Transcript High-Voltage Technology

High-Voltage Technology
16. High-voltage Cable
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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
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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
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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
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Mechanism of deterioration and failure(early 1930s, Robinson)
I. stop contraction voids occurring
II. 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
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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 – 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
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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
A pressure tank consists of a number of biscuits
Biscuit(consist of two
circular diaphragms)
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Filled with gas
Standard tank : 1 atm
Pre-pressurized tank
: 1.5 or 2 atm
Oil
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 system
Minimum 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 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
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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
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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
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Simple heat-transfer equation by conductor loss

WD 
  G2 WC  WD  WS 
TC  TA  G1 WC 
2 

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
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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
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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 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 r.m.s
kV peak
important
Vp  4.5Vw  10
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The dielectric of the oil-filled cable
 Required properties of the oil
a. Lowest viscosity volatility
b. A degree of gas absorption to soak up any residual gas
c. 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 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
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The dielectric power loss = V 2C
33 kV – 1% of full-load conductor loss
132 kV – 10%
275 kV – 50%
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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
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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 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
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• 750-850MVA, polythene alone
due to lower loss and thermal resistivity