Power Pow er Cable Insulation Insulation Design
• General design criteria • Differences between cables and overhead transmission transmission lines
• stress distribution in coaxial cable • Electric stress • Electrical insulation design developments in power cables • Future developments
General General design criteria of power cables The following factors govern the design of power cables:
• The cross-sectional area of the conductors chosen should be of the optimum size to carry the current without without overheating and should be within the required limits for voltage drop.
• The insulation applied to the cable must be adequate for continuous operation at the specific working voltage voltage with a high degree of thermal stability, safety and reliability.
• All materials used in the construction must be carefully selected in order to ensure a high level of chemical and physical stability throughout the life of the cable in the selected environment.
• The cable must be mechanically strong and sufficiently flexible to withstand the re-drumming operations in the manufacturer’s works, handing during transport or when t e ca e s nsta e y rect ur a , n trenc es pu e into ducts or laid on cable racks.
• Adequate external mechanical and/or chemical protection must be applied to the insulation and metal or outer sheathing to enable it to withstand the required environmental service conditions.
A typical XLPE power cable
Cable Components • Conductor (Copper and aluminium) Maximising the current carrying capacity by minimising the ac effect (skin and proximity effects) reven ng s or on o e con uc or ur ng bending operation on the cable
e
• Semiconducting screens (Carbon paper and carbon loaded polymer) To ensure a smooth interface between conducting and insulating area
• 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
Differences between cables and overhead transmission 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 problems caused by electrostatic fields
No direct safety threat
Suitable for rural area
Suitable for urban
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.
Electric stress distribution in coaxial cable • Under ac and impulse conditions, the stress distribution in a concentric cable is capacitance-determined. . The stress at radius x, Ex is given by
E x =
Q 2π xε r ε o
The voltage between conductor and outer dielectric screens or sheath is R
R
∫
V = E x dx = r
s nce then and
∫
Q 2π xε r ε o
dx =
Q 2πε r ε o
R r
ln( )
r
=
an
E x =
E r =
V xln( Rr )
V r ln( Rr )
=
r o ln( Rr )
V /
V / m
m
The stress shows its maximum value at the surface of the conductor The minimum value of Er is found from dEr/dr=0, and occurs when ln(R/r)=1, i.e. R=2.718r, when Er=V/r. This optimum relationship is often overridden by other considerations for conductor ra us.
E a =
V R − r
=
V Rm ln( R / r )
where Rm is defined as
Rm =
R − r ln( R / r )
Insulation Life • Factors affecting the life of an insulation system: -- Temperature which changes electrical properties such dielectric loss tanδ and also mechanical and chemical properties. -- Mechanical, due to differential ex ansions between the insulation and the surrounding sheath and also the conductor; due to forces set up on the conductor during short-circuit conditions. -- Presence of partial discharges. -- Oxidation. -- Treeing.
Experience over many years on samples and real cables has indicated that the life of a cable at constant temperature is governed by the empirical equation
n
tE = k
(const .)
This law is utilised b maintainin constant stress on the dielectric and measuring time to failure. Life under service conditions is obtained by extrapolating the straight line resulting from the plot of Log(E) against log(t). This assumes that the same mechanism which has operated at high stresses operates at the service stress.
Life-temperature relationships • Insulation life (time to failure) and temperature are related by the equation B T where
L=life in hours T=absolute temperature (K) A and B are constants for a material.
Insulation thickness • Impulse Voltage:
∆=
BILαβ E ai
BIL= Basic insulation voltage level (kV) Eai= average impulse breakdown stress (kV/mm)
α= impulse ageing coefficient (=1.2 for XLPE) β= temperature coefficient (=1.3 for XLPE)
• AC Voltage:
∆=
U i γ E a
Ui= ac testing voltage (~3U 0) (kV) Ea= long term average ac breakdown stress (kV/mm) =ac age ng coe c ent =
or
n=
Main steps in power cable manufacture
The design of cables is to a large extent regulated by a number of industry, national and international standards and guides Medium voltage up to 33 kV
High voltage 33 to 400 kV
Lapped paper, dried and impregnated with viscous or non-draining compound BS 6480[2], EA TS 09-12, IEC 60055 Extruded PVC Extruded vulcanised (EP) rubbers BS 6622, IEC 60502 Extruded crosslinked polyethylene BS 6622, IEC 60502
Lapped paper, dried and impregnated with low viscous fluid (oil) Eng. Recom C47, NGTS 3:5.1, IEC 60141-1 Extruded vulcanised (EP) rubbers Extruded crosslinked polyethylene TPS 2/12, IEC 60840, HD 632
Example: 500 kV XLPE Cable (Jcable 99 paper A1.1) • Insulation design
t AC =
( E 0 / 3 )k 1k 2 k 3
E L ( AC )
where tAC: Insulation thickness required for AC withstand voltage (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)
t imp =
' ' 1 2
' 3
LIWV ⋅ k k k E L (Im p )
where timp: Insulation thickness required for lightning impulse w t stan vo tage 26.7mm LIWV: Lightning impulse withstand voltage (1550kV) k1’: Allowance for repetitive application of lightning impulse (1.0), k2’: Temperature coefficient (1.25), k3’: Allowance for uncertain factors (1.1), EL(Imp): Lightning impulse design stress (80kV/mm)
Structure of 500kV XLPE Cable Nominal voltage
kV
Number of core Conductor
500 1
Nominal cross section
mm2
800
Outer diameter
mm
38.0
mm
Approx. 2.0
Thickness of conductor screen
. 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
2.8
Thickness of PVC covering
mm
6.0
diameter
mm
Approx. 133
weight
kg/m
Approx. 21.5
Overall Net
Future developments in power cables • High voltage extruded cables ≥ 500kV -- improve electrical performance of insulating materials
• High voltage DC power cables --
,
• Superconducting cables -- low dielectric loss (tanδ), higher electrical breakdown strength, less partial discharges, higher electrical treeing resistance and less thermal and chemical deterioration
