Siemens Power Cables & Their Applications

SIEMENS

CABLE BOOK

POWER CABLES

& THEIR APPLICATIONS

PART 1 VOLUME

I

I

-t

Power Cables and th eirApplication Part

1

Materials . Construction Criteria for Selection Prniont r I vJvvr Ple n nin n Laying and Installation . Accessories Measuring and Testing

Editor: Lothar Heinhold

i

3rd revised edition. 1990

Siemens Aktiengesellschaft

Observations on the German terms and the VDE Specifications

'

'Kabel' and oLeitungen'

Kabel' and 'Leitungen'

Porrer cables are used for rhe transmission of elecrrical energy or as control cables lor the purpuses of

measurement, control and monitorin-s in electric pouer installations. In German usage. a disrinction rs made rraditionally benr.een 'Kabel' and .Leitungen'.

'Leituneen' (literally'leads') are used. generally speaking. for wiring in equipment. in u.inng installations and for connections to moving or mobile cquipments and units. The terrn can thus be rranslated as 'insulatcd wires' or 'l.iring' or .flerible cables'

or'cords'. 'Kabel' (cables) are used principally for power rransmission and distribution in electricity supply-aurhoritv sys[ems. in indusrry and in mines etc.

$'ith the use of modern insulating and

sheathin_s materials rhe constructional differences between .Kabel' and 'Leirungen' are in many cases no longer discernl

ole.

The disrinction is therelore observed purelv in terms of rhe area of applicarion. as desiribed in DIN lDE 0:98 Part I for pouer cables and part 3 for s.iring and flexible cables, and in the desien specifica_ tions referred to lherein. e.g. DIN VDE Oij0for wiring and flexible cables and DIN VDE 0271 for pVC insulated cables.

Further factors in the choice between .Kabel' and 'Leitungen' are the equipment Specifications (e.g. DIN VDE 0700), the installation Specifications (e.g. DIN VDE 0100) or the operating stresses to be expected.

It can be taken as a rule of thumb that .Leirungen' must not be laid in the ground, and that cables of

flexible construction are classified as .Leituneen'. even if their rared voltase is higher rhan 0.6I kV - e.g. trailing cables. This apart, there are also types of 'Kabel' that are nor inrended for laying in the _eround (e.g. halogen-free cables with improved performance in condirions of fire to DIN VDE 0266. or ship lirin! cibles to DfN VDE 0261).r - .."

In the present translation the ierms 'cable' an,

'porver cable' have been used to include flexible anr_ u iring cables where there is no risk of confusion.

\-DE

Specifications

v

From considerations of consistencv in references an, for greater clrrity, the VDE Specificarions applicabl_ to po$er cables are eeneralll' quoted in accorcancc rvith the new pracrice as 'DIN VDE . . . .'.

This applies equalll ro rhe older specificationr ri hich still retain the designarion , VDE . . . ' or

'DIN

57

..

./VDE

..

.' in their tirles. Furrhermore

since these specifications are of lundamental significancc, the practice of quoring rhe date of publication has been dispensed wirh.

Insulated Wires and Flexible Cables

Constructional Elements

of Insulated Cables Conductors

ll

l.l l.:

Wiring Cables and Flexible Cables

l2

Porver Crbles

IJ

Insulation

l)

l5

3.1

l'7

3.1. r

tr.)

Poll mers Thermoplastics (Plastomers) Copolymers F)uoroplastics. Polvr rni l Chloridc {PVC) Pohcthylenc tPE) Cross- Linked Pol.vethylene (XLPE) Elastomers Thcrmoplastic Elastomers lTPE).Conducting Rubber.Natural R ubber (NR). Stl rene Butadienc Rubbcr (SBR).Nirrile Butadiene Rubber (NBR1. Butyl Rubber ( IIR ). E thylene- Pro py lene Rubber (EPR). Silicone Rubber (SiR).Ethl lene Vin;-i Acerrre (EVA) Thermosetting Polymers (Duromers) Chemicel Aging of Poh nrcrs Thc Intluence of Moisrure on Polyolefi ne Insulating !larerials Impregnatcd Paper Lirerature Referred to in Secrion 2

J

Protective She:lths

J.l 3.2 J.J

Thermoplastic Sheaths Elastomer Sheaths Sheathing Materials for Special

3'l

Purposes

lvletal Shearh

39 39

Protection against Corrosion

.ll

Cable rvith Lead Sheath Aluminium-Sheathed Cables

4l

Armour

43

Concentric Conductors

41

) 1.1

l.l.t l.

l.l

)t

2.) !.)

1t

-

8 3.1

I

3.!t

6

1

8.1.1 8.1.2 3.1.3 8.1.1 3.1.1 3.1.

::)

3.i

-r

3.1

io

J_1

t.1 '1 a

45

Conducting Layers Metallic Componen!s of Electrical

45

Screening

46

Longitudinally Water proof Screens.

47

.19

Core ldentificrrion of Clblcs

3l

ll lt.l I l.:

Application tnd Installxtion of Cablcs Rated Voltagc. Opcraring Volrilge Selcction of Conductor Cross-Sectional Area

.19 .19

54

)) 55 56

6: 1l

/)

86 33

89

Power Cables

lZ

National and Intcrnational Standards

0t

t2.l

VDE Specifications Standards oI Other Countries IEC and CENELEC Standards

01

12.2 12.3

94 94

l3

Tlpes of Construction of LowHigh-Voltage Cables

IJ.-t 13.2

General Type Designation Selection of Cables and Accessories

100

Power Cables for Special Applicatiom

124

t.)..)

Electrical Screening

National and International Standards VDESpecifications HarmonizedSrandards National Types IEC Standards Selection of Flcxible Cables Clbles ibr Fircd I nsrallations Fiexible Cablcs FLEXO Cords Flcxiblc Ceblcs lbr \lining and Industrv Halosen-Free SIENOPYR Wiring rnd Flexible Ca bles rritlt Improvco Perlormrnce in thc Evenr of Frrc

Dcfinition of Locltions to DIN !'DE 0100

3'7

38

.+9

t0

?7 30 J)

TJpes of Wires and Cables

14.1 14.2

and

Cable wirh Elastomer Insulation Shipboard Power Cable 14.2.L Constructionand Characteristics

102

t:+ 124 124

1.1.2.2 Application and Installation 1.1.3 Halogen-Free Cables with Improved Characteristics in the Case of Fire 1.1.3.1 Testing Performance under Conditions of Fire Spread of Fire.Corrosivity of Combusrion Gases. Smoke Density. Insulation Retention under Conditions of Fire Construction and Characteristics l+,_).J L:r1ing end I nsralla rion t.1.4 Cables for Mine Shafts and Galleries 14.5 R ivcr and Sea Crbles

125

l:)

18.4

ll5

18.4.1 18.4.2

l:8 129

Airport Cablcs

lJl

11.7

131

1{8

Cable ',vith Polymer Insulation and Lead Sheath I nsulated Overhead Line Cables

l5

High- and Extra-High-\roltage Cables

lJ+

1i.1

Cable with Polymcr Insulation

I

5.2

1

5.3

l

i.3.1

15.1.2 15.3.3

Lo*.Pressure Oil-Filled Cable wirh Leld or Aluminium Sheath Thermally Stable Cable in Stcel Pipe High-Pressure Oil-Filled Cable lnternai Gas-Prcssu re Crble External Gas-Pressure Cablc (Pressure Cable)

18.4.3

1i0

1r.6

1

18.2.4 Use of Tables 18.3 Calculation of Load Capacity

t.)!

18.-1..:l

J+

1i5

Ca b les

18.4.6

139

Planning of Cable Installations

16 17 17.1 17.2 17.3

Guide for Planning of Cable Installations

t4l

18.5 8.5.1

1

Cable Rated Voltages

t+o

18.5.2

.Allocation of Cable Rated Voltages Rated Lightning Impulse Withstand Voltage Voltage Stresses in the Event of Earth Fault

l+o

18.5.3 18.6

Current-Carrving Capacity in Normal Operation 18.1 Terms, Definitions and Regulations 18.2 Operating Conditions and Design Tables 18.2.1 Operating Conditions forlnstallations in Ground 18.2.2 Operating Conditions, Installation in

'| .11

147

1E

air 18.2.3 Project Design Tables

Load Capacity Installed in Ground/Air. Rating Factors for Installation in

Ground, lor Differing Air Temperatures and for Groups in Air

150 150

152

t52 157 159

Pipcs

Thernral Resisrances I{ and ?'i.Load Capacity for an Installarion of Pipes in Ground or Air or in Ducts Banks

1i8 139

18.6.1 18.6.2

18.6.3 18.6.4 18.6.5 18.6.6 18.7 18.8 19 19.1

19.2 19.2.1

l8t

ThermalResistances 184 Thermal Resistance of the Cable 18.1 Thermal Rcsistance of Air 186 Horizontal I nstallation . Vertical Installation . Atmospheric Pressure. .\mbicnt Temperature. Solar Radiarion. Arr;r n-genrent of Cables Thermal Resistance of the Soil . lgi Temperature Field of a Cable.Definition of Soil-Thermal Resistance . Daily Load Curve and Characteristic Diameter 'Drying-Out of the Soil and Boundary Isorherm d. Fictitious Soil-Thermal Resistance 7"j and ?"j".Load Capacirv v Grouping in the Ground . 107 Fictitious Additional Thennal Rcsistanccs AIj and AIi- duc to Grouping.Loud Capacity. Extension of the Dn .\rea.Current-C:rrrying Cupacity ol Dissimilar

18.-1.5 Installation in Ducts and

138

180

Soil-Thermal-Resisrivity

. l1-i -

. ll

{J

Cable in thc Ground. Phi'sical and Thermal Characteristics of Soil. Influcnce of Moisture Content.Msasurins. Basic Quantities for Calculation Bedding Matcrial.Sand.Gravel Mixtures. Sand-Ccmcnt Mixtures Calculation of Loud Caplcity Installation in Channcls and Tunncls ?-10 Unventilatcd Channels and Tunncls .,.0 Arransemcnt of Cablcs in Tunncls . 133

.

Channcls u'ith Forced Venrilation Load Capacity of a Cablc for ShortTime and Intermittcnt Operatron

General

.

215

. .

239

Calculation with Minimum Time Value Adiabatic Heat Rise . Root-Mean-Square Value of Current . Short-Time Operation Intermittent Operation . Symbols Used in Formulae in Section l8 Literature Referred to in Section 18. .

239

-

239

241

_

241

242

243

_

245

250

1

grlphitizing or conducting lacqucr or

I

tr

T

conduct-

ing adhesivc rvith weakly conducting tape applied cxtruded conducting luyers. lvhich serc either applied in a scparxte process or extruded in the sante process with the insulation.

R Flexible antl lYiritg Cables

Cables with PVC insulation manufactured by Siemens arc known by the trade name PROTODUR' They can be laid without special precautions in ambicnt temperatures above -5'C. If the cables are colder than this, they must be carefully warmed be, e installation. Flexible and wiring cables are generally of smaller diameter than porver cables' and are therefore subject to lower stresses in installation. so that with careful handling they can be laid at lorver .' lperatures. For countries such as Norway. Srveden or Finland. PVC compounds are available which afford the necessary bending capability down to low temperatures.

t

Because

to it: Lubric tut ts

I

T

I T T T

co[]pounds

T

Conduclor

T

Conducltng

I

I

Insulalinq compound

I

I Fig.2.6

Schematic arrangement of triple extrusion 19

1 -!I

2Insulation

According to the new specifications of only outer conducting layers perextruded rvith and bonded to the insulation are

DIN VDE

02731 . .87,

mitted.

The extruded conducting layers are very thin, and so firmly bonded to the insulation that they can be separated from it only with a scraper' In some counrries conducting layers are used whose adhesion is somervhat lower, so that - if necessary after scoring rvith a tool - they can be stripped by hand (cables rvith strippable conducting layers). Because of the force required in the stripping operation' such laycrs are made somewhat thicker. L

To ensure operational reiiability in medium' and highvoltage porver cables. it is particularly important' apart from using high-purity material and observing appropriate cleanliness in the nranulacturing processcs. that thc insulation and the conducting layers should be free of bubbles, and that therc should be good adhesion bctwecn the conducting laycr and the insulation. According to DiN VDE 0273 this must be checked on every manufactured length by means

of an ionization test. comparison with high polymers with polarized structures, such as PVC. high polymcrs with unpolariscd structures, such as PE and XLPE' are characterized by outstanding electrical charactcristics. They have, horvever, poor adhesion properties in relation to other materials, e.g. moulding compounds. This characteristic has to be takcn into account in the design of accessories.

In :

L

L Ui

li ll ll

I 1 n

For the lorv-voltage range a polycthylenc insulation compound has been successfully developed which bonds s'ell to accessory materials and thus ensures the water-tightness of joints.

PROTOTHEN.Y is not usual to use thermoplastic polyethylene in power cables for lJolIJ=0.611 kV, because of the high conductor temperatures to be expected under short-circuit conditions. For higber rated voltages' while it offers advantages in comparison with PVC and paper insulation because of its satisfactory dielectdc properties, it has declined in significance as power cable insulation, beceuse of its poor heat/presiure characteristics (Fig.2.1), in comparison with cross-linked polyethylene, and has been omitted from the new specification VDE DIN 0273/..87.

It

t0

Cross-Linked Polyethylene (XLPE)

PROTOTHEN.X The linear chain molecules of the polyethylene are knirted by the cross-linking into a three-dimensional network. There is thus obtained from the thermoplastic a material uhich at temperatures above the crlstallite mclting point cxhibits elastomcric propcrties By this mcans the dirnensional stability under heat As it and the mechanical properties are improved oC can be result, conductor temperltures up to 90 to 250 "C up and opcralion normal pcrmitted in ns. under short-circuit conditio There are thrce principal methods for cross-linking poll'cthylenc insulation matcrials :

Cross-linking bY Pcro.x idcs

Organic radical componcnts. in particular spccilic organic pcroxidcs. are incorporated. Thesc dccomposc at temperaturcs above thc cxtruding lemperaturc' into highly rcactive radicals. These radicals interlink rhe initially isolated polymer chains in the thermoplastic in such a rvay that i] spxce netuork results (Fig. 2.7). 'o'as crossFormerly, polyethylcne cable insulation

linkcd mainly by 'continuous vulcanization in

a

steam tube', in the so-called CV!)method (Fig.2.8)'

In this methoti the polycthylcne. mixcd u ith the pcroxide as a cross-linking initiator. is pressed onto thc conductor. by means of an extruder, at about 130 "C (below the temperature at rvhich the pcroxide dccomposes). Follouing this. in the same process, the insuiated core is passed through a tube, about 125 m long, Iilled with saturated steam at high pressure' At a pressure of 16 to 22 bar and a temperature ol aboui 200 to 220 "C, the organic peroxide decomposes into reactive primary radicals, which effect the cross-linking. The crosslinking process is followed immediately by a cooling stage. This must similarll take place under pressure in tubes 25 to 50 m long' to privent the formation of bubbles in the wlcanizec maierial through the presence of gaseous products

of the peroxide reaction' An alternatives to this 'classical' crossJinking pro cess, methods have been developed in which gase' or liquids, e.g. silicone oil or molten salrs (salt bath cross-linking) are used as media for the heat transfer I' Cv:

continuouJ nrlcanisation

f".

R-?-o-o-f?"'-R

Peroxide

CH,

CH.

cH.

Primary radical

t-R-9-9'

+ CH.

o

I

cH.

tR-C:O

CH"

+

+

- cH2-CH2-cH2-cH2-

cHr-cHr-cHz-cHr-

Po

I t

I

Lr|.

R-c-oH + I

cH4

I

+

- cH2-cH a -cHz-cH2O

CH.

- CHz-CH -CH2-CH2-

Polymer radrcal

-t

I

I

- cH,-tH-cH2-cH2- cH2-cH-cH2-cH2 -

Barliial combination during network formalion

Fig.2.7

i

I

Cross linked Pol'Tethylene

T

Cross-linking of Polyethylene by organrc perortdcs

T

I I

lnterml enl drive unil

T I

I

Tension

conlrcl unit

b

I."

Cooling

tit

l0ne

or

'f t

T Tube length approx 125 m

Fig.2.8

I

Continuous cross-linking in a steam tube (CV process) I

Y I I

2Insulation Compared to vulcanisalion with steam, these methods permit crossJinking at higher temperatures and lower pressures. Cross-linking by Electron Beants

The polymer chains are crosslinked directly by means of high-energy electron beams, without thc necessity for the heating stage which is essential with peroxides. It will be clear from consideration of the

reaction sequence in the cross-linking of polyethylene by electron irradiation, as illustrated in simplified form in Fig.2.9, that in this case also gaseous reacrion products are formed (mainly hydrogen). Cross-linking by Siloxane Bridges

Polyolefines can also be cross-linked by means of siloxane bridges, u hcreby suitable alkoxysilancs are radically grafted into the poll,mer chains. In the presence of moisture and a condcnsation catalt st. hvdro-

"- CH z-CHz-CH2-C H, *CHz-CHz-CHr-CH.^^,

lysis takes place to form silanol groups, which then condense to the interlinking siloxane bonds (Fig. 2.10). Because the grafted silane can contain up to three reactive alkoxy groups, this offers the possibility that bundled linking locations can be formed.

Although as regards the chemical structure of

the

cross-linking bridges the cross-linked polymer matrix appears to be quite different from those produced by the methods previously described, a combination of characteristics is nevertheless obtaincd which essentially corresponds to that of the crosslinked PE produced by the classical methods.

Like all polyolefines, crosslinked polyethylene is subject to a time and tenrperature-dependent oxidative decomposition, and it. has to be protected against this by the addition of anti-oxidants, so that il can uithstand continuous service at 90 "C over a lonq period of time (see page 27).

Polyethylene

t

leo I

Y

H.

+

-CHz-CH-CH2-CHr^ a

,

*cHz-cHz-cH2-cH.^, J

lao

Formation of polymer radicals

lI

Y

* C Hz-CH-CH2-CH.^,., H.*

a

a

-, CHz-CH-CH2-CH"I

Y

l

Eadical combination during nework lormarion

1)

* CH:-FH-CH2-CH, ^I

^^,CHz-CH-CH2-CH"-,

Cross.linked Polyethylene

Fig. 2.9 Cross-linking of Polyethylene by electron beams

Elastomers 2.1

HrC. H.C.

\

-CHr

+

2

/

cH,

OR

Hrc:cH-si-oR \

HrC.

Polyerhylene

cH,

OR

H.L

HrlGrairing

{Radical initiation)

/o^ "."a'cH-c{2-cq2-si-oR oR *"a.

RO

"tta. r, i'i2

Rojsi-cH,-cHr-c H cH,

cH"

+

Hydrolysis

H2O

(caralysrl

-2ROH

H.C CH. H

CHz

Fis.

2.10

Cross-linked Polyerhylene

Cross-linking of Polyethylene by Siloxane bridge method

2.1.2 Elastomers In contrast to the thermoplastics. the molecule chains l[ elastomers form an extensive meshed networli' This cross-linking, or vulcanization, gives rise to the elastic nature oIthe material: a large reversible extension in response to low tensile stressElastomeric materials are used lor insulation and for sheaths. They are applied mainly where the product has to be particularly flexible.

A wide range of

elastomers is nowadays available to the cable industry. This makes possible the manufacture of compounds with specif-rc properties, such as high abrasion and oil resistance, weather and heat resistance and flame resistance, combined with good overall elecrrical and mechanical chlracteristics.

The classical elastomeric material. natural rubber' hls declined in signific:.rnce in recent yerrs' In its place. the synthetic elastomers. produced by the copolymerization of ethylene and propylene, are conitantly finding new areas of application in cable engineering. These ethylene-propylene copolymers' known under the general term EPR, contaln no dou'

by ble bonds, and cannot, therefore, be crossJinked unsathe to rhe vulcanization methods appropriate turated rubbers (e.g. natural rubber, styrene butarubber). On ihe other hand, because of the

diene absence of iouble bonds in the main molecular greater chains, these elastomers have a significantly to resislance to thermooxidative decomposition and heat' and ozone rhe effect of ultra-violet radiation'

2 Insuladon IJ

Fig. 2.11

I

i-i*

I

H

Structural form of EPR and EpD

EPR

HH

tl

EP0i\il

with Erhylidiene

Y_Y tl

as Iercomp0nent

HH

With the incorporation of a dienet), EPDM elas-

ene butylene blocks, which are so struct.ured that etl

tomers are obtained (Fig. 2.1 1), in which the doublebond active in cross-linking is arranged not in rhe main chain but in a side -eroup.

ylene butylene chains contain styrene units as en blocks. Polyesters and polyurethanes wirh TPE prol' erties are also known.

Thermoplastic Elastomers (TPE)

:'

Technically interestine combinations of propcrtics can be obtained through thc admixture of rhermoplastic olefines. e._1. poll propylene rvith ethl lene propylene elaston.rers, or bl rhe direct production of socalled pollolciine block poll'nrers. Such copolvmers of ethl'lene and propllene *'ith a block structure consist of an EP elastomer phase uith crlstalline homopoll mer end blocks. *'hich represent the unstable reversible cross-linking centres. At temperatures above the cr]'stallite melting point. rhese materials have thermoplastic propcrties: belou thc cr1'stallite melting point the),behave as elastomers. Polymers of this kind are therefore called thermoplastic clastomers

(TPE):'. Another class of thcrmoplastic clastomers is represented by three-block polvmers o[ styrene and ethl l-

Other types of elastomers used in cable engineerir are polychloroprene, chlorosulphonated polyethl enc and chlorinated polycthylcne, which. because t thcir advantaceous properties in relation to enviror mental influences. are prcfcrably uscd as shearhin matcrials. Conducting Rubber

Through the addition of conducting fillers, e.g. ca. bon black. natural rubber and syntheric elastom. compounds riirh a resistivity of from a few Qcm u to several thousand C2cm can be produced. Conduc ine rubber compounds are generally used in the mor itoring of flexible and riiring cables in mines. an also for inner semiconductin-s layers and held limi ing in s,"-nthetic elastomer insulated high-r,ohag cables (Ozonex principle).

Natural Rubber (NR)

Natural rubber is obrained in various counrries i rhe equatorial belt from rhe rubber tree (hevea bras: The drencs used as tercomponenls are spccial hydrare mrlcrials *ilh double iinks tahich arc non{onjugatcd ln ror:re Fublicerions for thcmoFllsric cllslomcrs rh. !bbrclia(ion TPR

rj u*d

as prcviousl]

liensis). This tree contains in the cambium cells unde

its bark a milkl juice (latex), which flous our whe the bark is cut. The rubber is obtained from th through coagulation with chemicals, electro coagL

Elastomers 2.1 {

lation or by other methods. The resulting rubber is supplied to the manufacturer in smoked form as 'r*ok"d sheets' and in chemically bleached form as 'crepe'. Rubber is a hydrocarbon of high molecular weight with the monomer unit 1,4-polyisoprene' with the addition of vulcanization and aging-protection additives, specially selected fillers, and where appropriate by blending with synthetic elastomers, insuiating compounds for cables and compounds for the sheathing of flexible and wiring cables can be manufactured.

Unlike synthetic elastomers, natural rubber has to be subjected to a so-called mastication process during

rnufacture. to make it receptive to the additives a,rd to obtain the required plasticity in the compound. The significance ofnatural rubber in the cable industry has declined sharply in recent years in favour of synthetic eiastomers.

hand, i lower isoprene content lowers lhe rate of vulcanization and makes the product less elastic' The relatively smali number of double bonds makes butyl rubber less susceptible to the effects of oxygen and ozone. The main advantages are very low water absorption and low gas permeability. The good heat resistance permits operating temperatures uP to 90 "C with suitable compound structures. The mechanical properties can be improved by the addition of special active fillers; plasticizers, for example, im' prove the elastic properties, particularly aI low temperatures. Since EPR and EPDfvt synthetic elastomers have become available, butyl rubber ist used onl;' in special cases. Ethylene-Propylene Rubber (EPR)

EPR is uscd su

Styrene Butadiene Rubber (SBR)

SBR is a copolymer of styrene and butadiene' referred to as either a hot or a cold polymer according to the method of manufacture. Cold polymers. rvith the normal st)rene contcnt ol 2'lol, (by rveight) arc characterized in comparison rvith the so-called hotrubber t-vpes by higher tensilc and tclrr strength rnd bctter rvorking c h arac teristics : thel irrc thercfore prcferrcd as admixtures used in thc production of SBRNR compounds. SBR and SBR-NR mixturcs trc suit:rble for usc ts insulation in lorv-voltagc fleriblt' and,,r'irins cubles for operxting tenlpcrlltlres up to 60 "c. Nitrile Butadiene Rubber (NBR) Through the copolymerization of acryl nitrile nith hutadiene. ellstomers are obtained rvhich are distin.,-rishcd in comparison rvith the SBR types b-u" high oil rcsistance and good rveather resistance. For this rc:lson thev are preferably used for sheathing compo u nds.

By mixing rvith PVC, NBR-PVC blends are produced rvhich have better flame resistance.

Butyl Rubber (IIR)

Butyl rubber is a copolymer of isobutylene and lsoprene. To permit vulcanization, an unsaturated componcnr of 1.5 to 4.5% (by rveight) is introduced. Thc lorier the isoprcne content. thc less is the ertent to rvhich thc rubbcr lgcs under hc:.rt i on thc othcr

ls:r

general designation for the tr"o

b-types

ethylene-propylene rubber (EPR) and erhylene-propylene terpolymer rubber (EPDM).

Ethylene-propylene rubber (EPR) is a copolymer of lorv density rvithout C-C double bonds. i.e. it rs a completely saturatcd polymer rvhich. likc polyethylene. can only be cross-linked radically The further dcvelopment of this saturated rubber to EPDM (Fi-s. 2.11) through the incorporation of dienes rvith hrcral doublc bonds pcrmits a conventional sttlphur vulcanization as ',vell as radical cross-linking. e.g u irh pcroxidcs.

There is litrle ditterencc bcnvccn cross-linked EPR and EPDivl as regards mechanicll and electrical properties. Peroxide - i.e. radical - cross-linking. ho$ever. gives better long-term hext resistance and better heat pressure charucteristics than sulphur vul-.*caniza lio n.

Outstanding characteristics of these elastomers are resistance to ozone. oxtgen and ionization. good flexibility at low temperatures and high resistance to $eather and light. Because of their good dielectric properties EPR and EPDVI. depending on the struc' ture of the compound. are suitable for insulation at voltages up to 100 kV, rvith a maximum permissible oC. Such insuservice temperature between 80 and 90 Iating materials will wirhstand temperatures up to 250 "C without damage under short-circuit conditions. Cables rvith ethylene propylene rubber (EPR) manu-

rlcturcC by Siemens are knorvn by thc protected tradc name PROTOLON. 25

i

2lesulation Blending EPR with PE enables the mechanical strength and hardness to be increased significantly ('hard grade'). The insulating materials so produced closely resemble the elasticized polyethylenes in their combination of characteristics, i.e. they exhibit, as well as the improved mechanical characteristics, improved electrical characteristics, similar to those of polyethylene. They are known by the abbreviation HEPR.

Silicone Rubber (SiR)

Silicone rubber is produced by the polycondensation of hydrolyzed dimethyldichlorosilane and methylphenyldichlorosilane. The macromolecules in this case consist not of carbon chains, as in most other polymers, but of silicon-oxygen chains (Fig.2.1?), uhich is the reason for the rcmarkably high heat resistance.

During processing, fillers are added to the silicone rubber, together with organic peroxides for the purpose of crossJinking (vulcanization). The end product is characterized by a high heat resistance. Because

of the excellent insulation properties and the practically unvarying flexibility over the temperature range from -50 to +180"C, flexible and wiring cables insulated *irh siliconc rubber crn bc uscd contin'180'C (up uously at conductor tempctatutes up to to 250 "C for short periods).

Thc

silicone-rubber-bascd SInNOTHERM compounds manufactured by Siemens have outstanding eiectrical charactcristics r.vith good resistance to ozone. They are inscnsitivc to moisture and exhibit good rveather resistance. The-v are thus suitable for both insulation and sheathing. Another preferred arel of application is that of accessorics.

Ethylene Vinyl Acetate (EVA)

EVA is a copolymer which is used either as a t. moplastic (vinyl acetate content < 30%) or, with s able crossJinking, as an elastomer (see Fig.2.,l structural formula). The properties of an EVA cc lymer are in the main determined by the ratir vinyl to acetate content. Cross-linked EVA e tomers are characterised by good heat resistance permit conductor temperatures up to 120'C- T also exhibit excellent resistance to aging in hot and superheated steam, together with very satisfa ry heat/pressure characteristics (see page 16), part larly at high temperatures. In addition, EVA c, pounds have outstanding resistance to ozone and ygen, weather resistance and colour stability. The namic freezing point is in the region of -2{ - 30 "C. The application of EVA as an insula material is limited by its electrical characteristic the low-vohage range. The compounds are used heat-resistant non-sheathed cables and flexible co and for heating cables.

.

2.

1.3 Thermosetting Polymers (Duromers)

Unlike the elastomers, thcrmosetting polymers usually closely crosslinked and in general have be t. ',vear resistance and dimensional stability than moplastics and elastomers.

Thc application of the thermosetting pollmers as sulating materials is limited to the use of epo: and polyurethane rcsins for thc {iiling of cable ac sorics. Filling resins bascd on epoxides are preferr converted to the thermosetting state bv heat-cur Poll'urethane resin materials, on the other hil harden at room temperature, and so offer advanti. in application techniques. Both types o[ resin are : able for outstanding adhesive strength. particulr to metals.

A suitable choice of

resins and hardeners enabl uell balanced combination of thermomechanical

electrical pfoperties

to be obtained, together r

good chemical resistance.

ftttl QH" 9H. I

-Fo-gi-o-qi-o-fttrl I CH3 R L (R

Fig. _:o

=

CH, or C,H,)

2.12 Structural form of SiK

-Duroplastics ' Chemical Aging 2'2

2.2 Chemical Aging of PolYmers is understood lhe change in the Bv the term'aging' -maierial with time Polymers are subof a "ioo.it'i., iect'in use to chemical changes which have an adverse ,e-i;;;;; their mechanical and electrical characterisncs.

with The chemical aging processes are accelerated to necessary increasing temperature. It is therefore temperap.ot..t pily..tt which are exposed to high iur.t Uy means o[ stabilizers' in order to ensure an from adequare service life for the products made them.

t

particularly.'. sheaths tha( are exposed proto direct sunlight (UV radiation) must be further rected ag;rinst this effect by the addition of so-called lieht stalilizers to the compounds' Carbon black has

rlrtion lnd.

'.

vc-d

to be an excellent light stabilizer,

especialll"

lbr polyoleiines; an addition ol2 to 3% (by $cigh[) rineiu divided carbon black, well distributed' tords an effective Protectlon.

oi

al--

In the prescnce of atmospheric oxygen, thc chcmical aging of many polymers' e.g. the polyolefines' arises from oxidation processes rvhich are provoked' or accelerated. by heat and light' For thc purpose of stabilization. anti-oxidants are added to thc polymcrs ln a proportion, normally' oi 0.1 to 0 5% (b1" neight) Asins relcrions. espcciulll oxidution. itrc ltccclcrl(r'd cltall-ticall.v by the presence of some metals This is parricularl; marked in thc case of contact bct\\'cen coppcr and polyolefines. in this situation anti-oxii rnts oftcr no apprecicble proccction' tn practice. to avoid direct contact betlveen pollolchnc insulation and copper conductors. separators are otien introduced. e.g. plastic hlms rvith sufficient stahilitl in cont:-rct sith copper. or tinncd coppcr con-!.lctors itre used. In mediunl and high'voltage cable the tield-limitrng conducting layers act as separltors' Conductins compounds xre protected against the eft'ect of direct contact lvith copper by their high carbon black content. Where there is direct contact betrveen polyoleFtne insulating materials and copper conductors. metal deactivators are added to the insulating compounds to counl.eract the catalytic effect. It has been possible to demonstrate by practical aging tests that by this means a sen'ice life can be achier.ed which is compa-

Evaluation of the aging properties of polymer cable life' materials is based on two quantities: the sertice a material which for time *ii.h d"oot.t the period of up to remains serviceable' and the temPerature /imi'' ;hich a material can be used subject to given boundary conditions. These two quantities are interrelated' so that an increase in the temperature limit results in a reduction in the service [ife' In determining the corresponding pair of values for the service life and the temperature limit, the changes in the signihcant characleristics lvhich are necessary for the iunction of the material must be examined as functions of temperature and time up to point rvhere an end criterion is reached. The choice of characteristics and of the end criterion determine the re-

It

f i

I

sults.

In clble engineering the terr strength is in most clses adopted as the essential characteristic, and as an end criterion, on practical reasons' the attainmenl of a .."particular elongltion value, e.g. en=50% (residual elongation).

Tf f

T

DtN VDE 0304 contains guidclines for the dctermrnrtion of the thermal stability oI electrical insulating

i

mlrerials. the revised edition of December 1980 being a rcproduction oi IEC 216 (1974).

:

According to DIN VDE 030'1. under the designauon ' tcmpcraturc indcx ' (Tt ). a te mperaturc limit for a scrvicc liic ot'20000 h is stipulated Tli l6J' for example. signifies that the material for which it is quoted rcmains scrviccablc for 20000 h under lnl thermal strcJs a! tcmperaturcs up to l6'l 'C' To obtain the tempercture index (TI). expcrimentalll' determined pairs oI values for a rirnge of temperatures - the test temperaturc l' and the service life r5 - lre plotted on a grlph trith time on a loecrithmic horizonial aris and the reciprocal of the absolute temperature on the verticai axis. A straight line is drawn through the plotted points and bv- ertrapolation gives the required temperature index' ln the grrphs of Figs. 1.13 to I I6 the temperrture.values on the horizontal axis h:rve been converted to oegrees Celsius ("C) for erse of resdtng.

F

--

rrble riith that obtained in compounds not subjecr to contact $.ith coooer.

21

Chemical Aging 2.2

rc6.

h. 301 20

2

1

rcl o> Yeats

10s

b

4l

,l,I

2

1J

100

8l Months

2 101

1

6 4

?0 10

2

\

6

02 6

Days

2

\\. 2

10'

6 4 2

I

4b dc $

20

0.6 0.4

do120{0160 2oo 25o"c3so

40 60 8b 100 120140160 200 250 "C J50 TemPeralute-

Temperatute

Curvc

I

Curvc

2

..to.

Normll peroxide crosslinked compound for mcdium-voltage cablc Compound with metal dcactivator for lo$volrage clble with copper conductor

Fig. 2.15 Service life

of EVA insulation comPounds

2.1.1

,{vice

liie of XLPE insulation compounds

Fig. 2.15 shows thc temperature dependence of thc service life of an EVA insulating compound' Aging took place in contact with tinned conductors; the end crirerion was eq: JQo/o and the temperxture index (Ir) was 117 "C.

29

I

lDSUlaUOn

2.3 The Influence of Moisture on Polyolefine Insulating Materials

rc5 h

4 30 20

2

rcs

10

6

b

2

2

10"

6 4 ?

l.Months

2

l0j

,|

6 20

1 2

\

10

6 101

0ays

6 2 1

2

,

Practical experience and long-term tests on mod^, cables have shown that water has an adverse effer on polyolefine insulating materials such as pE anT XLPE. Using appropriate dyeing techniques, it;. possible to observe tree-like structures in such mater als subjected to electrical stresses. These originatd from practically unavoidable microscopically smalr fault locations and run in the direction of the electr field. This phenomenon, known as 'water treeing (WT) is quite distinct from 'eiecrrical' tree formatio=n (electrical treeing, ET) caused, for example, by ior izarion.

1o'

The mechanism whereby WT structures arise has nc so far been clearly explained. Because the WT growr is influenced by many lactors besides water and elecl tric flelds, and these processes rake a long time, inve: tigation is very difficulr and time-consuming. t-no

ing the finer points, WT structures can be c*ded into two groups (Fig.2.17):

6 1 2

D

1

tr

0.6

TemPeralute.....--..-.-

Fig. 2.I6 Service life of EPR insulation compounds

Fig. 2.16 shows the scrvice lifc characreristic for an EPR insulating compound for 0.6,/l kV cables. The values uere obtained from insulating coverings in direct contact rvith copper (l0yo elon-eated conduclors); the end criterion $as cR:100% and the rempirature index ( I/1 rres I I3 'C

'bow-tie trees' in rhe interior of the insulation _ 'vented trees' originaring from rhe boundaries of the insulation.

Because of the low concentration of moisture in rhF intcrior of the insularion, the growth of .bow-ti. trees' is slorved do*n. so that they usually remai

small (Figs. ?.18a and 2.18b). The serviceability oTcablcs is thcrcfore only rarch impaired b1..bo*,-tr. trces'.

'\'cntcd rrccs' (Figs.2.l9a ro :.190 r"quirc .norl

critical asscssmcnt. Thcsc can cxtcnd right rhrougi the insulation if sulficicnt rvatcr is available. In rhi \\a) thc clecrrical srability of rhe cablc is gradualll: rcduccd. until a breakdorvn of the cable is in, te,'

ourer conducting layel

Point oi

)K. Inner conducting layer

A\N

,,80w'tie

Conducror

Fig.2.11 Diasramatic example of WT structures

l0

Influence of Moisture 2'3

Fig. 2.194

" Vented tree ". grorving from the outer graphited conducting lay-er o[ a PE cable

Fis. 2.18

r

'' 6o*-tie rress" in a cable rvith XLPE insuhLion (magnihcation 1 : 100)

, t

Fig.2.l9b PE cable from the earll dlls of PE technLquc. " \'enrcd trcc". grosing from thc outer uraPhitcd conducring

la1-cr

,

8.

Fig. 2.18 b PE cablc' from the

'' B()\\-.!ic trccs" of

clrlv days of PE techniquc rvith hiei dcnsity (magniiiclrtion I : !60)

ll

Fig. 2.19d " Vented tree " (lcncrh approx. 700 pm) on rhe\.der extruded conducting layer of a 20 kV XLPE cable afler several years operation $ith water inside the cable (magnification I :135)

Fig.2.19c PE cabl: fronr rhc elrlr. davs of PE technique.

'\:nted rree" srorring from the outcr srlphited .-onductinc It\cr. {Thc picture ',r,rs constructed fronr i\\ o photociaphs

)

Fig. 2.19e " \/ented tree" (length approx. 50 pm) at rhe outer extruded conducting layer of a 20 kv XLPE cable after se\eral vears of operarion (maenification: 1 :1i5)

:r

Influence of Moisture 2.3

by the conversion of the water tree into an electrical tree (Fig. 2.19l). for Experiments on cables that had been in service of lack of a result as which' oUout.igttt years. in con' to the penetrated care in iristaliation, water had ductors and the screen regions, have confirmed the deterioresults of accelerated laboratory tests on the of insustrength ralion to be expected in the electrical lation. In this lonnection. Fi-s' 2'20 shows, by means of Weibull statistics (a method of evaluation speciall)" developed for the physics of breakdown mechanismsl, the determinid residual strength as dependent on the nature of the applied voluge' which by linear probresression of the measured values plotted in the ratec rhe of 2oh abilit-v diagram is established as 63 lvatet value. lt cin be seen from this illustration that in the conductor has a particularly unfavourable el'fect on the insulation [2. 1]. This knorvledge has given rise to the follo"vin-e measures for rhe construction. manufacture and installation of cables with PE or XLPE insulation:

a) minimization of fault locations in insulation

and

al the boundaries of the conducting layers' i'e': tr optimization of the purity of the insulating and conducting-layer materials and the cleanliness of the manuf;rcturing process:

D

extrudcd conductins lavers to bc preferred'

conrcnt and prcvcntion of ln' gress of moisture. i.c.:

b; rcduction of sltlcr

tr

tr

prevention of ingress of water into the conduc' tors and the screen region in mlnutacture' storage' transport and installation and in service (e.g. through subsequent damagc to the sheath). use

of mechanicitlll resistant outer sheath' e g'

of PE.

Fig.2.l9f Structure change from WT to ET at the top of a " Vented rree", XLPE cable aiter 6000h "Water treeing test" with 5 kV/mm and water in the conductor and following short-time stressing with approximately nine times operational field sfength. (The picture was construcred from trvo photographs)

D

provision of lcngthsise rvirtcr-tisht screen tegion to limit the ingress oI rvater in the event of damage to the sheath.

tr

in high-voltlge cable. for > 36i60 kV' the use

of a Iaminated aluminium sheath and a lengthrvise water-tight screen region' In addition to this. intensivc development is in progress to increase the resistance of XLPE insulating iompounds to WTt) bv means of additives [2 3]' r, In lFr hlc:trurc lLo rcicrrc,.l t..) r "WJtcr trcc rctrrdcnt

compouno

r\\ TP. rcmoourd)

-rJ

Probabiliry of failure P

aoa

/_"$_____

I 24

,rf:t I

ll

JI

Test series Percenr rared

kV/nn 7t )

varue %

12

100 157

0.1

100

lor

kV/mm

Vollaqe qradienr

rc2

a) a.c. \'oltage

I

102

f.-..........-

kvimm Voilage gradienl 6

b) Impulse voltage

Nerv condit.ion, dry. not prcstrcsscd

Water in the conductor and bclow thc shcath of thc cable after eight years in opcration Water under the sheath of thc cablc aftcr so.en )ears ln oPeratron

oJl 50

Ware..at the undxmt{:cd sherth aftcr ninc lcars in

operatron

t0

23 Jestseres Fig.2.20 Breakdosn stren,sth

1i 2)3|

eel:elr iared kV/mm. 150

varLe vo

of t0 kV pE cables.

\\'eibull-Disrribution : Probabilirv of lailure p relarir e to mcan voltage sradrent f (break-doun rolragc dir.ided bl thickncss of insulation)

.rl

4

100

,, i0 , 47

i

4

g9 | 116 59 I 78

0.r

tc

102

hV/mm

Voltage gradrenr F

c) d.c. r'oltale

-

Impregnated Paper ' Bibliography 2'5

_2.4 lmpregnated PaPer impregnated paper was used for conductor insulaoi th" last century' It made possible tion ai ttre "ni of cables for higher voltages' Be-the manufacture pirper lnsulilcause of its good dielectric properties' to lne tion is still indispensible for cables used al up -hiehest operating voltages customary today ln the loiv- and medium-voltage ranges' however - up to polymers to an ever30 kV - it has been replaced by development - increasing eKtent in recent decades' This to the highalso increasingly is noru bJing extended voltage range.

I G '. papcr consists of the purest possible -

long*ta-

It is oled ccilulos". obclincd from northcrn timbers' pro,lso knort n ls sudium ccllulosc papcr' lrom the insuarc ."ss by rrhr;h it is prepared. The conductors Ln ivith this special high-quality paper to the thick' n!-. required tbr the ratcd volta-ee In the casc ol prothc higher'voltagc cables, it is advantageous to scrcen I vide conducting pilpers on the conductor and of metallized paper on the core. rvhose thickness' up to ir certain iimit' according to DIN VDE 0225' is counred as part of the insulation thickness' The single- or multi-core clble asscmbly' accordins to the cable construclion. is dricd in an imprcgnltLtng txnk and then impregnatcd rvith a degasscd and dried impregnating medium f impregnrting compound') appropriate to lhe intended purpose of thc cablc' Papcr-insulated c:.rblcs arc dividcd according to thc ncthod of imprcgnation into mirss-tmpregniltco clblcs and oil-llllcd c0blcs. Cables lvhosc lnsulatlon I rillcd aftcr instlllation *ith nitrogen undcr pressltre crtblc 2q; knorr n its internrtl gls pressure

tion from the liquid to the semi-solid state' and are prevented from flowing in the permissible service temperalure range by the microwax structure'

- like all dielectric polybutene compounds - have outstanding propenies, even after long periods of service

In addition,

these non-draining compounds

For extra-high-voltage low-pressure oil-frlled cable'

a lorv-viscosity gas-absorbing impregnant is This may be e mineral oil rich in aromatic

used'

combenwith alkyl pounds. a naphtha-based mineral oil good )ene additivei or an alkyl benzene, ensuring gas absorption in an electric field at all service temOther i.r",ur.t. especially in regard to hydrogen' recharacteristics oI these impregnants are adequate in srvell to tendenc.v little sistance to oxidation and used' the presence o[ the sealing materials

2.5 Literature Referred to in Section

2

Krmmel, C : Sunderhauf. H: Lringswasserdichte Kunststoffkabel (Lengthrvise watertight clbles)' Elektrotechn. Z. (1952) No 4, pp' 173-176 f2.ll Kulkner, W: Miiller. U; Peschke' E F: Henkel' H.J.: Olshausen' R.v.: Water treeing in PE and XLPE insulated medium and high'voltage cables' Elektr.-Wirlsch. 8l (1932) No' 26, pp 9l l-9?2 VPE[1.i] Pcschke. E : Wicdenmann. R': Ein neues Water-tree-retardiermit \littclspannungskabel cndcr ilTR-)isolierung (A nerr XLPE mcdium(WTR) inr oltage cable wilh water-tree-retardant (19S7) No 6' 36 .uhtilon). Elektr.-Wirlsch

Il.ll

uou -r ollitgc. mcdium-r oltlge lnd urternitl gits prcssure cebles are impregnated rvith high-viscosity pol!-

'

'|tcnc compounds. r'hich have very lorv dielectrtc ,.sscs and erccllcnt irging charucteristics' trom .low to vcrl high opcrating tempcfatures. ln compilrtson ri ith thc oil-resin compounds produced tiom natural or svnrhetic hldrocirrbon restns.

The viscositl of the impregnating compound is chosen in such a way that small differences in level do not cause the compound to migrate.

For special cables to be installed on steep slopes. 'non-draining cables' and internal gas pressure cables. special compounds are used, known as nondraining or nd compounds. These consist oi polybutenc modificd by the addition ot' selected microcrys-

tallinc'r',lxcs: thcv shrink onlv slishtlv in thc transi-

i5

Experimental installation for the investigation of the influence of water on polymer insulating materials in medium- and high-voltage cables

,.:

;.i;:' ;;

i

\ '\

Protective Sheaths ' Thermoplastic Sheaths 3'1

3 Protective Sheaths

A distinction is made in the DIN-VDE specifications between sheaths and protective coverings or outer of thermoplastics or elastomers' -coverings Protective coverings and outer coverlngs serve as corrosion protection over a metal sheath or as light me-ct"nical protection for flexible and wiring cables' !i..*rexs shetths are dimensioned for greater mechanrc l stresses. Since the Droperties of these components ltre slmllcr 1 ior dimensions' only rhe collective term "'s..cxth' is uscd in rclation to cablcs in the follo* ing

-

section.

3.1 ThermoPlastic Sherths Pol-vvinyl Chloride (PVC)

PVC-based compounds arc uscd prcdomintntly as n sheathing material for po',ver clblcs and for llcxiblc lnd rriring cables becausc ol thc mlnl ad\'ltntil{cs

thcl ol'ilr. The thermoplastic sheath is extruded onto the c:Lblc corc assembly in a proccss providin-t a seamless sheaths havc a cleln. '"cr. Cablcs rvith PVC outer

iroth

surface.

The PVC compounds combine high tensile strength lnd elonr:arion. pressure stability even in high-tem' . .rrturc reglons. resistlnce to prlctic lly lll chenriclls in soils and most chemic:rls encountered in chemicll plants. and especially flame resistance and reslstlncc to aeing. The sheaths used in PROTODUR cablcs are characterized by their comparative hxrd' ncss. toughness rnd adequirte pliabiliry from the point of vierv of bending at low temperatures (see page 18).

The PVC outer sheath proved over many years for por"er and wiring cables in fired installations is also used. in a suitably softer form, for flexible cables. Llshr and medium PVC-sheathed flexible cords have bL'en introduced satisfactorily for household equipmcn! bccrusc ot'thcir cicar irnd durable colours and

smooth surlaces. PVC-shearhed flexible cords are nor suitlble for use at low temPeratures. in the open atr

or in heating appliances (e.g smoothing irons)' in rvhich the cable can come into contact with hot parts:

elasromer-sheathed cords should be used

in

these

cases.

Poll ethl lene (PE) Practical experience in supply'authority systems has shorvn rhat in many cases medium-voltage cables laid in the ground are subjected to considerably higher mechanical stresses than was originally assumed' Be' cause of rhe danger presented to cables by the pene' tration of moisture. an undamaged impervious outer sheath has a decisive effect on the life expectancl" of PE and XLPE insulation (see page 30). A mechanicalll resistant PE sheath is therefore increasinglvprelerre,i. especially' for medium- and high-voltage cables *ith XLPE insulation. A PE shearh is recommcndcd in thc ncrv spccification DIN VDE 0271 37 rbr XLPE cablcs laid in thc ground

Thc disadvantagcs of thcse mlterials. such as llamm.rbility. greater ditticulty of handling in instailatton' interior adhesion to the miltericls normally used in rueccsories lnd greater longitudinal shrinkagc' are lcccpted in vierv of their grei'tter hardness and abrasion ."ria,rnaa. From considerations of resistance to UV r;.-rdiltion and environmcntitl strcss cr:rcking' onl; black PE sheaths arc permitted. Thc mosl significant fuctor in the choice of the base poly-mers is the temperature to be expected in normal service. The screen temperature to be expected under fault condirions (see page 286) should be allowed for by suitable constructional measures.

Particularly advantageous is the combination of a PE sheath with the measures described in Section 7 3 tbr the sealing of the screen region of the cable againsr the ingress of moisture. ln connection with the leading-in or laying of cables in intcrior locations. ir must be rem!'mbcred th;lt PE 37

:

J

l'rotecrrve Sheaths

sheaths are not flame retardant. Where necessarv. appropriate fire protection measures should be adopted at the site, e.g. spraying or painting the cable with a flame retardant protective coating.

to weathering, chemicals and heat Siemens have developed special synthetic elastomer compounds for use as an outer sheath material.

_

Polychloroprene (PCP)

Poll'amide (PA) and Polyurethane (PUR) Polyamides are polycondensarion products with linear chain structures made up of dicarbon acids and diamines or aminocarbon acids. Polyurethanes are polyaddition products with a chain-formation to spatial structure of di-isocyanates or polyisocyanates and dialcohols or polyalcoholes respecrively. Flexible and wiring cables subjecred ro parricularly high mechanical stresses or to chemical influences, e.g. from benzene or agressive, mostly aromatic oils (e.g. coaltar oils), are provided with a protective layer of polyamide or polyurerhane or/er the sheath or the insulation. These two materials are distinguished mainly b1 outstanding mechanical propcrties and good resistance to oils, fats, ketones, esters and chlorinated hldrocurbons. Pollamide protective covcrings arc applied to, among others. flexible and wiring cables for use in mineral oil extraction and in aircraft. Poll'amides are not suitable lor use as insulating materials, on account of their poor dielectric characteristics. but because of their high abrasion rcsistance and touqhness, together with their good resistance to organic solvents and fuels thc! are used as sheathins materials for special flcrible and rviring cirblcs. Polvurerhane sheaths har.e high inrpact rcsistancc. high fleribility at low rentperarures and,sood abraslon rcslstance.

A polymer of 2-chlorine-butadien shows a good resis_ tance against the influences ol light, oxygen and ozone and a very good resistance against cold, heat and flames. Its excellent resistance against chemicals, which is very high for a elastomer deserves special mention.

It

has, therefore, particular advantages for use as

a basic material for sheathing compounds.

The cables and flexible cables manufactured b1,Siemens uith a sheath based on polychloroprene are kno* n under rhe rrade mark PROTOFIRM. The mechanical srrength of the vulcanized compound is very hi-lh. therefore. these cables have an increased service life under mechanical stresses of any \_J. PROTOFI RM sheaths also offer advantages u.hire good resistance to seathering. flame rctardance and a certain ammount of resistance to oil is reo uired. furthcrmore where a clastomer is preferred to pVC compounds bccausc of its higher flexibility, rcsistancc to abrasion and tear cxtension.

-

Thcsc shcaths, thercfore. arc particularly suited for flcxiblc cablcs in undcrground mining applicarions and loc:rtions rrith fire hlzard Chlorosulphonl

I Potvethllenc (CSII)

is produccd bl,chlorosulphonation of pol),crh1lc'nc. The parrly cr)srallinc polycthylcnc is in rhis CS i\'1

Poll propl lene (PP)

is of lcss importance because of its brittleness ar lo\\' tempcrature and its special sensitir._ itr to t hermo-oxida tir e deteriorltion. eipecilllv rvhen in contrcr uirh coppcr. can onll bc emploved undcr Poly'prop1.'lene

limired conditions.

3.2 Elastomer Sheaths

ln

the Federal Republic of Germany, apart from use in * iring and halogen-free coblei *ith im.ships proveo propertres under fire conditions, elastomer sheaths are only used for rviring and f,lexible cables. Because natural rubber has only limittcd rcsistancc -1O

process transferrcd into an anrorphous clastonter. The cross-linkins can be established by using eirt-^r radical or conventiontl special sulphur compounu-. CSNI is alailublc:rs an industrial prodtrct undcr the tradc mark HYPALON (N4anulacturer: Dupont de Nemours International S.A.). Both. propertics and the ranqc of applicarion corrcspond to thosc of polvchloroprc-nc (PCP). hos,cver. CSlvl has improved properties as regards colour lastness and resistance to heat.

-

-

Chlorinated Polyethl.lene (CM)

CM is a new sheath compound with characteristics virtualll identical wirh those of HYPALON but u,ith rcduced flcxibility at lou' temperirtures. When blcndc'd g ith oth!.r !'lastontcrs soccial comoounds

-

Sheaths of Rubber for Special Purposes'

_

-

-

EPR or EPDM can be produced, e.g. by the use of tl" at low temperarure is improved' 9r',by t'lexibility '"i'tiiur-i",adien rubber the oil and fuel resis-

"t" tance is imProved'

Nitril-ButNdien Rubber (NBR) immersed in oil Cables which are ro be permanently on o.. o-tia"a with an oil resistant sheath based predominantlv is rubber ttti", (NBR)' Nitril "iitii il"a"a'with and is known for its good "r.i ,..ir,on". to oil. This resistance to oil is based on ii. o"i".i,v of the nitril rubber molecule' Nitril ruboils U", it ,ft.t.f"re highl.v resistive to non'polarised highlv,olt.nt, but docs sri'eil considerably in "i. polarised solvents'

ivc

rylh*!::l

sheath NYBUY and PVC'insulated cables with lead as locutions with ;;;;J r- ntting stations as well cables also have' i"" ona explosion hazard: these mechanical damas protectio; against corrosion and ogi. uo oot.. sheath ol PVC' 17640 For the lead sheath a cable lead Kb-Pb to DIN vrthe agatnst is used which is sufficiently resistant materi bration whictr are normally present' The base at for this cable lead is pig lead Pb 99'94. tots DIN 1719. To avoid a coarce grain structure thts of copper blended with 0.03 to 0 05% (by weight) { I tole J, r,.

_ 3.3 Sheathing Nlaterials for Special Purposes cables Sicmcns have devclopcd cablcs and llexiblc (FlamcunO". ,ft" trade mark SIENOPYR FRNC n"i"tO.n,, Non Corrosive) which hlve particulirrly: namell importanr characteristics in the evcnt of tire'

>

reduced support

ol

combustion cvcn

rvhen

bunched

Trble 3.1 Cable lead to DIN 17640' A base metal of Pb 99.94 to DtN 1719 with an additional 0.001% lvlg must bc uscd

nation

bl thc usc ol'spccial shcathing mrtcrials

base matcrials are olelincopol-vmcrc such.:ls flamc-retardent qualities' - VA or EEA. To achicve thcsc mctcrii.tls being normally combustiblc' ccrtaln hvdratc containing mineral tlllers are used' Thcrefore - satisfl the aboic requirements itll other additivcs such as lnriitging a!:ents are halogen-frce'

Ahe

3.4 Nletal Sheath Lead Sheath

Insulation materials. sensitive to humidity' e'g tmprcgnated, paper are protected by a metal sheath' Since the bcginning of cable mlnufacturing leud, s hich is crsl io hlnJlc. hls bccn the proven mrtterill tbr this pLrrDosc. Lcud covc'rcd PVC'shelthed cablcs

Kb-Pb Te 0.0+

Abbreviation Uscd lbr

Cablc sheaths rvhich tre subjected to l high dcgrcc

Wcak ullol cable shcuths: busc metal

> tirmes do not contlin corrosivc sttbstanccs > grcrtlv rcduced snrokc dcvclopmcnt > rctcntion of instltiltion r', hicvcd

Tellurium lead

D.sig-

lbr manufacturc of liloy clblc shci.I

of vibrution

ths

Componenls in % (b! rveightt Cu

0.03

5b

L

Sn

0.0i to 0.05

-0.05

o.orto remlindcr to

rcmtindcr to t 00?6

Pb

\{lximum umount of rddiLi\cs in

70

Ag

0.001 0.001

I[

0.050

Fe

0.001

\tg Sb

0.001 0.005

Sn Zn

0.005 0.001

: Thir

"

lcrd smeLlers Cu uddi(ion crn bc omi(lcd by irgrconrcnt bct!\r'cn

rnd cablc m:rnuilcturlrl

uprol)05'; ' iirt-*"i".rrr riupumx\ rlso hrlc :rn so conicn( ol

l9

Table

3,2

Features of Lead and Aluminium

Features

Cable lead Kb-Pb

to DIN Density

Aluminium for

640

cable sheaths

g/cm3

Ten 0.6iI

4.2 Aluminium-Sheathed Cables Whilst being exceedingly durable when installed in free air, aluminium has to be protected by a water and ion-resistant anti-corrosive covering, if the cable is to be installed in the ground. In order to achieve a high degree of salety and mechanical strensth a multiJayer corrosion protection is required. According to DIN VDE 0255 Type A5 it consists of a plastic foil applied overlapped and bonded to the aluminium sheath and to the outer pVC sheath bv means of bitumen compound. Special tests show that the corrosion protection adheres well to the aluminium sheath and that in the event of a locally limired damage to the cable eventual corrosion on the outside of the aluminium sheath is practically limited to the exposed area.

.11

Armour

5

Armour

5

-fh.

orrnou, protects the cable against mechanical for rated stresses. In ,h. a"r" of polymeric cables above Lfo/U:0'611 kV it normally serves -voltages llso as an electrical screenlng' arPaper-insuhted lead-sheathed cables are normally apeacn tapes' steel *'ith trvo compounded -rnorrred in ol'. ln open helir in such a mlnner thill the ."cond tapc corers the g:rp left b;- thc firsc' -High-r'oh,rge crbles wirh polymer insulrrtion having

sysSingle-core cables in single'or three-phase a'c' **-, ur" not armoured as a rule, in order to avoid addirional losses. An armour of non-magnetic material. however, has to be provided wherever mecnanle'tcal demage or higher tensile srresses are to be pected du-ring or after laying of the clble Occasionaliy rntr.-itpr-.gnated or oil-filled cables are manuflactured sith an open armour of steel wires' inste:rd of a non-magnetic armour. tor reasons ot economlcs'

iallic a-opp". ,"ta.n as rvell as low-voltage cables s: PVC or XLPE insulation and alumintum-if a

-sheathed clbles do not require to be armoured they are sufficiently protected against damage and noi.uUj..t.a to tensiie stresses' The permissible pull - during ia1-ing of cablcs is shorvn on p'rge 406' An armour of flat-steel wires may also scrvc as a screen in multi-corc cablcs r"irh polymcr insullttion

-

not having a screen ofcopper' This design is common for PVC ilbles ior 3.5 6 kV. *here ;t scrccn lrounll each core separately is not rcquircd' irnd illso fLrr cubles to be installed in nenvorks rvhcre doubic carth-

earth-faults in earthcd ncutral s)sterns rcnticr ltn :lrmour ot'stccl rrircs ltdvltnt;tgcoLts in its firncrion as a commoll mctallic screen (scc also

Iiruits

-

or

^" ltc -n i

-

(;$les

!..,

).

lre to b!' itlbjectcd to highcr mcchlnl(especialll tensilc stress) must be 1r'

\\ hich

stresscs mourcd rrith salvanized stcel uircs' The right protilc I'g. tllt. round or "2"-rvire). dimensions lnd ...icnsth ol'thc $ires hits to bc choscn according to thc size :rnrl applicarion ot'thc clble' e g as rirer cable. submarine citble or shati cable (see pages 129 rrnd ll0t. A stccl t:tpc hclir prercnts bird-cageing of the * ires.

.+-)

o Loncentnc conductors

6

Concentric Conductors

The concentric conductors in low-voltage cables such as NYCY, 2XCY, NYCWY and 2XCWy are used as PE or PEN conductors (see page 397) and at the same time form touch protecdon. Accordine to the VDE specificarions these must be of copp-er. The cross-sectional area included in the type desienation. houever, relates only to the material used for the phase (main) conducrors.

in a cable

u.ith copper conductors. for cxamole. NYC\\'\' 3 x 95 SM,50 0.6. I kV. the value of diiecr currcnt reslsttnce of the conccntric copper conductor. to comph * ith thc abovc rc-,qulation nust not be greater than the maximum value of that of a cooper conduclor of 50 mmr. Similarly in a cable wiih aluminium conducrors NAyCWy 3 x 95 SM,,9i 0.6/l kV lhe value old.c. rcsisrance of rhe concenrric

copper conductor, to comply u,ith thc above recula_ tion, must not be greater than the maximum of that of an aluminium conductor of 95 mmr. ""alre The concentric conductor compriscs cirhcr

I hclicallr applied layer of copper sires or a ulve form llrcr of copper uires r CEAN DE R-crblr., ) c.g. NYCrif:

or 2XCWY. In addition a copper tapc is applicd

hclicalll to interconnect thc \\.ircs (transvcrse helical tapc). In rhe Fedcral Republic of Gcrmlnl. alumin_ runr rs not permitted for use as it concentric conduc_ tor.

tll

Concentric conductors are arranged under the outer polymer sheath to ensure they are protected against corrosron. ll armour is arranged above the concentric conducror a separarion sheaih limperviou, sh.eath) of PVC musr beapplied Uer*..n ttern. "*,.uJLJ (iyp. relerence desisnations for concentric conductor sLe page 101.)

Electrical Screening- Conducting Layers 7'1

7 Electrical Screening

with Electrical screening is necessary only for cables functions: Uo>0.6/1 kV and fulfils the following Potential grading and limiting of the electrical

>

I

> p

lelo

Conduction of charge and discharge currents .

ouch Pro tectlon

To satisfy these functions the screening normally" comorises a combination of conducting lay-ers rvith

,1lic

elements. One differentiates between cables ru-r non-radial characteristic fields (e'g' belted cables) and radial field cables. The rldial charactertstics of lines of held between conductor and screen is achieved by placing a conducting layer' I metal screen or a metal sheath over each individual core' Insulation is stressed only perpendicullr to the rvall (papcr thickness. In cables with laminated dielectric elcctriinsulation) this is the direction of the highest cal withstand. Interstices of the corcs in thcsc cablcs remain field-free (see Page 97). t) 7.1 Conducting LaYers

Thc magnirudc of clcct.ric strcss rnd thc dcgrcc of .sitivitl of thc insr-rlation mtterinl ilg lnst pilrtlal govern the tvpe of screening of the insulit-chrrrge !ron with conducting la]ers (Table 7.1). rble

(H-foil), if necessary in combination wirh conducting pop"t. it can also consist of a combination of aluminium tape with conducting Paper tapes' Cable with PVC Insulation

The "inner conducting layer" consists of a PVC

compound having a high carbon-black content' This is normally applied togerher with the insulation in a single production process so that both layers are bonded firmly without gcps or cavities'

For the " outer conducring layer" elastic conducting adhesives with a cover o[ conducting tapes (textile or carbon black paper) is a preflerred mcthod' Cable rvith PE or XLPE Insulation Bec::use

of the higher sensitivity of PE and XLPE

insulation to partial discharge the reliable ''lell adheilolc /. I Arrangement of conducting Layers above and beiorv lhc cable insulation I

belorv the in

conducting

laler over the conductor)

The "outer conducting layer" normaly consists of merallized paper. also known as H6chstidter Folie

I

mpregnated PlPer

t) belted cable radial field cable

Pvc

insulation (outer conductlng lir!'erl

kV

KV

3.7 l0 3.7 15

3.6i6

Lio,'L'

6i 10

EPR

6i10

PE

3.616

XLPE

3.6i6

r' tn thri boot rh\: rn]plcr rcrm .onducring h)_cr' hrs bccr ujcil ini(crd ol rcmrconllucrrng trt!r chosc in rh\: r\s0cct.i!c IEC jtinJrrJt

above the

rlted voltages excecding

Thc "inncr conducting la.ver" consists of scleral ielcrs ol'semi-conducting paper (llso kno"r'n as cat-

it

sulrtion

(inner

rrith Paper Insulation

bon black paper). This is often retlrred to as conductor smoothing because is used to smooth local peaks in the electric field rvhich could otherrvise oq' cur. e.g. because of irregularities in the surftrce of stranded conductors.

Conducting la;-ers required

Tl pe of insulution

wilh non_rldill tictd: p"missiblc only for rlled toit,rg.. Lo Ci _ Highlr flerible rCles 6 of tEC I jti) tbr s

j

Itc\lblc cables

-\'

Ntt. of corcs

l'rotcctile conductor \\'il hout green vellorr corc rlrtn grecn, r,ellorr core Sizc of conductor

cxrntptcs of t).pe desicnations

,i;i,T:"il:l-:heathed

clbre ror -ccncrar purposcs

r", r'llll;Y l;iil] 3::'t l1':^l **..-,n*,n.. *irh green vellou.core

;,_::::,i.-,

-R -K -F -IJ

Tinsel conductor

i,,1f; .... . .s,u 5uuq conductor

T T2 H H2

Conductors

!l:^i:1. lgi"', Flexible_(Clas

R S

H07RN_F 3c2.s

;:ii'il;:":lliiledcircurarcord.,-'l;;rl:llJlft1.;

rJ

Part 2

Part

3

Harmonized Standards 8.1

8.3

Table

Summary of cables to harmonized standards

Cables

to DIN VDE

0281

Type abbreviation

Rated voltage

No. of

Nominal

cores

cross-

Superseded types to VDE 0250

sectional area

UolU

mmz Single-core non-sheathed cables

for internal wiring - with solid conductur - with flexible conductor

H05V-U H05V-K

300/500

with rigid solid conductur wirh rigid stranded conductor with flexible conductor

H07V-U H07V-R H07V-K

45017 50

rat linsel cords

H03VH-Y

300,'300

H03VH-H

Single-core non'sheathed cables for general PurPoses

F

Flut non-sheathed cords

;

',r PVC-sheathed cords

-,,rcular

-

lli.r

t

Ordinary PVC'sheathed cords - circular

Flat PVC-sheathed flexible cables lo lilts and simihr aPPlication

0.5 to

NYFA, NYA NYFAF, NYAF

1

1.5 to l0 6 to 400 1.5 to 240

NYA NYA NYAF

1

-

NLYZ

300'300

1

0.5 and 0.75

NYZ

H03VV-F H03VVH2-F

3001300

2ro4 2

0.5 and 0.75 0.5 and 0.7i

NYLHY rd NYLHY fl

H05VV-F

300i 500

Ito)

0-75 to 4

1

H05VVH2-F

300/500

)

1

0.7 5

NYMHY rd NYMHY rd NYMHY N

HO5YVH6-F H07VvH6-F

300i 500

3to24

0.75 and

I 1 1

0.1

to

to

16

0.5 to

16

1.5

450r7i0

2.5

1

NY FLY NY FLY

Cuhles

to DIit' l/DE 0)31 llcut-resistant silicone

:tlili'b9_;.rided cords

HO5S.'-K

j00

500

H03RT-F l:oo':oo H05RR.F

300'500

narl- po I vchlo ropreneshclthcd cords

H05RN-F

300r500

v;- po lvchloroprencsheathed flexible cables

HOTRN-F

,..,

rd i nlr-v.. to

u

gh-rubber-

shcathed cords 'd

i

H ca

450,',750

I

I

l2and3 Ito)

0.7

i

to

1.5

0.75 to 2.5

N]GAFU NSA

NLH. NVIH

3and4

4and6

I

fandJ

0.75 and I 0.75 and I

Nivl Hdu Nlvl Hdu

4

0.7 5

N Nl Hou

.5 to 500

I

1

2and5 3and4

1to25 1to 300

4ro24 4to24

0.7 5

4to24 4to24

0.75

NNIH. NMH6U and NSHou

Rubber-insulated Iift cables for normal use

-

braided cables

armoured cables

HOsRT2D5-F HO7RT2D5-F

H05RND5-F HOTRND5-F

3o0i soo

450/750 300/5oo

45017i0

1

1

NFLG NFLG NFLCC N FLGC

5i

Table 8.4

Comparison of flexible cabres to harmonized shndards DIN vDE 02gr and Type of cable

Cables to

DIN VDE c:- -r^ ^-_-s rrux-snca[osq caDles Ior lnternal wiring ',r-yrE-!ur - with solid conductor - x'ith flexible conductor

*'irh rigid solid conductor

r'ith rigid stranded conductor uith flexible conductor

Flat tinsel cords

-----_.-.-

-.......--.........."o.d. ..---=-....."=.Light PVC-sheath.d "o.d, - circular flat Ordinarl' PvC-sheathed ;-d;- circular - flat Flat non-sheathed

Flat pvc-shearheo fl "*ibl. lifr and similar application

"ullill.-_.._.-

02g2 with IEC

Superseded 0281

Comparable construction

types to

DIN VDE

O25O

H05V-U H05V-K

NYFA, NYA NYFAF, NYAF

H()TY-U H07V-R H07V-K

NYA...e NYA...m NYAF NLYZ NYZ

Jrrrsrs-Lurs u(.)n-sncatneq caDtes lor general purposes

-

DIN vDE

H03VH-Y HO3VH-H

tough_rubbcr_sheathcd cords

c:rblcs

H05VVH6-F HOTVVHGF

NYMHY NYMHY NYFLY NYFLY

f'I

exist. nrmelr.

a number

' li';;,::i:i;J!:j:;:: na,iona, ,vpes which approvgd narional rypes u.hich are an addition ro the

I" ::HT ", 0.611 ky' Thc'se cables are suirable

for application in

3_phase

and single-phase installations having a maximum opcrarrng voltaee not e.\ceeding l0yo above the rated voltage ol the cable. Table I 1.2 Typical rated voltages of various cable types Rated voltage

Cabie type

r00/i00 v 300/500 450/750

single-phase installations where rhe star point is eflectiveiy earthed:

v v v

single-phase installations where rhe star point is not effecrively earrhed providing

Tinsel cords and flat non-sheathed cords FJat building u.ires

Light PVC-shearhed cables Heavy polychloroprene-sheathed flexible

cables

0.6/ 1kv | .8/ 3 kv

4 i 8kv 6 /10 kv

kv 12 120 kv l4 /25 kv 18 i30 kv :0 /35 kv

PROTOFI RM-sheathed cables Single-core poli chloroprene-sheathed

lor special purposes Neon lighting cables cables

8.7i t s

8E

a) in 3-phase and b) in 3-phase and

UolU

220i 380

The cable may be used:

Trailing cables

that any individual earrh laulr is not susrain.j

Ionger than 8 hours and the rotal ofall earth laulr._. tlmes per vear does not cxceed 125 hours. If this

situation can not be ensured than. ,o anru." u life of rhe cable. a cable having a higher

service

rated voltage should be selected.

for Direct Current Installatiotts For cables in d-c. installations the permanent perCables

missible operadng d.c- voltage between conductors must not exceed 1.5 times the rated a.c. voltage of the cable. In 2-wire earthed d.c. installationsthis value must be multiplied by a factor of 0.5.

Rated Opcrating Voltagc Conductor Cross Section I l '2

11.2 Selection of Conductor Cross-Sectional .{rex General

The temperature rise. respectively current-carrytngcaoacitv. of a cable is dependent upon the type ol construction. the characteristics ol the materials used and also operating conditions' In order to achieve a sat-e design and a full service life of a cable the

conductor cross-sectional area must be chosen such th:rt the requlrenent current-carry ing capacity

i

tr tr 1

thc

co

/"> loading

/o

nditions of

normal oPeration and shorr circuit

satistled. This rvill ensure that no part of the '-rble at any point in time is heated above the rated maximum permissible operating [empcrlturc rcspcctively short-circuit tcmperature. Current-CarrYing CaPacitY in r'r*ormal OPeration

The design of installation projecrs is simplified by using esrablished data collected over several decades in respect of current-carrt ing capacitl: undcr practlcal aoolications rvhich has now been incorporated in var'

ious regulations governing apparatus and installation. For electrical installations in buildings the standards for electrical installation of buildings DIN vDE 0100 apply for power installations up to '000V. In this standard. up to the present day' in /-ilrt 523 the types of installation rvere divided into ,-rree groups:

Group 1: insulated conductors and single-core cables in a conduit ,-iroup 2: multi-core cables for fixed installation Group 3: single-core cables for fixed installations and power cables. The xssociated values for current-carrying capacity had been determined originally for rubber insulated cables.

By todays standards, these groupings of types of installation appear extremely rough but have, however,

proved to be adequate for the methods practised at that time. Meanwhile, however, different installation practises have developed and modern, more sophisticated materials have become available. These developments have necessitated the evolution of more detaiied project planning.

In Februrry

1988. the specilication

DIN VDE 0:93

Part .1 " Recommended values for current-cilrr]-ing capacity lbr sheathed and non-sheathed cables tor tixed wirings. tlexible cabies and cords"' wus published. This pubiication contains comprehensive and detailed information on the relevant terms and regularions required to determine the cross-sectional area of conductors for normal operation and for shortcircuit conditions. Firstly the Precise operating condi' tions on rvhich this data was based were detlned and specilicd. Blscd on rhcsc rtferent'e !)perdtlttg conclirrr.,rs. rvhich teke into lcount thc rrpe oJ' operdtiotl as rlcll as iutullutiort tncf ttnbient c c'rtrtlilir'trls, tabu' lated data rvas prepared ol ratatl lalrrr's of currentcarr,ving clpacity | (rlted value). To clter tor conditions rvhich der iate from the ilgrced opcr:lting condirions. convcrsion fuctors were prepared. The relarionship:

I,:l,nf applics riherc fl/ is rhe product of ull conlerston tlctors rvhich arc aPPlicable. As J basls tbr ty.'pe of operation. t t.rttllrtti'.ltis ttperutitttr *as sclectcd. rvhich is operation at constant current lbr a durction sufficicnt for the cable to reach thermal equilibrium but otherwise not limited in time'

Short-tinc tnd' intermittentl operatiort e'g' for

the crane starting currents of motors or the operatlon or installations are described in Section 13 6'

The spec(ied untbient !empcratLrre for all applications is :O iC ind it is required that the room is sufficiently large and ventilated such that the ambient tcmperatu.. i. not noticexbly increased by the cable losses

The in.stallation conditions, in comparison to the pre' viously used groups I to 3. are more precisely defined and enlarged. One differentiates norv between:

\tethod of Installation

tYPe

A:

lnstallation in walls having low thermal conductivity' lvlethod of installation tYPe B I : Installation of single-core insulated cables in conduit or duct on or in a wall, Method of installation tYPe B 2 : Installation of multi03d CoDper conductor ainal-cross sectional afel rn mm-

Current-carrying caprcity in A 18.i l)

r.5 2.5 + 6 10 10

60 30

z) i5

1?6

101

50') 70')

153

"'

JJJ

95

I'

196

Typc designation and fur(hcr dctails

:r Not included in DIN VDE 0:98 I' Ratcd voltag. 1.6i6 kv

"'

Part

iI Secdon E.l l. Insuladon

ofc.oss-linkcd polyolcfine compound

Not included in DIN VDE 029E Part,t

r ur each of these reference operatittg corlditions the recommended rated values of current-carrying capacitv /. are shorvn in Tables I 1.3 and I 1 .4. The headings r ,.he tables include diagramatic representations of the installations for ease of understanding which, togelher with the footnotes, provide a detailed description. The current ( tr

two-core cables with two conductors loaded as well as for two loaded singletore insulated con-

ductors or two loaded single-core sheathed cables given in Table 1 1.3 columns 2. 4. 6 and 3 45 rvell as Table 1 1.4 columns 2 and lor

>

three-core cables with three conductors loaded as well as for three loaded single-core insulated con' ductors or three loaded single-core sheathed cables given in Tablell.3 columns 3. 5' 7 and 9 as well as Table 11.4 column 3.

Typc designadoo and furlher dcrrilr in seclioo 8.1 corducroG i! conduit ia cnclosed floor trcnch Also appliG lo iosulalcd coDducro6 io coDdui! i! ecatilatcd 0oor u.dcb Also appliG to ouili-corc cabl. iD opc! or vcotilatcd trench Also applies to idsulalcd co[duc!o.s, siogle-corc shothcd cablc, rnulti >

> > >

Additional S1'mbols for Cables with lmproled Characteristics in the Case of Fire

HX 'l FE

Insulation of crosslinked Halogen-free poIymer compound Shcath of cross-linked Halogen-free poll.mer compound Sheath of non-cross-linked Halogen-frce po-

lymer compound

Insulation retention (symbol appears after the designation of conducror)

;iymbols for Ships Cable

\{ G \J

Power supply ships cable ro Dsutatron of EpR Sbeath of CR Screen of copper braid

HNA standard

l

Slubols for Conrtuctor DF

RM SE

SM 100

The rated cross-sectional area of copper screens is given after an oblique sign 'i ' locared aftcr the symbols for the phase conductors e.g. NYSEY I x95 RMi 16 6 10 kV. The rated cross-sectional lrea of the concentric conductor is also rndicated folloriing I ' sign afrer the sl nrbols for the phase conductors c.g. NYCWY i x 95 SMi50 0.6 I k\/. Further Commonlv Used Sr mbots

Copper conduclor lnsulatron ol- impresnut!'d papcr (core. bclr ) Inner and outer conductins lavers in cables rrirh pollnrer insulation Inncr coverings Fillcrs of the interstices Inner beddings of fibreous materiais.

HX

OM

Flcxible circular conducror Stranded circular conductor conrpacted br cither squeezing through rollcrs or lhe us; of shaped wires (for thermall-"- stable cablc\ * ith paper insulation) Circular hollow conductor, lhe diamerer of the oil channei given in mm preceds the Jerter 'H' e. g. RMiV 14H Stranded conductor of oval cross-section

Solid circular conductor stranded circular conductor Jotrd sector shaped conductor stranded sector shaDed conducror

YV IYV O AA

Rcinlbrccd PVC shearh Reinforccd PE sheath Opcn armour (FO or RO) Double outcr pro(ccrive la1,cr ol'fibrous matcria

Tc sv

I

Lead sheath ol'lead Tellur alloy Special inrprcgnation for cable with paper insulation for steeply sloping cable runs

(sr':

nd

: non-draining compound

pagc 35)

see

Lctter Designation 13.2

Table

l-1.2

Summary ol'the main lcttcrs uscd for the typc dcsignation of cable

Construclion clenren(

r-

027 | . 0272. 02'7 3

High- and crtra high-voltaee ceblcs DIN VDE 0:56. 0257,0258

N

N

no le!ter

no lelter

Paper-insuhtcd

Poll"me

cables

insulated cables

DIN VDE O]5J Norm tl pc

DIN VDE

0265,

Conductor no letter of aluminium Insulation Paper rvirh mass impregnation Pcper oil impregnated - lvi!h high-pressure oil cables in jleel prpe Paper rvith mass impregnation

no lelter

o OI

-

Ibr external gr.s pressurised cable lor internJl g3s pressuriscd cablc \:C. polyvinylchloride PE. polrethylene XLPE. cross-linked pollcth-vlene

P

-

I

]Y 2X

Concentric copper conduclor longitu6110 kv (u_> Indoor sealing ends (examples)

Outdoor sealing cnds

Cablejoints

(cxamples)

(examples)

Straight

12

kv)

joint WP

Push-on sealing end FAE $ ith core sp.eading

Push-on scxling end IAES lvirh corc sprcirding

Srraight transition joint ctnncct paper insullted \\

ith i-core XLPE

5\l-wP to S.

L.

cablcs

Secling end FEP with porcelain insulators and core spreuding

l2l

lJ

I )?es

Tabte

ol uonstructlon ol Low- and Hlgn-voltage uables

13.3

Cables and associated accessories (continued)

Construction

N

2'l S(F) A 2X I -Al- 2 PROTO- 3 Copper scrcen 4 PROTOcoo- THEN-X- (longirudinal THENducror insulaiion (XLPE) wa|cr dght) shcalh (PE)

+ 's rtl

Prcferrcd application

Limircd applicarion

N2XS(R2Y

In unfavourablc insrallation conditions cspccially if, after mcchanical damagc ingress of water in longitudinal dircction musl be avoidcd longitudinally watertight cables with extrudcd fi lling compound and gap scaling in thc scrcen area offer advantage.

In cablc trunking and indo( It must bc noted that the pI sh,eath.is no-t.flamc retardar wnsn rnsta rng single-cors c:IOIeS ln arr adcquate fixin( must bc providcd bccause odlmamic effect of short-ci rc currcnts (s€e pagc 297). For lhe selection ofscreen cross-section the earth-faul rcspectively double ea h-p. condirions of the nerrvorl n be considcred.

Typcs used in countries where 3-core cables are rsquired

Thcse cablcs wirh sheu previously used in u..ran no works are in Gcrmany incrr ingly superscded by mech:Lr. cally superior rypcs wirh Pl shcuth.

NA2XS(F)2Y

4

5 Inn:r irnd

ourer Ialer

6 Gap sealing

(conducdng t:lpe q.ilh s*clling rapc)

cooductin_e

7 E{ruded fillcr

I

Designation, standards

IEC 502

sa I Cu*crecn over

PROTOTHE\--Xrnsulatior (XLPE)

DIN VDE O]7] N2XSEY

PROIODUR.

each individual

n*A2XSEY

shcath {PVC)

ffi .ri

56

.l Cu-conducror 5 Inncr rnd ourcr conducrinc

6 Conduclin! tapc

llrcr

7 Ertrudcd filler

-ffi \:\ I

SE \ F \ : Cu- I PROTO- I Flar srcel- 5 PROTO_ ;i{EN-X- scrccn DUR- $irc DURrntulstton ovcr cach shcath irrrnour shcrth ,\LPE) indilidual rpvc) lpYC) PROTO-

1

6 i 18

I

3{

IEC 502

N]XSEYFY NA2XSEYFY

N:XSEYRY

NA]XSE\'RY

5

8 Conducting 9 Trpc

lapc

Tl pes used in othercountrics with Bat steel-uire armour F or armour ofsteel round-\rire R $ hsre dilfi cult installarion and operating conditions exist. Preferred \r'ith PE sheath instead of PVC sheath for laling in

ground.

910

6 Cu-conducloa 7 Inr:i and ourer co:du.!ing hlc.

ll:

DIN VDE O]7]

10 Errruded llller

DIN VDE IL E )UI

O:73

A

Cables and Associated Accessories 133

U o; I U Indoor scaling cnds

Outdbor sealing cnds

(examplcs)

(examples)

2 l2

kV (U^>

24

kV)

Cablcjoints (cxrmplcs)

Brass straight

\-.

120

joint

EoD wirh transparent irst-rcsin insulttor

PLr;h-on strright

joint Ai!lS

Srrrighr joint wP

Plue-in termination WS

Srraight transition joint Sivl-wP for connccnng a papcr insulated S.L.-cable to i single-core X LPE cables

t2l

13 Types of Construction of Low- and High-Voltage Cables

Table

13.3

Cables and associated accessories (continued)

Construction

NA 1Al-

2 AluEridium- 3 Plasric

shcalh

coo-

mass

apc

4 PROTODUR-

cmbcdded

Dcsignation, standards

Prefcrrcd application

NKLEY NAKLEY

Cablcs prcviously used

for

urban nctworks; now bcing

rcioforc-cd

supcrscdcd by XLPE cablcs.

shcau

cluctor

Limitcd applica!ion

(Pvc)

Not suitablc for mcchanical strcsses and areas subjcct to subsidcncc; whcrc diffcrcnssl in lcvcl occur (e.g. stecp sloDe

cables wirh polymcr insulaiio must be used.

561 5 Conducting papcr 6 Insulatioo (irnprcgnaled papcr)

N2\

(conducting pap€r and

S

1 PROTOTHEN-X- 2

insuiation

Cu-scrccn

Al foil)

DIN VDE

0255

\

N2XSY

Thesc

3 PROTODUR-

NA]XSY

previously used in urban networks are in Gcrmany incrcasingly superseded by mechanically superior type

shcaft (PVC)

CXLPE)

:E

cablcswirh PVC shearh

with PE shearh.

ii

45

.1

I

conditions ol'the network ml

5 6

Cui conducror

\:\

7

lDoer and ourer

aooductinglayer

]\

I Al.

2 PROTOTHEN-XinsulalioD

duc!or

be

6 Conducliog 7

Tape

I

S :\ I Cu- a PROTOTHET'-

(XLPE) screen

EL )UI

N2XS2Y

ln ground for urban networks because ofextremely low di-

NA]XS]Y

shearh (PE)

electric losscs. To ease installation 3 cables can be layed up and supplied on a single drum.

4ffi and

ourcr

120

6 Conducdng

upe

7 Tape

If after mechanical da,nage i; gress

ofuater

is likely cable

having longiludinal water tightness in the scrcen area h,, hr

advanlages.

\\'hen used indoors it must b. b observed that the PE shealh r not flame retardant. \\'hen installing single.core cables in air adequate fixing must be provided because of the dynamic effecr ofshort-c: cuit currents (see pagc 297).

5561

conouctlng layer

considcred.

DIN VDE O]7]

l:14

5 lnn.i

ln ground ifbecause ofmcchanical stresses damagc to i. PVC sheath is likely. When instailing singlc-core cablcs in air adequare llxing must be providcd becausc of the dynamic effect ofshort-ci cuit currents (see pagc 297;. For the selection olscrcen cross sections. the e:trth-faui respectively double earth.fau

DIN VDE tEc 502

0273

Cables and Associated Accessories 133

url u>12 Indoor scaling ends (cxamples)

120

kv (u_ > 24 kv)

Cablc joints (examplcs)

Srraighr

joint with individual

lead inner casing

EoD wi(h rransparent cast.resin insulators

EoD wirh transparent cast-rcsrn insulators wlth lncreased short-circuir

withstand

Straight joint with steel inner casing for connecting H-cables to S.L. 18130 kV) are normally dimensioned and tested for use in netrvorks or planc with stress type A. If it is required that these cables will be opercted lor a limited time or longer rvith an errth fault on one phase. this must be taken into account rvhen dimensioning and testing the cable.

Ca

b Ie s,

insulation must be used (e. g. instead ol tis|L':6t10 kV use cable UolL-:8.7710 kV) (see Table 17.3). This type of belted cable is not used in Germany and there for no provision is made for it in VDE standards. For cables having rated volrages greater lhan lLolU:13'30 kV the insulation rvall thickness must be dimensioned appropriately. For medium- and high-voltage cables it must be noted that their service life is affected if for frequenr short periods and/or for longer periods the cables are operated with an earth fault on one phase. Low.l/oltage Cables, rvhich comply rvith the VDE and IEC standards are suitable for stress type C rvithout limitation. 149

l8

Current-Carrying Capacity in Normal Operation

18 Current-Carrying Capacity in Normal Operation

18.1 Terms, Delinitions and Regulations Basically the terms definitions and regulations laid down in DIN VDE 0298 Part 2 and DIN VDE 0289 Part 8 apply. Load Capacity is the short term to express current-carryins capacity. With load capacity the permissible current f. is being

pregnated cables, in addition, the remperature lise is limited to avoid the formation of voids in the insulation (Table 18.1).

Conduc tor C ross- Se c tional Areu must be selected such that in normal operation the loading 16 does nor exceed the load capacity 1. -eiven

t'lR 1e\

Ihs 1,.

defined under certain operaring conditions.

In addition to compliance with the above reeularions the following is also relevant: The lalues of current-carrying capacity for the reference operating conditions which are given in Tables 18.2 and 18.4 are rated values. These reference operaring conditions (in DIN VDE 0298 Part 2 named

as "normal" operating conditions) are in the same sense rated data to DIN IEC 50 (1 51). The followine equarion applies

1,=

I,nf,

(18.1)

*,here fI/ is the producr of all factors ',r'hich must be considered. For electricity utility operarion or other cyclic rypes of operarion the maximum load corresponds to load capacity which is defined as /, or 1..

ls the short term for current loadinc. Loading relates to lhe currents uhich a cable be required to mav carrl under specific operational conditions.

In normal operation loading is lhe operating currutt /0. in electricity utility operations or other cyclic types of operation the max. value of the loading is the operating current.

le O p era t ing

Te mp

er

atur e

is the maximum permissible temperature at the con-

ductor under normal operation. This value is used in the calculation ol load capacity for normal operation. This is included in DIN VDE 0298 pari 2 in respect of load duration (load factor). For mass-im-

li0

Tentperature Rise

of a cable is dependant upon construction, characteristics of materials used and operaring conditions. An additional temperature rise must be considered where grouping u ith other cables or heat input from heating pipes. solar radiation etc. occurs.

.\'onntl

O pcratiott

Normal operation includes all

t1.pes

of

operation.

such as. continuous operarion, short-time operation.

Loadirg

P ernt iss ib

Decisive for this are the most unfavourable operaring conditions at any point along the whole cable ' '' durine operation. This ensures that the conducosr is not heated at any time and at any point above the permissible operating temperature.

intermittent operation. clclic operation. utility s, plv operation, providing the permissible operatiig temperature is not exceeded.

OL'eruIran!s include both overload currents and short-circuit currents (DIN VDE 0100 Part 430 and Parr 200). These can cause, for a limited period, conductor temperatures u hich are higher than the permissible operating temperature. The cable in these cases must be protected against detremental temperature rise by overcurrent protection devices. If necessary the conductor cross-sectional area may have to be dimensioned to satisf)' the conditions of short-circuit stresses as discussed in Secrion 19.3.

-

_

Terms Deflnitions and-Regulations

Table

l8.l

l8.l

Permissible operating temperatutss and thermal resistivities

Type of construction

Standard

Permissible

Permissible temperature rise

Thermal

operating

installed in

resistivities

'c

Ground

Air

l\

l\

KmiW

XLPE cable

DIN VDE DIN VDE

0272, 0273

90

J.)

PE cable

DIN VDE

0273

70

3.5,'

PVC cable

DIN VDE DIN VDE

0265,

70

6.0:'

DIN VDE

O]55

Iass-impregnated cablc

0271

Belred cable

I kv

80 80 65

'3.6 6 kV rl^0.6 , 6l0kv

of

insulation

temperature

65 65

))

45

55 35

6i

))

65

55

6.0 6.0 6.0

Single-core cable, S.

L.

and H cablc 0.6 1 kv 61

10

80 80 10

kv

3.6.6

kV

kv 18, i0 kv l

6i

2,20

" Aiio roDlics

:r .\lso

60

45 35

.10

JI,,

for rll ou(cr rh\:rths ot PE

applics fo.

!ll

outcr shc:rrhs of PVC irnd proLcerirc co\crs ol jurc rcr!inS

lterloa

turrelrtr ciln occur bv operational overloadhat is otherrvise a fault-free circuit. For these conditions permissible temperatures have not yet reen defincd. These rvill be dependent on borh duration and frequencv of the overload occurances: these again at-fect the heat deformation characteristics and ing in

j-i

6.0 6.0 6.0 6.0 6.0

rr

accclerate a-seing.

Short-cirait currents flow when a fault of neglegible tmpedance occurs betrveen active conductors which in normal circumstances have different potentials. The permitted short-circuit temperatures are acceptable only for a duration of up to 5 seconds. In systems *ith an insulated neutral and in compensated networks, a line-to-earth short-circuit current is tcrmed earth-fault current. Such earth-fault current

c:ruse voltage stresses

in the fault-lree conductors (see Secrion l7). to an ertent thirt temperatures erceedins

\irh

bitunrinous compound

the permissible operating lemperltures cannot

be

permitted. Entergent'v Operation

is a type of operation quire common in USA and some other countries. Here currents are permitted which are higher than the load capacity in normal operation. The conductor "emergencv operating temperature" which may on some occasions signihcantly exceed the permissible operaring remperarure are limited in duration for the individual faults both during any one year and during the service life of the cable. A definition and the question of what values of emergency operating temperature are acceptable for the differenr tvpes of cable and also rvhat reCuction in service lit'e is to be agreed is currently under discussion in the relevant IEC rvorking sroups.

l5l

IE Current-Carrying Capacity in Normal Operation Type of Operation describes the temporal characteristics of the load capacity and the loading. Continuous Operation

is an operation with constant current for a duration sufficient for the cable to reach a thermally stable condition but is otherwise not limited in time.

18.2 Operating Conditions and Design Tables To assist in preparing a clear basis for design, regulatory and operating conditions are discussed under

tr type of operation, tr conditions of installation, tr ambient conditions.

Utility Supply Operation is described in Section 18.2.1.

Short Time and Intermittent Loadinp

18.2.1 Operating Conditions for Installations in Ground

is described in Section 18.6.

Type of Operation

The values included in rhe tables for installation in ground are based on the type of operation commonly experienced in electricity supply networks (supply utility loads). This load is defined by a 24 hour lc^4 diagram which illustrates maximum load and lu.-a factor (see Fig. 18. t ).

Load,har load 100

en --1

0.6

I

---Fig.

12

16

20 hours

24

Time-......*

Relation of load to maximum load in % Relation of average load to maximum load

l8.l

Daily load plot and determination of load factor rr (Example) I

-sl

Operating Condirions installed in Ground 18.2

Ivlaximum load and load lactor of the given load are determined from the daily load plot or reflerence load plot. The daily load plot (24 hour load plot) is the shape of the load over 24 hours under normal operation. The reference load plot is the average load shape of selected, similar daily load plots. The highest value of the maximum load read from the daily load plot is taken as operating current .Ib. If the load fluctuates within time bands which are less than 15 minutes, then the mean value ofthe load peak over a 15 minutes period is taken as maximum load, i.e. a mean value must be determined over the range of time which contains the peak, this being then termed maximum load.

'

he load factor nr is determined by plotring the load erpressed as percent of maximum load on squared ^

paper (see Fig. 18.1). The load facror nr results in total area belorv the curve which is equal to the '-\e;er of the rectansular shape. By counting squares belorv the load curve the area can be determined reasonably accurltely. This arca should be entered on the diagram. thus enabling direcr reading of rhe relationship between average load and ma.rimum load and hence load factor rn provided thar, as in Fi_q. 18.1, the scale is selected such thar 100% load is equal to unity on the load lactor sclle (see example 18.1, page 180).

The average load is the mean valuc ol'rhc daily load plot; the load factor being the quotienr from the avcrage load divided b-"" the maximum load.

For this calculated load factor the given maximum load /o must not excced thc Ioad capLrcity 1..

the commonly used depth of lay for low-voltage and medium-voltage cables (0.7 to 1.2 m) it is therefore assumed that the necessary slight reduction in load capacity is compensated for by the slightly more favourable conditions.

For these reasons when the depth of lay varies within that range any variation in load capacity is ignored.

The quantities for cable load capacity are for the arrangements shown in Table 18.2 for one multi-core or one single-core cable in a d.c. system or for three single-core cables in a 3-phase system. With larger numbers of cables a reduction factor from Tables 18.15 to 18.21 must be applied. These reduction factors were derived for cables of equal size arranged side by side in one plane and loaded identically with the same maximum load and load factor. For cables of different construcrions and,/or operaring with different load factor it is necessary to form appropriate reduction factors for erch form of construction and/ or load lactor for the toral number of cables in the trench and thus establish the lactors most unfavourable for all cables. Crossing of cable runs can cause difficulties especially rvhen these are denselv packed. At such points the cables must be laid rvith a sufficiently rvide vertical and horizontal spacing. In addition !o this the heat dissipation musr be assisted by using the mosr far ourable bedding material. A calcularion ol conductor heat output and temperaLure rise is adlisable

ll

8.11.

In situations of great grouping and rvhere there is limited space, a sufficientll large bricked pit can eleviate heat build-up. This pit can enable the cables to cross in air and the resultant temperature rise of the air in the pit and also the temperature rise of conductors can be calculated as indicated in Section 18.5.

Installation Conditions

The depth oJ luv ol a cable in ground is generally taken as 0.7 m rvhich is the distance below the eround surface to the axis of the cable or the centie o[ a bunch oI cables. If one calculares the load caoacitv of a cable laid in the ground it is found this reduces as depth increases. assuming the same temperature and soil-rhermal-resistivity. With increasing depth of lav horvever, the ambient temDerature is reduced and so. normally. is the soil-rhermal-resistivity since the deeper regions of the ground are more moist and remrtin morc- consistant thirn the surtirce llvers. For

The load caytcitt, oJ' tnuki-core PVC cfules is calculated by multiplying rhe load capacity for 3-core cables in Table 18.5 by the rating factors for laying in the ground given in Table 18.25.

In the -eround, cables are normally embedded in

a

layer of sand or a layer ol sieved soil and are covered with either bricks or tiles of concrete or plastic. Tbese bedding and covering arrangemenrs (see Table 18.2) do not affect the load capacity. When inverted 'U'-shaped cover plates are installed, air may be trapped and therefore it is advisable to use a reduction trctor of 0.9 in the c:rse. I

)J

18 Current-Carrying Capacity in Normal Operation Table

18.2

Operating conditions, installation in ground r) S ite operating conditions

Refe r e nc e op er ating c on dit ions

to evaluate the rated currents

and calculation of current-carrying capacity

.f.

L-r,nr Type of operation

Load factor of0.7 and maximum Ioad from tables for insrallation in sround

Rating factors /, to Table 18.1 5 or 18.1 6 , to Table 18.17 to 18.21

I ns t a I lat ion conditions Depth of lay 0.7 m

For depth of lay up to 1.2 m no conversion necessary

Arrangement:

I

/n \v

multi-core cable

1 single-core cable in d.c. system

Rating lactors for multi-core cables to Table 18.25 for grouping or bunched

t.l

3 single-core

cables in 3-phase system side by side rvith clearance

ol/cm

/, /,

to Table 18.15 or 18.16 to Table 18.17 to 18.21

Calculation refer Section 18.4.4

3

single-core cables in 3-phase system bunched 2)

Rating factors for

Embedded in sand or soil backfill and if necessary with a cover of bricks, concrete plates or flat to slightly curved thin plastic plates

' U '-shaped cover rvith trapped I

air/=

0.9

nstalled in pipes/= 0.85

Calculation refcr Scction 18.4.6 .4nbient conclitiorrs Ground temperature at installation depth ?0 "C

Rating factors

Soil-thermal resistivitl of moist area

1Km'W

./, to Table

18.1-5

./r to Table

18.1 7

or I 8.1 6 to 18.21

Cclculation refer to Section 18.J.3 Soil-thermal resistivitv of dry area 1.5

Km W

Protection from external heating

e.

g. from heating ducts

See

Section

16.

Table 16.1

Jointhg and earthnrg of metal sheaths or screens at both ends (see Section 21) t'

Sire operarin8 coDdirions

for installarion in ground musl alwlys be calculrred using the two rating fcclors

thc specific grouDd thermrl resislivity nnd on thc .aring factor: '?' Cabl.s touchiog io lrianSular

12cm

T. -r-E+Yr---v'---Fr--.=l. .22d

d

fe---e---e--51 +a__-@---@--.

::-.rna:idspaccs.:?hcr.nuchgroupingoccuGlhe:osscsofthEc.tblca:ncrcas.lhcairrcmpemturclodlhereforcddirional olllenog air tcmpcrarurcs faor! Iublc 18.22 musr bc applied

r3dDg fac(o6 for

179

l8

Current-Carrying Capacity in Norn:al Operation

Table 18.25 Rating factorsr), multi-core cables with conductor cross-sectional area of 1.5 to l0 mm2. Installation in eround or in air

18.2.4 Use of Tables

If the transmitted power is knorvn the operalinq current

1b

(loading) can be calculated using the equations

from Table 18.26 where Uo is the operating voltage Number of Ioaded

lnstalled in

of the network and cos

the oower factor.

co

cores

Air 5

7 10

t9 1A

0.70 0.60 0.50

0.75 0.65 0.55

0.45

0.50

0.40 0.35

0..15

0..10

0.30

0.35

0.25

0.i0

5'Thcse facrors arc to bc applied to ratings in Tablc 18.5. multi-corc cables in rhc ground and to ratings in Tablc 18.6. multicore cabl€s in air. bolh in 3-phasc operation

Table 18.25 Equations for the calculation of operating current /o from the transmitted power Type of

Apparen t

Active

Network

Power S

Power P

P II

Direct current

s tl

Single-phase a.c.

Reactive Power Q var

P

a

U" cos,/

Uo sin

r

s Three phase

I

V3un

g

=o-

J L hsln

q4

From the 24 hour day load diagram and as referred to in Sections 18.1 and 18.2.3 the maximum load is also the operating current /0. Where the installation is to be in ground the 24 hour load diagram is to be used to determine the load factor nr. Where the installation is to be in air this is not required.

\/ Example

l8.l

. In a three-phase network with Ub= l0 kV an apparent power of l0 MVA is to be transmitted. The operating current /b is determined from f-

s

10

x

106

vA

V)vt fxl0x103V

=

577

A.

From the 24 hour load diagram (Fig. 18.6) with the maximum load equal to operating current I6-- 577 A, the average load is first calculated. This is done by taking the area below the load curve plotted from current and time values and calculating an average value over the 24 hour period: 180

Calculation of Load CapacitY 18.3

+h

-100

-\ + 160 A

+-l

h

260A+577A

577

A+400 A

l

.100

+-+ h

A +450 A

450

A+300 A = +01 .{.

From this the load lactor becomes ,' =

9= J/l

o

O.t.

The load capacity of two cables

NA2XSzY

3

x

185

SE/25 6i l0 kV

Load

is required to be determined when installed in ducts under the following operating conditions:

577

500

Load factor m= 1.0 Soil-thermal resistivity

400 300

Ground temperature

Qs

9E

:

=

1.5

Km/W.

30 oC.

-10

The rating factors for these conditions: r00

4

12

8

16

20

Hours 24

Time-=_

Schematic daily load diagram

Fig. 18.6

The load capacity per cable becomes

o

The calculated operlring current /o:577 A rvith the load factor nr:0.7 is to be transmitted using XLPE cables type

NA2XS2Y

3

x .../...

6110

KmAV and nr:0.7, the rating factor r.om Table 18.15:/, = 1.6, rating factor from Ta_.'lr 2 cables, the group -

t

1g.20,

1.0

f.:0.85.

In order ,o rnut. a direct comparison with the tabur"ted currents I. the calculation is made with a Iicti.-.rus value of operating current /br. With N:2 parallel connected cables

'br

(where tors).

- .,\i n/ fI/

Jt/ 2x1.0x0.85

fac-

From Part 2, Table 5.6.5 two cables with A.luminium conductors and a cross-sectional area of 185 mm2 will be adequate. The load capacity for one cable is:

x

1.0 x

,= I ,nJ'=

1.17 x 0.8

I x 0.72 x 0.3 5:

172 A.

0.85-

295 A.

18.3 Calculation of Load Capacity

A cable is heated by

losses generated by current in the conductors and, when on a,c., by losses generated

in the metal coverings as well as by dielectric losses in the insulation. The dielectric losses can be ignored, however, in PVC cables up to Uo/[.r = 3.5i6 kv, in mass-impregnated cables up to Uolu = 18i30 kV and in cables with PE or XLPE insulation up to U olU = 6ai 1 10 kV. Under steady-state conditions the dissipated heat is equal to the sum of all losses in the cable. Heat losses are conducted to the surface of a cable and thence, when a cable is in air, transmitted to the ambient by convection and radiation (Sec-

= 339 A per cable

is the product of all relevant rating

I.: I,ttf:347

I

kv

under the specified operating conditions in Table 18.2. From Part 2, Table 5.6:5 it is found that the largest cross-sectional area is not sufficient to carry 577 A: therefore 2 cables in parallel are required:

For gr=

from Table 18.15 li =0.81. and for trvo cables from Table 18.20 l:.=0.72. for laying in pipe from Table I 8.2 /R = 0.35.

tion 18.4.2). Where a cable is installed in the ground, the heat loss is conducted from the cable surface through the surrounding soil to the atmosphere (Section 18.4.3). The difference between conductor temperature and ambient temperature is approximately proportional to the total losses. The law of heat flow is analogous to Ohm's law, where the heat flow @ corresponds to electric current I, the temperature dif' ':ce 4 ^ --rresr.."'1" to ''-llrge dit::ene lr ^ the lotal thermal resistance 2. ./ corresponds ao elestn181

l8

Current-Carry ing Capacitl in Normal Operarion

in the analogy by currents ied in at

cal resistance R thus:

U: IR

fro m

A3.=rP1;r

the analog)'

(

18.r)

The heat florv @ (losses) is the sum of the heat losses Pi attributed to load current and the losses Pi related to the supply voltage. For heat to be transferred from its place of origin to the ambient it must overcome the thermal resistance ( of the cable and the thermal resistance d to the ambienr. In considering heat transfer from a cable surface to the ambient 7l may be the thermal resistance of the air 7i' or the thermal resistance of the ground 7i. Using the analogy between the florv of heat and the flo*' of electric current (Equation 18.2) an aquivalent . , :ircuit dia.gram can be dra*n (Fig. 18.7) for heat from a cable and the resulting temoerature rises produced. Heat transfer by radiation and convecrion from a cable installed in free air is represented by two resistors connected in parallel u'ith ,.'each other but in series rvith the thermal resistances of the cable. When installed in the ground the tl'o resistors are replaced by a single resistor being the soil-thermal resistance. losses flou inq

The heat losses Pj which are related to load current arise in the conductor, in the metal parts and in the armour, rvhereas the dielectric heat losses P! are generated in the insulation. These losses are reoresented

Conductor ienperature

appropriate points. Due to these losses the conductor temperature 3'- is increased by A3. and the surface temperature of the cable So is increased by A3o relative to the ambient temperature 9u.

For a cable with current flowins in n conductors the to current are

losses due

P'i: n I2 R*, and the dielectric losses (see Section 22) are / II \:

P;=nuc'b\f3)

equatron

R*. = Ri, + A R' =

R.:,(

I

+r',-:i

o)( I

+i.

r

+i,:) ( 3V I

rvhilst the d.c. resistance at permissible operating tenrperatu re 3.. is

R'"= R':o

Il

+ r.o (J1. _ l0)].

and the additional resistance

AR',:R;.-R;

(

r8.6)

is

(18.7)

Conductor lempetature

Thermal resistance

ol insularion

Condunor losses Thernral resislance

lr'l,

of insulation

Sheath losses

Iiin

Sheath losses

Ihermal resislancc oi

Thermal resistance ol

inner Iayers

inner layers

Il

Ar,'rour iosses Thermal resistance oi ourer shearhs

du

Thelmal resistance, of the ground

Ij

I;

Toral losses

Amour iosses

fj

Thermal resistances corresponding ro convecfl0n ano

radiation

fj

Thermal resistance oi outer shearhs

Ij

P,'+ PJ

Toral losses

Ambient lemperature

Ambienl temperalure

a) Cable in free air

b) Cable in ground

182

(l 8.1)

tan a.

The effective resistance (a.c. resistance) R! (see Section 20) is practically constant at the permissible operating temperature and can be expressed by the

Conductor losses

0ielecrric

(18.3)

P,'+ Pj

Fig. 18.7 Equivalent circuit for heat flow in a cable

Calculation of Load Capacity 18.3 measurable rise in conductor resistance caused by current dependant a.c. losses. These losses lrise in each conductor due to skin effect and proximity eifect (-v. and.r,,) and by induction and eddy currents in the metal sheath (,i,) as well as by eddy currents and mxgnetic reversal in the armour (1..). If these factors are incorporated in equation 18.2 for rhe temperature rise of each conductor the following .^"" rinn r nnlicc'

giving

a

LlL=lrz Rr+ejlf ri+ +urR;(l +)-,)+Pol + U'r R;( I + ). | +

).2)

rrlj+

+

P',r)n( Tj

{13.3)

+ 7l).

:lne

actual thermal resistance of the cable (see also Section 18.4.1)is given by:

^

TK=(Tiltr)+I!+1r:.

(t8.9)

The partial resistances of the insulation are represented by I/ and for the inner and outer protective covers as ?j and Tj respectivelv. (The tliermal resistances of the metallic elements are small enoush to be ignored).

To make the equations clearer and to simplify their application in design work, fictitious thermal resistances are introduced. The fictitious thermal resistance lii for heat losses due to the current. resuits from equation 18.2 and equation 18.8 with

lL+r rKi-

tl

+ ).,) Ti

l+i.r+).-

+T:

Where the individual thermal resistances. Ioss lactors. or effective resistances are not given. tlrey can be derived using the methods provided in the literarure referred to later It8.2, 13.7 and 13.3].

In the following the effective resistances are calculated or derived for the permissible operating temperature 91,.

If the operating voltage Uo is liable to deviate significantly from the rated voltage U of the cable then the dielectric losses must be calculated usinu Li. rather than [.I in equation 13.-1. The thermal resistance of the surroundings Ti is governed by operating conditions described in Secrion 13.2. For ittstallution in J)'ee air the thermal resistlnce of the air T,- is calculated as shown in Section i3.J.2 lnd has been used to determine the load capacity in air under specilied conditions rvirh an ambient temperature of j0'C. as can be seen in the tebles and text in Section 13.2: I-

31,-i0-A3,r r R",(7ir+

(13.11)

TL")

The load capacity for installation arrangements other than in free air or lor groups, is calculated using the rating lactors (Table 18.23 and 18.24). Rating factors /for ambient temperatures I, other than 30"C are calculated by using equations 18.2 and 18.14, assuming constant effective resistance and. thermal resistance (see also Table 18.22) with

(18.r0)

3. -ln-43.

-,d the lictitious thermal resistance fKd relating to the dielectric losses from equation 18.4, assumes that these originate at a mid point in the insulation. with

(1s.r5)

.

r,i":ft+r;+r!.

(r8.ll)

From these relationships the load capacity 1" can be found for a permissible operating temperature 3Lr and an ambient temperature 3u

Normally the dieletric temperature rise A3o in cables up to U: 30 kV is neglegible apart from PVC cables rvith rated voltages of U>10 kV. For these cables however it is common practice when calculating rating factors in air to neglect the dielectric heat rise which with the exception of a few cases is little more than 2 K.

For installations in the grountl Ij represents the thermal resistance of the soil. As indicated in Section 18.4.3 the equation 18.12 has to be extended because

In

R'*,(7-r, + Ij.)

with the temperature rise due to dielectric

Aid:4(7id+I4).

(18. r 2)

losses (18.13)

of drying out of the soil and cyclic loading Values for load capacity can be taken from the tables in Section 18.2. The load capacity for non-specified operating conditions must be calculated according to Sections 13.4.3 to 18.4.5 or alternarively by the use of conversion facrors in Tables 18.15 ro 18.2t. 183

l8

Curren t-Carry ing Capacirl in

Normll Opcnrtion

18..{ Thermal Resistances

0uter shearh

fj

lvletal shealh or screen

18.4.1 Thermal Resistance of the Cable

The thermal resistance of the cable ft takes into consideration the thermal insulating effect of electrical insulation and cable sheaths (Fig. 18.8) and must be calculated by using construction data and thermal resistivities [18.2, 18.7, 18.8].

For single-core cables with a metal sheath for example:

ri= =

Qt O: dL _ lr d, d

T;+

l':j

jrr"**9r"* :n aL :it

dtr

(18.r6)

thermal resistivity of insulation thermal resistivity of outer sheath material cond uctor diameter diameter over insulation or under metal sheath or screen diameter over metal sheath or screen overall diameter

The thermal insulating effect of metal covers is very small and can be ignored. Values for the thermal resistivity of materials used in cables can be found in Table 18.1. These values are assumed to be constant over the temperature range up to the permissible conductor operating temperature and so is the resulting thermal resistance.

The fictitious thermal resisiances 7ii to equation 18.10 and Qo to equation l8.l I for commonly used - cable types of constructions are shown in Fig. 18.9.

184

Fig. 18.8 Thermal resistances

Iianddofa

single-core cable

Thermal Resistance of the Cable 18.'l

Example 18.2 The cable data mentioned in the examples are taken from Part 2 (English version is in preparation). These values were calculated on the basis of the latest constructional design of the relevant cables and there[ore they may slightly deviate from the data indicated in the Tables 18.5 to 18.14 in resDect of the currentcarrying capacities.

0.6 0.5 0.4 0.3

0.7

Ibble

-

05

The conductor resistances for the cable selected for lhe example

04

NAIXS2Y I x

0.1

rre taken from Part

PVC

:

150 Rlvl/25 12/20 kV 2. Table 5.6.6 a and b:

Direct current resistance ol conductor ar 20

'c

Rlo=0.106Qkm

Eflective resistance at 90'C - bunched installed in ground or air - side by side installed in sround

A 02

Rl".=0.169 a km

R",=0.185Okm

The specific details of construction are: -,1 qL

Diameter of aluminium conductor

r-.J< rrrlr -- r.t --

Thickness of inner conducting layer

0.7 mm

Thickness oI insulation of XLPE

5.5 mm

Thickness of outer conducting layer including the protective cover under the screen

0.8 mm

Diameter under the screen

r/r = 28.5 mm

Diameter of single screen wire

0.-i mm

Increase in length due to helically wound construc:=OOS tiY" t tion of screen wire

:0.2 mm b:5.0 mm

Thickness of transverse helical tape ?5

70

95

120 150

185 240 500 400 500 mmz

Conductot cross'sectional araa

q--

Cables with or without common screen Cables with individual core screens Single-core cables

Fig. 18.9

-

Fictitious thermal resistances of commonly used cable constructions. 7ii, from equation 18.10 and ?xo for PVC-cable tor U6lU:6110 kV from equation 18.11. Cables with XLPE insulation have been calculated with PVC sheaths

rJ

Width of transrerse helical tape Increase in length due to helically wound construc::0.30 (i0%) tion of transverse tape Geometric cross-sectional area of screen4r:25 mm2 Electrical conductivity of screen, mean value

Diameter over screen

z:56.

106

dv= 299 mm

Thickness of protective layers and separating layer above the screen

sheath Outer overall diameter

I Qm

0.-1

Thickness ofouter PE

mm

2.5 mm

d:

35.7 mm

185

l8

Curren t-Carrying Capacttl in Normal Opcration

Using the thermal resistivities given in Table I 8.1

rve

18.4.2 Thermal Resistance of Arr

Horizontal Installation in Free Air

-" "._=0.176'i':', {lg.16) _ " - In I,, = sr ln "' "r W dt 2n "- 14.5 2n

/,

15

157

Km

ri=irlni=iln]=0099 - ./7t dM Jlt /9.9 T

K= ri + lrj =

*-, W

0.376 + 0.099 = 0.475

(18.t6)

Km/w.

Heat from cables installed in air is dissipated by convection and radiation. In the equivalent circuit, Fig. 18.7, the thermal resistance Tt" ofair is indicated by trvo thermal resistances in parallel representing convection and radiation. The thermal resistance of air can be expressed by [18.7; 18.9]:

(18.16)

For the calculation of the fictitious thermal resistance l'*, of rhe cable, the sheath loss factor /., must be used in the calculation according to [18.7]. zl., and T', are zero since armour and protective cover beirveen screen and armour are missing. For a trefoil installation in the ground this gives:

A+(l +r.,)?l ,l (1+i.r + /-:)

+

rj

0 176

(18.10)

= r-::;0.01 60 - -0.099--0.{69 -

tt

-

Consider a cable which is not influenced by other sources of heat (solar radiation) and rvhich does not increase the temperature of its surroundings. If such a cable is arranged horizontally in free air, so that it dissipates its losses into its surroundings by natural conlection and unhindered radiation, the coefficient of heat transfer z*, in dry air at an atmospheric p sure of l01J hPa. is:

0.0185 ,,, 1l: /i,, + ri kij

Km.w. u

These values. together with values for other types of insrallation, are shown for comparison in Table 18.27. Values for the fictitious thermal resistance of the cables l'i, differ from one another due to their dependance on the magnitude of the sheath loss factor i-r.

(18.17)

dlftz*+ f,t,)

108(*)r

ith

k':0.919

3

+J. JOv

30+3u

.

k"=1.033-#, A3o=

3o-3,

/tR rq\

(18.20)

and the rhermal transfer coefficient:. for radiation co a[(273 + 3o)a

Table 18.27

Comparison of fictitious thermal resistances ?ii between calculated lalues from equation 18.10 and graphical results from Fig. 18.9

Arrangement

)-\ to

I8.71

7ii to equation 18.10

In ground,

0.0160

0.469

0.0163

0.601 ,)

0.0116

0.448

Tiii to Fig. 18.9 l)

bunched

In free air, bunched

In ground, side by side "

1r

valucs for cabks with PVC shlath Calcularcd to Scciion 18.4.2

186

:0.545

-(273 + 3u)*] ,

A9o

(t8.ll) -

where o:5.67 x l0-E W/m2 Ka (Stephan-BolzmaYn constant) and eo the emissivity of the cable surface. With the factors k' and k" for the mean temperature account is taken to tbe variable quantities of the air.

Fig. 18.10 [18.10; l8.ll] facilitates the selection of auxiliary values forf,/* and k for the arrangements selected as specified operating conditions (Table 18.4) [18.7; 18.10].

-

-

The cable shown in Fig. 18.10a radiates freely in all directions. The heat is transferred by radiation from the cable surface to the walls of the room in which the cable is situated. A decisive factor in the temperature rise of the cable surface at a constant rate of loss is the temperature of these walls which normally one would expect to be at ambient temperature.

{hermal

/,=1

Resistance of

Air l8'{

Fig. 13.l0d illustrates free heat dissipation by conlection. The heatcd air initially florvs around the cable (laminar limiting layer) then rises uprvards in laminar form mixing with cooler air from the surroundings in an area of turbulence. A decisive factor in the temperature rise of the cable surface is, in this instance. apart from the cable diameter and amount of losses, the temperature of the surrounding air. The selected clearances shown in Fig. 18.10e which are equal to the cable diameter do not obstruct the heat flow since the thickness of the {lowing lir stream is comparatively small.

for one outel cable: /.=t arc sin {d/2a}/180", ior the cenlle cable: [=1-2 arc sin ldl| ali180'

-

Heat dissipation by radiation

In the bunched arrangement the cooling area of the cable is reduced to approximately t$o thirds. Bi/ reducing the cooling surface area the thermal flow rvithin the cable is also hindered and because of this the thermal resistance of the cable is effectively increased [13.10]. This restriction in heat florv was taken lnto account rvhen calculating the \tlues shorvn in

OJ

itr v{fir

6 6665 uf f -1t'l

Fi-s. 18.9.

The temperature rise of the cable surface is:

,-'l

tLrLr-rU-rrt,d, -----;;---;irl(if ,Lu

a ,to

Heat dissipation by convection

rLu

, -1-

D'

'r'

(l 3.12)

and the temperature rise A3o of the conductor caused by dielectric losses is:

Fig. 18.10 Heat dissipation, installed in free air

A 3d

= P;{7id + Ti,).

(18.23)

The thermal resistance of the cable Tiu can be calculated e.g.:

'- -e emissivitv of a cable surface can be taken €^0.95.

as

The same considerations also apply for the arrange.-ents 18.10b and 18.i0c. However any obstruction .- the thermal transfer must be considered. In Fig. 18.10b three single-core cables of a three-phase system are shown where only the thermal radiation from the centre cable is indicated. It is seen that the neighbouring cables obstruct heat transfer to the surroundings in the areas shown shaded. The reduction in heat dissipation is approximately directly proportional to the part of the cable surface embraced by the shaded angles.

In

Fig. 18.10c three single-core cables are shown bunched in trefoil. The obstruction in this arrangement is greater than that ofFig. 18.10b since approximately one third of the cable surface considered does not radiate heat to the surroundines.

tr

Calculation of temperature rise A36 to equation 18.22 and 18.23 with Tr_"=0.5 KmrW;

tr

Calculation of thermal resistance l"r, to equation 13.17

D

to

18.21

the calculations must be repeated n times until the difference between (?f,)" and (7t.)"-t is suffiently small.

For a multi-core cable without dielectric losses and with a 30'C ambient temperature, the external thermal resistance can be reasonably accurately obtained from the curves shown in Fig. 18.11a. Where the dielectric loss can be ignored one obtains from equation 18.22

AgL,_

?ii+?i.

A3o ri"

(18.22a)

By a graphical method, assuming a cable having a fictitious thermal resi":: '! " . . i KmAM with 187

18 Currcnt-Carrying Capacity in Normal Operation

0uter diamerer d

)ter

dr-

i\m

-24 26. 28

l0

45 50

80Temperature iise of cable 40

20 20

30

J0

40 Temperature rise of

-i-

a9" .c60 +

50

60

5U

t

0ri i

'l

0

02-l

\___________ Temperaturd rrse

ol

cable surlace

ior 20 Jo 30

50 510

50

Temperature tise of conduclot

odJ

\

061

700

r50 __

A3o-

"c

60

d8gl-_ |

\^

081

, Km

|(m

I

lwl 12'

1.0

r;-rili0us Thermal resislance 0f cable

Fig. 18.1I

/ii

a

Thermal resistance of air for a cable instailed horizontally in free air (3u= 30'C; €o = 0.95)

188

|

lFicririous Thermal resistance oi cable I'i,

\v' Fig. 18.11b Thermal resistance of air for three cables bunched in free air installed horizontally (9u=30'C;

so

= 0.95)

-

Thermal Resistance of

--.1:-.--_ ';i a ir

$_" "

ii,

Air

18..1

l8

a permissible temperature rise of A3.. = $Q K. entering these values as coordinates in Fig. 13.lla gives the point P. Through point P a straight line must be drawn such that point P', rvith the thermal resistance oI air Tt" and the temperature difference value A3o as coordinates, lies on the curve corresponding to the cable diameter d=32 mm.

20

The following values are obtained from the graph:

0uter drametet

0l I

mm to

22.

t":0.7

24 26

KmrW,

A30:40 K

28

For bunchetl single-core cables [18.10] the thermal resistance of the insulation and the outer sherth is increased due to obstructed heat dissipation. For cables to Fig. 18.8 without a thermally conducting metal sheath one derives:

Temperature

5r0

20

510

20

i0

t), :n dt

tl , L' = f i-L ln l-L ' '-

rise ol cable surface

40

50

f,,

rise oi conducror A $L

02

-

,/rr

_:_+_

0.6

us15)

(18.26)

T'r-f*tf-t-,. Jtt'l 'l

JlltM

^.1 rl:+ln+, :JT AL

0.8

(

13.27)

I

r\t

1.0

-fiorlr{s

'..

( 13.2.1)

.

:ft

For cables rvith metallic covering and hence improved heat dissiparion the follorving applies rvith additional reference to Table 13.29:

0.4

rm

,). :-Ltn ,l

*S,.''"(f) ':f

40

J0 Temperature

T: = f ' "'

lhermal fesislance of cable

Il,

Fig. 18.1I c Tr"ermal resistance of air for three single-core cables .-.-talled side by side in free air

-

914 o 11.otr, 'vv

gtt dt 'i f,n

+X

L

0 3.28)

"ry1

The thermal resistance of the outer protective covering Ij is calculated using equation 18.24 with equa-

tion

18.25.

The thermal resistivity gM of the metallic covering has to be taken into account and mav be selected from Table 18.28. Table 18.28 Thermal resistivity

Material

g'

Thermal resistivity

9,,,

KmAv Copper

Aluminium

l04.8 . l02.7.

28.7. 10-

3 3

l

19.1 '1-3

t89

l8

Current-Carrying Capacityjn Normal Operation

Table 18.29 Valucs required for the calculation of the effcctive thermal thickness of a sheath or screen Sheath-. screen factor

Thermal effective thickness of sheath or screen

Mean diame ter of metal shcath or screen

Metal sheath

du-du

,)r, 7r

n Tapes with spacing

dn. +:)

d"-J

nb(1

Wire screen with a 570 increase tn length due to the helix and with n transverse helical taPes Tape overlapped (as roof tiles)

tdu.

l-

nb(l +:)

t1-7

{=

,i)t'

dn-

6

I

-.t

dM-l(t

I

-t ---ubtr + =t 0ll f.| ------:

d"

applied rvith:=0.05

Two tapes aPPlied without 3pacing.

I

I

+:'

rT,,/r'

- l,)

-

rr Ar approximatc consideralion hcal dissipation lhrough thc \\'ires

Expression:

r.lq

b rr z d 6"

Diameter over the metal sheath or screcn (lransversc hclical tapc) ,){ : 24 or

dr d respectively 96 > 9. : 9o=

9r'-Pi Tit-

9.=3e+A9'. )n)

PiTi.d,

(18.64) (18.65)

Thermal Resistance of the Soil 18.'l

Fig. 18.23 Fictitious soil-thermal resistance f. at nr= 1.0, l'ly at nr=0.7 relative to outer diameter d of cable and depth of iay /r for a soil-thermal resistivity of s. = 2.5 Km/w and pE = 1.0 Kmiw

1.0

I

.

---70mm.

6.0

(9 q/ i9

5,J 4.0 3.0

1.5

1.0

2.5 2.0

ta

1.0

08 0.6 0.5

40 50 50 80 100 150 Outet diameter

200mm

oi cable d

-

The ohmic losses in equation 18.64 must be determined using the load capacity calculated for dryingout the soil. If the surface temDerature is found to be less than the temperarure of the boundary isotherm, the calculation for load capacity must be rePeated but under the assumption that the soil does not dry-out. The calculation routine described above

To simplify this calculation the characteristic diameter is to be determined using the thermal resistivity of the moist area. A comparison of diameters is therefore avoided and the result is on the safe side since the lower thermal resistivity results in a maximum value for the characteristic diameter.

ior capaclly Jo < J! must be satlslied. 203

l8

urrent-Carry ing Capacitl'in Normal Operation

C

Example 18.7 Three single-core cables

\A2XS2Y I x

150

RM/25 12/20 kV

are installed in ground under different oPerating conditions. Dimensions and thermal resistane of the cable can be taken from example 18.2 on page 185

or from Part 2, Table 5.6.6a. Bntched installatiotr for the speciJied operatittg conditions to Table 18.2.

Type of operation: Supply utility operation with nr=0.7 or any equivalent load variation (Fig. 18.6) rvith a frequency of load cycles x: 1. u

= 0.3

rrr

+0.7 rnr: 0.J x 0.7+0.7 x 0.7tr = 0.553,

r/i n o\o '

,'

:0.186

(18.51) m

(18.51)

,

Krn

2x0.7 35.7x 10-3

(13..14)

-l

(

, )h 2x0.7 JT "^".Kt ir ft-' ^" = 7= lstlo.l: T:,=

*[lnk+ ts

=f

3(p-

ttn 78.42+3

^ r',: h]nk*z

1)

(1

lnk"+2 lnk,]

15

fr

[n

8.18)

(18.61)

l tn q 6q+) ln ta ttt= -.. I aas .- Kt (0.553-1/... w

tn,t"J =

13.55)

78.+2+2ln

,

Km

39.:3]:4.656;.

(18.62)

{l-07) 100-25K. t3_:tr, (l-rn)3 100=15 '- r

(18.56)

.

is made with t ,: specilied operating conditions to Table gives in equation 18.63 the rated value ,f the load capacity /. with P: = 0. f'Kt = 0 469 KmTw as in Sectio 18.4.1 and R*.:0.269 Q/km as in Part 2, Ta : 5.6.6a. Since the calculation

18.2, this

o3-. A, ,,:t,/ry4+''r '' V nR*,(T'11;+r',,) t@= / txo.z6ex l0-3(0.469+3.445) '--"' 320

30=91.-Prfli: St,-n Il R-,- =90-1

(18.63)

x3202 x0.269 x10-3 x0.469=77'1 "C' (18 64)

with

P:: 3,:

0 and Pi to equation 18.3 Se

+ A9, = 20 +25 =

45'C.

The assumption that 90> 9, is 204

there:

(18.65)

verified.

Thermal Resistance of the Soil 18.'l

Btutchetl tnstulltrtiort

For

rn: I

then

4:

rlitlt lr =

The same vrlue is obtained using the rating factors of Table 18.15 (/i =0.93) andTable 18.19(/::0.3,i)

I

I and T'." = 1'

l'=i

rl-lllon

A3.:t5-" ;""":15K. ao, . - F*-Wfr.,tsll____;r*tr,.,*nrl _ ,,=

(18.s6)

(18.63)

For nr: 0.7 3o=

The current-carrying capacity 1.:120A is identical to rhe ouantitv siven in DIN VDE 0298 Part 2 as ti )e seen in Table 18.11From the quantities for /, and I. the rating factor is

90- I x 353r

:74.1"C.

V

90-10+[(].5,1]-11 l5 _1(q l _r/ !1' = [/ lro%9.10=l{0..t69+4556)--ra

xf 2= 0.93 x 0.35 = 0.79.

i = lOr- ?5 = 45 oa and for

rrr:

1.0

3o= 90- I x 277: x 0.285 x = 80.2 'C, .'l =1n-! I\= ltoa 3o

l0-r

x0.'l-18 (

13.64)

> 3, .

159

The same value is obtained Table 18.l5 Ur

:

f: f, f.: x

by using

factors from

0.93) and Table 18.171/,=6.371 t";,1t

Calttltttiotr of diunterer d, untl tlepth of lay h, of the tlrr areu Jbr u bunclrcd installatiott und lor m= 1.0 Assuming: r/, > d,

0.93 x 0.87 = 0.8

1.

ho=tn

The individual thermal resistance can also, in this case, be calculated using the equations in Fig. 18.16 and Fig. 18.22. It is however easier to take these from the graphs in Fig. 18.23 and 18.9 or Part 2, Table 5.6.6 b giving

= 0.448 KmAV.

ls

-"r =.I/rn

h. = ho

For R;.:9.235Q/km (according to Part 2, Table nr

x

l0-i l)i

, | 2rA3, l k,:expl;_r*l

r/,

T',:3.794Kmlw, T'.r: 2.583 Km7-W,

",'.6 b) and

ltl-r::VO;,-(:S.l

=0.7 m,

(18.41)

Insrallatiott in Ground Side by Side

'l'kr

x l0-3 x 0.448 (13.64)

In both cases therelore

.1 . l, ,t1,3:o

. '.

x 0.285

with

= 0.7 is

dy

Qr.

l3 x

lx

1.0x1592 x0.269x l0-r_l

/. 570 ;'' , = 4x0.7 ;i=la;-l )./u--l ;

(Fig. l s.la)

I r;)

= 0.51 m, (13.66)

t,2-Ll < -nz Ll ^4J,M:--:-_-:0.7 0.74 = ;:i ; K;-t ).iu--l =

m.

(

13.67)

= 0.286 m the assumption d, > d, is proven.

go-20+[(2.trt-lJx _.r,.r ,__r/ ' t* 0.285 x l0-r10.4-+3+2.583) ^ v

and for

rz:

t.o

_,1 eo-2GtdtD-lJts

,-

/

r x0.285x t0-r(0.lt48+3.794)

(18'63)

^--. (18.63)

The two quantities give a resulting rating factor of

I

277 )i!

205

l8

Curren t-Carry ing Capacity in Normal Operation

Diameter of the Dry Area

tI-:/+ll^-

The diameter of the dry area rvith respect to the characteristic diameter can be determined once the load capacity is knou'n. For this calculation the necessary'

the depth of the boundary isotherm is given by

/',:ro#

geometric factors k, for a multi-core cable as well as for three single-core cables are shown in Fig. 18.24. The diameter for the dry area is obtained from

G^,

d.,dv

',,="'pl::*l

t'

k,=exp

d,'dy

- d,.dy ,-

1"9

[(?

i,=exp

d|.dr.2a

d..22'd, k,=exp

lzrat

/5aE

r o\ Pov 0t>0,>G \//o \ lJ._', t,=e,o

{tff

v/

-tr-r, ) (tn

k,) /

lFJfl))

+(r-p

1'tn

t,)rf 4+g)]

) P,tn kt)

/

lil

lP'+

€Fi

{pPJP;)l

2a-d^t

d,t

r,,=",p

t(ff

?a-d,r-da

dr.\a-d,1

-(t-r,)

@r t({p

r,="'olffii)

'

d,t

- (p4 = P)

/

2 tn k

"l

tpl! ftll

t@{::,

d,1

rv- (P,+{) 2ln

n J$, ;+3 Gpl 4ln rrl=expL-----:--+E-r ,

2

/p

t.,

.,2r!t|,/pp+(1-p)frlnit-(pPJff)2lnt.' k1=ex9t_________FE-|

Fig. 18.24 Geometric constants of the dry area for one three-core cable and three single-core cabies 206

(18.67)

and the depth of the line source /to is derived from equation 18.38.

/6\--4

d, d.!>

13.93)

(I (

20 30 40 50 60 80 100, cm 200

Fig. 18.36 Equivalent radius r, = r.lrl2 of a duct bank rvith dimensions r and _r' in Fig. 13.3.1, where ;s 4 -1' provided rhat

,r'-rSl

Lastly it must also be investigated rvherher the assumption r/, > ri u applies: J,

18.93 a)

=

.l lrs

L.

;;f-:,

(18.66a)

rvit h

dB (

'ir!,', from

(l 8.93 b)

equation 18.93;

18.92b)

k, = exp

2r /3, q, N, (p Pi+ Pi)

(18.e4)

and Pi to equation 18.3 as well as Pj ro equarion

".ngle-core cables and d,> dR> d"

18.4.

from equation 13.92.

r*: Pri

l0

8.92 a)

?'i,* from equation 13.92

Tf

r0

(I

8.92 c)

(18.93c)

.

If d,