m , a ls o c a u s e d c o n c e r n r e g a r d i n g m e c h a n i c a l s tr e ss e s in j o i n ts .W i t h i n t h e j o i n t t h e r e c a n b e ' b o w i n g ' o f t h e c o r e s i n th e j o i n t s l e e v e a f t e r h e a t i n gc y cl es . W h i l s t t h e se p r o b l e m s c a n r e a d i l y b e o v e r c o m e b y t h e u s e o f j o i n t s o f th e c a s tr e s i n fi ll ed t y p e , t h e i n c r e a s e i n c o s t is n o t a l w a y s c o n s i d e r e d t o b e j u s t i f i e d . T h o s eu t il it ie s w h i c h u s e d s m o o t h s h e a t h e d c a b l e s c o n t i n u e d w i t h b i t u m e n f il le d j o i n t s o n t h eb a s i s t h a t t h e c a b l e s w e r e i n s t a l l e d i n o p e n - r i n g c i r c u i t s a n d v e r y s e l d o m r e a c h e d f u l ld e s i g n l o a d .M a n u f a c t u r i n g t e c h n i q u e is p a r t i c u l a rl y i m p o r t a n t f o r c a b le s w i t h c o r r u g a t e d s h e a t h sb e c a u s e o f t h e c o n s i d e r a b l e i n c r e a s e i n s p a ce b e t w e e n t h e i n s u l a t i o n a n d t h e i n si d e o ft h e s h e a t h . I f th i s is c o m p l e t e l y f il le d w i t h i m p r e g n a t i n g c o m p o u n d , i t is p o s s i b l e t og e n e r a t e v e r y h i g h i n t e r n a l pr e s su r e s a t o p e r a t i n g t e m p e r a t u r e . I t is c o n s e q u e n t l yn e c es s ar y t o c o n t r o l t h e a m o u n t o f c o m p o u n d b e tw e e n t h a t n e c e s s ar y to p r e v e n ti o n i s a t i o n w i t h i n t h e i n s u l a t i o n a n d t h a t p r o d u c i n g m a x i m u m p r e s s u r e o n h e a t i n g . I na d d i t i o n , i f t h e a l u m i n i u m s h e a t h s h o u l d b e p u n c t u r e d a n d w a t e r e n t e r e d t h e c a b l e , a llt h e c a bl e a ff e c te d w o u l d h a v e t o b e r e p l a c e d . S u ff ic ie n t i m p r e g n a t i n g c o m p o u n dt h e r e f o r e n e e d s to b e p r e s e n t t o p r e v e n t w a t e r f r o m p e n e t r a t i n g a l o n g t h e c a b le . T h e

: ~ i ~ i ~ ? : :

Fig. 19 .5 3-core, 11 kV, bel ted PIA S cable with corr uga ted a lum inium sheath

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Paper Insulated Distribution Cables 323annular corrugation design used for these solid type cables (fig. 19.5) has positiveadvantages. In the event of local puncture of the sheath, the small clearance at the rootof the corrugation, containing impregnating compound, is an obstacle to transmissionof water along the cable. Furthermore, it is much easier to cut around one crest of acorrugation rib when a piece of sheath has to be removed for jointing or terminating.

The economic advantage of the aluminium sheathed design (EA09-12) hasundoubtedly been the main reason for the continued use of medium voltage distributionpaper cables in the UK, whereas XLPE insulation has proceeded to oust paper in manyother countries. XLPE cables are becoming competitive in price and their otherattraction, that they do not require such highly skilled jointers, is having more impactas the traditional skills become less available.

A R M O U RThe choice between steel tape (STA) and galvanised steel wire (GSW) armour formulticore cables is very much related to local practice. Generally in the UK STA isadopted for 0.6/1 kV and GSW is favoured for higher voltages because of its betterresistance to mechanical damage and provision of improved longitudinal cable strengthfor handling purposes. However, in continental Europe and many other countries, steeltape armour, sometimes galvanised, tends to be used more extensively throughout thevoltage range.Aluminium sheaths are not armoured and are finished with an extruded layer ofpolymeric material, commonly PVC. Lead sheathed and armoured cables are now alsooften finished with extruded PVC or MDPE but bi tuminous corrosion protection is alsoemployed (chapter 5).

OPERATIONAL AND INSTALLATION PARAMETERSContinuous operat ing temperatureConductor temperature limits as a basis for cable ratings are referenced in chapter 8.Table 19.2 summarises the requirements for paper cables.The temperature of 80C for 1 kV cables is based on the physical and electricaldegradation of the dielectric materials. At the higher voltages the limit is primarilyassociated with the imposition of conditions to prevent increase in ionisation within the

Table 19.2 Conductor temperature limits for paper cablesVoltage (kV) C a b l e Maximum conductord e s i g n temperature (C)0.6/1, 1.9/3.3, 3.8/6.6 Belted 806.35/11 Belted 656.35/11, 8.7/15 Screened 7012.7/22, 19/33 Screened 65

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3 2 4 Electric Cables Handbookd i el ec tr ic . M o s t d i s tr i b u t io n c a b le s o p e r a t e w i t h c y c li c l o a d i n g p r e d o m i n a n t l y o n a d a i lyb as is . D u r i n g h e a t i n g t h e re is e x p a n s i o n o f th e i m p r e g n a n t a n d f o r v a r i o u s r e a s o n s t h ec o m p o u n d m a y n o t f l ow b a c k b e t w e e n t h e l a ye rs o f p a p e r d u r i n g c o o li n g , t h u s c r e a t in gv o i d s in w h i c h d i s ch a r g e m a y o c c u r . L e a d s h e a t h s r e a d il y e x p a n d u n d e r p r e s s u r e b u t d on o t c o n t r a c t w h e n t h e p r e s su r e is r e d u c e d , t h u s r e d u c i n g t h e l i k e li h o o d o f th ec o m p o u n d r e t u r n i n g i n t o t h e i n su l a t io n . A l l th e s e ef fe c ts h a v e a b e a r i n g o n t h em a x i m u m t e m p e r a t u r e w h i c h c a n b e a d o p t e d . T h e t e m p e r a t u r e i s l o w e r f o r b e l t e dc a b le s , o w i n g t o t h e n o n - r a d i a l f i e ld , a n d r e d u c e s w i t h i n c r e a s in g v o l t a g e b e c a u s e o f t h em o r e s e r io u s e f f e ct o f i o n i s a t i o n o n c a b l e l if e.

T e c h n i c a l l y i t h a s b e e n p r o v e d f e as i bl e to d e v e l o p s a t i s f a c t o r y i m p r e g n a t i n g c o m -p o u n d s a n d c a b l e c o n s t ru c t i o n t o p e r m i t a t e m p e r a t u r e o f 8 5 C u p t o 3 3 k V b u t s u c hc a b l e s h a v e n o t b e e n b r o u g h t i n t o c o m m e r c i a l u s e .

Bending radi iM a t e r ia l s a n d m e t h o d s o f m a n u f a c t u r e h a v e t o b e c h o s e n t o m i t ig a t e a g a i n s t d i el e c tr icd i s t u r b a n c e o n b e n d i n g . I E C 5 5 a n d m o s t n a t i o n a l s t a n d a r d s p r e s c r ib e th e m i n i m u mb e n d i n g r a d i i q u o t e d i n a p p e n d i x A 1 9 .

T h e s tif fn es s o f im p r e g n a n t s c h a n g e s w i t h t e m p e r a t u r e t o a n e x t e n t w h i c h v a r ie s w i t hc o m p o s i t i o n a n d t y p es o f c o m p o u n d u s ed . I f ca b le s a r e t o b e i n st a ll e d i n c o l d c l im a t e s ,i t is u s u a l t o p r e s c r i b e a b e n d i n g t e s t t o b e c a r r i e d o u t a t a n a p p r o p r i a t e t e m p e r a t u r e , t oe n s u r e t h a t t h e r e is n o d i s r u p t i o n o f t h e in s u l a t i o n .

L I F E A N D P E R F O R M A N C E C H A R A C T E R I S T I C ST h e s e rv i ce p e r f o r m a n c e o f p a p e r c a b l e s h a s b e e n s o g o o d t h a t i t is a c c e p t e d a s t h es t a n d a r d b y w h i c h o t h e r t yp e s a r e j u d g e d . A p a r t f r o m m e c h a n i c a l d a m a g e , o n e o f th em a i n c a us e s o f ca b l e r e p l a c e m e n t i s t h e e n t r y o f w a t e r a n d t h e t r a v e l o f th e w a t e rt h r o u g h a c o n s i d e r a b l e l e n g t h o f c a b l e. E l e c t ri c a l f a il u r e s in t h e d i e l e c t ri c a r e r a r e u p t o6 k V . A t 11 k V a n d a b o v e t h e y a re m o r e u s u a l ly a s s o c i a te d w i t h t h e d i s c h a r g e a n dc a r b o n t r a c k i n g ( ' t r e e i n g ' ) m e c h a n i s m d i s c u s s e d i n c h a p t e r 2 .

A n i n te r e s ti n g t y p e o f b r e a k d o w n w a s k n o w n f o r m a n y y e a r s a s 'p i n h o l e t y p e f a il u reb e l o w t e r m i n a t i o n s o n 11 k V b e l t e d c a b le ' . I t w a s c o m m o n i n S o u t h A f r i c a b u t a ls oa r o s e e l s e w h e r e . A l t h o u g h n o t n e c e s s a r i l y a s s o c i a t e d w i t h h i g h a l t i t u d e s t h e i n c i d e n c ew a s g r e a t e r a t h ei g h ts o f a r o u n d 1 00 0 m . T h e f a i lu r e s in v a r i a b l y o c c u r r e d i n t h e v e r ti c a lc a b le b e l o w t e r m i n a t i o n s a n d a t a d i st a n c e o f 1 t o 15 m f r o m t h e t e r m i n a t i o n . T h e r e w a si n t en s e p i n h o l e t y p e e r o s i o n o f t h e d i e le c t r ic p a p e r s , w i t h n o c a r b o n i s a t i o n b u t w i t hg r e e n d e p o s it s b o t h o n th e c o n d u c t o r a n d o n a d j a c e n t i n s u la t in g p a p e r s . A f t e r l e n g t h yi n v e st ig a t io n s i t w a s e s ta b li sh e d t h a t t h e b r e a k d o w n s t a rt e d f r o m d r a i n a g e o f o i l - r o s i ni m p r e g n a n t w h i c h a l l o w e d a ir to e n t e r t h e c a b l e t h r o u g h i n c o m p l e t e s e al in g o f t h e w i rei n te r s ti c e s o f th e c o n d u c t o r i n t h e t e r m i n a t i o n . D i s c h a r g e t h e n o c c u r r e d i n th e a i r -f il le dv o i d s i n th e d i e l e c tr i c i n th e r e g i o n o f h i g h e s t s t re s s, i .e . w h e r e t h e c o n d u c t o r s a p p r o a c he a c h o t h e r m o s t c l o se l y a n d w h e r e t h e r a d ii o f th e s h a p e d c o n d u c t o r s a r e s m a ll es t. T h ed i s ch a r g e s re s u lt e d in th e f o r m a t i o n o f o x i d e s o f n i t r o g e n a n d o z o n e w h i c h a t t a c k e d t h ep a p e r t o p r o d u c e t h e p i n h o le s a n d l e d t o p r o d u c t i o n o f m o i s t u r e . T h i s c o m b i n e d w i t ht h e o x i d e s o f n i t r o g e n t o f o r m n i t r ic a c i d w h i c h r e a c t e d w i t h t h e c o p p e r t o g i v e t h eg r e e n c o p p e r n i tr a te .

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Paper Insulated Distribution Cables 325The solution was to use hermetically sealed termination boxes. Also the use of non-

draining cables mitigates against creation of voids and the suction of air into the cable.

TESTSTesting procedure and requirements are reviewed and discussed in chapter 28.