1. High-Voltage Technology
16. High-voltage Cable

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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
Hydraulic design
A pressure tank consists of a number of biscuits
Biscuit(consist of two
circular diaphragms)

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

10. 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)

11. 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

12. 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

13. 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

14. 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

15. 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
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

16. 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

17. 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

18. 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

19. 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

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

21. 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

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

23. 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

24. MVA, polythene alone
due to lower loss and thermal resistivity