Underground System Design TADP 547 Basic Cable Design I
Transcript of Underground System Design TADP 547 Basic Cable Design I
Underground System Design
TADP 547
Basic Cable
Design I
Presentation 4.1
Instructor: Frank Frentzas
Basic Cable Design
Power cable design involves designing a system that has
the required current capacity, and sufficient insulation to
withstand both normal and surge voltage levels that will be
experienced during operation. In addition, the cable system
must be able to withstand the installation and operation
environment.
Fundamental design criteria include insulation voltage
withstand, along with normal and emergency allowable
conductor temperatures.
In addition to the above criteria, other design aspects that
need to be considered include the following:
System Design Criteria
Conduit-to-cable size ratio for single conductor
cables or pipe-to-cable ratio for pipe type systems.
Grounding design of sheaths to minimize standing
voltage and circulating currents, thus maximizing
system current carrying capacity.
For oil filled cables the control of hydraulic pressure
to avoid cable sheath rupture and high pressure on
accessories during thermal expansion, and low
pressure during cable cooling cycles.
System Design Criteria (cont.)
Thermomechanical forces generated during thermal
expansion of the cable conductor means joints and
terminations must be designed to withstand these
forces. (A typical cable with a copper conductor can
produce several thousand pounds of thrust - larger
conductors produce larger forces.)
Design of supports and cable clamps to withstand
both thermal expansion forces and short circuit
forces.
System Design Criteria (cont.)
Size of manholes or joint bay to accommodate
joints during installation and normal operation.
For pipe type and metallic sheath cables with a
corrosion protection system must be considered.
Soil, cable depth, and other cable crossings or
heat sources in the cable environment.
Length of cable circuit must be considered since it
will affect the capacitance, inductance and surge
impedance of the system.
Electrical Field Stress
Defined as the maximum voltage insulation can
withstand before electrical breakdown or failure.
If voltage stress is not controlled it can lead to the
overall breakdown of cable insulation.
Cables should be designed to operate with a safe
electrical field stress based on operating voltage.
Higher operating voltages generally result in high
electrical field stress levels.
Allowable Electrical Stress
Semi-conductive shields are used to reduce field
stress levels in the conductor insulation.
AEIC standards provide recommend stress levels
for cables at various voltages and insulation types.
For 138 kV cable, allowable field stress is 8 kV/mm
at conductor or inner shield and 4 kV/mm at the
insulation or outer shield.
Allowable stress for 345 kV system is 12 kV/mm at
conductor and 6 kV/mm at insulation shield.
Maximum Internal Stress
Provided by industry standards such as AEIC, ICEA, IEEE,
and IEC, as shown in Table 4-2 from ICEA S108-720 below:
Allowable Stress
Table 2.3 – 1 Allowable field stress for nominal internal and external
stress (from AEIC CS9)
Rated
Voltage
kV
Conductor
Size kcmil
Conductor
Size mm2
Max.
Insulation
Eccentricity %
Nominal Internal
AC Stress Limit
V/mil (kV/mm)
Nominal
External AC
Stress Limit
V/mil (kV/mm)
69 wet 500-4000 240-2000 12 100 (4.0) 50 (2.0)
69 dry 500-4000 240-2000 12 150 (6.0) 75 (3.0)
115 750-4000 400-2000 12 200 (8.0) 100 (4.0)
138 750-4000 400-2000 12 200 (8.0) 100 (4.0)
161 750-4000 400-2000 10 225 (9.0) 100 (4.0)
230 1000-5000 500-2500 10 275 (11.0) 125 (5.0)
345 1000-5000 500-2500 10 350 (14.0) 150 (6.0)
Table 2.3 - 1 Rated Voltage, Conductor Size Range, Insulation Eccentricity Limits,
Nominal Internal AC Stress Limits and Nominal External AC Stress Limits
Design Stress Calculations
Electrical field stress can be calculated using the
following:
and
Where,
V = rated voltage line to ground
Einner = electrical stress at inner insulation surface
Eouter = electrical stress at outer insulation surface
R = insulation radius
r = conductor radius
Cable Diagram
Electrical Stress - Accessories
Field stress can have a significant effect on
accessories (splices and terminations) since cables
can withstand higher stress levels.
Splices and terminations are designed for a smooth
transition of field stress between the cable and
accessories.
If not controlled by joint design or termination
stress cone, a high electrical stress will eventually
cause a breakdown and failure at the interface
between cable and joint or termination.
Typical Electrical Stress on a Splice
Cable Capacitance
A cable can be viewed as a large capacitor in which
conductor and outer sheath are two parallel plates
separated by insulation.
The actual capacitance will depend upon the cable’s
geometry and type of insulation used.
Capacitance for a single conductor cable can be
calculated using the following equation:
Capacitance Calculation
Where,
C – capacitance
– dielectric constant
T – insulation thickness
d – diameter over conductor shield
Charging Current
Is the current that flows when voltage is applied
to a cable conductor.
Charging current is caused by the cable’s
capacitive reactance and decreases
exponentially with time.
As a rule of thumb, a typical cable can draw 1
amp per 1000 feet.
Charging Current (cont.)
Charging current can be calculated using:
Where,
Ic – charging current per 1000 feet
f – frequency
C – capacitance
V – line-to-ground voltage
Dielectric Loss
When voltage is applied to a perfect dielectric
no power loss occurs since the capacitance
current (Ic) leads the applied voltage by 90°.
For practical dielectric materials there is always
a small current (Iv) in phase with the applied
voltage.
Summing the above two vectors results in a
current (I) that leads the voltage by less than
90°, as shown in the following diagram:
Dielectric Loss Diagram
Iv
Iv
Ic I
V
Fd
d = 900 - Fd
Dielectric Power Factor
The parameter cos Fd is known as the power factor and it
provides a useful measure of the cable’s dielectric quality.
The power factor is also referred to as tan d, and should not
be confused with the supply power factor.
Under normal operating conditions cos Fd should be kept
very small, otherwise the resulting power loss can cause the
insulation to increase in temperature.
An increase in material temperature can cause more power
loss, which contributes to a further temperature increase.
Should the cable continue to operate under this condition the
temperature will continue rising until the insulation breaks
down or fails.
Typical Values (tan d)
Typical tan d values for cable insulation are listed
below:
XLPE – 0.00015 to 0.00035
Oil – 0.0018 to 0.003
EPR – 0.001 to 0.003
Skin Effect
For DC circuits, current flows uniformly throughout
the conductor cross section.
For AC circuits, as frequency increases the non-
uniformity of current density becomes large.
Current flows more densely towards the outer
conductor surface than at the center.
The phenomenon that causes this non-uniform
current distribution is known as Skin Effect.
Skin Effect Losses
Skin effect occurs because magnetic flux linkage
of current near conductor center is greater than
that of current flowing at outer conductor surface.
Skin effect is a function of frequency, conductor
size, and relative resistance of conductor material.
Skin Effect increases as conductor size and
frequency increase.
Skin effect decreases as the conductors relative
resistance decreases.
Skin Effect Reduction
Skin effect can be reduced with suitable conductor
construction - such as conductor stranding and
use of segmented conductors.
In some underground cable designs the center
strand is omitted and replaced with a non-
conductive core since the center stand only carries
a very small current.
Proximity Effect
Proximity effect is similar to the skin effect.
Proximity effect is the additional losses caused by
magnetic fields from parallel conductors - through
eddy currents and current displacement effects in
conductors and cable sheaths.
Since three conductor cables are only used for
medium cross sections, and single conductor
cables with large cross sections have sufficient
axial spacing, these losses have less influence in
cable installations.