_ good (elec6089) power cable insulation design

7/30/2019 _ Good (ELEC6089) Power Cable Insulation Design

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Power Cable Insulation Design

General design criteria

Differences between cables and overheadtransmission lines

Electric stress distribution in coaxial cable Electrical insulation design Future developments in power cables


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General design criteria of power cables

The following factors govern the design of powercables:

The cross-sectional area of the conductors chosenshould be of the optimum size to carry the

current without overheating and should be withinthe required limits for voltage drop.

The insulation applied to the cable must beadequate for continuous operation at the specificworking voltage with a high degree of thermal

stability, safety and reliability.


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All materials used in the construction must be carefully

selected in order to ensure a high level of chemical andphysical stability throughout the life of the cable in theselected environment.

The cable must be mechanically strong and sufficientlyflexible to withstand the re-drumming operations in themanufacturers works, handing during transport or when

t e ca e s nsta e y rect ur a , n trenc es pu einto ducts or laid on cable racks.

Adequate external mechanical and/or chemical protection

must be applied to the insulation and metal or outersheathing to enable it to withstand the requiredenvironmental service conditions.


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A typical XLPE power cable


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Cable Components Conductor (Copper and aluminium)Maximising the current carrying capacity byminimising the ac effect (skin and proximityeffects)

reven ng s or on o e con uc or ur ng ebending operation on the cable

Semiconducting screens (Carbon paper andcarbon loaded polymer)To ensure a smooth interface between conductingand insulating area


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Insulation (Paper, PVC, XLPE and EPR)Isolate high voltage conductor from the earth

Metallic sheath (Lead, lead alloy and corrugated

aluminium alloy )Pressure retaining in the case of SCOF cable

(XLPE)

Other protection (PVC and HDPE)

To prevent metal sheath against corrosion


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Differences between cables and overheadtransmission lines

Overhead lines Cables

Cheap (air is a good insulation) Expensive (insulating materials)

Easy to maintain High repair cost

u e no se em ss on ess space requ re

Radio and TV interference Well screened

Emission of ozone and oxides of

nitrogen

Environmental clean

Safety and comfort problemscaused by electrostatic fields

No direct safety threat

Suitable for rural area Suitable for urban


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Different types of cables Conventional ac cables

-- High-pressure oil-filled (pipe-type) system-- Self-contained low-pressure oil-filled system--

Conventional dc cables Compressed gas insulated (CGI) cables

Cryogenic cables and superconducting cables.


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Electric stress distribution in coaxialcable

Under ac and impulse conditions, the stressdistribution in a concentric cable iscapacitance-determined.

.The stress at radius x, Ex is given by

or xQ

x E 2=


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The voltage between conductor and outerdielectric screens or sheath is

)ln(22 r RQ

R

r xQ

R

r x

or or dxdx E V === s nce an

then

and

= mr Ror

)ln(=

V E r

R x

V

x/

)ln(=

mV E r

Rr V

r / )ln(=


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The stress shows its maximum value at the surface

of the conductorThe minimum value of E r is found from dE r/dr=0,and occurs when ln(R/r)=1, i.e. R=2.718r, whenEr=V/r. This optimum relationship is oftenoverridden by other considerations for conductor

ra us.

where R m is defined as) / ln( r R R

V r R

V a m

E ==

) / ln( r Rr R

m R=


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Insulation Life

Factors affecting the life of an insulation system:-- Temperature which changes electrical properties such

dielectric loss tan and also mechanical and chemicalproperties.-- Mechanical, due to differential ex ansions between

the insulation and the surrounding sheath and also theconductor; due to forces set up on the conductor duringshort-circuit conditions.

-- Presence of partial discharges.-- Oxidation.-- Treeing.


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Experience over many years on samples and real cables

has indicated that the life of a cable at constanttemperature is governed by the empirical equation

This law is utilised b maintainin constant stress on the

.)(const k tE n =

dielectric and measuring time to failure.Life under service conditions is obtained byextrapolating the straight line resulting from the plot of

Log(E) against log(t). This assumes that the samemechanism which has operated at high stressesoperates at the service stress.


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Life-temperature relationships

Insulation life (time to failure) and temperature are

related by the equation

B

where L=life in hours

T=absolute temperature (K)A and B are constants for a material.

T


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Insulation thickness Impulse Voltage :

ai E BIL

=

BIL= Basic insulation voltage level (kV)Eai= average impulse breakdown stress

(kV/mm) = impulse ageing coefficient (=1.2 for XLPE)= temperature coefficient (=1.3 for XLPE)


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AC Voltage:

Ui= ac testing voltage (~3U 0) (kV)

Ea= long term average ac breakdown stress (kV/mm)

a

i

E U =

=ac age ng coe c ent = or n=


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Main steps in power cable manufacture


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The design of cables is to a large extent regulated by anumber of industry, national and internationalstandards and guides

Medium voltage up to 33 kV High voltage 33 to 400 kV

Lapped paper, dried andimpregnated with viscous ornon-draining compound BS

Lapped paper, dried andimpregnated with low viscousfluid (oil) Eng. Recom C47,

6480[2], EA TS 09-12, IEC60055Extruded PVCExtruded vulcanised (EP)rubbers BS 6622, IEC 60502Extruded crosslinkedpolyethylene BS 6622, IEC

60502

NGTS 3:5.1, IEC 60141-1

Extruded vulcanised (EP)rubbers

Extruded crosslinkedpolyethylene TPS 2/12, IEC

60840, HD 632


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Example: 500 kV XLPE Cable(Jcable 99 paper A1.1)

Insulation design

)(

3210 )3 / (

AC L AC E

k k k E t =

where t AC: Insulation thickness required for AC withstandvoltage (24.1mm),

E0: Maximum line-to-line voltage (550kV),

k1: Deterioration coefficient (2.3),k2: Temperature coefficient (1.2),k3: Allowance for uncertain factors (1.1),

EL(AC): AC design stress (40kV/mm)


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wheret imp : Insulation thickness required for lightning impulse

)(Im

'3

'2

'1

p Limp E

k k k LIWV t =

w t stan vo tage 26.7mmLIWV: Lightning impulse withstand voltage (1550kV)k1: Allowance for repetitive application of lightningimpulse (1.0),k2: Temperature coefficient (1.25),k3: Allowance for uncertain factors (1.1),

EL(Imp): Lightning impulse design stress (80kV/mm)


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Structure of 500kV XLPE CableNominal voltage kV 500

Number of core 1

Conductor Nominal cross section mm 2 800

Outer diameter mm 38.0

Thickness of conductor screen mm Approx. 2.0

.

Outer diameter of insulation mm 102

Thickness of insulation screen mm Approx. 1.0

Thickness of cushion layer mm Approx. 3.0

Thickness of aluminium sheath mm 2.8Thickness of PVC covering mm 6.0

Overall diameter mm Approx. 133

Net weight kg/m Approx. 21.5


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Future developments in power cables

High voltage extruded cables 500kV-- improve electrical performance of insulatingmaterials

High voltage DC power cables -- ,

Superconducting cables-- low dielectric loss (tan ), higher electrical breakdownstrength, less partial discharges, higher electricaltreeing resistance and less thermal and chemicaldeterioration

_ good (elec6089) power cable insulation design

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