High-Voltage TechnologyHigh-Voltage Technology

16. High-voltage Cable

Cost : Underground power-cable > Overhead-line

Example : dissipation of heat Overhead-line : directly to free air Underground power-cable :

cable insulation, sheath, serving -> ground

Working temperature Overhead-line : limited only by mechanical considerations Underground power-cable : limited by the characteristics

of insulation materials

Carry same current – cable resistance per unit length is smaller than overhead line (increase cost)

Required mainly for reasons of amenity

16.1 Introduction

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16.2 Development of the underground cable

Withstand higher working voltage

paper

Spark discharge -> failure

(earth potential)

Voltage limit of cable : reached at 66kV But conductor stress was not higher than about 40kV/cm Higher voltage cables require large insulation thicknesses

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Impregnating compound 70 X 10-5/K

Copper 5 X 10-5/K

Lead 9 X 10-5/K

Paper fibres 15 X 10-5/K

Thermal expansion coefficient

Weakness : voids(formed by the differential expansions and contractions)

Excess impregnating compound can be accommodated by distension of the lead sheath On cooling, lead sheath exerts little pressure to force excess compound Lack of compound in nearby conductor causes contraction void

Void : locally high electric stress, electrically weak, ionization at low electric stress

At high conductor-stress Ionization causes waxing, local deterioration of the paper Deterioration forms treeing pattern of discharge tracking, cable failure

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Mechanism of deterioration and failure(early 1930s, Robinson)I.stop contraction voids occurringII.fill voids with high-pressure gas

Oil-filled cableDielectric is impregnated with fluid oil(maintained by externally-connected pressure tank)Oil passage :

along the conductors or between the cores

Always fully impregnated and contraction voids cannot form

Operating pressure : not exceed about 75lb/in2

Pressure limitation defines the low-pressure oil-filled cable system

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Möllerhoj cable(three-phase cable) : low-pressure oil-filled cable

Arranged in flat formation, flat sides of the lead sheath (as elastic diaphragms) Self-compensating(expansion and contraction)

A – conductorB – cabonized paperC – cellulose paperD – cabonized paperE – metal-foil screeningF – lead-alloy sheathG – oil-impregnated paperH – circumferential copper tapeI – corrugated bronze tapeJ – copper binding wireK – for submarine cablesL – armoring wiresM – impregnated jute and bituminous compound

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I. Oil-filled cable system

II. Oilostatic system contained in pipe filled with viscous oil at high pressure

(up to about 1000 lb/in2) the use of a viscous oil without oil impedance problems

III. Gas compression cable system consist of an impregnated core sheathed with either

lead or polythene, the sheath being slightly oval space between inner(as elastic diaphragm) and outer

sheaths filled with high-pressure gas (200 lb/in2)

IV. Pipe-line system high oil pressure(200 lb/in2 or more) increase in impulse strength and AC strength

3 core 33kV gas-filled cable

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16.3 supertension oil-filled cable system

Oil-filled cable Ever fall below atmospheric pressure Dielectric and oil contain the minimum practical quantities

of air and moisture Manufacture : vacuum-dried, vacuum impregnated, de-

gassified oil

16.3.1 Hydraulic design

Biscuit(consist of two circular diaphragms)

A pressure tank consists of a number of biscuits

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Filled with gasStandard tank : 1 atmPre-pressurized tank

: 1.5 or 2 atmOil

Oil

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Little gain in oil capacity for increases of pressure above about 30 or so lb/in2

The limiting oil-pressure in the systemMinimum static : (3 lb/in2) Maximum static : (75 lb/in2) Maximum transient : (115 lb/in2)

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To withstand the internal oil–pressure of the cable lead or lead-alloy sheaths are reinforced with bronze or steel tapes

Plain aluminum tubeSheath thickness required to withstand the buckling force of bending is much larger than the thickness required to contain the internal oil-pressureBy using an oversize tube and corrugating

sheath flexibility is increased by the bending movements being accommodated by the ribs of the corrugations and the sheath thickness can be considerably reduced

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Oil-filled cable is manufactured in discrete lengths coiled on to drums

Required insulating Straight joints Trifurcating joints Termination

Hydraulic barrierEnsure that the transient pressure at any point in the installation is held below the specified limit

• Oil pressure-tanks are connected at the low-pressure side of a stop joint• But at times it may be necessary to connect to a lower point of

the cable (when no site is available at a lowest pressure point for pressure

tanks)• The head of oil can then be accommodated by pre-pressuring the

biscuits of the pressure tank so that zero useful oil content occurs at higher pressure

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16.3.2 Thermal design

Burying an oil-filled cable is not simple

SandTo avoid damage

Cover tileTo protect from mechanical damage from any excavation

Minimum 27 in

Minimum 36 in(safety reason)

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SDCD

CAC WWWGW

WGTT

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Simple heat-transfer equation by conductor loss

WC : conductor loss WD: dielectric loss WS : sheath loss

TC: conductor temperature(maximum 85℃) TA: air temperature

G1: thermal resistance of the cable G2: thermal resistance of the heat path(ground to air)

controllable

Require the use of empirical formulate of the thermal resistivity

Estimated by consideration of the nature and structure of the ground But major variation by ground surface type

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132 kV systemInstall cost of three-core cable is lower than three single-core cable Because of decreasing as conductor size and increasing cable voltage

150 to 200 kV systemSingle-core cables are used exclusively Because of impracticable to manufacture three-core cable

16.3.3 Electrical design

• Cable must be coiled, a stranded conductor(smooth surface) is used Since it has minimum value of electric stress on the conductor surface

• The surface is smoothed by lapping the conductor with several layers

Metallized paper Carbon paper Metallized carbon paper

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Metallized paper No conductivity normal to its surface Three-layer structure is required to smooth electrode surface to the dielectric

Carbon paper Advantage of conductivity through the paper and a conductor screen Advantage oil-cleansing characteristic Disadvantage is that the particle activity at the surface facing the dielectric causes a rise in power factor with increasing voltage

Metallized carbon paper Through-conductivity of carbon paper But power-factor increment effect is little at metal surface to the dielectric

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New design of oil-filled cable must pass CEGB type approval tests

bending

loading cycle, thermal stability AC test voltage 1.5 times of working voltage1.33 times of working voltage (275 kV and 400 kV cable)Thermal stability test (only 132 kV cable and above)

impulse test

R.m.s. value system voltage

Vs

R.m.s. value working voltage

Vw

Test value impulse voltage

Vp

Ratio

Vp/Vw

33 19 194 10.2

66 38 342 9.1

132 76 640 8.4

275 160 1050 6.6

400 230 1425 6.2

kV r.m.s kV r.m.s kV peak

Comparison of test impulse voltages and working voltage

105.4 wp VV

important

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The dielectric of the oil-filled cable Required properties of the oil

a. Lowest viscosity volatilityb. A degree of gas absorption to soak up any residual gasc. Low loss angle and high chemical stability under

temperature and stress Properties of paper still are considerable

For the same thickness, double-ply paper is more uniform in structure than single-

ply

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The butt-gap width controlled by the cable-bending requirementsBut consideration of buckling force

The butt-gap depth is the thickness of the paper tapePaper thickness is reduced by decreasing the tensionBut consideration slack

The dielectric power loss =33 kV – 1% of full-load conductor loss132 kV – 10% 275 kV – 50%

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CV 2

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Decreasing the conductor stress would reduce the cable capacitanceBut increased the thermal resistance, size, cost

Lower density paper - reduce the capacitanceBut consideration of mechanical and impulse strength

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Reducing loss angle ->reduce the thermal instability

Hand applied insulation must be operated at lower stresses

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16.3.4 recent and future developments

• Power cable developmentNew material of superior performance (same job with the same reliability, lower cost)Exploit the potential of existing materials

•PolytheneHigh electric strength, low premittivity, dielectric loss angle, thermal resistivity, costBut it is too small to attract supply engineersWeak resistance to electrical discharge extruded wall must be free from voids internally requirement of freedom from void at insulation/electrode interface

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• 750-850MVA, polythene alone due to lower loss and thermal resistivity