High-Voltage Technology 16. High-voltage Cable

16.1 Introduction 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

16.2 Development of the underground cable Withstand higher working voltage Spark discharge -> failure paper (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

Impregnating compound Thermal expansion coefficient 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 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

Mechanism of deterioration and failure(early 1930s, Robinson) stop contraction voids occurring fill voids with high-pressure gas Oil-filled cable Dielectric 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

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 – conductor B – cabonized paper C – cellulose paper D – cabonized paper E – metal-foil screening F – lead-alloy sheath G – oil-impregnated paper H – circumferential copper tape I – corrugated bronze tape J – copper binding wire K – for submarine cables L – armoring wires M – impregnated jute and bituminous compound

Oil-filled cable system 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 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) Pipe-line system high oil pressure(200 lb/in2 or more) increase in impulse strength and AC strength 3 core 33kV gas-filled cable

16.3 supertension oil-filled cable system 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 A pressure tank consists of a number of biscuits Biscuit(consist of two circular diaphragms)

Filled with gas Standard tank : 1 atm Pre-pressurized tank : 1.5 or 2 atm Oil Oil

The limiting oil-pressure in the system Minimum static : (3 lb/in2) Little gain in oil capacity for increases of pressure above about 30 or so lb/in2 The limiting oil-pressure in the system Minimum static : (3 lb/in2) Maximum static : (75 lb/in2) Maximum transient : (115 lb/in2)

To withstand the internal oil–pressure of the cable lead or lead-alloy sheaths are reinforced with bronze or steel tapes Plain aluminum tube Sheath thickness required to withstand the buckling force of bending is much larger than the thickness required to contain the internal oil-pressure By 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

Oil-filled cable is manufactured in discrete lengths coiled on to drums Required insulating Straight joints Trifurcating joints Termination Hydraulic barrier Ensure 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

16.3.2 Thermal design Burying an oil-filled cable is not simple Cover tile To protect from mechanical damage from any excavation Minimum 36 in (safety reason) Sand To avoid damage Minimum 27 in

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) Require the use of empirical formulate of the thermal resistivity controllable Estimated by consideration of the nature and structure of the ground But major variation by ground surface type

16.3.3 Electrical design 132 kV system Install 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 system Single-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

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

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 voltage 1.33 times of working voltage (275 kV and 400 kV cable) Thermal stability test (only 132 kV cable and above) impulse test Comparison of test impulse voltages and working voltage 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 peak important

The dielectric of the oil-filled cable Required properties of the oil Lowest viscosity volatility A degree of gas absorption to soak up any residual gas 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

The butt-gap width controlled by the cable-bending requirements But consideration of buckling force The butt-gap depth is the thickness of the paper tape Paper thickness is reduced by decreasing the tension But consideration slack

The dielectric power loss = 33 kV – 1% of full-load conductor loss 132 kV – 10% 275 kV – 50%

Decreasing the conductor stress would reduce the cable capacitance But increased the thermal resistance, size, cost Lower density paper - reduce the capacitance But consideration of mechanical and impulse strength

Reducing loss angle ->reduce the thermal instability Hand applied insulation must be operated at lower stresses

16.3.4 recent and future developments Power cable development New material of superior performance (same job with the same reliability, lower cost) Exploit the potential of existing materials Polythene High electric strength, low premittivity, dielectric loss angle, thermal resistivity, cost But it is too small to attract supply engineers Weak resistance to electrical discharge extruded wall must be free from voids internally requirement of freedom from void at insulation/electrode interface

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